Gustavo C Rezende

Stéphane Le Calvé
email: slecalve@unistra.fr

Jürgen J Brandner
email: juergen.brandner@kit.edu

David Newport
email: david.newport@ul.ie

Micro Milled Microfluidic Photoionization Detector for Volatile Organic Compounds

Keywords: photoionization detector, microfluidics, microfabrication, volatile organic compound (VOC) detection, toluene

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Introduction

Volatile organic compounds (VOCs) are a class of carbon-containing chemicals with a high vapor pressure at ambient temperature. Typical indoor sources of VOCs are varnishes, paints, solvents, cleaning materials, etc. In addition, outdoor sources, such as automobile and industrial waste, also contribute to both indoor and outdoor VOC pollution [START_REF] Wallace | Emissions of Volatile Organic Compounds from Building Materials and Consumer Products[END_REF]. Many VOCs are harmful to humans, including benzene, which is carcinogenic and has no safe recommended level of exposure [START_REF]Air Quality Guidelines for Europe[END_REF][START_REF] Who | WHO Guidelines for Indoor Air Quality: Selected Pollutants; WHO Regional Office for Europe[END_REF]. The European Commission established, with effect from 2010, a regulation for benzene exposure at a maximum limit of 5 µg/m 3 (1.6 ppb) [START_REF]Relating to Limit Values for Benzene and Carbon Monoxide in Ambient Air[END_REF]. In addition, within the European Union, regulations and guidelines are established to maintain a healthy indoor air quality [START_REF]Relating to Limit Values for Benzene and Carbon Monoxide in Ambient Air[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].

Air must be monitored in order to identify and quantify pollutants so that appropriate action can be taken to clean the air. Gas chromatographs (GCs) equipped with detectors are suitable gas analyzers to accomplish this task because they can separate, identify and quantify different chemicals, including VOCs. GCs in the market with low detection limits are still heavy, bulky, slow and lab-based [START_REF]BTEX Analyzer with PID Detector: ChromaPID[END_REF][START_REF]Benzene/Toluene/Xylene Analyser[END_REF][START_REF] Baseline | [END_REF], while current regulations raise the demand for efficient gas analyzers with enhanced portability, reduced resource consumption, improved robustness, high analysis speed, low cost and reduced detection limits. One possible way to achieve these improvements is through miniaturization of all the gas analyzer components, including the detector itself.

Among the existing detectors and techniques to detect VOCs [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], the photoionization detector (PID) is commonly used with a GC and it is suitable for miniaturization [START_REF] Rezende | Micro photoionization detectors[END_REF]. The PID uses ionization of gaseous compounds by light as a working principle to quantify chemicals in the gas samples. This detector can be classified according to the ionization source. When the ionization source is unseparated from the ionization chamber by a window, the ionization source is usually a discharge in a noble gas, such as helium, and is referred to as a discharge photo ionization detector (D-PID); or when the ionization source has no fluidic connection to the ionization chamber, which is known as the lamp photo ionization detector (L-PID). In both devices, a gas sample containing the species to be detected flows through the ionization chamber, where photons emitted by the ionization source reach the sample molecules. As a general rule, if the ionization energy of the photon is greater than the ionization potential of the molecule, ionization occurs. The electrodes establish an electric field in the ionization chamber where the ionized molecules generate an ionization current proportional to their concentration. Based on an external calibration, the electrical signal can be expressed as a chemical compound concentration.

Commercial detectors either have large ionization chamber volumes around 50-100 µL [START_REF]IO Analytical[END_REF][START_REF]Photoionization Detector, SRI Instruments[END_REF] or membranes, which increase the ionization chamber fill time [START_REF] Baseline | [END_REF][START_REF]Ion Science[END_REF]. For photoionization detectors applied to GC, reducing the external size of the PID is not the main point to ensure high efficiency, because existing portable PIDs are already small (20 mm) and lightweight (8 g) [START_REF] Baseline | [END_REF][START_REF]Ion Science[END_REF]. The challenge, therefore, is to develop a micro PID (µPID) detector with a rapid ionization chamber fill time, low gas flow rate and high sensitivity that is compatible for the development of a portable µGC equipped with a small carrier gas cylinder. This can be achieved with a miniaturized flow-through PID ionization chamber.

Reducing the ionization chamber volume of the PID can play an important role in overall GC-PID miniaturization. A small ionization chamber can improve both the gas analyzer performance and the portability. Ionization chamber miniaturization results in a higher surface to volume ratio, which should translate into a more sensitive signal. A small ionization chamber should also result in a higher signal-to-noise ratio. In addition to that, the smaller the ionization chamber volume is, the smaller the gas sample volume can be (without depleting signal intensity), and for a fixed carrier gas flow rate, this means also a faster analysis time and lower consumption of the carrier gas. Low carrier gas consumption is important to enable reduction in carrier gas cylinder size (reducing gas analyzer weight) and maintain carrier gas cylinder autonomy, both important qualities for portable GC-PID.

Recent scientific publications reveal innovations in the miniaturization of photoionization detectors, also in reducing the size of the ionization chamber. Table 1 shows the main publications, the type of PID, materials, design features and ionization chamber volume. These improvements in the ionization chamber volume were made possible by using silicon or etching based microfabrication techniques. Such techniques are often complex and expensive. This work proposes a new µPID prototype that uses simpler fabrication techniques, allows easy mount-dismount of the device components, easy exchange of parts and small ionization chamber volume.

Micro PID

This section describes the µPID design fabricated by micro milling and electrical discharge machining. The components are simply assembled together, dispensing any chemical or physical bonding process. Initially, the design, main components, fabrication and assembly are presented (divided in two sections: Core and shell), followed by a description of the electrode coating.

Design, Fabrication and Assembly

The detector has two main structures: The core, which is the heart of the device, responsible for the detection; and the shell, whose main functions are to clamp together all parts and enable the world-to-chip connections (electronic and fluidic).

Core

The components of the core when assembled yield the microchannel where the gas sample flows, which is the ionization chamber, where the detection of the sample occurs. The main parts of the core are numbered in Figure 1. The top and bottom poly(methyl methacrylate) (PMMA) are made using micro milling and the electrodes are fabricated with electrical discharge machining (both processes with tolerances ±0.02 mm). PMMA was chosen due to its transparency (to facilitate visual inspection of the assembly), its structural stability, chemical inertness to the VOC gas samples used and its low porosity (minimal sorption-desorption of the gasses passing through the ionization chamber).

Copper was chosen for the electrodes owing to its high electrical conductivity and ready availability. The O-rings are made of Viton (ERIKS, Utrecht, The Netherlands), since its softness is required to avoid breaking the UV window which is fragile material (MgF 2 ). In addition, a commercial UV lamp (Baseline MOCON, Lyons, CO, USA) with 10.6 eV output was used.

The bottom layer of PMMA constitutes the bottom wall of the microchannel, which contains two micro-pores of 500 µm diameter to form the inlet/outlet of the channel. It also has two holes to fit the electronic connectors to the copper electrodes. The electrodes fit on top of the bottom PMMA to form the lateral wall of the microchannel. The elevation in the bottom PMMA serves both to fit the electrodes to the right position in order to constitute the channel and to electrically isolate the two sides of the electrodes. To enclose the microchannel (ionization chamber), the UV lamp is placed on top of the copper electrodes, two O-rings with 1 mm cross section fit in the pockets designed on the top PMMA, which closes the channel and ensures that the lamp is fitted to the right position in the center of the fluidic path between the inlet/outlet pores. The top layer of PMMA is designed to fit tightly to the bottom PMMA layer to ensure alignment of the components and reduce the probability of leaks.

micro-pores of 500 µm diameter to form the inlet/outlet of the channel. It also has two holes to fit the electronic connectors to the copper electrodes. The electrodes fit on top of the bottom PMMA to form the lateral wall of the microchannel. The elevation in the bottom PMMA serves both to fit the electrodes to the right position in order to constitute the channel and to electrically isolate the two sides of the electrodes. To enclose the microchannel (ionization chamber), the UV lamp is placed on top of the copper electrodes, two O-rings with 1 mm cross section fit in the pockets designed on the top PMMA, which closes the channel and ensures that the lamp is fitted to the right position in the center of the fluidic path between the inlet/outlet pores. The top layer of PMMA is designed to fit tightly to the bottom PMMA layer to ensure alignment of the components and reduce the probability of leaks.

Shell

Another important structure is the shell of the device, which mechanically presses all the components together to minimize leakage and facilitates world-to-chip connectivity (fluidic and electronic). The electronic connections are to power the UV lamp, apply voltage on the electrodes and acquire the photoionization current. The fluidic connections connect the sample inlet and sample outlet. The main components of the shell are indicated in Figure 2. The top and bottom shells were made of PVC and fabricated using micro milling. The bottom shell has a groove to fit the spring and copper tape, which makes the electronic connection to the copper electrodes. A wire is soldered to the copper tape to connect the power supply and photoionization current signal acquisition. The bottom shell also has O-ring pockets (O-rings dimensions are: 3 mm outside diameter, 1 mm cross section and 1 mm internal diameter) on its top surface to connect the inlet/outlet gas sample to the ionization chamber. The O-rings are also made of Viton. The bottom surface of the bottom shell

Shell

Another important structure is the shell of the device, which mechanically presses all the components together to minimize leakage and facilitates world-to-chip connectivity (fluidic and electronic). The electronic connections are to power the UV lamp, apply voltage on the electrodes and acquire the photoionization current. The fluidic connections connect the sample inlet and sample outlet. The main components of the shell are indicated in Figure 2. The top and bottom shells were made of PVC and fabricated using micro milling. The bottom shell has a groove to fit the spring and copper tape, which makes the electronic connection to the copper electrodes. A wire is soldered to the copper tape to connect the power supply and photoionization current signal acquisition. The bottom shell also has O-ring pockets (O-rings dimensions are: 3 mm outside diameter, 1 mm cross section and 1 mm internal diameter) on its top surface to connect the inlet/outlet gas sample to the ionization chamber. The O-rings are also made of Viton. The bottom surface of the bottom shell contains M3 threads to assemble the FESTO connector type QSM-M3-2 (Festo AG & Co. KG, Esslingen am Neckar, Germany), which is a plug in connector to 2 mm outside diameter tubing.

