Maëva Millan

C Szopa

A Buch

M Cabane

S Teinturier

P Mahaffy

S S Johnson

Multi-column gas chromatography analysis of chlorohydrocarbons with the SAM experiment onboard NASA's Mars Curiosity rover

Keywords: Sample Analysis at Mars, chlorinated hydrocarbons, gas chromatography, mass spectrometry, efficiency, separation

The Sample Analysis at Mars (SAM) instrument is a gas chromatograph-mass spectrometer onboard the NASA Curiosity rover, currently operating on the surface of Mars. Organic compounds are of major importance with regard to questions of habitability and the potential presence of life on Mars, and one of the mission's main objectives is to analyze the organic content of soil and rock samples. In SAM's first chromatographic measurements, however, unexpected chlorine-bearing organic molecules were detected. These molecules have different origins but the presence of perchlorates and chlorates detected at the surface of Mars suggests that reactivity between organic molecules and thermal decomposition products from oxychlorines is one of the major sources of the chlorinated organic molecules. Here we perform a comprehensive and systematic study of the separation of volatile chlorohydrocarbons with the chromatographic columns used in the SAM instrument. Despite the constrained operating conditions of the flight instrument, we demonstrate that SAM's capillary chromatographic columns allow for effective separation and identification of a wide range of chlorine-bearing species. We also show that instrumental limitations prevent the detection of certain molecules, obscuring our ability to make definitive conclusions about the origin of these organic materials.

Highlights

 In situ analysis of organic molecules on Mars with GCMS  Chlorohydrocarbons unexpectedly produced during heating of solid samples on Mars  Multi-columns chromatograph for separation and identification of chlorohydrocarbons

Introduction

Since the successful landing of NASA's Curiosity rover in Gale Crater, Mars, on August 6 th , 2012 [START_REF] Grotzinger | Mars Science Laboratory Mission and Science Investigation[END_REF], the Sample Analysis at Mars (SAM) experiment [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF] onboard the rover has analyzed the molecular composition of surface samples collected with the rover robotic arm. Organic molecules in rocks and soil are of particularly high interest because they may be related to an ancient biological activity and/or prebiotic chemistry. Along with the presence of liquid water and a source of energy, the presence of organic molecules is necessary to make an environment habitable according to our current knowledge of life [START_REF] Westall | The Importance of Water for Life[END_REF]. It is now well accepted that liquid water and energy were present in the past history of Mars, demonstrated in particular by the Mars Exploration Rovers [START_REF] Bibring | Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data[END_REF][START_REF] Carter | Widespread surface weathering on early Mars: A case for a warmer and wetter climate[END_REF]. But no organic molecules had been detected on the surface of Mars before the Mars Science Laboratory mission. Despite the fact that meteoritic infall at the surface of Mars is a likely continuous source of organic material [START_REF] Flynn | The delivery of organic matter from asteroids and comets to the early surface of Mars[END_REF], the conclusion of the in situ measurements performed with the Viking landing probes in the late 1970's was organic molecules were not present in the regolith. Chlorohydrocarbons were detected by Viking, but at the time, the science team believed the chlorinated molecules were sourced from terrestrial contamination [START_REF] Biemann | The search for organic substances and inorganic volatile compounds in the surface of Mars[END_REF]. We have now discovered that the martian soil contains various oxychlorine species [START_REF] Hecht | Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site[END_REF][START_REF] Sutter | Evolved gas analyses of sedimentary rocks and eolian sediment in Gale Crater, Mars: Results of the Curiosity rover's sample analysis at Mars instrument from Yellowknife Bay to the Namib Dune[END_REF]. Because these molecules are strong oxidants, capable of chlorinating organic molecules under high temperature, this result has led to a reconsideration of the conclusions of the Viking GCMS measurements. It is now believed that the chlorohydrocarbons detected could have actually been produced in the pyrolytic process of extraction of the organic materials present in the presence of oxychlorine [START_REF] Navarro-Gonzalez | Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars[END_REF]. Moreover, the SAM experiment has shown, for the first time, that organic matter is definitively present on Mars [START_REF] Freissinet | Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars[END_REF]. This was accomplished through the detection of chlorohydrocarbons mostly produced in the pyrolytic oven of the SAM instrument as part of reactions occurring between indigenous organic molecules in the mudstones collected in Gale crater and thermal decomposition products from oxychlorines also present in the sample. Chlorobenzene, the most emblematic molecule of this discovery, has now been retrieved in the chromatograms recorded by the Viking GCMS [START_REF] Guzman | Identification of Chlorobenzene in the Viking Gas Chromatograph-Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars[END_REF] whereas it was not reported in the original data analyses suggesting the presence of organic materials indigenous to the samples collected by Viking landers.

