Sulaiman Khan
email: sulaiman.khan@ul.ie

Stéphane Le Calvé
email: slecalve@unistra.fr

David Newport
email: david.newport@ul.ie

A REVIEW OF OPTICAL INTERFEROMETRY TECHNIQUES FOR VOC DETECTION

Keywords: Gas sensor, optical gas sensor, Fabry-Perot interferometer, Sagnac interferometry, Mach-Zehnder interferometer, Zeolite

Exposure to volatile organic compounds (VOCs) is widely associated with adverse health effects.

Detection and monitoring of VOCs are important for maintaining safe and healthy industrial and domestic environments. Interferometry is a highly-sensitive optical measurement technique that has been widely applied to a vast range of physical parameters from the speed of light to temperature and has also been used to detect VOCs at the sub-ppm range. Owing to the vast range of interferometer arrangements and processing techniques, this review assesses the different approaches adopted in detecting VOCs. Different interferometry arrangements including the Fabry-Perot interferometry, Sagnac interferometry and Mach-Zehnder interferometry are reviewed for VOC detection, including the different sensing films and materials employed. We present the basis of each technique, applications and limitations. The different interferometry techniques are summarized by comparing the sensitivity, limit of detection, linearity, response time and the challenges of current interferometry techniques.

Lastly, prospects to realize a miniaturized, high-sensitive and multiplex interferometric sensors based on the recent technology are suggested.

Introduction

VOCs are organic compounds that have a high vapour pressure at room temperature, i.e. they readily evaporate into a gaseous phase at room temperature. The United States Environmental Protection Agency (EPA) defines VOCs as "organic chemical compounds whose composition makes it possible for them to evaporate under normal indoor atmospheric conditions of temperature and pressure" [START_REF]Technical overview of volatile organic compounds[END_REF].

The EU uses the boiling point for the definition with a VOC being "any organic compound having an initial boiling point less than or equal to 25 o C measured at standard atmospheric pressure of 101.3 kPa" [START_REF]Directive 2004/42/CE of the European Parliament and of the Council of 21 April 2004 on the limitation of emissions of volatile organic compounds due to the use of organic solvents in certain paints and varnishes and vehicle refinishing products and amendi[END_REF]. Common VOCs are acetaldehyde, acetone, benzene, carbon tetrachloride, ethyl acetate, heptane, hexane, isopropyl alcohol, formaldehyde, naphthalene, styrene, toluene and xylenes [START_REF] Koppmann | Chemistry of Volatile Organic Compounds in the Atmosphere[END_REF][START_REF] Khan | Removal of Volatile Organic Compounds from polluted air[END_REF]. Most VOCs, such as the aromatic compounds, alcohols, ketones and aldehyde are toxic. Benzene, Toluene, Ethylbenzene and Xylene (BTEX) are aromatic hydrocarbons, considered as some of the most hazardous pollutants among VOCs.

VOCs can be found in both indoor and outdoor environments. Outdoors, the common sources are emission from automobiles, vegetation, petroleum products and incomplete fuel combustion [START_REF] Gallego | Indoor and outdoor BTX levels in Barcelona City metropolitan area and Catalan rural areas[END_REF][START_REF] Pilidis | BTX measurements in a medium-sized European city[END_REF]. The natural sources are mainly emissions from terrestrial and ocean sources [START_REF] Zhang | Adsorption of VOCs onto engineered carbon materials: A review[END_REF]. BTEX levels are typically greater in indoor environments [START_REF] Schneider | Indoor and outdoor BTX levels in German cities[END_REF]. In public places, the sources of BTEX are coal burning, cigarette smoking, combustion and cleaning products, laser printing, floor adhesives, paint, wood panelling and traffic emissions [START_REF] Wallace | Major sources of benzene exposure[END_REF] [10] [START_REF] Ayoko | Volatile Organic Compounds in Indoor Environments[END_REF].

Exposure to BTEX and other VOCs is one of the reasons for sick building syndrome [START_REF] Kawamura | Development of a novel hand-held toluene gas sensor: Possible use in the prevention and control of sick building syndrome[END_REF]. Benzene is particularly toxic and carcinogenic [16.17]. Acute occupational exposure to benzene can cause narcosis, headache, dizziness, drowsiness, confusion, tremor and loss of consciousness [START_REF] Who | Exposure to Benzene: A Major public health concern[END_REF]. [START_REF] Baan | A review of human carcinogens--Part F: chemical agents and related occupations[END_REF] [START_REF]Benzene (IARC Summary &amp[END_REF]. The International Agency for Research on Cancer has classified benzene as carcinogenic to humans [START_REF] Pyatt | A review of the potential association between childhood leukemia and benzene[END_REF].

Exposure to toluene can affect the central nervous system, liver, kidney and skin [START_REF] Patnaik | A comprehensive guide to the hazardous properties of chemical substances[END_REF]. Exposure to xylene at a low level can cause fatigue, tremor, nervous system, respiratory, kidney and cardiovascularrelated problems [START_REF] Kandyala | Xylene: An overview of its health hazards and preventive measures[END_REF]. Inhalation of acetone is associated with headache, dizziness and dermatitis. The symptoms of cyclohexane exposure include irritation of respiratory tract, skin and eyes. Formaldehyde is toxic and allergenic [START_REF] Kim | Effect of formaldehyde on the expression of adhesion molecules in nasal microvascular endothelial cells: The role of formaldehyde in the pathogenesis of sick building syndrome[END_REF]. It is widely used in industry due to its high reactivity and low cost. Highlevel exposure can cause nasopharyngeal cancer and can damage cells and tissues [START_REF] Castro-Hurtado | Conductometric formaldehyde gas sensors. A review: From conventional films to nanostructured materials[END_REF]. The World Health Organisation has set its maximum exposure limit of 100 µg/m 3 (81.4 ppb) averaged over 30 min [START_REF] Bunkoed | Sol-gel based sensor for selective formaldehyde determination[END_REF]. Methanol exposure can cause nausea, abdominal pain, shortness of breath and dizziness [START_REF] Patnaik | A comprehensive guide to the hazardous properties of chemical substances[END_REF].

Different developed countries and organization have set increasingly more stringent legislation and regulation for exposure limits to harmful VOCs. The EU adopted a maximum exposure limit of 5µg/m 3 (1.6ppb) for benzene effective from 2010 [START_REF] Eu | European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe[END_REF]. Different organizations, for instance, National Institute of Occupational Safety and Health (NIOSH) and Occupational Safety and Health Administration (OSHA) have established exposure limits to different VOCs as summarized in Table 1 [START_REF] Barson | NIOSH-Pocket Guide to Chemical Hazards[END_REF]. The recommended exposure threshold values range from sub-ppb (0.6 ppb for benzene) to 1000 ppm for ethanol. Detection of VOCs at trace level requires a highly sensitive and accurate sensing method. Different sensing techniques have been reported to detect and monitor VOCs, for instance Photo-ionization detectors (PID) [START_REF] Spinelle | Review of Portable and Low-Cost Sensors for the Ambient Air Monitoring of Benzene and Other Volatile Organic Compounds[END_REF], electrochemical gas sensors [START_REF] Sekhar | Detection of Harmful Benzene, Toluene, Ethylbenzene, Xylenes (BTEX) Vapors Using Electrochemical Gas Sensors[END_REF], micro gas chromatography (µ-GC) [START_REF] Haghighi | Through the years with on-a-chip gas chromatography: a review[END_REF] , piezoelectric-based gas sensors i.e. surface acoustic wave [START_REF] Zhu | Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor detection[END_REF], quartz crystal microbalances [START_REF] Kumar | Phthalocyanines based QCM sensors for aromatic hydrocarbons monitoring: Role of metal atoms and substituents on response to toluene[END_REF], and tuning fork [START_REF] Chen | A new sensor for the assessment of personal exposure to volatile organic compounds[END_REF], gravimetric-based gas sensors [START_REF] Fanget | Gas sensors based on gravimetric detection -A review[END_REF], metal-oxide semiconductor (MOS) gas sensors [START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A review[END_REF] and optical sensor such as colorimetric gas sensors [START_REF] Askim | Optical sensor arrays for chemical sensing: the optoelectronic nose[END_REF], non-dispersive infrared gas sensors [START_REF] Chen | A new sensor for the assessment of personal exposure to volatile organic compounds[END_REF] and UV spectrophotometry gas sensors [START_REF] Allouch | Portable, miniature, fast and high sensitive real-time analyzers: BTEX detection[END_REF]. Each technique has its advantages and shortcomings depending upon the nature of molecules and the environment. MOS sensors usually operate at high temperature with high sensitivity to external humidity and gases like CO2. Electrochemical sensors have a National Institute of Occupational Safety and Health (NIOSH)-recommended exposure limit is an exposure for 8 or 10-h time weighted-average. b Occupational Safety and Health Administration (OSHA) permissible exposure limit are expressed as a time-weighted average; the concentration of a substance to which most workers can be exposed without adverse effect averaged over a normal 8-h workday or a 40-h workweek. c American Conference of Governmental and Industrial Hygienists (ACGIH) threshold limit value are expressed as a timeweighted average; the concentration of a substance to which most workers can be exposed without adverse effects. During the last few decades, tremendous advancements have been observed in the area of optical sensors for gas detection applications. Optical gas sensors can detect gas molecules with high sensitivity and selectivity. They are non-destructive, have quick response times and do not generally suffer from drift.

They have little cross-response to other gases, are inherently reliable and have high specificity [START_REF] Hodgkinson | Optical gas sensing: a review[END_REF].

Various types of optical detection are available with direct spectrometry, and reagent/film mediated optical sensor are the common types of sensors used for sensing applications [35][36]. In film/reagent mediated sensing system, a change in the optical response of an intermediate agent is used to quantify the analytes absorbed/adsorbed on the sensing surface and then to deduce the gas concentrations once calibration is performed [START_REF] Mcdonagh | Optical chemical sensors[END_REF]. Interferometry applied gas detection can be considered as a film-mediated optical sensing method because it is the variation in the optical path length of the sensing film which is monitored.

Interferometry is an important investigative and detection technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, spectroscopy, biosensing, surface profiling and chemical sensing [START_REF] Bunch | The history of science and technology[END_REF][38] [START_REF] Hirsch | Low-Coherence Interferometric Fiber-Optic Sensors with Potential Applications as Biosensors[END_REF]. There is a wide range of interferometric configurations and processing techniques that have been deployed for VOCs detection. techniques applied as well as the different sensing films and materials used will be discussed. Points for improvement and possible miniaturization will be considered in perspective.