The top shell fits the top PMMA and positions the core so that it is aligned with the fluidic connections. It also encloses the UV lamp for safety measures. Two lateral pins are designed to slide freely and fit an electronic spring pin that reaches the lateral electrodes of the UV lamp. The spring allows freedom of the pin movement after touching the lamp, which is important to avoid impact and load being transferred to the fragile glass lamp. When the pins reach the right position, two M3 screws on top of the pins aid in fixing the pin to the right position. The lateral pins also ensure complete enclosing of the UV lamp to avoid its light. The top and bottom shells have passing holes for M3 screws that press all the parts at appropriate pressure to ensure fluidic sealing and structural stability, meaning no part would move unnecessarily during operation of the device.

stability, meaning no part would move unnecessarily during operation of the device.

Using no bonding might give rise to undesirable leakage. However, in comparison with other µPID designs [START_REF] Zhu | Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor 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][START_REF] Narayanan | Characterization of a micro-helium discharge detector for gas chromatography[END_REF][START_REF] Akbar | Chip-scale gas chromatography: from injection through detection[END_REF][START_REF] Zhou | A fully automated portable gas chromatography system for sensitive and rapid quantification of volatile organic compounds in water[END_REF], the device presented here has the following advantages: (1) Easy prototyping, since the fabrication techniques are more straightforward than other approaches adopted in the literature; (2) Lower fabrication cost, this device does not require clean-room complex and high-cost facilities and processes. In addition, the materials used are cheaper compared to silicon wafers and it is easy to exchange different microchannel geometries, if needed; (3) Easy mountdismount, which facilitates maintenance and ease of substitution of components such as the UV lamp (e.g. to use lamps with different energies).

Ionization Chamber

The ionization chamber of the µPID is the microchannel formed between the bottom PMMA, top PMMA, lamp and electrodes. This design proposed can produce an ionization chamber with volume up to 100 times lower compared to some commercial PIDs. The height of the channel, 500 µm, is the thickness of the copper plate and the width is initially chosen to keep the aspect ratio of the microchannel close to 1, since a wider channel could increase unnecessarily the chamber volume and a smaller width could reduce excessively the UV illumination area. The length and shape of the microchannel are designed to maximize the use of the illumination diameter of the commercial UV lamp (estimated illumination diameter ~6 mm).

Important channel properties are: (1) Width, representing the distance between the copper electrodes; (2) Height, directly proportional to the electrode area; (3) Electrode area, which depends on the shape and the height of the microchannel; (4) Lamp illumination area, surface of the channel illuminated by the UV lamp and (5) Volume of the channel from inlet to outlet, which corresponds to the ionization chamber volume. Figure 3 shows three different electrodes/channel designs and Table 2 presents their dimensions. The three shapes displayed have the objective to change gradually the area of the electrodes and ionization chamber volume. Using no bonding might give rise to undesirable leakage. However, in comparison with other µPID designs [START_REF] Zhu | Flow-through microfluidic photoionization detectors for rapid and highly sensitive vapor 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][START_REF] Narayanan | Characterization of a micro-helium discharge detector for gas chromatography[END_REF][START_REF] Akbar | Chip-scale gas chromatography: from injection through detection[END_REF][START_REF] Zhou | A fully automated portable gas chromatography system for sensitive and rapid quantification of volatile organic compounds in water[END_REF], the device presented here has the following advantages: (1) Easy prototyping, since the fabrication techniques are more straightforward than other approaches adopted in the literature; (2) Lower fabrication cost, this device does not require clean-room complex and high-cost facilities and processes. In addition, the materials used are cheaper compared to silicon wafers and it is easy to exchange different microchannel geometries, if needed; (3) Easy mount-dismount, which facilitates maintenance and ease of substitution of components such as the UV lamp (e.g., to use lamps with different energies).

Ionization Chamber

The ionization chamber of the µPID is the microchannel formed between the bottom PMMA, top PMMA, lamp and electrodes. This design proposed can produce an ionization chamber with volume up to 100 times lower compared to some commercial PIDs. The height of the channel, 500 µm, is the thickness of the copper plate and the width is initially chosen to keep the aspect ratio of the microchannel close to 1, since a wider channel could increase unnecessarily the chamber volume and a smaller width could reduce excessively the UV illumination area. The length and shape of the microchannel are designed to maximize the use of the illumination diameter of the commercial UV lamp (estimated illumination diameter ~6 mm).

Important channel properties are: (1) Width, representing the distance between the copper electrodes; (2) Height, directly proportional to the electrode area; (3) Electrode area, which depends on the shape and the height of the microchannel; (4) Lamp illumination area, surface of the channel illuminated by the UV lamp and (5) Volume of the channel from inlet to outlet, which corresponds to the ionization chamber volume. Figure 3 shows three different electrodes/channel designs and Table 2 presents their dimensions. The three shapes displayed have the objective to change gradually the area of the electrodes and ionization chamber volume.

Coating Shield

When the copper electrodes are exposed to radiation at a frequency higher than the copper work function, electrons are ejected from the surface and cause signal noise. Therefore, it is desirable to protect the electrodes from the high-energy radiation coming from the UV lamp. A possible solution is to create an UV light coating shield on the copper surface. Ideally, a good shield will filter out all the radiation with an energy greater than the work function of the metal used as electrodes (in the case of copper, 4.48-4.94 eV [START_REF] Haynes | CRC Handbook of Chemistry and Physics[END_REF]). In addition to that, it should be thin to ensure that the thickness of the coating does not increase the volume of the ionization chamber. Moreover, it must not cover the area of the electrode used for ion and electron collection. Other important characteristics are: Chemical compatibility with the electrode materials and gas samples, resistance to temperature oscillation, humidity and mechanical deformation. Two materials were considered in this paper for use as an electrode shield: Diamond-like carbon (DLC) and polymethyl methacrylate (PMMA).

DLC Coating

Diamond-like carbon is a metastable form of amorphous carbon that can be used as a protective coating. It can be applied as thin film with dimensions ranging from a few nm to µm [START_REF] Rahman | Nanostructures in Electronics and Photonics[END_REF]. Even quite thin films can block unwanted radiation, which renders it potentially suitable as an electrode coating in this application. A 0.1 µm thickness layer of DLC can have 5-18% UV transparency at 350-450 nm wavelength range and 0.2 µm DLC thickness yields 2-8 % UV transparency [START_REF] Rahman | Nanostructures in Electronics and Photonics[END_REF]. Also, DLC preferentially blocks short wavelengths [START_REF] Mednikarov | Optical properties of diamond-like carbon and nanocrystalline diamond films[END_REF][START_REF] Lin | Optical properties of diamond-like carbon films for antireflection coating by RF magnetron sputtering method[END_REF].

A first sample of DLC coating was produced on one side of a 1 mm thick copper plate. Before the coating was applied, the surfaces were polished and etched (to remove impurities) for better adhesion of the DLC. The coating was made using plasma enhanced chemical vapor deposition and had 1 µm thickness. Figure 4 shows the difference between the coated and uncoated side. It can be seen that the DLC coating yields a grey surface on top of the copper. This sample started to peel off after a few weeks, likely due to exposure to sunlight and summer temperatures/humidity. Figure 4c shows the state of the peeled of sample. The scanning electron microscopy (SEM) images on Figure 5 shows that the DLC coating detached from copper like a peeled off thin foil, suggesting poor adhesion between the coating and substrate. A second sample was produced on the same type of substrate, this time incorporating a titanium adhesive layer between the DLC and copper to increase adhesion. The new sample had considerably better stability and did not fail (crack or break) during the test period.

Coating Shield

When the copper electrodes are exposed to radiation at a frequency higher than the copper work function, electrons are ejected from the surface and cause signal noise. Therefore, it is desirable to protect the electrodes from the high-energy radiation coming from the UV lamp. A possible solution is to create an UV light coating shield on the copper surface. Ideally, a good shield will filter out all the radiation with an energy greater than the work function of the metal used as electrodes (in the case of copper, 4.48-4.94 eV [START_REF] Haynes | CRC Handbook of Chemistry and Physics[END_REF]). In addition to that, it should be thin to ensure that the thickness of the coating does not increase the volume of the ionization chamber. Moreover, it must not cover the area of the electrode used for ion and electron collection. Other important characteristics are: Chemical compatibility with the electrode materials and gas samples, resistance to temperature oscillation, humidity and mechanical deformation. Two materials were considered in this paper for use as an electrode shield: Diamond-like carbon (DLC) and polymethyl methacrylate (PMMA).