Even though the detection of chlorohydrocarbons from Mars sample analyzed with pyro-GC-MS is now understood, the SAM gas chromatograph was developed before oxychlorines were discovered on Mars. At that time, the whole family of chlorohydrocarbons was not considered as a major source of analytes. Thus, the instrument's ability to separate and analyze chlorohydrocarbons with the different chromatographic columns used for the SAM gas chromatograph, under constrained flight operating conditions, was never characterized. Finally, from our knowledge, only a limited number of studies dealing with the separation of a limited variety of volatile chlorohydrocarbons was reported in the literature. For all these reasons, we have undertaken a systematic study of the analysis of chlorohydrocarbons, including linear and aromatic ones, with five of the six columns used in the SAM gas chromatograph specifically dedicated to analyze organic compounds. This laboratory work is primarily dedicated to properly process and interpret the data collected with the SAM instrument on Mars, but it could also be used as a reference work for other applications.

Material and methods

Samples: the chlorohydrocarbons

26 chlorinated hydrocarbons, listed in Table 1, were selected for this study. Most of them were detected in the first fifteen samples of sand and rocks analyzed with the SAM GC-MS experiment [START_REF] Freissinet | Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars[END_REF][START_REF] Glavin | Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater[END_REF][START_REF] Szopa | Detection of indigenous organic matter at the surface of Mars by the SAM instrument onboard Curiosity[END_REF]. The others, belonging to similar chemical categories (aromatics, etc.), were thought to be potentially present as well, or relevant to the search for organics in the future Martian samples [START_REF] Glavin | Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater[END_REF]15). All the chlorohydrocarbons were supplied by Sigma Aldrich and were of pure analytical grade (>99.9%). They were injected into the GC-MS both as separate concentrated standards as well as mixed together in equimolar ratios.

GC columns and stationary phases

There are six columns on the SAM chromatograph. The PLOT Carbobond column (Agilent), located in channel 3 (GC-3), was not included in this study because it is fully devoted to the analysis of atmospheric and inorganic light molecules (< C 2 ). Thus, GC-3 is not relevant for the analysis of chlorohydrocarbons. Of the remaining five, three have been used on Mars: the MXT-20 (GC-1), the Chirasil-β dex (GC-4) and the MXT-CLP (GC-5), located in channels 1, 4 and 5 respectively and the other two: the MXT-5 (GC-2) and PLOT MXT-Q (GC-6) columns located in channels 2 and 6, have not been used so far. To perform the laboratory experiments, five replicates of each SAM flight capillary column were purchased in 2007. The columns were each tested and qualified in the laboratory under the same conditions and with the same analytes as the columns used to build the SAM GC instrument. The best columns were selected to be integrated into three different models of the SAM GC. The two columns remaining were kept at Laboratoire Atmosphères Milieux Observations Spatiales (France) under controlled storage conditions for running laboratory tests which could not be completed with the integrated models of SAM. All the tested columns were 30 m long, with a 0.25 mm internal diameter, and were supplied by Restek (Bellefonte, PA, USA). Their film thickness was 0.25 μm except for the PLOT MXT-Q column, which has a 10 µm thick stationary phase. Their specifications are listed in Table 2.