Interferometry

Optical interferometry is a well-known investigative technique applied to a number of applications for highly precise measurements. Examples range from Michelson and Morley's aether drift experiment to the extraordinary sensitivity of the modern gravitational-wave detector [START_REF] Riles | Recent searches for continuous gravitational waves[END_REF]. During the last few decades, interferometry has been applied to several applications in nano-science and technology for highly sensitive measurements due to being in-situ, non-contact and non-destructive. Recent advancements in computation, data acquisition and data analysis have made it possible to achieve a highly-precise and sensitive measurement using interferometry. Fringe analysis and phase retrieval processes became faster with the use of digital computers and further enhancement in future is foreseen [41][42]. For instance, the displacement sensitivity of pico-meter has been demonstrated using interferometry [43][44]. Interferometry has also been applied for highly precise measurement of temperature [START_REF] Garvey | Full field measurement at the micro-scale using micro-interferometry[END_REF], pressure [START_REF] Newport | Digital interferometry: techniques and trends for fluid measurement[END_REF], gas concentrations [START_REF] Hubert | Hydrogen sensors -A review[END_REF], growth rate in crystallization [START_REF] Bommareddi | Applications of Optical Interferometer Techniques for Precision Measurements of Changes in Temperature[END_REF], refractive index of materials [START_REF] Zhang | Ultrahigh-accuracy measurement of refractive index curves of optical materials using interferometry technology[END_REF], acoustic field [START_REF] Wang | Application of optical interferometry in focused acoustic field measurement[END_REF], surface profile [START_REF] Wang | Review of surface profile measurement techniques based on optical interferometry[END_REF], vibration [START_REF] Aizen | A compact real-time high-speed highresolution vibrometer, surface profiler and dynamic focus tracker using three wavelengths parallel phase-shift interferometry[END_REF] and bio-sensing [START_REF] Makarona | Point-of-Need bioanalytics based on planar optical interferometry[END_REF][54],

for different applications.

Generally, for sensing application, two-beam amplitude-division interferometry is employed [START_REF] Merzkirch | Flow visualization[END_REF]. The working principle of two-beam amplitude-division interferometry is that two light beams of the same frequency, constant phase difference and same direction are recombined to form an interference pattern, which can be expressed as,

Where I1 and I2 are the beam intensities and ϕ is the phase difference between the two beams, which is given by,

Where n and L is the refractive index and the length over which the phase change occurs respectively.

N value varies with technique. For FP interferometer N=4 and Mach-Zehnder Interferometer N=2.

Interferometry quantifies the phase changes induced by the measurand, i.e. temperature, pressure, strain, concentration and refractive index. The objective of interferometry is to measure the phase difference and relate it with the measurand.

Interferometry for VOC detection assesses changes in sensing materials, typically films. The sensing film is usually a polymer or microporous-silicates, selected according to the desired target molecules.

When a VOC interacts with the gas sensing film, it changes volume (i.e. thickness) and/or optical properties (i.e. refractive index) of the film due to absorption/adsorption. These perturbations change the effective optical path length, which leads to a quantifiable phase shift, as shown in figure 1. By demodulating the phase shift, the concentration of VOCs can be inferred. There are a number of optical configurations for interferometry. The common interferometry techniques employed for VOC detection are the Fabry-Perot, Sagnac, Mach-Zehnder and Pohl interferometers.

Fabry-Perot interferometer

The Fabry-Perot (FP) interferometer, also called the Fabry-Perot etalon, is composed of two mirrors of reflectance R1 and R2 separated by the cavity of length L, as shown in Figure 2(A). The FP interferometer works as a reflective device and is approximately equivalent to a two-beam interferometer. It is very sensitive to perturbation that changes the optical path length between the two mirrors. The FP interferometer is compact, has a fast response and is readily integrated [START_REF] Yin | Fiber Optic Sensors[END_REF]. It has been applied to several applications such as temperature, pressure, humidity, stress and strain, vibration, bio-sensing, gas detection, magnetic fields, current and airflow measurement [START_REF] Huang | Research Progress on F-P Interference-Based Fiber-Optic Sensors[END_REF]. The reflectance and transmittance for low-finesse FP is given by [START_REF] Paper | Fiber Optic Smart Structures[END_REF],

(

RFP is the ratio of power reflected by FP interferometer to the incident power, where ϕ is a phase shift in the interferometer which is represented as,

Where L is the length of the FP cavity and n is the refractive index of the FP cavity. For resonant wavelength, the reflection spectrum satisfies the condition,

Where k = 1, 2, 3,... and nL is the optical path length of the FP cavity. The dip wavelength λdip is given by ( 6)

VOCs concentration can be measured by monitoring the wavelength shift of the reflective resonant spectrum. The experimental setup used for gas sensing consists of a light source and detector, fibercoupler and gas enclosure, where the FP sensor is exposed to gas molecules, as shown in figure 2(B).

Sagnac interferometer (SI)

The Sagnac interferometer is used in filtering, communications and sensor applications due to its simple design, ease of fabrication and lower susceptibility to external noise [START_REF] Fu | Pressure sensor realized with polarization-maintaining photonic crystal fiberbased Sagnac interferometer[END_REF]. It has been applied for gas detection [START_REF] Hubert | Hydrogen sensors -A review[END_REF], pressure [START_REF] Fu | Pressure sensor realized with polarization-maintaining photonic crystal fiberbased Sagnac interferometer[END_REF], temperature [START_REF] Shao | Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer[END_REF], humidity [START_REF] Ascorbe | Recent Developments in Fiber Optics Humidity Sensors[END_REF], and strain measurements [START_REF] Gu | Temperature compensated strain sensor based on cascaded sagnac interferometers and all-solid birefringent hybrid photonic crystal fibers[END_REF]. The Sagnac interferometer is usually applied in a ring configuration, or fiber loop mirror for gas sensing, as shown in Figure 3(B). A fiber coupler split the input light in two and then recombines the two counterpropagating beams. A section of birefringent fiber (Polarization-Maintaining Fiber (PMT)) is inserted in the loop which causes interference between the two counter-propagating waves when it interacts with measurand as shown in figure 3(A). The output of the transmitted intensity can be expressed as [START_REF] Zhang | Highly-Sensitive Temperature Sensor Using a Hi-Bi Fiber Tip Probe[END_REF], Where L is the length of the birefringent fiber, B is the refractive index difference between the fast axis and slow axis of birefringent fiber, λ is the wavelength of light beam in free space. When the phase difference satisfies the condition i.e.

then a resonance dip can be observed in the transmitted spectrum [START_REF] Xu | Sagnac interferometer hydrogen sensor based on panda fiber with Pt-loaded WO_3/SiO_2 coating[END_REF], which is given by ( 9)

Where k is an integer value. The wavelength spacing between the transmission dips can be expressed as, [START_REF] Ilgen | Aromatic hydrocarbons in the atmospheric environment: Part I. Indoor versus outdoor sources, the influence of traffic[END_REF] For VOC detection, the B and L values of the sensing film change upon exposure to VOCs. Thus the wavelength shift of polarization mode interference is modulated by the change in PMF length and/or refractive index. By demodulating the wavelength shift, the sensitivity towards target VOCs can be determined.

Mach-Zehnder interferometer

The Mach-Zehnder (MZ) Interferometer was developed independently by Ludwig Zehnder [START_REF] Zehnder | Ein neuer interferenzrefraktor[END_REF] and

Ludwig Mach [START_REF]Uber einen interferenzrefraktor[END_REF]. It is one of the most common interferometric techniques employed for sensing applications [START_REF] Faryad | Measurement Science and Technology Integrated optical sensors for the chemical domain Recent citations Cuma Tyszkiewicz et al-Przemyslaw Struk et al-Integrated optical sensors for the chemical domain[END_REF]. The MZ interferometer provides a flexible and robust platform for sensing applications that can be fiber-coupled and implemented on a microchip. There are three different variants of the MZ interferometer: Open path, fiber-based, and planar waveguide-based interferometers are shown in Figure 4. The MZ interferometer has been exploited for temperature [START_REF] Baby | Measurement of Temperature Distribution around a Vertical Fin by Mach-zehnder Interferometry[END_REF][69], pressure [START_REF] Statkiewicz-Barabach | Hydrostatic Pressure and Temperature Measurements Using an In-Line Mach-Zehnder Interferometer Based on a Two-Mode Highly Birefringent Microstructured Fiber[END_REF], strain [START_REF] Van-Pham | Formation and relaxation of the elastic strain generated by photocuring in polymer blends monitored by Mach-Zehnder interferometry[END_REF][72], seismology [START_REF] Kamenev | Fiber-optic seismometer on the basis of Mach-Zehnder interferometer[END_REF], vibration [START_REF] Liu | Distributed Fiber-Optic Sensors for Vibration Detection[END_REF], current [START_REF] Kumar | Simultaneous measurement of current and temperature by using an all-fiber interferometric cost-effective and non-destructive sensing scheme[END_REF], magnetic field [START_REF] Luo | A novel optical fiber magnetic field sensor based on Mach-Zehnder interferometer integrated with magnetic fluid[END_REF], water salinity [START_REF] Qian | Review of salinity measurement technology based on optical fiber sensor[END_REF] and bio-sensing applications [START_REF] Bastos | Integrated Optical Mach-Zehnder Interferometer Based on Organic-Inorganic Hybrids for Photonics-on-a-Chip Biosensing Applications[END_REF].

In MZ interferometers, a coherent collimated light beam is split into two; one acts as a reference arm while the other acts as a sensing arm. The two light beams are then recombined to obtain an interference signal, as shown in figure 4. The sensing arm is exposed to the measurand (temperature, pressure, gas molecules) which causes modulation of the interference signal. The phase difference for MZ interferometer is given by [START_REF] Ayoko | Volatile Organic Compounds in Indoor Environments[END_REF] Where n1 and n2 are effective refractive indexes of the two optical paths, L1 and L2 are transmission length of the two optical paths and ϕo is the initial phase. The phase difference is then demodulated to measure the concentration of gas molecules.

Pohl interferometer

The Pohl interferometer is an amplitude splitting interferometer in which light beam from a point source is reflected from the surfaces with different optical properties (refractive index, reflectivity) forming an interference pattern [START_REF] Barthwal | Construction of an inexpensive molecular iodine spectrometer using a self-developed Pohl wavemeter around 670 nm wavelength[END_REF]. The Pohl configuration offers a simple, quick and efficient approach for measuring the wavelength [START_REF] Hariharan | Basics of Interferometry[END_REF]. It has been applied for shop testing conditions, phase measurement and parallelism measurement of transparent surfaces [START_REF] Gresty | Optical Shop Testing[END_REF][81] [START_REF] Wasilik | Measurement of Parallelism of the Surfaces of a Transparent Sample Using Two-Beam Nonlocalized Fringes Produced by a Laser[END_REF]. For sensing applications, a simple Pohl configuration is constructed by depositing a sensing film on a substrate. For instance in Figure 5, the light reflected from the three interfaces form an interference pattern on the screen, which can be represented as [START_REF] Martínez-Hipatl | Detection of volatile organic compounds by an interferometric sensor[END_REF],

where k is the wavenumber, y is the position of the fringe on the screen, θi is an angle between I1 and I3

and Δϕ is the phase difference, given by.

(

) 14
Where n2 and n3 are the refractive indexes of glass substrate and PDMS film respectively. When VOCs interact with sensing film, d1 and n3 changes, which causes a change in Δϕ inducing a shift in the fringes pattern. properties in contrast to polymers with organic backbone, with a large free volume due to absence of crystallites which facilitate diffusion [START_REF] Clarson | Siloxane polymers[END_REF]. It undergoes volume and/or refractive index changes when it interacts with VOCs. Although the swelling of PDMS is not desired in many microfluidics applications, this property can be exploited for sensing applications when employed as a sensing film [START_REF] St-Gelais | A Fabry-Perot refractometer for chemical vapor sensing by solid-phase microextraction[END_REF][90].