DLC Coating

Diamond-like carbon is a metastable form of amorphous carbon that can be used as a protective coating. It can be applied as thin film with dimensions ranging from a few nm to µm [START_REF] Rahman | Nanostructures in Electronics and Photonics[END_REF]. Even quite thin films can block unwanted radiation, which renders it potentially suitable as an electrode coating in this application. A 0.1 µm thickness layer of DLC can have 5-18% UV transparency at 350-450 nm wavelength range and 0.2 µm DLC thickness yields 2-8 % UV transparency [START_REF] Rahman | Nanostructures in Electronics and Photonics[END_REF]. Also, DLC preferentially blocks short wavelengths [START_REF] Mednikarov | Optical properties of diamond-like carbon and nanocrystalline diamond films[END_REF][START_REF] Lin | Optical properties of diamond-like carbon films for antireflection coating by RF magnetron sputtering method[END_REF].

A first sample of DLC coating was produced on one side of a 1 mm thick copper plate. Before the coating was applied, the surfaces were polished and etched (to remove impurities) for better adhesion of the DLC. The coating was made using plasma enhanced chemical vapor deposition and had 1 µm thickness. Figure 4 shows the difference between the coated and uncoated side. It can be seen that the DLC coating yields a grey surface on top of the copper. This sample started to peel off after a few weeks, likely due to exposure to sunlight and summer temperatures/humidity. Figure 4c shows the state of the peeled of sample. The scanning electron microscopy (SEM) images on Figure 5 shows that the DLC coating detached from copper like a peeled off thin foil, suggesting poor adhesion between the coating and substrate. A second sample was produced on the same type of substrate, this time incorporating a titanium adhesive layer between the DLC and copper to increase adhesion. The new sample had considerably better stability and did not fail (crack or break) during the test period.

PMMA Coating

The second type of electrode coating considered is PMMA because it does not transmit wavelengths below 360 nm [START_REF] Joram | Transmission Curves of Plexiglass (PMMA) and Optical Grease[END_REF], which should be sufficient to avoid photoelectric effects in the copper electrodes. PMMA is a relatively cheap material compared to DLC and can be deposited as a top layer on the electrodes by spin coating, a process that is not expensive and does not require complex machinery. Disadvantages to be considered are: (1) Difficulty to produce perfectly flat surfaces to avoid leakage; (2) difficulty to produce very thin layers (< 0.1 µm) and ( 3) during fabrication, the PMMA might run inside of the channel, isolating the electrodes from the ions, then reducing the active area inducing a lower signal.

Results

Toluene was chosen as the representative compound of the VOCs for all the following aiming at characterizing the µPID signal where different parameters were changed, such as voltage applied to the electrodes or total gas flow rate.

Experimental Setup and Material

An experimental setup was built to evaluate the µPID prototype performance, represented in Figure 6. A gas sample supply, containing either pure nitrogen or toluene 100 ppm was connected to a mass flow controller (MFC), Bronkhorst El Flow Select F-201CV-050-AAD-33-V (Bronkhorst High-Tech BV., AK Ruurlo, The Netherlands), with a full scale of 20 mL/min and error of 0.5% of reading plus 0.1% of full scale. 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), Bronkhorst El Flow Select F-100D-AAD-33-V, full scale 20 mL/min and same error as the MFC. To power up the UV lamp, the lateral electrodes of the lamp are plugged inside the lamp drive circuit of the commercial Baseline MOCON PID. This circuit is powered by a power supply at 4V and yields a current of about 35 mA. The power supply was also used to provide the voltage on the electrodes in the ionization chamber and the digital electrometer Keithley 616A (Keithley Instruments, Solon, OH, USA) was used to measure the current signal.

PMMA Coating

The second type of electrode coating considered is PMMA because it does not transmit wavelengths below 360 nm [START_REF] Joram | Transmission Curves of Plexiglass (PMMA) and Optical Grease[END_REF], which should be sufficient to avoid photoelectric effects in the copper electrodes. PMMA is a relatively cheap material compared to DLC and can be deposited as a top layer on the electrodes by spin coating, a process that is not expensive and does not require complex machinery. Disadvantages to be considered are: (1) Difficulty to produce perfectly flat surfaces to avoid leakage; (2) difficulty to produce very thin layers (< 0.1 µm) and (3) during fabrication, the PMMA might run inside of the channel, isolating the electrodes from the ions, then reducing the active area inducing a lower signal.

Results

Toluene was chosen as the representative compound of the VOCs for all the following experiments aiming at characterizing the µPID signal where different parameters were changed, such as voltage applied to the electrodes or total gas flow rate.

Experimental Setup and Material

An experimental setup was built to evaluate the µPID prototype performance, represented in Figure 6. A gas sample supply, containing either pure nitrogen or toluene 100 ppm was connected to a mass flow controller (MFC), Bronkhorst El Flow Select F-201CV-050-AAD-33-V (Bronkhorst High-Tech BV., AK Ruurlo, The Netherlands), with a full scale of 20 mL/min and error of 0.5% of reading plus 0.1% of full scale. 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), Bronkhorst El Flow Select F-100D-AAD-33-V, full scale 20 mL/min and same error as the MFC. To power up the UV lamp, the lateral electrodes of the lamp are plugged inside the lamp drive circuit of the commercial Baseline MOCON PID. This circuit is powered by a power supply at 4V and yields a current of about 35 mA. The power supply was also used to provide the voltage on the electrodes in the ionization chamber and the digital electrometer Keithley 616A (Keithley Instruments, Solon, OH, USA) was used to measure the current signal.

Optical Microscope Images

In order to investigate the effects of the fabrication and design, digital optical microscope images were taken from the microchannel formed by the electrodes and the bottom PMMA. Figure 7a shows the image of channel 1 previously used in experiments with the UV lamp. It is possible to notice

Optical Microscope Images

In order to investigate the effects of the fabrication and design, digital optical microscope images were taken from the microchannel formed by the electrodes and the bottom PMMA. Figure 7a shows the image of channel 1 previously used in experiments with the UV lamp. It is possible to notice round circles marked by the use of the lamp, the central circle suggest that UV light is more intense within a diameter of approximately 4.5 mm. The image zoomed on the fitting of PMMA and electrodes (Figure 7b) reveal imperfections of the fitting between them, which can lead to gaps ranging from 15-60 µm. The consequence of those gaps will be leakage and an undesirable variation in the distance between the electrodes. The fitting of electrodes presented in Figure 7a presented an average channel width of 504 µm (5.5% error), while the channel 2 shown in Figure 7c presented considerable deviations in the nominal value of channel width, varying from 454 µm to 548 µm.

Optical Microscope Images

In order to investigate the effects of the fabrication and design, digital optical microscope images were taken from the microchannel formed by the electrodes and the bottom PMMA. Figure 7a shows the image of channel 1 previously used in experiments with the UV lamp. It is possible to notice round circles marked by the use of the lamp, the central circle suggest that UV light is more intense within a diameter of approximately 4.5 mm. The image zoomed on the fitting of PMMA and electrodes (Figure 7b) reveal imperfections of the fitting between them, which can lead to gaps ranging from 15-60 µm. The consequence of those gaps will be leakage and an undesirable variation in the distance between the electrodes. The fitting of electrodes presented in Figure 7a presented an average channel width of 504 µm (5.5% error), while the channel 2 shown in Figure 7c presented considerable deviations in the nominal value of channel width, varying from 454 µm to 548 µm.

Experimental Results and Discussion

The signals from the µPID were obtained without an amplifier, which should be developed in the future in order to improve the signal/noise ratio and thus the sensitivity. The multimeter (Keithley 616A) acquired µPID signals ranging from about 70 pA to 3.5 nA. The current standard uncertainty is calculated considering two main factors: (i) Multimeter resolution ( ), which varies from 0.1 pA to 10 pA depending on the current measurement range and (ii) measurement repeatability, thus the combined standard uncertainty is given by =

The standard uncertainty calculated due to resolution and repeatability are obtained with

= /√3 (2)
And

= ( )/ √ (3)

Experimental Results and Discussion

The signals from the µPID were obtained without an amplifier, which should be developed in the future in order to improve the signal/noise ratio and thus the sensitivity. The multimeter (Keithley 616A) acquired µPID signals ranging from about 70 pA to 3.5 nA. The current standard uncertainty is calculated considering two main factors: (i) Multimeter resolution (R), which varies from 0.1 pA to 10 pA depending on the current measurement range and (ii) measurement repeatability, thus the combined standard uncertainty is given by

u 2 C = u 2 1 + u 2 2 (1)
The standard uncertainty calculated due to resolution and repeatability are obtained with

u 1 = R/ √ 3 (2)
and

u 2 = σ(X)/ √ n (3)
where σ(X) is the standard deviation of the sample and n are the number of samples. The expanded uncertainty (U) is calculated by

U = t • u C ( 4
)
where t is the Student's distribution factor for 95% confidence interval with v degrees of freedom calculated by the Welch-Satterthwaite Equation.