Gas chromatograph-mass spectrometer

Measurements were performed with two instruments: a GC Trace chromatograph coupled to an ITQ MS, and a GC trace chromatograph coupled to an ISQ MS (both instruments from the ThermoFisher company). A split/splitless injector was used in split mode with a 1:50 mean split ratio. Low volume liquid and gas syringes (Hamilton), with 0.5 μL and 1 mL volumes respectively, were used to inject the different samples in the injector. The temperature of the injector, the GC-MS transfer line, and the ionization source were set at 250°C in order to prevent condensation of tested molecules in the instrument, as all tested compounds except pentachlorobenzene and hexachlorobenzene have lower boiling points. The MS was set to scan all the ions produced from the electron impact ionization source (electron energy of 70 eV) in the m/z 10 to 300 range that covers all the ions produced from the tested compounds according to the NIST MS spectrum library.

The carrier gas was helium (purity ≥ 99.999%) to match the SAM GC-MS experiment on Mars. The temperatures of the columns and carrier gas flow conditions used are given in the following section.

Analytical approach

 Reproducing the SAM-flight pressure in laboratory

The carrier gas pressure drop between the injector and the detector of the chromatographic column is a key parameter of the chromatographic analysis. During the development of the SAM flight instrument, a 1.3 bar inlet carrier gas pressure was originally chosen as a good tradeoff between chromatographic efficiency and instrumental design constraints, essentially driven by the requirement that the pressure of the MS chamber remain low. However, during the last test campaign of the instrument before its integration into the rover, the inlet pressure was decreased to 0.9 bar to limit the risk of failure of the MS instrument.

The carrier gas tank pressure was predicted to decrease with time over the course of the mission, but this change had an immediate effect on retention times and column efficiency.

With our laboratory instruments, it was not possible to set the absolute inlet pressure to 0.9 bar as it is below ambient laboratory pressure (1 bar). For that reason, we altered the flight carrier gas flow by using a deactivated fused silica capillary tube, 30 cm long and with a 50 μm internal diameter connected between the split/splitless injector and the column, following the method suggested by DeZeeuw et al. [START_REF] Dezeeuw | A simple way to speed up separation by GC-MS using short 0.53 mm columns and vacuum outlet conditions[END_REF]. The appropriate tube length (L tube ) was estimated using the Poiseuille's law, described by the following equation: with η, the helium viscosity, at 1.96.10 -5 Pa.s at 20°C, L col and L tube as the respective lengths of the column and the tube, r as their radius (0.125 mm for the column and 50 μm for the restrictor tube), and Q as the helium flow rate calculated from the mean gas velocity of 16.28 cm.s -1 and 16.66 cm.s -1 observed in flight for the column from the GC-4 and GC-5 channels respectively. These velocities were calculated from the deadtime measured in flight chromatograms (3.07 min and 3.00 min respectively). For the GC-1 channel, the deadtime was difficult to determine because of the absence of a precise injection system in the flight experiment. For this reason, we used a 3.00 min estimate corresponding to a 16.66 cm.s -1 carrier gas mean velocity. The pressure drop ΔP was set to 100 kPa for the calculation as this corresponds approximately to the pressure drop in the SAM flight model instrument.

The restrictor tube length was calculated for four different internal diameters values varying from 5 to 50 μm. The 50 μm diameter tube was chosen for its realistic and reasonable tube length to cut and add to the column inside the GC oven. The tube length calculated was 30 cm. For the following set of experiments to work under SAM-flight pressure, a 30 cm long and 50 μm internal diameter restriction tube was used with each column replicate already used on Mars (GC-1, GC-4 and GC-5) in addition to the columns that have not yet been used (GC-2 and GC-6). This method enabled us to mimic the inlet gas vector flow of the SAM instrument.

Once the flow restrictor was installed, we studied the efficiency of the columns as a function of the inlet pressure by varying the inlet pressure from 1.1 bar, i.e. the minimum pressure that can be set on the laboratory instrument, up to 5 bars. For the study of the analytical capabilities of the column under SAM like operating conditions, the inlet pressure of the laboratory GC was