Zeolite sensitive thin film

Zeolites are microporous alumino-silicate crystal with uniform nanometric-scale pores systems. It discriminates molecules through size exclusion or shape selectivity. It can selectively adsorb molecules depending on the crystal structure, framework Si/Al ratio and type of framework cations [START_REF] Zhang | Zeolite-Fiber Integrated Optical Chemical Sensors for Detection of Dissolved Organics in Water[END_REF], and has excellent chemical and optical properties. Zeolites have been extensively investigated for use in separation [START_REF] Bowen | Fundamentals and applications of pervaporation through zeolite membranes[END_REF] and electrochemical sensors [93][94]. It is an excellent material for optical sensors due to a high surface-to-mass ratio, chemical selectivity and flexibility for surface chemistry modification.

The optical properties of zeolites changes when molecular species are loaded on it depending on the type, amount and state of the molecules [START_REF] Thomson | Handbook of Zeolite Science and Technology Edited by[END_REF]. Zeolites exhibit different sensitivities towards different molecules depending on the molecule sieve. Thin films of zeolite integrated with different optical methods have been successfully applied for trace organic vapours detection [START_REF] Wu | Characteristic study on volatile organic compounds optical fiber sensor with zeolite thin film-coated spherical end[END_REF][97].

Application of Interferometry for VOC detection

Different interferometric techniques have been applied for detection of various gases, for instance, hydrogen [START_REF] Hubert | Hydrogen sensors -A review[END_REF], CO2 [START_REF] Ma | CO2 Gas Sensing Using Optical Fiber Fabry-Perot Interferometer Based on Polyethyleneimine/Poly(Vinyl Alcohol) Coating[END_REF], ammonia [START_REF] Hao | Graphene-Based Ammonia-Gas Sensor Using In-Fiber Mach-Zehnder Interferometer[END_REF], N2O [START_REF] Raymond | Miniature cavity for in situ millimeter wave gas sensing: N2O and CH3OH detection[END_REF], HS [START_REF] Deng | Trace hydrogen sulfide gas sensor based on tungsten sulfide membrane-coated thin-core fiber modal interferometer[END_REF] and a number of VOCs. Recently, there is a growly interest in the use of interferometry for VOC detection. We review here different interferometric approaches deployed for VOC detection, as summarized in Table 3. Fibre-based interferometry is discussed first followed by chip-based interferometers and then other types of interferometers.

An FP gas sensor was developed by Liu et al. [START_REF] Liu | Highly versatile fiber-based optical Fabry-Pérot gas sensor[END_REF] using a metal-coated FP cavity for detection of VOCs such as hexanol, methanol and acetone. The interferometer was fabricated by depositing silver and a gas-sensing film sequentially at the end of an optical fiber, and FP cavity was realized by silverpolymer and polymer-air interfaces as shown in figure 6. Polyethylene glycol (PEG-400) and Norland optical adhesive (NOA-81) were applied as a sensing film due its optical and chemical properties. The sensitivity of 3.5 picometers(pm)/ppm with a detection limit of 1 ppm was reported using PEG-400 as a sensing film. The design was extended to integrate with µGC for on-column detection of toluene, decane, methanol and Dimethyl-methyl-phosphonate (DMMP) [START_REF] Liu | Fabry-Pérot cavity sensors for multipoint on-column micro gas chromatography detection[END_REF]. The sensitivity was enhanced to sub-nano gram level, and limit of detection down to 50 pg (105.6ppb for µGC with length and diameter of 1.9m and 0.25mm respectively) was achieved for DMMP. A sensitivity of 4.75 mV/ng and 77 mV/ng was reported for decane and DMMP respectively. A PMMA (Polymethyl methacrylate) functionalized FP interferometer was developed by Yu et al. [START_REF] Yu | Highly sensitive and selective fiber-optic Fabry-Perot volatile organic compounds sensor based on a PMMA film[END_REF] for ethanol and acetone detection with good selectivity. The FP cavity was created by coating PMMA on the end of a glass tube and then inserting the SM fiber into the tube, as shown in figure 11. A sensitivity of 2.7 pm/ppm and 2.17 pm/ppm was measured for ethanol and acetone respectively. The sensor distinguished different VOCs with good repeatability and showed low sensitivity towards inorganic molecules such as NH3, CO and H2. A Sagnac interferometer functionalized with PDMS was designed by Wu et al. [START_REF] Wu | Sagnac interferometer fiber volatile organic compounds sensor based on PDMS[END_REF] for detection of 2propanol with a limit of detection of 19.3ppm. A PMF (length =8 cm) functionalized with PDMS was inserted between the two ends of SM fiber and was exposed to VOCs. The mixing ratio of the PDMS precursor and curing agent was investigated, and sensitivity of 0.001nm/ppm and 0.002 nm/ppm was found for mixing ratios of 10:1 and 5:1 respectively. A sensitivity of 1.03 pm/ppm was observed for concentrations in the 0-6000 ppm range with a response time of 1 min.

A chip-size VOCs sensor was developed by Reddy et al. [START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF] using FP array inside a μGC microfluidic channel. The FP array was integrated with the μGC column and tested for four VOCs mixture: acetone, methanol, heptane and toluene as shown in figure 12. Each FP sensor (200 μm × 200 μm with a depth of 1.3 μm) was fabricated inside a microfluidic channel by coating a thin polymer layer into pre-etched wells in a silicon substrate as shown in figure 13. Four polymers (i.e. OV-1(PDMS), OV-73 (diphenyldimethylsilicone), OV-215(trifluoropropylmethylsilicone) and OV-1701 (dimethylphenyl cyano substituted)) were coated as a sensing film, and each polymer exhibited a different response to different analytes. The VOCs mixture was separated by μGC and then injected into the sensor module. The airpolymer and polymer-substrate interface formed an FP cavity whose resonance length changed when exposed to VOCs. The sensor had good sensitivity, and detection limits of 0.79pg (28 ppb) and 0.64pg (25 ppb) were obtained for toluene and heptane respectively. Pattern analysis was used to analyse the data from the multiple FP sensors to discriminate the different VOCs. The sensitivity was improved compared to two FP sensors array with PDMS and SU-8 layer developed by the same group [START_REF] Reddy | On-chip Fabry-Pérot interferometric sensors for micro-gas chromatography detection[END_REF]. In that study, a sensitivity of 2900 μV/ng g (limit of detection of 1.7ppm) was achieved for toluene using a PDMS layer. The CMOS imager provides quantitative information about the polymer response to the vapour analytes. Reprinted from [START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF] with permission from Royal Society of Chemistry. A novel on-chip VOCs sensor was demonstrated by St-Gelais et al. [START_REF] St-Gelais | Gas sensing using polymer-functionalized deformable Fabry-Perot interferometers[END_REF] using deformable silicon FP interferometer. The FP interferometers were functionalized with PDMS and PDMSpolydiphenylsiloxane copolymer (PDMS-PDPS) and were tested to detect m-xylene and cyclohexane.

A deformable FP sensor was constructed using silicon air Bragg reflectors with a sensing polymer between the two mirrors as shown in figure 14. The monolithic integration of the microfluidics system and optical fiber alignment grooves was realized by the vertical mirrors and in-plane optical axis. A detailed analytical model with numerical analysis was developed, and it was found that mechanical g μV/ng(micro-volt/nano-gram)

deformation was the dominant sensing mechanism. The sensor demonstrated high sensitivity of 0.023 nm/ppm for m-xylene. A limit of detection of 34ppm was achieved which was limited by the flowmeters used in the study. According to the author, a limit of detection of 1.6 ppm and 6.3 ppm can be obtained for m-xylene and cyclohexane respectively. The on-chip interferometer can potentially be extended to multiple parallel interferometers without the need of laborious alignment process. Martinez et al. [START_REF] Martínez-Hipatl | Detection of volatile organic compounds by an interferometric sensor[END_REF] developed an open path interferometer using Pohl configuration for detection of alcohol, ether and alkanes. A PDMS film (thickness, 8 μm) was deposited on the backside of glass while front side was exposed to a light beam. The reflected light from three interfaces, i.e. air-glass, glass-PDMS and PDMS-air formed an interference pattern. The sensor showed a sensitivity of 0.14 nW/ppm h and 0.12 nW/ppm for propyl acetate and octane respectively for concentration range 0-24000 ppm with good reproducibility. The sensor demonstrated good selectivity towards esters compared to alcohols.

The response of the sensor was dominated by the volume change in comparison to refractive index change. It was found that a higher molecular volume caused a higher volume change of PDMS due to molecular diffusion.

Spectral interferometry was used to detect alkanes and halogenated hydrocarbons by employing a thin film of polysiloxane with good linearity [START_REF] Gauglitz | Chemical and biochemical sensors based on interferometry at thin (multi-) layers[END_REF]. The detection limits down to 100 ppm with time response less than 1 sec was demonstrated. A good linearity of 40-2600 ppm and up to 10 4 ppm was obtained using DMPS layer for tetrachloroethene and perfluroethylene respectively.

A Michelson interferometer was tested by Marzuarman et al. [START_REF] Marzuarman | Investigation of michelson interferometer for volatile organic compound sensor[END_REF] for detection of benzene and ethanol without employing sensing film. The number of fringes shift (pixels) was used to calculate the concentration of gas molecules. A sensitivity of 0.006 pixels/ppm and 0.015 pixels/ppm with good linearity of 1611-32210 ppm and 964-19290 ppm was found for ethanol and benzene respectively.

Comparison and challenges in the interferometric sensors

Different interferometry techniques have been applied as discussed for trace level detection of VOCs with good sensitivity, i.e. limit of detection from 28ppb to 20ppm have been obtained. Most of the techniques are based on FP interferometry. Optical-fiber and chip-based interferometers were designed to achieve miniaturization, robustness, multiplexing, ease of fabrication and integration. A comprehensive comparison of different interferometric techniques is summarized in Table 3. Different performance parameters, i.e. sensitivity, linearity and time response of each technique with sensing film employed are presented. Most of the techniques used are fiber-base interferometry due to its ease of operation, fabrication and integration with the different component of the interferometer. A detection limit down to 28 ppb was demonstrated for toluene using FP interferometer coupled with µGC column [START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF].

Interferometric based VOC detection techniques are simpler and have a flexible structure, good mechanical properties, easy fabrication process without the need of different optical fiber processes (i.e.

polishing, chemical etching, and tapering process). However, demodulation of interference spectrum and periodic output signal are the drawbacks with the use of interferometry for gas sensing applications.

For instance, the existence of multi-mode in MZ interferometer will make the extraction of peaks from the spectrum prone to errors. In Sagnac interferometer, the fiber is prone to external factors like temperature, humidity and vibration which can introduce errors in the measurement. In FP interferometer, the disadvantages are the large transmission losses, cavity size limitation due to coupling losses and offset of the fiber end-faces.