Figure 8 shows the µPID signal for channel 3 using toluene 100 ppm gas sample when voltage and flow rate are changed (leakage as a function of flow rate is presented in the supplementary file Figure S1). A range of voltages was applied from 0.1 V to 30 V and flow rates of 0.5, 10 and Micromachines 2019, 10, 228 9 of 12 20 mL/min were used. To obtain the data corresponding to the three flow rates, the µPID signal was stabilized, which lasted about 2h for 0.5 mL/min and 1h for 10 mL/min and 20 mL/min, this time being necessary to remove humidity traces which may perturb the signal. After stabilization, the voltage was varied from 0.1 to 30 V for each flow rate. Each point displayed on Figure 8 corresponds to an average of 10 signal measurements taken within 2 min interval at each flow rate and voltage. The repeatability is calculated individually for each point and corresponds to the standard deviation of the 10 measurements. The relative uncertainty of the µPID signal (U/Signal) varies from 7.5% to 0.1% at low and high voltages respectively. signal measurements taken within 2 min interval at each flow rate and voltage. The repeatability is calculated individually for each point and corresponds to the standard deviation of the 10 measurements. The relative uncertainty of the µPID signal ( / ) varies from 7.5% to 0.1% at low and high voltages respectively. The three flow rate datasets (0.5, 10 and 20 mL/min) present similar curve shape and the signal increases with flow rate. Two linear regions of the data can be identified: (i) 0.1 V to 2.5 V and (ii) 5V to 30 V. The slope of the first region is ~ 25 higher than the second region. At 5 V, signals for 0.5, 10 and 20 mL/min are about 1.5, 1.8 and 2.2 nA, meaning that low flow rates might not be so detrimental for signal generation.

For gas analyzer portability, low flow rate and low voltage are desirable specifications, which means longer duration of portable carrier gas cylinder and longer battery life. As an initial analysis from Figure 8, a voltage of 5 V and flow rate 2 mL/min, might be a good choice to yield best cost benefit of portability vs. µPID signal level. However, since the currents generated by the device are very low, and those experiments were performed without a signal amplifier, the flow rate and voltages yielding the highest signal possible were chosen for the rest of the work and in particularly to obtain the results shown in Figures 9 and10. In order to compare the performance of the three channels fabricated, Figure 9 presents the current signal for nitrogen and toluene 100 ppm for each of the three channels. The gas sample was injected at 20 mL/min and 30 V was applied on the electrodes. The current was measured after signal stabilization and five measurements were taken, the values displayed on Figure 9 are the average. The same procedure was adopted to calculate the uncertainties of this data. However, the repeatability corresponds to the average of the five measurements taken after stabilization. The values of uncertainties are given in the table of Figure 9 and the highest yielded value was 1.9%. The three flow rate datasets (0.5, 10 and 20 mL/min) present similar curve shape and the signal increases with flow rate. Two linear regions of the data can be identified: (i) 0.1 V to 2.5 V and (ii) 5V to 30 V. The slope of the first region is ~25 higher than the second region. At 5 V, signals for 0.5, 10 and 20 mL/min are about 1.5, 1.8 and 2.2 nA, meaning that low flow rates might not be so detrimental for signal generation.

For gas analyzer portability, low flow rate and low voltage are desirable specifications, which means longer duration of portable carrier gas cylinder and longer battery life. As an initial analysis from Figure 8, a voltage of 5 V and flow rate 2 mL/min, might be a good choice to yield best cost benefit of portability vs. µPID signal level. However, since the currents generated by the device are very low, and those experiments were performed without a signal amplifier, the flow rate and voltages yielding the highest signal possible were chosen for the rest of the work and in particularly to obtain the results shown in Figures 9 and10.

Micromachines 2019, 10, x 10 of 12

When comparing the signal for the three channels, it is evident that higher area yields higher signal. Also, the difference between the nitrogen and toluene 100 ppm signal increases, meaning higher sensitivity and possible better detection limit. Channels 1 and 2 have both 500 µm as nominal channel width, and it is interesting to notice that the proportion of area increase from channel 1 to channel 2 is similar to the signal increase observed, about 2 times higher. It is important to note that the signal of channel 3 for toluene 100 ppm was taken on a different day, yielding a level slightly higher than that observed in Figure 8. Because the signal is higher with channel 3, the following measurements were performed in this configuration. Figure 10 shows signal of channel 3 for various toluene concentrations, ranging from 1 to 100 ppm. Again, the same conditions of voltage and flow were applied to get this data (30 V, 20 mL/min). Before and after measuring the signal of toluene, the µPID was purged with nitrogen. The data shown corresponds to the values of the signal for toluene at each concentration subtracted by stable. However, the fabrication technique should be improved for adhesion only on top surface of the electrodes.

Optical microscope images of microchannels shows that electrodes might not fit well on bottom PMMA, possibly leading to distortion in the width of the microchannel and moderate leakage. µPID signal level increases with flow rate, voltage and electrode area. Channel 3 has highest signal and electrode area. However, it has the highest ionization chamber volume. The device presents an estimated detection limit of 0.6 ppm for toluene. Future design improvements should be done to mitigate leakage, improve electrodes fitting on bottom PMMA. Future evaluations will be done on response time, residence time, possible adsorption of analytes on the interior surfaces, influence of pressure and humidity on the signal. The device will also be compared to commercial detectors and coupled to a separation column and pre-concentrator.

Figure 1 .

 1 Figure 1. Assembled and exploded view of the detector core. (1) Top poly(methyl methacrylate) (PMMA), (2) O-ring 1, (3) UV lamp, (4) O-ring 2, (5) shield, (6) copper electrodes, (7) bottom PMMA, (8) electrode connection holes, (9) elevation structure, (10) micro-pores.

Figure 1 .

 1 Figure 1. Assembled and exploded view of the detector core. (1) Top poly(methyl methacrylate) (PMMA), (2) O-ring 1, (3) UV lamp, (4) O-ring 2, (5) shield, (6) copper electrodes, (7) bottom PMMA, (8) electrode connection holes, (9) elevation structure, (10) micro-pores.

Figure 2 .

 2 Figure 2. Isometric and cut view of the shell. (1) Bottom shell, (2) top shell, (3) lateral pin, (4) electrodes connection, (5) fluidic connections, (6) M3 pin fix screws, (7) M3 screw.

Figure 2 .

 2 Figure 2. Isometric and cut view of the shell. (1) Bottom shell, (2) top shell, (3) lateral pin, (4) electrodes connection, (5) fluidic connections, (6) M3 pin fix screws, (7) M3 screw.

Figure

  Figure Microchannels for ionization chamber.

Figure 4 .

 4 Figure 4. Diamond-like carbon (DLC) coating before and after failure. (a) Uncoated side before failure, (b) Coated side before failure, (c) Coated side after failure (4.5 weeks).

Figure 4 .

 4 Figure 4. Diamond-like carbon (DLC) coating before and after failure. (a) Uncoated side before failure, (b) Coated side before failure, (c) Coated side after failure (4.5 weeks).

Figure 4 .Figure 5 .

 45 Figure 4. Diamond-like carbon (DLC) coating before and after failure. (a) Uncoated side before failure, (b) Coated side before failure, (c) Coated side after failure (4.5 weeks).

Figure 5 .

 5 Figure 5. Scanning electron microscopy (SEM) images of the DLC coating failure.

Micromachines 2019, 10 , x 8 of 12 Figure 6 .

 10126 Figure 6. Experimental setup used for signal measurements from µPID prototype.

Figure 6 .

 6 Figure 6. Experimental setup used for signal measurements from µPID prototype.

Figure 6 .

 6 Figure 6. Experimental setup used for signal measurements from µPID prototype.

Figure 7 .

 7 Figure 7. Digital optical microscope images of copper electrodes assembled on bottom polymethyl methacrylate (PMMA). (a) Channel 1; (b) Zoom at bottom PMMA and copper electrodes assembly; (c) Channel 2.

Figure 7 .

 7 Figure 7. Digital optical microscope images of copper electrodes assembled on bottom polymethyl methacrylate (PMMA). (a) Channel 1; (b) Zoom at bottom PMMA and copper electrodes assembly; (c) Channel 2.

Figure 8 .

 8 Figure 8. Channel 3 signal with variation of voltage on copper electrodes for various gas sample flowrates. Uncertainty bars are not displayed in the graphic since their size is small compared to the symbols representing the points.

Figure 8 .

 8 Figure 8. Channel 3 signal with variation of voltage on copper electrodes for various gas sample flow-rates. Uncertainty bars are not displayed in the graphic since their size is small compared to the symbols representing the points.

Figure 9 .

 9 Figure 9. Signal for Channels 1, 2 and 3 using nitrogen and toluene 100 ppm as gas samples. Flow rate 20 mL/min and 30 V on electrodes. Relative uncertainty for signals of Channel 1 with nitrogen and toluene 100 ppm are 1.9% and 0.3%. Relative uncertainty for signals of Channel 2 with nitrogen and toluene 100 ppm are 0.8% and 0.2% respectively. Relative uncertainty for signals of Channel 3 with nitrogen and toluene 100 ppm are both 0.4%.

Figure 9 .

 9 Figure 9. Signal for Channels 1, 2 and 3 using nitrogen and toluene 100 ppm as gas samples. Flow rate 20 mL/min and 30 V on electrodes. Relative uncertainty for signals of Channel 1 with nitrogen and toluene 100 ppm are 1.9% and 0.3%. Relative uncertainty for signals of Channel 2 with nitrogen and toluene 100 ppm are 0.8% and 0.2% respectively. Relative uncertainty for signals of Channel 3 with nitrogen and toluene 100 ppm are both 0.4%.