 Reproducing the SAM-flight temperatures of the GC columns in the laboratory

To study the efficiency of the columns as well as their thermodynamics, we chose isothermal columns temperatures between 30°C and 200°C, the minimum and maximum temperatures the columns can reach in flight. To study the separation efficiency of the chlorohydrocarbons, we performed the laboratory experiments under SAM-like operating conditions, as close as possible of the flight temperature profiles. We applied identical temperature programs for the GC-1, GC-4 and GC-5 columns used on Mars to the columns we tested in the laboratory. For the GC-2 and GC-6 columns, standard temperature programs were applied considering their thermal constraints on SAM, so as to mimic the flight conditions for future analyses on Mars. These conditions are detailed in Table 5. Depending on the nature of the molecules analyzed and the column, the final temperature was held between 5 and 40 minutes. When the highest temperature possible is reached for the column considered in flight, it immediately stops heating, as it is generally not possible to maintain a long plateau at the final temperature because of thermal constraints.

While the temperature programs of the three columns already used on Mars were mostly reproduced, some slight differences exist because the flight and laboratory set ups are not strictly identical. A comparison between the flight and laboratory temperature programs applied to the columns are presented in Figure 1. The slight difference observed between the flight and the laboratory temperature profiles is primarily due to the fact that temperature regulation of the columns is less accurate in the flight model because of the heating method used. However, the profiles are sufficiently similar to consider that the laboratory method mimics the flight temperature programs of the SAM-GC columns.

Analytical performances of the SAM-flight GC columns

The investigation proceeded in three steps: first, an efficiency study as a function of the operating conditions to estimate the relevance of the conditions finally used in the flight model;

second, a thermodynamic study to estimate the possible evolution of the elution order of the chlorohydrocarbons targeted within the temperature conditions; and third, the measurement of the retention times and determination of the separation power of each column for the chlorohydrocarbons studied. This last step was of critical importance in developing a reference database for analyzing the flight data.

Efficiencies of the SAM-GC columns on chlorohydrocarbons

The column's efficiencies were studied as a function of the temperature and the inlet carrier gas pressure, as the outlet column pressure is constant in the SAM instrument (0 bar).

The main objective was to determine the optimal carrier gas flow rate conditions to analyze chlorohydrocarbons. To perform these measurements, the carrier gas inlet pressure was varied from 110 kPa, the minimum absolute pressure possible on a GC laboratory, to 500 kPa. These pressures correspond to 7 to 39 cm.s -1 mean linear velocities. Two isothermal temperature conditions were used, 35°C and 185°C. These temperatures were representative of the extreme temperatures used in the SAM GC instrument, taking into account the environmental conditions of the Mars surface and the instrumental constraints. Three light chlorohydrocarbons detected in the martian samples were selected for the low column temperature: dichloromethane, 1,2dichloropropane (chiral), and chlorobenzene. The Chirasil-β dex (GC-4) column was used for the separation of the following chiral molecules: the 3-chloro-2-methyl-1-propene, also detected on Mars and the 1-chlorobutane, that was selected to replace the chlorobenzene, this last not being eluted at such low temperature. At high temperature, the same compounds were selected in addition to 1,2,4-trichlorobenzene and 1-chlorooctane (these two compounds have not been detected in the martian samples so far).

A previous study of the MXT-CLP (GC-5) column efficiency was performed [START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF] but the organic molecules targeted (alcohols, alkanes and aromatic hydrocarbons) did not include chlorohydrocarbons. For this study, the GC-5 column efficiency was evaluated on a variety of organic compounds including the major molecules detected by SAM at that time. The chlorohydrocarbons studied have different physical and chemical properties (molecular weight, boiling point, etc., see Table 1) and thus different retention time properties. The columns efficiency, estimated with the number of theoretical plates value N, was evaluated using the retention times and peak widths derived from the Total Ion Current (TIC) given by the laboratory MS, using the formula N=5.54(t R /w h ) 2 with t R being the retention time and w h the peak width at half height. The Height Equivalent to a Theoritical Plate (HETP or H in mm) was calculated using the formula H=L/N with L the column length and N the efficiency. The deadtime, required to calculate the mean carrier gas velocity, was measured using the elution time of air (essentially composed of N 2 , O 2 et CO 2 which are not retained on the studied columns), injected separately from the other compounds under the same operating conditions.