Existing challenges of the sensors

Sensing film

The sensitivity of an interferometer depends on the interaction between the analyte and the sensing film, which in turns depend on several factors including the polarity of the gas and sensing film, the volatility of the analyte, the presence of functional groups, the gas molecular weight and size. Uniformity, surface roughness, thickness, thermal stability and development process of the sensing film also affect the response of film-mediated sensors [START_REF] Liu | Characteristics of an Optical Fiber Hydrogen Gas Sensor Based on a Palladium and Yttrium Alloy Thin Film[END_REF]. For instance, the mixing ratio of PDMS elastomer and hardener affects the absorption of gases into the PDMS. Ning et al. [START_REF] Ning | PDMS-coated fiber volatile organic compounds sensors[END_REF] investigated the effect of mixing ratio of elastomer and hardener for PDMS and found that better sensitivity is achieved at a ratio of 5:1 as shown in figure 16. The curing temperature can also change the optical properties of PDMS [START_REF] Wang | Fabrication and characterization of PDMS thin film[END_REF]. The thermal and chemical stabilities of sensing film can affect the lifetime of the sensor.

The partition coefficient (Kpa) of each polymer-analyte combination is taken into account for selecting a sensing film. A higher Kpa value yields a higher sensitivity, but it also increases the retention time (lower diffusion constants) [START_REF] Martos | Estimation of Air/Coating Distribution Coefficients for Solid Phase Microextraction Using Retention Indexes from Linear Temperature-Programmed Capillary Gas Chromatography[END_REF]. St-Gelais et al. [START_REF] St-Gelais | Gas sensing using polymer-functionalized deformable Fabry-Perot interferometers[END_REF] demonstrated that PDMS has a faster time response towards cyclohexane than for m-xylene. However, the sensitivity is decreased four-fold for cyclohexane than m-xylene. The disparity is due to the difference in the partition coefficient values of the film and analyte. Sensitivity is improved by using sensing film with a uniform surface and high surface area for absorption/desorption. For instance, the sensitivity of an FP interferometer was improved by using an arc-shaped head which increased the interfacial area between the zeolite and fiber head. One of the solutions to enhance the gas detection specificity is to use a polymer matrix with different properties, as different polymers respond differently according to the polarity, partition coefficient and refractive index. These polymer matrices can be tailored according to the desired class of analytes.

Figure 16. Comparison of wavelength change for the PDMS with different development process, i.e. mixing ratio of elastomer and hardener. Reprinted from [START_REF] Ning | PDMS-coated fiber volatile organic compounds sensors[END_REF] with permission from Optical Society (OSA) publishing.

Slow time response

The time response and recovery time is a drawback for continuous real-time gas monitoring in interferometric film-mediated sensors. For PDMS, the recovery time is slow which can affect the repeatability and continuous use of the sensor. The time response and repeatability can be improved by using thinner PDMS film or by optimizing the development process of PDMS film. The sensor response is mainly determined by the gas diffusion into the polymer. A thinner sensing film will absorb/desorb the analyte quickly as the diffusion time inside the solid sensing film is proportional to the square of length. For example, the Sagnac interferometer developed by Ning et al. had a slow time response compare to the FP interferometer due to the high volume of PDMS on it [START_REF] Ning | PDMS-coated fiber volatile organic compounds sensors[END_REF].

Temperature and humidity influences

The physiochemical properties of sensing film are sensitive to temperature, and a variation can induce uncertainties in the measurement. The diffusion and partition coefficient of PDMS depends upon temperature [START_REF] Seethapathy | Applications of polydimethylsiloxane in analytical chemistry: A review[END_REF]. Humidity has a negative effect on both the sensitivity and response rate of the sensing film. The adsorbed H2O into PDMS or zeolite reduces the diffusion rate of VOCs into the sensing film.

For a durable sensor, suitable and robust packaging is needed.

Nonlinear output

Non-Linear response of a sensor is not a desired characteristic since the calibration becomes complicated while the quantification becomes risky. In addition, linearity is often subjected to environmental factors like temperature, pressure, humidity and external vibration. For example a non-linear behaviour was observed with FP interferometer for different toluene concentration as shown in figure 17 [START_REF] Kacik | Toluene optical fibre sensor based on air microcavity in PDMS[END_REF]. The reason was attributed to the non-linear relationship between the toluene absorption and physiochemical properties of the sensing film. The film thickness can also cause changes in the response time.

Figure 17. Variation of sensitivity with the gas concentration of toluene. Reprinted from [START_REF] Kacik | Toluene optical fibre sensor based on air microcavity in PDMS[END_REF] with permission from Elsevier.

Data acquisition and analysis

The data analysis of an interferometry based sensor can be resource-intensive. For most of the interferometric techniques, a high-resolution Optical Spectrum Analyzers (OSA) were employed, which is not a cost-effective approach to meet the challenge of miniaturization and portability. Using a high sensitive OSA at a portable scale is a large challenge for future requirements.

Direct phase retrieval techniques, i.e. phase stepping interferometry and heterodyne interferometry is a good candidate for phase measurement [START_REF] Garvey | Full field measurement at the micro-scale using micro-interferometry[END_REF]. A resolution up to λ/1000 can be achieved with heterodyne interferometry [START_REF] Kafri | Fundamental limit on accuracy in interferometry[END_REF]. In phase stepping technique a known phase steps are introduced into one of the beams, and the phase is retrieved by different phase-retrieval methods, for example, three-step method, four-step method, five-step method and Carre method [START_REF] Malacara | Interferogram Analysis For Optical Testing[END_REF]. Phase stepping technique is suitable for liquid flow measurement and is not recommended for gas analysis. In heterodyne interferometry, a frequency shift is introduced into two beams using acoustics optics modulators. A modulated signal with a frequency difference between the two beams is acquired either by CMOS or CCD detector. Recently such a heterodyne interferometry approach has been explored for VOC detection [START_REF] Khan | Study on Absorption of VOCs into PDMS Film using Heterodyne Interferometry for Application of VOCs[END_REF].

Future Prospects and Conclusion

The ideal features for sensors are simplification, miniaturization and multiplexing with high sensitivity and selectivity, wide operating range and good precision. In film-mediated sensors, selectivity is considered as one of the main challenges for real-life applications where the sensing film can absorb/adsorb a number of molecules. The recent progress in artificial intelligence, machine learning and data analysis offers a solution to tackle the selectivity of sensors [START_REF] Hsieh | Intelligent gas-sensing systems and their applications[END_REF]. For instance, the different VOCs can be differentiated by using inverse matrix methods [START_REF] Ning | PDMS-coated fiber volatile organic compounds sensors[END_REF] or by employing artificial neural networks [START_REF] Chen | Concentration Estimator of Mixed VOC Gases Using Sensor Array with Neural Networks and Decision Tree Learning[END_REF]. The addition of a separation column and pre-concentration with the gas sensor can also enhance the sensitivity and selectivity of sensor. The higher sensitivity achieved by Reddy at al. and Liu et al. are attributed to the integration of interferometer with µGC columns [START_REF] Reddy | On-chip Fabry-Pérot interferometric sensors for micro-gas chromatography detection[END_REF] [103] [START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF]. In order to achieve high accuracy and precision of measurement, the effects of external factors such as temperature, pressure, humidity, gas dynamics around the sensor have to be well-understood. Most of the interferometric techniques are single-point which means that the sensor is targeting the molecules in a limited domain. For real-life application, a multipoint detection of gas molecules covering the whole gas domain and a maximum interaction of gas molecules with a sensor surface is desired. The different technique can be employed for enhancing the interaction of gas molecules with sensor. Kacik et al.

increase the interaction of gas molecules with the sensing film by slowly rotating the gas chamber having the FP sensor [START_REF] Kacik | Toluene optical fibre sensor based on air microcavity in PDMS[END_REF]. Some of the new methods such as slow light [START_REF] Zhang | Multi-component gas sensing based on slotted photonic crystal waveguide with liquid infiltration[END_REF], whispering gallery mode [START_REF] Del | Whispering gallery mode microresonators: Fundamentals and applications[END_REF] and Vernier effect [START_REF] Quan | Ultra-high sensitivity Fabry-Perot interferometer gas refractive index fiber sensor based on photonic crystal fiber and Vernier effect[END_REF] can provide a solution to increase the interaction of gas molecules with light beam. The exposure limits for carcinogenic VOCs like benzene and formaldehyde are in the sub-ppb range as shown in table 1. It can be challenging to detect such a low concentration with only interferometry. However, detection of such an ultra-low concentration can be realized by employing interferometry with a pre-processing of the gas mixture for instance separation or pre-concentration.

The recent advancement in material science has given an opportunity to deploy new materials as a sensing film. Recently state-of-art materials such as metal-organic framework (MOF) have shown tremendous potential for gas sensing applications. It can be tailored according to desired applications and can be exploited for gas sensing using interferometric sensors [START_REF] Iswarya | Metal Organic Framework (MOF-5) For Sensing of Volatile Organic Compounds[END_REF][132] [START_REF] Lopez-Torres | Enhancement of the Sensitivity of a Volatile Organic Compounds MOF-Sensor by Means of Its Structure[END_REF].

In this study, different interferometry technique for the detection of VOCs are discussed. The sensor design, working principle and different analytical performance parameters including sensitivity, linearity, and response time were presented in details. Different sensing films and materials employed for the target VOCs, pros and cons of each technique were outlined. New approaches for improving the performance of sensor concerning the material selection, data acquisition and analysis are suggested.

Interferometry has good potential to achieve a highly precise and sensitive gas sensor for domestic and industrial applications. Today the emerging growth in the area of electronics, optics, materials science and data analysis have open new avenues for research and can be extended to interferometry to realize an interferometric sensor with high precision and sensitivity. 0.006 pixels/ppm for ethanol and 0.015 pixels/ppm for benzene. NA No sensing film was [START_REF] Marzuarman | Investigation of michelson interferometer for volatile organic compound sensor[END_REF]

Figure 1 .

 1 Figure 1. Mechanism of interferometry for gas detection. The sensing film is exposed to VOCs which changes its thickness and refractive index resulting in a shift of wavelength.

Figure 2 .

 2 Figure 2. (A) FP interferometer having reflective surfaces with reflectivity of R1 and R2, respectively. Examples of intrinsic and extrinsic FP interferometers. (B) The experimental setup employed for detection of VOCs using FP interferometer.FP interferometer can be categorized into either intrinsic or extrinsic. In both types, a fiber is coupled to transport light from the emitter to the interferometer and then to a photodetector. In an intrinsic FP interferometer, the two mirrors are separated by the length of the fiber, and the measurand interacts with light propagating in the fiber to change the optical path length. In an extrinsic interferometer, the two mirrors are separated by an air gap or other material. The measurand changes the optical path length in a medium other than the fiber. Examples of intrinsic and extrinsic FP interferometers are given in Figure2(A).

Figure 3 .

 3 Figure 3. (A) Schematic of Sagnac interferometer. A Sagnac interferometer is constructed by employing a Polarization Maintaining (PM) fiber in the fiber loop. (B) Schematics of the experimental setup used for VOC detection using Sagnac interferometer.

Figure 4 :

 4 Figure 4: Schematic of Mach-Zehnder Interferometer (A) open space interferometer. (B) Chip-based MZ interferometer. (C) Fiber-based MZ interferometer.

Figure 5 .

 5 Figure 5. (A) Pohl optical arrangement for VOC detection. (B) The interference pattern is formed by light beams reflected from air-substrate interface, substrate-sensing film interface and air-sensing film interface.