Table 1 .

 1 Main works on microfluidic photoionization detector (µPIDs).

	Reference	Ionization Source	Manufacturing Main Materials	Design Main Features and Dimensions	Ionization Chamber
	[14]	UV Lamp, 10.6 eV	-	Introduced nozzle inside a conventional ionization chamber.	10 µL
	[15]	UV Lamp, 10.6 eV	Highly doped p-type <100> single-sided polished conductive Si wafers with resistivity 0.001-0.005 thick Pyrex glass wafers. Ω.cm and 380 µm thickness; 500 µm	Ionization chamber is a microchannel with cross-section 150 µm (width), by lamp is 2.4 mm × 2.4 mm. × 15 mm. Microchannel area covered 380 µm (depth) and length 2.3 cm. Entire overall channel size is 15 mm	1.3 µL
	[16]	UV Lamp, 10.6 eV	Conductive p-type <100> silicon wafer and glass.	Channel etched 380 µm (width) × 380 µm (depth) × 2 cm (length).	0.5 µL
				Micro separation column fabricated
	[17]	Helium discharge	Silicon and glass architecture.	on the same chip. Overall size (1.5 cm	Not mentioned
				× 3 cm)
			500 µm thick p-type <100> double	Microchannels formed by Si and glass.
			side polished Si wafer with 500 nm	Three main channels: 1) Auxiliary
	[18]	Helium discharge	thick thermal oxide layers; 100 µm	helium; 2) Analytes; 3) Outlet channel.	1.4 µL
			thick Borofloat 33 glass wafer; 500 µm	Cross-section 380 µm (width) and 500
			thick Borofloat 33 glass wafer.	µm (depth);
			Two (bottom and top) Borosilicate
	[19]	Helium discharge	glass wafers 700 µm thickness and	Channel etched 250 µm (depth).	Not mentioned
			100 mm diameter used as substrate.
	This work	UV Lamp, 10.6 eV	Micromilled PMMA and PVC. Copper plate.	Modular assembly of components. vary between 400 and 500 µm. No use of glue. Microchannels width	1.75-6.42 µL

Table 2 .

 2 Ionization chamber properties.

	Channel n	w [µm]	A electrodes [mm 2 ]	V inlet/outlet [µL]	A/A 1	V/V 1
	1	500	5.98	1.75	1.0	1.0
	2	500	12.67	2.75	2.1	1.6
	3	400	31.17	6.42	5.2	3.7

Figure 3 .

 3 Microchannels for ionization chamber.

Table 2 .

 2 Ionization chamber properties.

	Channel n	w [µm]	Aelectrodes [mm²]	Vinlet/outlet [µL]	A/A1	V/V1
	1	500	5.98	1.75	1.0	1.0
	2	500	12.67	2.75	2.1	1.6
	3	400	31.17	6.42	5.2	3.7

Micromachines 2019, 10, 228; doi:10.3390/mi10040228 www.mdpi.com/journal/micromachines

Micromachines 2019, 10, x

Micromachines 2019,[START_REF] Rezende | Micro photoionization detectors[END_REF] 228

Acknowledgments:

The authors are grateful to the Bernal Institute, the University of Limerick, Karlsruhe Institute of Technology, Centre national de la recherche scientifique (CNRS), the University of Strasbourg and In'Air Solutions.

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

as in Figures 8 and9, yielding a maximum of error of 2%.

To estimate the detection limit, the maximum variation of the blank within a 2 min interval was obtained (after stabilizing) ∆ = 0.005 nA. The detection limit for toluene is the concentration corresponding to 3 × ∆ (0.015 nA), which is estimated from a rule of three with the lowest concentration investigated, i.e. 2.5 ppm and 0.063 nA from the graph. The resulting estimated detection limit is therefore 0.6 ppm. In order to compare the performance of the three channels fabricated, Figure 9 presents the current signal for nitrogen and toluene 100 ppm for each of the three channels. The gas sample was injected at 20 mL/min and 30 V was applied on the electrodes. The current was measured after signal stabilization and five measurements were taken, the values displayed on Figure 9 are the average. The same procedure was adopted to calculate the uncertainties of this data. However, the repeatability corresponds to the average of the five measurements taken after stabilization. The values of uncertainties are given in the table of Figure 9 and the highest yielded value was 1.9%.

When comparing the signal for the three channels, it is evident that higher area yields higher signal. Also, the difference between the nitrogen and toluene 100 ppm signal increases, meaning higher sensitivity and possible better detection limit. Channels 1 and 2 have both 500 µm as nominal channel width, and it is interesting to notice that the proportion of area increase from channel 1 to channel 2 is similar to the signal increase observed, about 2 times higher. It is important to note that the signal of channel 3 for toluene 100 ppm was taken on a different day, yielding a level slightly higher than that observed in Figure 8.

Because the signal is higher with channel 3, the following measurements were performed in this configuration. Figure 10 shows signal of channel 3 for various toluene concentrations, ranging from 1 to 100 ppm. Again, the same conditions of voltage and flow were applied to get this data (30 V, 20 mL/min). Before and after measuring the signal of toluene, the µPID was purged with nitrogen. The data shown corresponds to the values of the signal for toluene at each concentration subtracted by the average of the purge signal (pure N 2 ) obtained before and after the toluene signal acquisition. It can be noticed that up to 90 ppm, the response of the µPID is linear and a linear fit can be obtained (r 2 = 0.9996), which is presented in the graphic. The relative error is calculated with same procedure as in Figures 8 and9, yielding a maximum of error of 2%.

To estimate the detection limit, the maximum variation of the blank within a 2 min interval was obtained (after stabilizing) ∆S max = 0.005 nA. The detection limit for toluene is the concentration corresponding to 3 × ∆S max (0.015 nA), which is estimated from a rule of three with the lowest concentration investigated, i.e., 2.5 ppm and 0.063 nA from the graph. The resulting estimated detection limit is therefore 0.6 ppm.

Conclusions

This paper presents the design, fabrication and characterization of micro milled µPID. The design's simple construction allows easy mount-dismount for components change, cleanse or replacement. The ionization chamber volume ranges from 1.75 to 6.42 µL, depending on the electrodes. Two materials have been proposed for electrode shielding. DLC needs to have a titanium adhesive layer for improved stabilization on the surface of copper electrodes. PMMA structures on copper electrodes are Supplementary Materials: The following is available online at http://www.mdpi.com/2072-666X/10/4/228/s1, Figure S1: Leakage for three channel designs. Uncertainty varies from 1% at high flow rates to 5% at low flow-rates.

Conflicts of Interest:

The authors declare no conflict of interest.


Sulaiman Khan
email: sulaiman.khan@ul.ie

David Newport
email: david.newport@ul.ie

Stéphane Le Calvé
email: slecalve@unistra.fr

Development of a Toluene Detector Based on Deep UV Absorption Spectrophotometry Using Glass and Aluminum Capillary Tube Gas Cells with a LED Source

Keywords: ultraviolet light-emitting diode (UV LED), spectrophotometry, UV absorption, gas sensors, Benzene, toluene, ethylbenzene and xylene (BTEX), toluene, hollow core waveguides, capillary tubes

come

Development of a Toluene Detector Based on Deep UV

Absorption Spectrophotometry Using Glass and Aluminum Capillary Tube Gas Cells with a LED Source

Introduction

Monitoring of air quality in indoor spaces is critical for healthy living. Nowadays most of our daily activities are based in indoor spaces where exposure to various indoor air pollutants is inevitable [START_REF] Koppmann | Chemistry of Volatile Organic Compounds in the Atmosphere[END_REF]. Indoor air can contain various volatile organic compounds (VOCs) among other pollutants such as air-borne particles, microorganisms, household odours, and gases. VOCs are organic compounds with a high vapour pressure at room temperature; they readily evaporate into a gaseous phase at room temperature. Some of the common VOCs are acetaldehyde, acetone, benzene, carbon tetra chloride, 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]. Benzene, toluene, ethylbenzene and xylene (BTEX) are aromatic hydrocarbons and are some of the most hazardous pollutants among VOCs. Toluene is a colourless VOC with a sweet, pungent odour, density of 0.866 g•cm -3 at 20 • C, and boiling point of 110.7 • C [START_REF] Patnaik | A Comprehensive Guide to the Hazardous Properties of Chemical Substances[END_REF]. Its sources of generation in indoor spaces are common household items, i.e., cleaning products, paint thinners, adhesives, synthetic fragrances, nail polish, and cigarette smoke. Automobile emissions are the main source of toluene in outdoor air environments [START_REF]Agency for Toxic Substances and Disease Registry (ATSDR)[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]. The American Conference of Governmental Industrial Hygienists (ACGIH) have established the threshold limit value (TLV) of 50 ppm for toluene for 8 h exposure. In addition, the Occupational Safety and Health Administration (OSHA) recommends permissible exposure limits (PEL) for toluene of 200 ppm as the 8 h time-weighted average (TWA) concentration [START_REF] Barsan | NIOSH-Pocket Guide to Chemical Hazards[END_REF].