Finally, from the experimental points, the experimental evolution of H as a function of the mean linear velocity (cm.s -1 ) was fitted with the Van-Deemter Golay theoretical equation given as follows:

A polynomial function was used to fit the efficiency of the GC-6 columns because the conditions of the Van-Deemter Golay equation were not met. This can be explained by its stationary phase, mostly dedicated to separate light organic molecules (<C 5 or <100 g.mol -1 ). This is the case for dichloromethane (M = 84 g.mol -1 ) but not the two other molecules (M ~112 g.mol -1 ). These heavier molecules have higher retention times compared to the analytical capabilities of the GC-6 column inducing broader tailing peaks from a longer time spent into the column. For this reason, the best fit was obtained with a second-degree polynomial function for this column.

Generally, HETP were optimal to few millimeters depending on the compounds, but the range of optimal velocities were similar for all the compounds when looking at a given column.

Laboratory efficiencies were compared, when possible, to the flight efficiencies currently measured on the SAM-flight instrument of the chlorohydrocarbons detected which was the case for the MXT-CLP (GC-5), MXT-20 (GC-1), and Chirasil-β Dex (GC-4) columns.

For dichloromethane, the HETP minimum value was 1.7 mm for =12.5 cm.s -1 . In the 9-15 cm.s -1 velocity range, the HETP value did not increase more than 10. For chlorobenzene and 1,2-dichloropropane, the HETP minimum values were 2.8 and 3.1 mm respectively at 9 cm.s -1 and 10 cm.s -1 carrier gas speeds. HETP values did not increase more than 10% of the minimum in the 7 to 12 cm.s -1 velocity range.

No analyses of a Mars sample have been performed on the SAM flight model with the GC-6 column so far. However, a calibration test was performed on Mars with the GC-6 channel and the deadtime measured was 3.5 min at 40°C. This corresponds to a 14.3 cm.s -1 mean carrier gas velocity. However, no measurement of the deadtime was available on flight at 185°C temperature column to calculate the carrier gas velocity at this temperature. But, the inlet column pressure is known (0.9 bar), as well as the deadtime measured on the columns that have been used in flight, and equal to about 3.9 min at 185°C. Knowing these values, we calculated the supposed average velocity of the carrier gas that would be equal to 12.82 cm.s -1 at 185°C for the current inlet column pressure, called " SAM theoretical" (Figure 2), value compatible with the carrier gas velocity of He in flight measured at 40°C column temperature.

From these measurements, we concluded that the optimal efficiency of the GC-6 column decreased with the retention times of the analyzed compounds, likely because of the increase of the peaks tailing with their retention due to the nature of the stationary phase. Moreover, when comparing the optimal velocity range of the column with the actual mean linear velocity measured in the flight model, we noticed that the flight operating carrier gas flow was near optimal for this column and the tested compounds. However, the decrease of carrier gas velocity during the mission should not significantly influence the efficiency down to 9 cm.s -1 .

Figure 3 shows the evolution of the HETP as a function of the mean carrier gas velocity for all the SAM GC columns bonded with liquid stationary phases, for the two extreme temperatures used in the SAM GC flight model. Most curves show the usual behavior of the evolution of HETP with , yet we observed one phenomenon that we are unsure how to explain:

that those obtained with the MXT-CLP columns at 185°C were rather flat for higher mean carrier gas velocities. All the values used to discuss the efficiencies of the columns bonded with liquid stationary phases are summarize in Table 3.

The MXT-20 column (GC-1) plot was consistent with the theoretical evolution of the Van-Deemter-Golay equation at high temperature for each molecule. The minimum H values were between 0.5 mm for dichloromethane and 1 mm for the other compounds at low temperature. The optimal velocity range was 15-20 cm.s -1 for dichloromethane and stayed optimal for the other compounds even at high carried gas speeds. At high temperature, plots had a minimum of 0.8 mm for the more retained compounds, to 1.4 mm for the less retained. The optimal velocity range was the same for each molecule and between 15 and 20 cm.s -1 .

In conclusion, the optimal velocity range for the GC-1 column did not depend on the temperature. The efficiency did not improve when the temperature increases and was slightly degraded (of ~0.4 mm), meaning the exchange between the mobile and stationary phases was more difficult at high temperature. At both temperatures, GC-1 was convenient and efficient for the analysis of low and high molecular weights chlorinated compounds and on a wide range of velocities with a slight better efficiency at low temperature.