Figure 6 .

 6 Figure 6. (A) Schematic of the FP sensor integrated with µGC column, light source and detector. (B) Schematic of the FP sensor formed by coating sequentially the end of the fiber with silver and polymer. Adapted from [103].with permission American Chemical Society.Ning et al.[START_REF] Ning | PDMS-coated fiber volatile organic compounds sensors[END_REF] demonstrated a sensitive FP and Sagnac Interferometer functionalized with PDMS for simultaneous multiple VOC detection. Ethanol and 2-propanol were detected and differentiated using a second-order inverse matrix method. An in-line FP sensor was fabricated by using a single-mode hollow optical fiber (diameter = 125 μm and length = 50μm) with a PDMS (thickness=15μm) coating on the tip of fiber. The FP cavity was formed by the fiber end face and the interface between air and PDMS. A sensitivity of 1.17×10 -3 nm/ppm e and 1.61×10 -3 nm/ppm was reported for ethanol and 2-propanol respectively. A Sagnac interferometer was fabricated by splicing a polarization-maintaining fiber (PMF) with a length of 10 cm between the SM fibers and coated with PDMS similar to Figure3. The Sagnac interferometer showed a non-linear response for different concentration of VOCs. A sensitivity of 9.02×10 -4 nm/ppm and 2.71×10 -3 nm/ppm was measured for ethanol and 2-propanol respectively. The diffusion of 2-propanol from the PDMS was a drawback of Sagnac interferometer for repeated use. The

Figure 7 .Figure 8 .

 78 Figure 7. Schematic of the experimental setup with zeolite coated FP interferometer. BBS and OSA represent BroadBand Light Source and Optical Spectrum Analyser, respectively. Reprinted from the [105] with permission from Elsevier. Wu et al. [106] applied a zeolite coated FP interferometer for 2-propanol and formaldehyde detection. The fiber spherical shape (diameter, 192μm) and zeolite coating (thickness, 25 μm) constituted the FP cavity as shown in figure 8(A). The sensitivity of 281.9 pm/ppm f and 4.99 pm/ppm was found for 2propanol and formaldehyde respectively. The sensor was 56 times more sensitive towards 2-propanol than formaldehyde owing to their molecular sizes in comparison with the zeolite size. The sensor was tested for a mixture of 2-propanol with different concentrations of formaldehyde (0-150 ppm) as shown in figure 8 (B) and good stability and repeatability was reported.

Figure 9 .

 9 Figure 9. (a) Schematic of the FP cavity. Air cavity inside PDMS constitutes the FP cavity. (b) Micrograph of the fabricated PDMS FP sensor. Reprinted from [108] with permission from Elsevier.

Figure 10 .

 10 Figure 10. (a) Schematic of MZ interferometer. PDMS microfiber works as a sensing beam. (b) Micrograph of fabricated MZ interferometer. Reprinted from [107] with permission from Elsevier.

Figure 11 .

 11 Figure 11. Schematic of FP interferometer. The FP cavity is formed by the end of fiber and PMMA film. Reprinted from[START_REF] Yu | Highly sensitive and selective fiber-optic Fabry-Perot volatile organic compounds sensor based on a PMMA film[END_REF] with permission from the Optical Society(OSA) publishing.

Figure 12 .

 12 Figure 12. (A) Schematics of FP sensors array integrated with µGC. (B) Schematic of the optical detection setup. The laser is integrated with the FP sensor using optical fiber.The CMOS imager provides quantitative information about the polymer response to the vapour analytes. Reprinted from[START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF] with permission from Royal Society of Chemistry.

Figure 13 .

 13 Figure 13. (A) Cross-section view of FP sensors array (B) Top-view of array (C) Image of the sensors array with overall size of 8 mm × 6 mm (D) Image of 4 wells with separation distance of 800 um. Reprinted from[START_REF] Reddy | Rapid, sensitive, and multiplexed on-chip optical sensors for micro-gas chromatography[END_REF] with permission from Royal Society of Chemistry.

Figure 14 .

 14 Figure 14. (A) Optical micrograph of a polymer-coated device showing monolithically integrated microfluidics system and the optical fiber alignment groove with FP interferometer. (c) Schematics of FP interferometer with working principle. A shift in the resonance wavelength is observed upon absorption of the sample into the polymer. Adopted from [113] with permission from Elsevier.A miniaturized complementary metal-oxide semiconductor (CMOS)-MEMS based FP interferometer with photocurrent readout was designed by Maruyama et al.[START_REF] Maruyama | Optical-interferometry-based CMOS-MEMS sensor transduced by stress-induced nanomechanical deflection[END_REF] for detection of ethanol.Polychloro-para-xylene (Parylene-C) was used as a sensing film which bulged upon exposure to ethanol.An FP cavity was formed by the thin flexible Parylene-C film (350nm), air gap (300nm) and the silicon oxide film (400nm) on the top of the silicon substrate as shown in figure15. A novel signal transduction was employed by using the FP interferometer to convert the mechanical deformation of sensing film to a change in light transmittance detected by a photodiode. The sensor size was 5 mm 2 with a LED source.A signal processing circuit was integrated to quantify the exposed gas molecules. The sensor demonstrated a good sensitivity of 50nm deformation of the film for 80 ppm of ethanol.

Figure 15 .

 15 Figure 15. (a) Schematics of the MEMS FP interferometer sensor. (b) The working mechanism of the sensor. The deformation model of sensing film due to Coulomb repulsion. Reprinted from [114].

Table 1 :

 1 The different exposure limits recommended by various organizations

	VOCs	NIOSH-Recommended Exposure Limit a (ppm)	OSHA -Permissible Exposure Limit b (ppm)	ACGIH-Threshold Limit Value c (ppm)	ANSES (VGAI) -Long exposure d (ppm)
	Benzene	0.1	1	0.5	0.0006
	Toluene	100	200	20	5.31
	Ethylbenzene	10	10	10	0.345
	Xylene(m-,o-,p-)	100	100	100	-
	Formaldehyde	0.016	0.75	0.3	0.024
	Acetone	250	1000	250	-
	Ethanol	1000	1000	1000	-
	Methanol	200	200	250	-
	Isopropanol	400	400	200	-

  issues of ageing and zero drift. PIDs have disadvantages of selectivity, cost, and complex electronics. Piezoelectric-based sensors show large measurement-noise and have sensitivity to humidity and temperature. The advantages and disadvantages of different techniques are summarized in Table2

d National Agency for Food Safety, Environment and Labor (ANSES) Interior Air Quality Guide Values (VGAI) France.

major

Table 2 .

 2 Pros and Cons of different gas sensing techniques.

3 Interferometry for VOC detection 3.1 Sensing films for Interferometry Optical

  properties and volume of a sensing film change when it is exposed to gas molecules.Interferometry quantifies these variations in terms of phase difference inferred from changes in the optical path length. The commonly used sensing films for VOCs are PDMS and zeolite. Other types of

	sensing films including polymethyl-methacrylate (PMMA), polychloro-para-xylylene (Parylene-C),
	SU-8, polyethylene glycol (PEG 400), PEG-1000, Norland optical adhesive (NOA 81) have also been
	reported for VOCs applications.
	3.1.1 PDMS sensitive thin film
	Poly(dimethylsiloxane) (PDMS), -[O-Si(CH3)2] is a silicone-based elastomer and is widely used in
	microfluidics and biomedical applications due to its features like optical transparency, chemical

inertness, biocompatibility, low cost, flexibility, elasticity and tunable permeability

[START_REF] Whitesides | Soft Lithography in Biology and Biochemistry[END_REF]

[START_REF] Bhattacharjee | The upcoming 3D-printing revolution in microfluidics[END_REF]

. It is optically transparent in the wavelength range 230-800 nm

[START_REF] Wilbur | Elastomeric optics[END_REF]

[START_REF] Sia | Microfluidic devices fabricated in Poly(dimethylsiloxane) for biological studies[END_REF]

. PDMS has relatively stable physical

Table 3 .

 3 Comparison of different interferometry techniques for VOCs detection.

	S No	Interferometer	Type	Molecules detected	Sensing film	Linear response range	Sensitivity	Time response	Remarks	Ref
	1	FP Interferometer	Fiber-based	Hexanol, methanol and acetone	Polyethylene glycol(PEG 400) and Norland optical adhesive (NOA 81)	0-4000 ppm	3.53pm/ppm for methanol with PEG sensing 10ppm) film. Limit of detection(LOD):1ppm and 0.1 pm/ppm with NOA sensing film (LOD:	10 seconds for pulse mode	The sensor was tested in continuous and pulse mode	[102]
	2	FP Interferometer	Fiber-based	Toluene, decane, methanol and Dimethyl-methylphosphonate (DMMP)	PEG 1000 and PDMS	0-20 ng for DMMP. 0-50 ng for decane using PEG 1000	4.75 mV/ng for decane and 77 mV/ng for DMMP (LOD :50 pg(105.6 ppb))	-	FP sensor can be easy to as per analytes. integrate with the GC column and can be tailored	[103]
	3	FP and Sagnac interferometer	Fiber-based	Ethanol and 2-propanol	PDMS	0-6800 ppm	9.02 x 10 -4 to 5.14x10 -4 nm/ppm for ethanol. and 2.71 x 10 -3 to 7.78 x10 -4 nm/ppm for 2-propanol	15 sec for FP interferometer, 40 min for Sagnac interferometer	Multiple VOC were matrix inverse algorithm. simultaneously measure by using a second-order	[104]
	4	FP interferometer	Fiber based	Isopropanol	Zeolite	0ppm-70ppm	0.92 nm/ppm	2 sec	-	[105]
	5	FP interferometer	Fiber based	Isopropanol and formaldehyde	Zeolite	0-1000 ppm for formaldehyde	281.9 pm/ppm for isopropanol and 4.99 pm/ppm for formaldehyde	2 sec	-	[106]
	6	FP interferometer	Fiber based	Toluene	PDMS	0.833 g.m -3 to 140.8 g-m -3 .	1.4 nm/g-m -3	Response time 5 sec Recovery time 40 sec	-	[108]
	7	FP Interferometer	fiber based	Ethanol and acetone	Polymethyl methacrylate (PMMA) film	0 ppm -1800 ppm for Ethanol and acetone	2.7 pm/ppm for ethanol and 2.17 pm/ppm for acetone	NA	Good selectivity. compounds Insensitive to inorganic	[109]
	8	Sagnac interferometer	Fiber based	Isopropanol	PDMS	0ppm-6000 ppm	1.03 pm/ppm LOD: 19.3 ppm	1 min	-	[110]
					1. OV-1 (PDMS)	10ng-100 ng for
					2. OV-73	methanol.
	9	FP interferometer	Chip-based with μGC	Acetone, methanol, heptane and Toluene	one) (diphenyldimethylsilicone) 3. OV-215 (trifluoropropylmethylsilic	heptane. 0.5ng -10 ng for acetone. 0.1ng -1ng for	LOD for toluene, 0.79 pg (28 ppb) using OV-1.	Sub second	The interferometer was employed with µ-GC	[117]
					4. OV-1701 (dimethylphenyl	0.1 to 1 ng for
					cyano substituted)	toluene. (Approx.*)
	10	FP interferometer	Chip-based	Toluene and acetone PDMS and SU-8	0-20ng for toluene 0-150 ng for acetone using PDMS.	2900uV/ng(LOD 1.7 ppm) for toluene and 46 uV/ng (LOD 202 ppm)using PDMS. SU-8 4uV/ng(LOD 2336 ppm) for acetone using	0.9 sec for toluene PDMS 0.5 sec for acetone using	-	[112]

h nW/ppm(nano-watt/parts-per-million)

Acknowledgments

This work is supported by European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Innovative Training Network-MIGRATE (Miniaturized Gas flow foR Applications with Enhanced Thermal Effects), grant agreement No. 643095 [H2020-MSCA-ITN-2014].