Detection of aromatics VOCs at ppm and sub-ppm ranges requires a sensitive and accurate method. Different techniques have been applied for the detection of different VOCs, for instance, 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], photoionization detectors [START_REF] Rezende | Micro photoionization detectors[END_REF], piezoelectric-based gas sensors, i.e., surface acoustic wave [START_REF] Jakubik | Surface acoustic wave-based gas sensors[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 forks [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 gas sensors [START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A review[END_REF], and optical sensors 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 ultraviolet (UV) spectrophotometry gas sensors [START_REF] Allouch | miniature, fast and high sensitive real-time analyzers: BTEX detection[END_REF]. Among these, optical gas sensors are highly sensitive, they have minimal drift issues and rapid time responses, and they facilitate real time and in situ measurements without changing the chemical nature of gases.

UV spectrophotometry is a non-destructive, rapid time response with minimal cross-responses to other gases as long as its design is carefully considered. The technique involves direct measurement of a molecular absorption at a specific wavelength which offers an inherently reliable approach for gas sensing with excellent selectivity [START_REF] Hodgkinson | Optical gas sensing: A review[END_REF]. BTEX gases absorb strongly in the deep UV range, i.e., 250-270 nm [START_REF] Tunnicliff | Toluene, Ethyl benzene, 0-Xylene, m-Xylene, and p-Xylene: Determination by ultravoilet spectrophotometry[END_REF], indicating they can be detected using deep UV spectrophotometry.

Recently, deep ultraviolet-light-emitting diodes (UV-LEDs) with narrow bandwidths (<30 nm) have been developed, which matches with the absorption band of many molecules. It provides a portable source without the need of monochromators or filters. LEDs have good stability, robustness, flexibility in output intensity, low power consumption and low heat generation [START_REF] Bui | Analytical devices based on light-emitting diodes-A review of the state-of-the-art[END_REF]. In order to broaden the emission band, an array of LEDs of different wavelength can be used to cover a broad range of molecules [START_REF] Bui | Molecular absorption measurements with an optical fibre coupled array of ultra-violet light-emitting diodes[END_REF]. UV-LEDs have been applied in the detection of different gases, for instance, NO 2 [START_REF] Hawe | Hazardous gas detection using an integrating sphere as a multipass gas absorption cell[END_REF], O 3 [START_REF] Aoyagi | High-Sensitivity Ozone Sensing Using 280 nm Deep Ultraviolet Light-Emitting Diode for Detection of Natural Hazard Ozone[END_REF], SO 2 [START_REF] Degner | Real time exhaust gas sensor with high resolution for onboard sensing of harmful components[END_REF], and BTEX [START_REF] Bui | A deep-UV light-emitting diode-based absorption detector for benzene, toluene, ethylbenzene, and the xylene compounds[END_REF].

In this work, we have applied UV spectrophotometry to detect toluene using a fiber-coupled deep UV-LED with aluminium-based hollow core waveguides (HCWs) coupled to mini spectrometer. We have assembled the fluid and optic parts using 3D connectors, which makes the alignment and sealing of the setup easier.

Materials and Methods

Spectrophotometry

UV absorption spectrophotometry is a direct optical gas detection method that is based on the unique absorption spectra (fingerprints) at a specific wavelength. In spectrophotometry, the molecular absorption level is measured according to the Beer-Lambert law:

A = σcl = log I 0 I ( 1
)
where A is absorbance, σ (cm 2 /molecule) is the absorption cross-section, c (molecules/cm 3 ) is concentration of gas molecules, and l (cm) is length of gas cell. I 0 and I are the transmitted intensities recorded for the background gas (i.e., nitrogen) and toluene gas concentrations in the gas cell, respectively. The constant σ is the absorption cross-section, which is a molecule-specific property and has a constant value at a specific wavelength. It represents the effective area of a molecule that is needed for a photon to transverse the molecule. The sensitivity depends on the optical path length, which is defined by the design of the absorption gas cell. The absorption cell can be single pass, multi-pass, or a resonant cavity. The single pass cell is relatively easy to manufacture, has a quick time response and is relatively easy to couple with light sources and detectors compared to multi-pass or resonant cells. HCW offer a compact and efficient alternative to gas cells. They provide a compact platform for the interaction of photons and gas molecules to realize a quantitative and molecular specific absorption spectroscopy. The HCWs guide the radiation in a leaky-mode and the radiation is propagated by metallic reflection inside the coaxial hollow core. The transmission of UV in HCW depends on material, size and geometry of HCW. HCW with smaller diameter have higher attenuation losses compared to the higher diameter. They have been applied in IR spectrometry and other sensing applications, e.g., biomedical and toxicology.

Instrumentation

A fiber-coupled deep UV-LED (Mightex Systems, Pleasanton, CA, USA) with a peak at 260 nm and power range from 45-80 µW was used. A mini-spectrometer (Hamamatsu mini-spectrometer C10082CH, Iwata, Japan) with a spectral detection range from 200-800 nm and integration time of 10-10,000 ms was used to record the intensity of the transmitted UV light. Two HCWs: one glass capillary tube with an inner wall coated with aluminium (Doku Engineering, Japan) and the other aluminium capillary tube as a waveguide (Advent, London, UK), were tested. The gas cell was coupled with the source and detector using optical fibers (Ocean Optics, Largo, FL, USA) for UV applications (range 200-1100 nm) with a core diameter of 400 µm. The optics and fluidics connections were 3D printed. The connectors were designed using Solidworks 2018 and 3D printed (Ultimaker 3, Geldermalsen, The Netherlands) using Acrylonitrile Butadiene Styrene (ABS). Toluene was delivered from a gas cylinder with concentration of 100 ppm ± 2% (Air Products, Aubervilliers, France). The gaseous flow was controlled via two mass flow controllers (Bronkhorst, Gelderland, The Netherlands) with a full-scale range of 20 mL/min ± 0.5% and 50 mL/min ± 0.5%.

Experimental Setup

The schematic of experimental setup is shown in Figure 1a. In order to ensure a stable intensity, a constant current was supplied to the LED and the emission intensity was varied by changing the input current (0-30 mA). The opto-mechanical components were aligned using 3D printed holders on the optical breadboard to avoid baseline shifts due to mechanical movement. The gas cell was thermally insulated to minimize thermal fluctuations.

Micromachines 2019, 10, x FOR PEER REVIEW 3 of 9

They have been applied in IR spectrometry and other sensing applications, e.g., biomedical and toxicology.

Instrumentation

A fiber-coupled deep UV-LED (Mightex Systems, Pleasanton, CA, USA) with a peak at 260 nm and power range from 45-80 µW was used. A mini-spectrometer (Hamamatsu mini-spectrometer C10082CH, Iwata, Japan) with a spectral detection range from 200-800 nm and integration time of 10-10,000 ms was used to record the intensity of the transmitted UV light. Two HCWs: one glass capillary tube with an inner wall coated with aluminium (Doku Engineering, Japan) and the other aluminium capillary tube as a waveguide (Advent, London, UK), were tested. The gas cell was coupled with the source and detector using optical fibers (Ocean Optics, Largo, FL, USA) for UV applications (range 200-1100 nm) with a core diameter of 400 µm. The optics and fluidics connections were 3D printed. The connectors were designed using Solidworks 2018 and 3D printed (Ultimaker 3, Geldermalsen, The Netherlands) using Acrylonitrile Butadiene Styrene (ABS). Toluene was delivered from a gas cylinder with concentration of 100 ppm ± 2% (Air Products, Aubervilliers, France). The gaseous flow was controlled via two mass flow controllers (Bronkhorst, Gelderland, The Netherlands) with a full-scale range of 20 mL/min ± 0.5% and 50 mL/min ± 0.5%.

Experimental Setup

The schematic of experimental setup is shown in Figure 1a. In order to ensure a stable intensity, a constant current was supplied to the LED and the emission intensity was varied by changing the input current (0-30 mA). The opto-mechanical components were aligned using 3D printed holders on the optical breadboard to avoid baseline shifts due to mechanical movement. The gas cell was thermally insulated to minimize thermal fluctuations.

Different concentrations of toluene were generated in nitrogen using a configuration of mass flow controller (MFC) as shown in Figure 1b. The desired concentration of toluene was obtained by mixing toluene with nitrogen using MFC-2 and MFC-1, respectively. A total gas flow of 40 mL/min was injected into the waveguide gas cell. The sealing of the opto-fluidics connector was tested, and less than 3% leakage was found.

Results and Discussion

The glass HCW and aluminium HCW were investigated for gas sensing applications. The glass HCW was composed of a glass capillary tube with a thin inner coating of aluminium. The aluminium HCW was a capillary tube made of tempered aluminium with 99.7% purity. The lengths of the glass HCW (inner diameter, 1mm; internal gas cell volume, 27 µL) and aluminium HCW (inner dimeter, 2 mm; internal gas cell volume, 157 µL) installed were 34 cm and 50 cm, respectively. Aluminium was selected because it had good reflective and transmittance properties (i.e., attenuation loss, 0.2 dB/m) Different concentrations of toluene were generated in nitrogen using a configuration of mass flow controller (MFC) as shown in Figure 1b. The desired concentration of toluene was obtained by mixing toluene with nitrogen using MFC-2 and MFC-1, respectively. A total gas flow of 40 mL/min was injected into the waveguide gas cell. The sealing of the opto-fluidics connector was tested, and less than 3% leakage was found.