The GC-1 column has since been used on Mars on the Windjana sample and in parallel with the GC-4 column on the Ogunquit Beach (OG) sample. However, the lack of efficient inlet injection system did not allow an ideal chromatographic analysis with symmetric and Gaussian peaks. In addition, without the injection time given by the thermal desorption for other columns used in SAM, it is impossible to precisely determine its deadtime. If the carrier gas velocity is estimated using the estimated deadtime, it equals 3.0 and 3.9 minutes at 35 and 185°C

respectively. This corresponds to flight linear velocities of 16.66 and 12.82 cm.s -1 . Compared to laboratory efficiencies, the flight efficiency was slightly degraded of 15% at high temperature.

At low temperature, the efficiency was not degraded at SAM-flight pressure. At high temperature, it will have to be considered that GC peaks shape can be degraded, possibly resulting in a limitation of the separation power. This confirms that the GC-1 column has better analysis capabilities at low temperature regarding its efficiency and use at the in situ flightpressure.

The MXT-5 (GC-2) column had experimental data close to the theoretical fit of the Golay equation. At low temperature, efficiencies values were similar for all compounds and between 1 and 1.3 mm, corresponding to optimal carrier gas velocities between 15 and 20 cm.s -1 . At high temperature, efficiencies were degraded and H values increase from 1.8 to 3 mm depending on the compounds. The limitation of experimental data at high pressure made the fitting of the plots difficult. The optimal velocity ranges varied from one compound to another of a few centimeters per second. For light compounds (dichloromethane, dichloropropane, and chlorobenzene), the optimal velocities ranged between 13 and 17 cm.s -1 . The optimal velocities of 1,2,4trichlorobenzene and 1-chlorooctane were between 13 and 19 cm.s -1 and 10 and 15 cm.s -1 respectively. In conclusion, this column was shown to be more efficient for the analyses of high molecular weight aliphatic chlorohydrocarbons (no loss of efficiency). Its efficiency decreased to 25% for aromatics.

The MXT-5 (GC-2) column has not yet been used in flight. However, we were able to theoretically estimate its efficiency degradation under SAM-flight conditions. As for GC-1, the efficiency of GC-2 was optimal at low temperature, even at SAM-flight pressure. At high temperature, the efficiency was degraded for most compounds, except for chlorooctane. The current carries gas velocity is thus ideal for the analysis of aliphatic high molecular weight compounds at high temperature. For low molecular weight organo-chlorinated or aromatic compounds, the flight-pressure was 20% weaker than the optimal one. As GC-1, GC-2 had better analysis capabilities at low temperature but was globally less efficient for the analysis of the targeted compounds, thus using GC-1 under these circumstances will be preferable.

The Chirasil-β Dex (GC-4) column is dedicated to the analysis and separation of chiral molecules. From all selected compounds, 1,2-dichloropropane is the only chiral one. Both isomers of dichloropropane are partially co-eluted at low temperature and totally at high temperature.

At low temperature, efficiencies follow the theoretical evolution of Golay, except for dichloromethane. For this last, except at low velocities, points are irregular and do not present a minimum. Those results are difficult to interpret since all the analyses were performed in the same conditions for all molecules. For 1,2 dichloropane, despite it includes enantiomers, its efficiency evolves as the other compounds. However, a higher HETP of 2.8 mm (compared to 1.2 mm for chlorobutane and 3-chloro-2-methyl-1-propene) is obtained. These observations result from the likely slight separation of the enantiomers achieved on the column, broadening the peak observed on the chromatogram and resulting from the coelution of the peaks of the enantiomers. The results obtained for this compound should therefore be taken as indicative of the efficiency trend. The optimal gas velocities are between 13 and 18 cm.s -1 compared to 15-20 cm.s -1 for dichloropropane. At high temperature, plots are atypical since even with the Golay's fits, they tend to parabolic equations implying steep slopes at high velocities. Consequently, the optimal carrier gas velocities range is reduced compared to the other columns at the same temperature. Efficiencies improve with the temperature increase with H values between 0.9 and 3.5 mm. In conclusion, the GC-4 column is more efficient for the highest molecular weight chlorohydrocarbons. However, the optimal gas velocities range is the same for all compounds and between 10 and 13 cm.s -1 , which is significantly reduced compared to the optimal range of the other columns.