Gustavo Coelho Rezende

Stéphane Le Calvé

Jürgen J Brandner

David Newport

CHARACTERIZATION OF A MODULAR MICROFLUIDIC PHOTOIONIZATION DETECTOR

Keywords: Photoionization Detector, Miniaturization, Microfabrication, Gas Chromatography, VOC detection, Air Quality

Photoionization detectors with a small ionization chamber can contribute to overall gas analyser miniaturization. This work reports the characterization of a microfluidic photoionization detector prototype (µPID) which is constructed to be modular for easy replacement of the components and maintenance. The device is fabricated by micromilling and electrical discharge machining, dispensing clean room fabrication techniques. The µPID ionization chamber is a microchannel and four channel designs are presented and tested in experiments in order to evaluate the influence of the geometrical parameters on the detector performance. The chamber volumes of the channel designs range from 1.1 to 6.7 µL. Experimental characterization of the prototype is performed when it is used without and with a portable gas chromatograph (GC) for volatile organic compounds (VOCs) analysis. When a sample of 100 ppm toluene is injected directly into the µPID, it can generate a current signal up to ~4 nA. When used without a GC, the device showed a linear response for an injection of toluene gas concentrations ranging from 1 to 100 ppm. A combination of high illumination area and electrode area resulted in the highest signal in the µPID with a detection limit of ~40 ppb of toluene. When integrated to the portable GC, the detection limit reached for toluene is ~140 ppb. The detection limit for toluene was further reduced to low ppb levels (~5 ppb) when a preconcentrator was integrated into the sampling loop of the portable GC.

Introduction

Indoor environments, where people spend close to 90 % of their time [START_REF] Klepeis | The National Human Activity Pattern Survey (NHAPS): a resource for assessing exposure to environmental pollutants[END_REF][START_REF]Indoor air pollution: new EU research reveals higher risks than previously thought[END_REF], can have Volatile Organic Compounds (VOCs) contaminating the air. VOCs are a class of carbon-containing chemicals with a high vapour pressure at ambient temperature. Those chemicals are released by varnishes, paints, solvents, cleaning materials, etc. In addition, outdoor VOCs emission from automobile or industrial waste can contribute to indoor pollution [START_REF] Wallace | Emissions of volatile organic compounds from building materials and consumer products[END_REF]. Many VOCs can cause health problems, including benzene, which is carcinogenic and has no safe recommended level of exposure [START_REF] Who | Air quality guidelines for Europe[END_REF][START_REF] Who | WHO Guidelines for Indoor Air Quality: Selected Pollutants[END_REF]. Those facts raised the awareness of authorities in many countries, which issued regulations to limit the exposure to such chemicals [START_REF][END_REF][START_REF]Décret no 2011-1727 du 2 décembre 2011 relatif aux valeurs-guides pour l'air intérieur pour le formaldéhyde et le benzène[END_REF].

Photoionization detectors (PIDs) can measure the presence of VOCs in air samples. They measure the photoionization current generated when the VOCs are hit by photons with larger energy than the molecule ionization potential [START_REF] Poole | Gas Chromatography[END_REF]. The PID is a concentration sensitive detector, the current generated being proportional to the concentration of VOCs inside its ionization chamber [START_REF] Poole | Gas Chromatography[END_REF]. The main components of the detector are: 1) ionization source, which emits the high energy light to ionize the sample; 2) electrodes, which generate an electric field for the collection of the electrons and ions; 3) ionization chamber, which is where the gas sample flows through and is detected.

In order to identify the different VOCs present in air samples, the PID can be integrated to a gas chromatograph (GC). This type of gas analyser (GC-PID) can separate, identify and quantify different chemicals. Commercial GC-PIDs with low detection limit are still lab-based, slow, bulky and heavy [START_REF]BTEX analyzer with PID detector: chromaPID, Chromatotec[END_REF][10][START_REF] Baseline | [END_REF]; therefore, not ideal to comply with the recent regulations demands which require portability and fast analysis. An efficient gas analyser with low resource consumption, improved robustness, high analysis speed, low cost, improved portability and low detection limit can be produced with miniaturization of the main GC components, including the detector.

An important part of the PID to be miniaturized is the ionization chamber, whose volume influences directly the response time. In addition, a small ionization chamber allows a reduction in the sample volume without depleting the signal intensity. With a smaller sample volume, at a constant sample flow rate, it is possible to have a faster analysis time and increase the autonomy of the carrier gas cylinder used with the GC-PID; thus, increasing the portability of the gas analyser. Commercial PIDs are not ideal for a miniaturized gas analyser since they have a large ionization chamber, with volumes ranging from 50 -100 µL [START_REF] Khan | Development of a Toluene Detector Based on Deep UV Absorption Spectrophotometry Using Glass and Aluminum Capillary Tube Gas Cells with a LED Source[END_REF][START_REF] Rezende | Micro photoionization detectors[END_REF][START_REF]Model 4430, Photoionization Detector Operator's Manual[END_REF][START_REF]PID detector[END_REF][START_REF] Liaud | Experimental performances study of a transportable GC-PID and two thermo-desorption based methods coupled to FID and MS detection to assess BTEX exposure at sub-ppb level in air[END_REF]. Some commercial PIDs can have small external size (20 mm) but they have an ionization chamber whose design is not optimized for integration into a portable gas analyser due to the presence of a membrane which can delay the fill time of the ionization chamber [START_REF]Model 4430, Photoionization Detector Operator's Manual[END_REF]17].

Recent developments in PID miniaturization produced devices with ionization chamber volume close to 1 µL [START_REF] Sun | An improved photoionization detector with a micro gas chromatography column for portable rapid gas chromatography system[END_REF][START_REF] Zhu | Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor detection[END_REF][START_REF] Lee | Fully Automated Portable Comprehensive 2-Dimensional Gas Chromatography Device[END_REF][START_REF] Akbar | Chip-scale gas chromatography: From injection through detection[END_REF][START_REF] Zhu | Low-Power Miniaturized Helium Dielectric Barrier Discharge Photoionization Detectors for Highly Sensitive Vapor Detection[END_REF][START_REF] Narayanan | A micro-discharge photoionization detector for micro-gas chromatography[END_REF]. The new designs used mostly lithography based fabrication techniques with silicon and glass as main materials. As an alternative to those fabrication techniques, Rezende et al. (2019) [START_REF] Rezende | Micro Milled Microfluidic Photoionization Detector for Volatile Organic Compounds[END_REF] proposed a µPID fabricated by micromilling and electrical discharge machining. The main features of this design were its relatively low prototyping cost, no need for clean room or photomasks for fabrication, and its modular construction, allowing easy replacement of the components and maintenance of the detector. The present work shows the characterization of the µPID previously described by Rezende et al. (2019) [START_REF] Rezende | Micro Milled Microfluidic Photoionization Detector for Volatile Organic Compounds[END_REF], including experiments without and with a portable GC. Four ionization chamber geometries shaped as microchannels are evaluated; the performance of the GC-µPID is investigated and compared to a commercial PID integrated to the same portable GC system. In addition, the µPID response is evaluated when a preconcentrator (PC) is integrated into the GC sampling loop.

Experimental Setup and Materials

Chemicals

Main chemicals used in the experiments are: 1) Nitrogen gas at 99.99 % purity; 2) Toluene gas mixture in nitrogen at 10 ppm and 100 ppm; 3) BTEX mixture with nitrogen as balance gas containing either 5 ppb or 10 ppm of benzene, toluene, ethylbenzene, o-xylene, m-xylene, pxylene.

Microchannels designs

The µPID design tested for this work is detailed in Rezende et al. (2019) [START_REF] Rezende | Micro Milled Microfluidic Photoionization Detector for Volatile Organic Compounds[END_REF]. The ionization chamber of the µPID is a microchannel and Figure 1 shows the design of the four microchannel geometries used inside the detector (channel A, B, C and D). The channel designs presented have the objective to investigate the influence of channel geometry on the detector performance. Figure 2 presents the geometrical parameters of the microchannel, which are: channel width (𝑊), height (𝐻), length (𝐿), illumination area (𝐴 𝑖 ), electrode area (𝐴 𝑒 ) and ionization chamber volume (𝑉 𝑐 ).

In theory, the geometrical parameters influence the µPID performance as following: 1) Width (𝑊): equals the distance between the copper electrodes; therefore, it is inversely proportional to the electric field intensity, and consequently, to the detector sensitivity; 2) Height (𝐻): shallow channels yield more uniform sample ionization, but less electrode area; 3) Length (𝐿): for the microchannel design, the length influences the electrode area, illumination area and ionization chamber volume; 4) Electrode area (𝐴 𝑒 ): depends on the shape and the height of the microchannel and it is directly proportional to sensitivity; 5) Lamp illumination area (𝐴 𝑖 ): top surface of the channel illuminated by the UV lamp. Higher illumination area means more ionized molecules; and 6) Ionization chamber volume (𝑉 𝑐 ): volume of the microchannel from inlet to outlet. Large volumes can deplete the signal if the sample volume is small compared to the chamber volume (supplementary material S.4).

From channel A to B, the main objective is to evaluate the effect of width change, which modifies the illumination area and ionization chamber volume consequently, but keeps a constant electrode area. Channels B, C and D are designed to investigate the influence of increased illumination area and electrode area. Notice that the width of channel D is 400 µm instead of 500 µm (as in channel B and C), this was done so that the channel D ionization chamber would fit inside the lamp illumination diameter, which is 6 mm as indicated by the dashed circle on all microchannels in Figure 1. Table 1 gives the values of the geometries for all the channels. Notice that channel D has electrode area of 32.6 mm 2 at least 4 times higher than channels A and B. In addition, the illumination area of 13.3 mm 2 of channel D is around 6 and 4 times higher than those values for channels A and B, respectively. With a value of 6.7 µL, channel D has the highest ionization chamber volume, almost 7 times higher than the chamber volume of channel A.

Setup for direct injection into µPID

Figure 3 shows an experimental setup built to evaluate the µPID prototype. Two gas sample supplies are used, one containing a VOC pollutant and another with nitrogen. Both are isolated by ball valves (BV1 and BV2) and connected to a mass flow controller (MFC). The MFC regulates the flow rate of gas sample into the µPID and the flow rate leaving the device is read by a mass flow meter (MFM). The MFC and MFM (F-100D-AAD-33-V) are from Bronkhorst and the model of MFC used varies depending on the flow rate required for the experiment. All tubing connecting the gas flow are made of PTFE. For safety reasons, the experimental setup is installed inside a fume hood to avoid spreading of the pollutant gases into the test room. At the exit of the fluidic path, the gas sample is left to atmospheric pressure.