Results and Discussion

The glass HCW and aluminium HCW were investigated for gas sensing applications. The glass HCW was composed of a glass capillary tube with a thin inner coating of aluminium. The aluminium HCW was a capillary tube made of tempered aluminium with 99.7% purity. The lengths of the glass HCW (inner diameter, 1mm; internal gas cell volume, 27 µL) and aluminium HCW (inner dimeter, 2 mm; internal gas cell volume, 157 µL) installed were 34 cm and 50 cm, respectively. Aluminium was selected because it had good reflective and transmittance properties (i.e., attenuation loss, 0.2 dB/m) for deep UV application, compared to other reflective metals like silver [START_REF] Matsuura | Hollow optical fibers for ultraviolet and vacuum ultraviolet light[END_REF]. It also has good chemical compatibility with toluene.

The absorption spectra of toluene and the emission band of UV-LED was measured and compared, as shown in Figure 2. Toluene had three peaks for a wavelength range of 250 to 270 nm. The bandwidth (full width at half maximum (FWHM)) of the LED is 10 nm, centred at 260 nm, covering the absorption spectra of toluene, which implies that the setup can be applied for measuring the absorbance of toluene.

Micromachines 2019, 10, x FOR PEER REVIEW 4 of 9

for deep UV application, compared to other reflective metals like silver [START_REF] Matsuura | Hollow optical fibers for ultraviolet and vacuum ultraviolet light[END_REF]. It also has good chemical compatibility with toluene.

The absorption spectra of toluene and the emission band of UV-LED was measured and compared, as shown in Figure 2. Toluene had three peaks for a wavelength range of 250 to 270 nm. The bandwidth (full width at half maximum (FWHM)) of the LED is 10 nm, centred at 260 nm, covering the absorption spectra of toluene, which implies that the setup can be applied for measuring the absorbance of toluene. The measurement sequence was started by flushing nitrogen through the gas cell, followed by injecting different concentrations of toluene for 1 to 2 min until a stable signal was obtained as shown in Figure 2b. The recorded intensities for nitrogen and toluene concentrations were used as a reference and measured intensity, respectively. A dark intensity was recorded and subtracted from both references, i.e., I0 and the measured I intensities to calculate the absorbance according to Equation ( 1).

The setup with the glass HCW was tested for a concentration range of 10-100 ppm. The absorbance increased linearly with an increasing toluene concentration according to the Beer-Lambert law, as shown in Figure 3. The measurement sequence was started by flushing nitrogen through the gas cell, followed by injecting different concentrations of toluene for 1 to 2 min until a stable signal was obtained as shown in Figure 2b. The recorded intensities for nitrogen and toluene concentrations were used as a reference and measured intensity, respectively. A dark intensity was recorded and subtracted from both references, i.e., I 0 and the measured I intensities to calculate the absorbance according to Equation (1).

The setup with the glass HCW was tested for a concentration range of 10-100 ppm. The absorbance increased linearly with an increasing toluene concentration according to the Beer-Lambert law, as shown in Figure 3.

for deep UV application, compared to other reflective metals like silver [START_REF] Matsuura | Hollow optical fibers for ultraviolet and vacuum ultraviolet light[END_REF]. It also has good chemical compatibility with toluene.

The absorption spectra of toluene and the emission band of UV-LED was measured and compared, as shown in Figure 2. Toluene had three peaks for a wavelength range of 250 to 270 nm. The bandwidth (full width at half maximum (FWHM)) of the LED is 10 nm, centred at 260 nm, covering the absorption spectra of toluene, which implies that the setup can be applied for measuring the absorbance of toluene. The measurement sequence was started by flushing nitrogen through the gas cell, followed by injecting different concentrations of toluene for 1 to 2 min until a stable signal was obtained as shown in Figure 2b. The recorded intensities for nitrogen and toluene concentrations were used as a reference and measured intensity, respectively. A dark intensity was recorded and subtracted from both references, i.e., I0 and the measured I intensities to calculate the absorbance according to Equation ( 1).

The setup with the glass HCW was tested for a concentration range of 10-100 ppm. The absorbance increased linearly with an increasing toluene concentration according to the Beer-Lambert law, as shown in Figure 3. The absorbance peaks at λ 1 = 260.3 nm, λ 2 = 263.1 nm, and λ 3 = 267.2 nm were plotted for different concentrations of toluene, and a linear relation was obtained, as shown in Figure 4. The combined uncertainties in the toluene concentration were calculated according to BIPM guidelines [START_REF] Jcgm | Evaluation of Measurement Data-Guide to the Expression of Uncertainty in Measurement (Évaluation des Données de Mesure-Guide pour L'expression de L'incertitude de Mesure[END_REF], by taking into account the uncertainties of MFCs and gas cylinders. The sensitivity was calculated by taking the slope of the plot, representing absorbance vs. concentration. Sensitivities of 0.20 mA•U/ppm, 0.15 mA•U/ppm, and 0.19 mA•U/ppm were obtained for λ 1 , λ 2 , and λ 3 respectively. According to the absorption cross-section values at λ 1 , λ 2 , and λ 3 , the absorbance values should be higher for λ 3 , but relative lower values were observed due to the emission profile of LED and limited resolution of spectrometer. A limit of detection of 8.2 ppm was calculated for λ 1 from standard deviation of the calibration data assuming that the data is normally (Gaussian) distributed, using the equations [START_REF] Loock | Detection limits of chemical sensors: Applications and misapplications[END_REF].

x LOD = S y t r (2)

x LOD is the limit of detection, where r and S y represent sensitivity of the linear fit and average standard deviation respectively.

r = ∆y ∆x = n ∑(x i y i ) -∑ x i ∑ y i D (3)
S y = ∑(y i -r x i -b) 2 n -2 ( 4
)
where b is intercept of the calibration curve i.e. the signal offset.

b = n ∑ x 2 i ∑ y i -∑(x i y i ) ∑ x i D (5) D = n ∑ x 2 i -∑ x i 2 ( 6
)
where t is Student t-function. x i and y i are the calibration curve points and n is the number of data points on calibration curve. The same experiment was repeated for the aluminium HCW and a good linearity was found for different concentrations of toluene (20-100 ppm) as shown in Figure 5. Sensitivities of 0.32 mA•U/ppm, 0.23 mA•U/ppm and 0.30 mA•U/ppm were obtained for λ 1 , λ 2 and λ 3 respectively. A limit of detection of 12.5 ppm was calculated for λ 1 .

Micromachines 2019, 10, x FOR PEER REVIEW 5 of 9

uncertainties in the toluene concentration were calculated according to BIPM guidelines [START_REF] Jcgm | Evaluation of Measurement Data-Guide to the Expression of Uncertainty in Measurement (Évaluation des Données de Mesure-Guide pour L'expression de L'incertitude de Mesure[END_REF], by taking into account the uncertainties of MFCs and gas cylinders. The sensitivity was calculated by taking the slope of the plot, representing absorbance vs. concentration. Sensitivities of 0.20 mA•U/ppm, 0.15 mA•U/ppm, and 0.19 mA•U/ppm were obtained for λ1, λ2, and λ3 respectively. According to the absorption cross-section values at λ1, λ2, and λ3, the absorbance values should be higher for λ3, but relative lower values were observed due to the emission profile of LED and limited resolution of spectrometer. A limit of detection of 8.2 ppm was calculated for λ1 from standard deviation of the calibration data assuming that the data is normally (Gaussian) distributed, using the equations [START_REF] Loock | Detection limits of chemical sensors: Applications and misapplications[END_REF].

𝑥 = 𝑆 𝑡 𝑟 (2)
xLOD is the limit of detection, where r and Sy represent sensitivity of the linear fit and average standard deviation respectively.

𝑟 = ∆𝑦 ∆𝑥 = 𝑛 ∑ 𝑥 𝑦 ∑𝑥 ∑𝑦 𝐷 (3)
𝑆 = ∑ 𝑦 𝑟 𝑥 𝑏 𝑛 2 (4)
where b is intercept of the calibration curve i.e. the signal offset.

𝑏 = 𝑛 ∑𝑥 ∑𝑦 ∑ 𝑥 𝑦 ∑𝑥 𝐷 (5)
𝐷 = 𝑛∑𝑥 ∑𝑥 ( 6
)
where t is Student t-function. xi and yi are the calibration curve points and n is the number of data points on calibration curve. The same experiment was repeated for the aluminium HCW and a good linearity was found for different concentrations of toluene (20-100 ppm) as shown in Figure 5. Sensitivities of 0.32 mA•U/ppm, 0.23 mA•U/ppm and 0.30 mA•U/ppm were obtained for λ1, λ2 and λ3 respectively. A limit of detection of 12.5 ppm was calculated for λ1. The repeatability and reproducibility of the system were tested for a toluene concentration of 30 ppm using aluminium HCW by varying the gas cell flow rate, LED current input, and the integration time of the spectrometer. A good repeatability (relative standard deviation, RSD = 2.5%) was found for five experiments using a flow rate of 20 mL/min with LED current input of 30 mA and integration time of 100 ms (experiment 2 in Figure 6). Reproducibility with RSD = 4.8% was observed for five different experiments at different conditions, as shown in Figure 6. The repeatability and reproducibility of the system were tested for a toluene concentration of 30 ppm using aluminium HCW by varying the gas cell flow rate, LED current input, and the integration time of the spectrometer. A good repeatability (relative standard deviation, RSD = 2.5%) was found for five experiments using a flow rate of 20 mL/min with LED current input of 30 mA and integration time of 100 ms (experiment 2 in Figure 6). Reproducibility with RSD = 4.8% was observed for five different experiments at different conditions, as shown in Figure 6.