The µPID 10.6 eV lamp is powered by the UV lamp drive circuit from a commercial PID (piD-TECH® eVx™, 045-010), which is supplied with 4 V and 35 mA from the power supply 1. To measure the current inside the ionization chamber, the power supply 2 applies a voltage between the copper electrodes (30 V), and the current generated by the PID detection goes through the current preamplifier (Low-Noise Current Preamplifier, Stanford Research Systems, SR570), which can filter and amplify low current signals. After amplifying the current signal to voltage signal, the voltage magnitude is measured by an analog-to-digital converter (ADC) from National Instruments (NI-USB-6001). A MATLAB algorithm reads and stores the digital signal in a computer. Due to the low magnitude of the current measured, a Faraday cage enclosing the µPID is used in the experiments to mitigate electromagnetic noise influence.

Setup of µPID coupled to GC for BTEX separation

The µPID is also tested with a portable GC [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF] by substituting the commercial PID (Baseline MOCON, piD-TECH Blue, part number 045-014) for the µPID; yielding a GC-µPID system with the schematic shown in Figure 4a. The analyser works in three steps: sampling, separation and detection. A BTEX gas mixture is connected to the inlet of the gas analyser which uses a mini diaphragm air pump (270 EC, Schwarzer) and a solenoid 6-ports valve (MTV-6LL-N32UF-1, Takasago) equipped with a PEEK 200 µL sampling loop which is filled with the sample and then injected into the column for analysis as described in Figures 4b and4c.

At the sampling stage illustrated by Figure 4b, the gas sample is directed to a waste, so that the 200 µL sampling loop is filled with the gaseous pollutant mixture, which occurs in less than 10 seconds. After filling the sampling loop with a BTEX mixture, the injection stage can start. The injection is initiated by the BTEX analyser controller panel, and makes the valve switch position to reach the configuration displayed in Figure 4c, so that the carrier gas pushes the sample downstream. The optimal time for the injection stage was verified in previous studies and is 20 seconds [START_REF] Nasreddine | Development of a novel portable miniaturized GC for near real-time low level detection of BTEX[END_REF].

The 6-ports valve is connected directly to the separation column to minimize dead volume. The separation column is a 20 m long capillary from Restek with RXi-624 stationary phase and 1 µm film thickness. Internal and external diameters are 0.18 mm and 0.74 mm, respectively. The separation column is coiled in a 7.5 cm diameter inside a 10 cm × 10 cm × 1 cm aluminium plate which serves as an oven. It is heated by an electric resistor that controls the temperature with a commercial controller (CAL 3300). To improve the oven performance the column is covered by a 2 cm thick cork insulation layer. The temperature set at the column for all the experiments is 65 °C and is constant for the analysis duration (isocratic mode).

The µPID is located in the outlet of the separation column (Figure 4a). The electronic connections for the µPID operation are the same as demonstrated in Figure 3, and a MFM can be placed after the separation column and the µPID to verify the absence of leakage. The flow rate through the system is ensured by the pressure regulator (PR) from Bronkhorst, set at 4 bars, which yield a flow rate measured at the end of the separation column of 2.39 mLn/min.

Results for Sample Direct Injection into the µPID

Influence of microchannel design

In order to verify the influence of different microchannel designs on the detector signal, the response using channels A, B, C and D were obtained with direct injection of nitrogen and 100 ppm of toluene (nitrogen balance gas) at 50 mLn/min. Figure S.1 shows the signal (current measurement) as a function of time for all channels. For each channel, three datasets (n = 3) measured in a sequence are shown to demonstrate the repeatability of the results (black, red and blue lines). The signal was obtained by applying 30 V on the signal electrodes. The current preamplifier was set for a low-pass first order RC filter with cut-off frequency (𝑓 𝑐 ) of 10 Hz, 12 dB attenuation and 1 nA/V amplification. The amplification and attenuation values used in the preamplifier will remain constant for all results obtained in this work.

The following procedure was used to obtain the signal: i) channel purged with nitrogen during 5 min; ii) Start the data acquisition (t = 0 s on the plots of Figure S.1) and continue the purge with nitrogen for 1 minute; iii) Close nitrogen supply valve and wait for the MFC to measure 0 mLn/min (which lasts about 1 minute); iv) at 2 min of test, open the toluene 100 ppm connection valve for pollutant signal acquisition during 3 min; v) close the toluene 100 ppm valve and wait for the MFC to display 0 mLn/min; vi) open the nitrogen valve to purge the channel so that the signal level returns to the baseline.

For each dataset, the signal representing the stable value for the baseline (𝑆 𝑏 ) and the pollutant (𝑆 𝑝 ) are obtained at 35 s and 270 s of experiment, respectively. The signal at 35 and 270 s is considered stable with less than 0.2 % variation in a 5 s interval. The difference between the pollutant and the baseline signal is the signal rise:

𝑆 𝑟 = 𝑆 𝑝 -𝑆 𝑏 . (1)
The average value of the signal at the baseline (𝑆 𝑏 ), pollutant (𝑆 𝑝 ) and rise (𝑆 𝑟 ) level for all channels are displayed in Figure 5. The 𝑆 𝑟 is what ultimately permits to estimate the sensitivity level of the detector. As illustrated by Figures 5b and5c, the highest variation for 𝑆 𝑝 and 𝑆 𝑟 is found between channel A and D, where the channel D can have values up to 3 times higher. Regarding 𝑆 𝑏 (see Figure 5a), the largest difference is found between channel B and D, where the baseline of channel D is higher than double the channel B baseline. The error bars displayed in Figure 5 are 3.5 % of the measured value. This error value stems from the maximum error observed in the experiments for direct injection of sample. The response time, which is the time it takes for the signal to go from baseline level to pollutant level, is less than 1 s for all channels.

(a) (b) (c) Table 1 can be used as a reference to investigate the influence on the measured signal caused by the geometry changes of each channel. From channel A to channel B, the signal level doubles and the main geometrical change between those two designs is the width of the channel (𝑊), which changes the illumination area (𝐴 𝑖 ) consequently. The change in the two parameters, 𝑊 and 𝐴 𝑖 , observed from channel A to channel B have opposite effects on the signal level. Channel A has smaller 𝑊 which means it has a stronger electric field compared to channel B, this increases the ion and electron collection efficiency and contributes positively for the sensitivity of the detector. On the other hand, channel B, despite the larger 𝑊, has a larger illumination area (𝐴 𝑖 ), which also contributes positively to the sensitivity of the µPID. The increase in signal rise (𝑆 𝑟 ) from channel A to B observed in Figure 5c shows that, for the investigated channel dimensions, illumination area is more important than width for signal level.

Figure 5a shows that the baseline level for channel A is higher than channel B, which is likely caused by a larger portion of the electrodes being hit by photons on channel A. For channels A, B, C and D, the portion of the electrodes top surface illuminated by the lamp diminishes with increasing ionization chamber volume; however, for channels B, C and D, baseline signal increases with less electrode surface area illuminated by the lamp and channel A has an inverse effect. An explanation for this phenomenon can be that channels B, C and D generate higher current for the residual ionisable chemical components that can be inside the ionization chamber even when nitrogen is used, since those channel designs have higher sensitivity due to the larger electrode area and illumination area compared to channel A.

For channels B, C and D, it is possible to attribute the increase in signal rise (𝑆 𝑟 ) to the simultaneous increase in illumination area and electrode area. A change in channel D compared to channels B and C is the width, which is 100 µm lower compared to channels B and C. This has also a positive impact in the channel D sensitivity and has contributed to establish it as the most sensitive channel design from the four presented in this work. Because of the changes in the channel shapes, ionization chamber volume increases as illumination area increases. 𝑉 𝑐 variation should not have a significant impact on the signal presented in Figure 5 because the PID is a concentration sensitive detector. A 𝑉 𝑐 variation can cause signal intensity change if the sample volume is small compared to the ionization chamber volume (see Supplementary material S.4).

Calibration curve and detection limit

A range of toluene concentrations (1 to 100 ppm) were injected into the µPID to obtain a response curve. The detector (using channel D) was subjected to a series of gas purge with nitrogen followed by injection of toluene. The range of concentrations were achieved by diluting the concentration level of the toluene 100 ppm gas cylinder with a mixed nitrogen flow. The nitrogen and toluene flow rates summed up 20 mLn/min and a potential of 30 V was applied on the electrodes to yield fast stabilization and a stable signal, respectively.

The µPID signal rise (𝑆 𝑟 ) as a function of toluene concentration (𝐶) at the range 1 to 100 ppm follows a linear growth, as indicated in Figure 6, which is estimated from a linear fit function with forced zero intercept. The linear fit function angular coefficient (𝑎) and the coefficient of determination (𝑟 2 ) are 24.67 pA/ppm and 0.9997, respectively. The concentration uncertainty presented in Figure 6 is obtained from the toluene gas cylinder concentration error and from the mixing of nitrogen and toluene flows that yielded the dilution. The resulting concentration uncertainty has values from 5 to 12.5 % at the highest and lowest concentrations, respectively.

Figure 6 -Channel D response curve for 1 to 100 ppm toluene concentration range at 30 V and 20 mLn/min. Vertical error bar is 3.5 %, which stems from the maximum error observed in the experiments for direct injection of sample. The horizontal error bar varies from 5 % to 12.5 % where the highest error is obtained for lowest concentration. This value range is obtained by the toluene gas cylinder concentration error and from the mixing of nitrogen and toluene flows that yielded the dilution.

The detection limit in concentration units of the µPID (𝐶 𝐷𝐿 ) is defined as the concentration that yields a signal rise three times higher than the noise measurement at the baseline (𝑁). Considering the linear nature of the signal for the concentration range measured, 𝐶 𝐷𝐿 can be estimated from the linear fit of the response curve with forced zero intercept, therefore:

𝐶 𝐷𝐿 = (3. 𝑁)/𝑎 (2)
The sensitivity (𝑎), which is also the slope of the linear fit, depends essentially on the design of the device, while the noise can be treated with filters and signal post-processing. The lowest baseline noise measured is 0.3 pA and this noise value was obtained using a low pass first order RC filter from the current preamplifier with a cut-off frequency (𝑓 𝑐 ) of 0.03 Hz in combination with a digital moving average filter. Considering the lowest baseline noise, the detection limit of toluene is ~40 ppb for the sample direct injection test presented in Figure 5.

Results for µPID Integrated in the GC

Influence of microchannel design on the chromatogram and comparison with a commercial PID

Experiments with GC-µPID were also performed for channels A, B, C, D and a commercial portable PID (Baseline MOCON, piD-TECH eVx Green, part number 045-010). The commercial PID tested is not the same one originally used with the portable GC because the original commercial PID had a measurement range (0.5 ppb to 2 ppm) lower than the sample concentrations applied to test the µPID (1 ppm to 10 ppm); therefore, it would not be able to read the sample at the same concentration levels injected into the µPID.

Figure 7 shows the chromatogram with 10 ppm BTEX sample for channel D. The chromatograms for all other channels and the commercial PID are presented in section S.3 of the Supplementary material.