represent standard deviations in absorbance values and combined uncertainties in the generated concentrations defined by the uncertainties of mass flow controllers (MFCs) and gas cylinder concentrations, respectively. The repeatability and reproducibility of the system were tested for a toluene concentration of 30 ppm using aluminium HCW by varying the gas cell flow rate, LED current input, and the integration time of the spectrometer. A good repeatability (relative standard deviation, RSD = 2.5%) was found for five experiments using a flow rate of 20 mL/min with LED current input of 30 mA and integration time of 100 ms (experiment 2 in Figure 6). Reproducibility with RSD = 4.8% was observed for five different experiments at different conditions, as shown in Figure 6. The performance of the two HCWs for toluene detection applications is compared and summarized in Table 1. The aluminium HCW had a higher sensitivity compared to the glass HCW at the different peak wavelengths. Also, the aluminium HCW had good mechanical properties and can be easily assembled with fluidic and optical components. There were low coupling optical losses associated with the aluminium HCW, resulting in improved sensitivity. On the other hand, the glass HCW was fragile, and it was challenging to obtain a smooth end surface for alignment with the LED source and detector. The aluminium HCW offered a simple, cost-effective, and robust approach for absorption spectrometry with a good level of sensitivity. On the other hand, the glass HCW was a good candidate for application where a low gas cell volume is needed, for example integration of the detector with a µ-GC.

By comparing the performance of the sensor with other toluene sensing methods, the UV absorption detector based on HCW had a good sensitivity and linearity at room temperature. For instance, resistive gas sensors have a linearity range between 10-100 ppm, but they operate at high temperature [START_REF] Mirzaei | Resistive-based gas sensors for detection of benzene, toluene and xylene (BTX) gases: A review[END_REF]. Fiber optic toluene sensors have limited linearity and selectivity. Optical fiber sensors based on a long period grating for toluene has an operating range of 0-60 ppm [START_REF] Yin | Recent development of fiber-optic chemical sensors and biosensors: Mechanisms, materials, micro/nano-fabrications and applications[END_REF], and Fabry Perot fiber sensors face the issue of non-linearity [START_REF] Kacik | Toluene optical fibre sensor based on air microcavity in PDMS[END_REF].

The setup has demonstrated a good sensitivity for toluene and has potential to be used for detecting BTEX molecules. The setup can be applied for ultra-low and high-selective detection of BTEX molecule mixtures by coupling it with a pre-concentration unit with/without a GC-column for separation purposes.

Conclusions and Future Scope

In this study we have demonstrated a simple and sensitive deep UV absorption spectrophotometry for detection of air-borne toluene. A fiber-coupled deep UV-LED was employed as a light source, and a spectrometer was used as detector with a gas cell in between. 3D printed opto-fluidics connectors were designed to integrate the gas flow with a UV source and detector. A glass HCW with aluminium coating and an aluminium HCW were tested as a gas cell. The setup was tested for different toluene concentrations (10-100 ppm), and a linear relationship was observed with sensitivities of 0.20 mA•U/ppm and 0.32 mA•U/ppm for glass HCW and aluminium HCW, respectively, at 260 nm. The limits of detection of 8.15 ppm and 12.45 ppm were calculated for glass HCW and aluminium HCW, respectively. The sensitivity and selectivity of the setup can be improved by coupling it with a pre-concentration unit and a micro GC column, respectively. This study provides a guide for the design of aluminium-based HCWs for UV spectrophotometry and can be applied to detect a number of molecules which show UV absorption for example, ozone, benzene, xylenes, NO 2 and SO 2 .

Figure 1 .

 1 Figure 1. (a) Experimental setup for toluene detection. (b) Toluene concentration generation setup.

Figure 1 .

 1 Figure 1. (a) Experimental setup for toluene detection. (b) Toluene concentration generation setup.

Figure 2 .

 2 Figure 2. (a) Absorption spectra of toluene (100 ppm), background gas, i.e., nitrogen, and emission spectrum of deep UV-LED. (b) Schematics of flushing routine of nitrogen and toluene for measuring reference and measured intensity respectively.

Figure 3 .

 3 Figure 3. Absorbance of toluene at different wavelengths for different concentrations of toluene using the glass hollow core waveguide (HCW). The absorbance peaks at λ1 = 260.3 nm, λ2 = 263.1 nm, and λ3 = 267.2 nm were plotted for different concentrations of toluene, and a linear relation was obtained, as shown in Figure 4. The combined

Figure 2 .

 2 Figure 2. (a) Absorption spectra of toluene (100 ppm), background gas, i.e., nitrogen, and emission spectrum of deep UV-LED. (b) Schematics of flushing routine of nitrogen and toluene for measuring reference and measured intensity respectively.

Figure 2 .

 2 Figure 2. (a) Absorption spectra of toluene (100 ppm), background gas, i.e., nitrogen, and emission spectrum of deep UV-LED. (b) Schematics of flushing routine of nitrogen and toluene for measuring reference and measured intensity respectively.

Figure 3 .

 3 Figure 3. Absorbance of toluene at different wavelengths for different concentrations of toluene using the glass hollow core waveguide (HCW). The absorbance peaks at λ1 = 260.3 nm, λ2 = 263.1 nm, and λ3 = 267.2 nm were plotted for different concentrations of toluene, and a linear relation was obtained, as shown in Figure 4. The combined

Figure 3 .

 3 Figure 3. Absorbance of toluene at different wavelengths for different concentrations of toluene using the glass hollow core waveguide (HCW).

Figure 2 .

 2 Figure 2. Absorbance vs. toluene concentration using the glass HCW at different wavelengths: (a) λ1 = 260.34 nm, (b) λ2 = 263.17 nm, (c) λ3 = 267.18 nm. The vertical and the horizontal error bars

Figure 4 .

 4 Figure 4. Absorbance vs. toluene concentration using the glass HCW at different wavelengths: (a) λ 1 = 260.34 nm, (b) λ 2 = 263.17 nm, (c) λ 3 = 267.18 nm. The vertical and the horizontal error bars represent standard deviations in absorbance values and combined uncertainties in the generated concentrations defined by the uncertainties of mass flow controllers (MFCs) and gas cylinder concentrations, respectively.

Figure 3 .

 3 Figure 3. Absorbance vs. toluene concentration using the aluminium HCW at different wavelengths: (a) λ1 = 260.34 nm, (b) λ2 = 263.17 nm, and (c) λ3 = 267.18 nm. The vertical and the horizontal error bars represent standard deviations in absorbance values and combined uncertainties in the generated concentrations defined by the uncertainties of MFCs and gas cylinders, respectively.

Figure 4 .

 4 Figure 4. Absorbance for different experimental conditions for repeatability and reproducibility. (1) flowrate, 40 mL/min; (2) flowrate, 20 mL/min; (3) flowrate, 10 mL/min; (4) Detector integration time, 50 ms; and (5) LED current input, 15 mA.

Figure 5 .

 5 Figure 5. Absorbance vs. toluene concentration using the aluminium HCW at different wavelengths: (a) λ 1 = 260.34 nm, (b) λ 2 = 263.17 nm, and (c) λ 3 = 267.18 nm. The vertical and the horizontal error bars represent standard deviations in absorbance values and combined uncertainties in the generated concentrations defined by the uncertainties of MFCs and gas cylinders, respectively.

Figure 3 .

 3 Figure 3. Absorbance vs. toluene concentration using the aluminium HCW at different wavelengths: (a) λ1 = 260.34 nm, (b) λ2 = 263.17 nm, and (c) λ3 = 267.18 nm. The vertical and the horizontal error bars represent standard deviations in absorbance values and combined uncertainties in the generated concentrations defined by the uncertainties of MFCs and gas cylinders, respectively.

Figure 4 .

 4 Figure 4. Absorbance for different experimental conditions for repeatability and reproducibility. (1) flowrate, 40 mL/min; (2) flowrate, 20 mL/min; (3) flowrate, 10 mL/min; (4) Detector integration time, 50 ms; and (5) LED current input, 15 mA.

Figure 6 .

 6 Figure 6. Absorbance for different experimental conditions for repeatability and reproducibility. (1) flowrate, 40 mL/min; (2) flowrate, 20 mL/min; (3) flowrate, 10 mL/min; (4) Detector integration time, 50 ms; and (5) LED current input, 15 mA.

Table 1 .

 1 Comparison of aluminium and glass HCWs for toluene detection.

	HCW	Length (cm)	Wavelength (nm)	Sensitivity (mA•U•ppm -1 )	Sensitivity/Length (mA•U•ppm -1 •cm -1 )
			260.34	0.32	0.00640
	Aluminium	50	263.76	0.23	0.00460
			267.18	0.30	0.00600
			260.34	0.20	0.00588
	Glass	34	263.76	0.15	0.00441
			267.18	0.19	0.00559

Micromachines 2019, 10, 193; doi:10.3390/mi10030193 www.mdpi.com/journal/micromachines

Micromachines 2019, 10, x FOR PEER REVIEW

Funding: This work was funded 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]. This work has also received funding from the Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No 687014 (MACAO Project).

Author Contributions: Conceptualization, experiments, data analysis and writing by S.K.; supervision and review by D.N. and S.L.C.

Conflicts of Interest:

The authors declare no conflict of interest.