Optimal filter cut-off frequency applied in the µPID, which generated the best signal-to-noise ratio was 0.3 Hz. Retention time for µPID and commercial PID are essentially the same with a difference lower than 1 %. The peaks, in the order that they leave the separation column are: 1 st ) benzene; 2 nd ) toluene; 3 rd ) ethylbenzene; 4 th ) m-xylene and p-xylene (coelluted in the same peak); 5 th ) o-xylene. Retention times for benzene, toluene, ethylbenzene, m,p-xylenes and o-xylene peaks are 88 s, 189 s, 382 s, 412 s and 495 s, respectively.

The detection limits of BTEX for all channels and commercial PID are displayed in Figure 8. Since only the 10 ppm BTEX signal level was measured, the detection limit is estimated assuming a linear response curve with zero intercept. For the microchannels, the noise considered for the detection limit calculation is the same (0.3 pA, the lowest noise) and the detection limit magnitude follows the order (𝐶 𝐷𝐿 ) 𝐶ℎ𝐴 > (𝐶 𝐷𝐿 ) 𝐶ℎ𝐵 > (𝐶 𝐷𝐿 ) 𝐶ℎ𝐶 > (𝐶 𝐷𝐿 ) 𝐶ℎ𝐷 . The detection limit for channel D calculated by this way reach values of 72, 127, 429, 316 and 448 ppb of benzene, toluene, ethylbenzene, m,p-xylenes and o-xylene, respectively. Notice that the detection limit of benzene for the µPID channels B (134 ppb), C (107 ppb) and D (72 ppb) are better compared to the commercial PID (157 ppb) whereas the commercial PID exhibits better sensitivity for ethylbenzene and xylenes.

The peak width at half height (𝑤 1/2 ) is compared on Figure 9 for the microchannels and the commercial PID. For the benzene peak, the commercial PID has a negative impact on the width, yielding a slightly larger benzene peak (15 % larger than the channel D, for example). However, for toluene and ethylbenzene, the peak width has no conclusive difference from the microchannels and the commercial PID, because the average differences uncertainties are all overlapped. The xylenes (m,p,o-xylenes) are sharper for the commercial PID. No significant 𝑤 1/2 variation was observed between the microchannel designs, this can be explained by the fact that the volume of the ionization chamber is small compared to the sample volume eluting from the separation column, as analysed in the supplementary material section S.4.

Response curve

The response curve of the GC-µPID (using channel D) for each peak was obtained when applying concentrations ranging from 1 ppm to 10 ppm of BTEX into the gas analyser. The concentration range was obtained using the dilution method described in Section 3.2 and injecting 200 µL of sample from the sampling loop. The peak heights are calculated from the average of two consecutive experiments for each concentration. The height error is 6.5 %, same as mentioned in section 4.1, and the concentration error has a maximum value of 7.3 %. The response for this range of BTEX concentration is not linear, as shown in Figure 10. The concentration detection limit (𝐶 𝐷𝐿 ) can be estimated using Equation 2and the signal corresponding to the lowest concentration injected (1 ppm). Considering the noise for the digitally smoothened signal (0.3 pA), the detection limits in concentration and mass units for the four peaks are presented in Table 2. The detection limit in mass units (𝑚 𝐷𝐿 ) is obtained by applying Equation 3:

𝑚 𝐷𝐿 = 𝐶 𝐷𝐿 . 𝑝 𝑎𝑡𝑚 . 𝑉 𝑅 𝑖 . 𝑇 , (3)
where 𝑝 𝑎𝑡𝑚 is the atmospheric pressure (101325 Pa), 𝑉 is the sampling loop volume (200 µL), 𝑅 𝑖 is the specific gas constant for each chemical compound and 𝑇 is the ambient temperature (300 K).

Notice that the detection limit for toluene is approximately 3.5 times higher than when the detector was tested with direct injection. This can happen because, when the GC system is used, the concentration of the sample spreads while flowing through the separation column and reaches the detector at a lower level.

Preconcentrator integrated with the GC-µPID sampling loop

As previously demonstrated by Lara-Ibeas et al. ( 2019), the use of a preconcentration unit (PC) can improve the sensitivity and consequently decrease the detection limit of the gas analyser [START_REF] Lara-Lbeas | Sub-ppb level detection of BTEX Gaseous mixtures with a compact prototype GC equipped with a preconcentration unit[END_REF]. An experiment with a PC prototype (developed at ICPEES) was performed as an attempt to improve the detection limit of the GC-µPID. The PC uses an adsorbent to trap the sample and a heating system to release the concentrated sample into the gas analyser. The sampling flow rate for the preconcentration process was 20 mLn/min applied during 5 min at ambient temperature (~30 °C), the total sample volume being therefore 100 mL. The desorption temperature applied was 270 °C for ~70 s with the sample being injected during 75 s. The PC was integrated into the sampling loop of the gas analyser.

Unfortunately, the preconcentrator prototype suffered a contamination problem due to the presence of unexpected impurities in nitrogen even before the tests with the µPID. This is identified by an unwanted peak measured by the experiment with nitrogen (Figure 11). After a series of purges with nitrogen, a 5 ppb BTEX sample is injected into the gas analyser with the preconcentrator.

Figures 11a and11b show the chromatograms for the injection of nitrogen and 5 ppb of BTEX, respectively. Since the preconcentrator is applied in this operation, the separation process was adapted and it is 8 min longer compared to the separation process without the preconcentrator. During the purge test with nitrogen (Figure 11a), it is possible to identify the contamination peak, which hides the benzene signal when the 5 ppb BTEX sample is injected.

Conclusions & Recommendations

This work presented the characterization of a µPID used with and without a portable GC prototype. The modular feature of the µPID made it possible to test four different channel geometries, with ionization chamber volume ranging from 1.1 to 6.7 µL, and use the same UV lamp. The influence of the geometrical parameters was investigated and showed that the electrode and illumination areas play an important role in the signal magnitude, while the width of the channel has probably a small influence in the signal of the µPID.

When a sample of 100 ppm toluene was injected directly into the µPID at 50 mLn/min and 30 V applied on the electrodes, it produced a current signal up to ~4 nA (channel D). The signal response time was less than 1 s for all channel designs tested. The higher baseline level for channel A compared to channel B can be due to a larger portion of the electrodes being hit by photons, causing the photoelectric effect. Lamp deterioration can cause significant drop in 𝑆 𝑟 , near 3 times lower after ~700 h of use. This highlights the importance of the modular feature of the µPID design showed in this work, because the UV lamp can be replaced or removed for maintenance. Considering the linear fit of the response curve from 1 to 100 ppm of toluene and the lowest noise obtained (0.3 pA), the detection limit for sample direct injection was ~40 ppb.

The response curve of the GC-µPID obtained for channel D from the injection of a 1 to 10 ppm BTEX sample showed non-linear behaviour, which was more pronounced in the last three peaks of the chromatogram. The detection limit obtained for benzene, toluene, ethylbenzene, m,p-xylenes and oxylene peaks are 73, 138, 308, 262 and 254 ppb, respectively. For the four microchannels tested, no significant variation of peak width was observed and can be explained for the relatively high sample volume eluting from the separation column. Despite the contamination issues in the preconcentrator prototype, it was still possible to reach low ppb detection limit for toluene when the PC was integrated into the GC sampling system.

Future work could be done to develop a portable amplifier for the µPID so it can be easily integrated into the GC prototype for field analysis. Practical study of the influence of humidity in the sensor response is necessary to apply filters. Another important future step is to integrate the µPID into a labon-a-chip gas chromatograph with preconcentrator, this will enable to increase the portability of the whole gas analyser system. A modular µGC-µPID system should be built in the future and can improve maintenance of overall gas analyser and increase the flexibility of the application, since specific parts can be added according to application needs.

ACKNOWLEDGEMENTS:

This study was developed during the ITN Research Project, MIGRATE, supported by European Community H2020 Framework under the Grant Agreement No. 643095.

Figure 1 -

 1 Figure 1 -Microchannel designs used in the µPID (channel A, B, C and D).

Figure 2 -

 2 Figure 2 -Main microchannel geometrical parameters.

Figure 3 -

 3 Figure 3 -Experimental setup used for the experiments with the µPID.

Figure 4 -

 4 Figure 4 -Schematics of the experiment integrating the µPID with a portable GC. a) Setup main components, including pressure regulator (PR) and mass flow controller (MFC); b) 6-ports valve sampling stage; c) 6-ports valve analysis stage.

Figure 5 -

 5 Figure 5 -Average signal level for all channels. a) Baseline signal (𝑆 𝑏 ); b) Pollutant signal (𝑆 𝑝 ); c) Signal rise (𝑆 𝑟 ).

Figure 7 -

 7 Figure 7 -Chromatogram for 10 ppm BTEX injection using channel D.

Figure 8 -

 8 Figure 8 -Estimated concentration detection limit (𝐶 𝐷𝐿 ) for all microchannels (30 V) and commercial PID with a 10 ppm BTEX sample: a) benzene, b) toluene, c) ethylbenzene, d) m,p-xylene and e) o-xylene. Detection limit error is 8.9 %.

Figure 9 -

 9 Figure 9 -Chromatographic peak width at half height (𝑤 1/2 ) for all channels and commercial PID with a 10 ppm BTEX sample and 30 V on electrodes: a) benzene, b) toluene, c) ethylbenzene, d) m,p-xylene and e) o-xylene.

Figure 10 -

 10 Figure 10 -Calibration curve for GC-µPID (channel D) chromatographic peaks with samples of BTEX at concentration ranging from 1 to 10 ppm and 30 V applied on the electrodes. a) Benzene; b) toluene; c) ethylbenzene; d) m,p-xylenes; e) o-xylene.

FigureFigure 11 -

 11 Figure11bshows the peaks corresponding to the 5 ppb BTEX injection. The pattern of the last three peaks, corresponding to ethylbenzene, m,p-xylene and o-xylene, can be recognized starting at 𝑡 = 700 s. However, the ethylbenzene and o-xylene signal heights are close to the noise level with 𝑆𝑁𝑅 < 3. This means that only toluene and m,p-xylenes peaks are detectable for this experiment, their detection limits being close to 5 ppb (1.87 ng and 2.16 ng with a sample volume of 100 mL, for toluene and m,pxylenes, respectively). Nonetheless, this shows the potential to a low ppb level detection limit when the µPID is used in a portable GC equipped with a preconcentrator.

Table 1 -

 1 Magnitude of the microchannels geometrical parameters.

	Parameter	Channel A	Channel B	Channel C	Channel D
	𝑯 [mm]	0.50	0.50	0.50	0.50
	𝑾 [mm]	0.25	0.50	0.50	0.40
	𝑳 [mm]	7.0	7.0	14.0	32.9
	𝑨 𝒆 [mm²]	7.0	7.0	12.9	32.6
	𝑨 𝒊 [mm²]	2.2	3.5	6.9	13.3
	𝑽 𝒄	[µL]	1.1	1.8	3.5	6.7

Table 2 -

 2 Detection limit for each species in ppb and pg. Error of 8.9 %.

		Benzene Toluene	Ethylbenzene	m,p-Xylene	o-Xylene
	D.L. [ppb]	73	138	308	262	254
	D.L. [pg]	46	103	266	226	219