The GC-4 column is now the privileged column for flight-GC analyses and was recently used in parallel of GC-1 to analyze the OG samples. Using the chromatogram from the Green Horn (GH) sample, the column deadtime was evaluated at 3.07 minutes. At both temperatures, the velocity range is optimal for the chlorinated compounds analyzed in the laboratory. At high temperature, the flight-mean carrier velocity is below the limit to which the loss of efficiency would be significant. At low temperature, Figure 3 shows that, despite the current flightoperating conditions, the efficiency is not degraded and consistent with the flight one. To confirm this hypothesis, the efficiency observed for dichloromethane, detected in GH, was measured and equals 1.5 mm. This value is consistent with the H value of chlorobutane and 3chloro-2-methyl-1-propene, indicating that the laboratory conditions are representative of the flight ones. Moreover, when we compare the flight H value (1.5 mm) to the laboratory one for the dichloromethane (2.1 mm), we see that GC-4 is even more efficient in flight compared to the laboratory. In conclusion, the flight operating conditions of GC-4 are optimal at low temperature and correspond to the maximal column efficiency. The efficiency loss is negligible at high temperature. Despite the instrumental constraints, GC-4 allows analyses in optimal conditions, at both temperatures considered in flight.

The MXT-CLP (GC-5) column allowed the analyses and detection of all the chlorohydrocarbons detected in the martian samples so far. The efficiency of this column was previously studied for non-chlorinated compounds [START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF], but it was not clear how the column efficiency behaves in the presence of chlorinated molecules. At 35°C, we found that 1,2dichloropropane and chlorobenzene had the same behavior, and that the theoretical Golay curves fit correctly with the experimental data. However, dichloromethane efficiency is almost linear with no increase at the highest carrier gas velocities. Thus, dichloromethane is not well retained by the column and has a retention time close to the deadtime. The H values measured were: 2.3 mm for dichloromethane and 1.6 mm for the other molecules. The optimal range of velocity for all three compounds was between 13 and 19 cm.s -1 . Efficiencies were much improved at 185°C.

Minimal H values were between 0.5 mm for the more retained chlorohydrocarbons to 0.8 mm for the less retained. The best efficiency was obtained for a carrier gas velocity of 24 cm.s -1 and stayed mostly constant at high velocities, is consistent with previous studies [START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF] and also indicating that GC-5 appears to have the same behavior regarding all types of molecules.

A comparison between the flight and optimal laboratory efficiencies showed that the analysis conditions were quite optimal at low temperature but degraded at high temperature. The efficiency loss was evaluated at 4% at low temperature and 46% at high temperature (Table 3).

The consequence is a degradation of the flight chromatograms compared to the optimal, through broader peaks and a more limited separation power of the column. To confirm that efficiencies are representative of SAM-flight operating conditions, we calculated the column efficiency for the dichloromethane detected in the Cumberland (CB) sample. At the SAM carrier gas velocity of 16.66 cm.s -1 , the H value for dichloromethane is 2.65 mm, corresponding exactly to the laboratory efficiency for this compound. For chlorinated compounds, laboratory operating conditions are perfectly consistent with the flight ones and analyses were performed in analytical conditions close to the optimal, especially at low temperature.

A previous study demonstrated that the lowest carrier gas speed on SAM does not have a great influence on the GC-5 flight column efficiency regarding alcohols and alkanes [START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF] at low temperature, even with an efficiency loss of ~33%. This study demonstrated that the efficiency loss in the case of chlorohydrocarbons was even lower (4%) and that GC-5 was best adapted for their analysis at low temperature. At high temperature, efficiency losses did not depend on the nature of the compound and were about 47% for all the compounds considered in both studies.

In conclusion, all five columns tested had similar ranges of optimal carrier gas velocities at low temperature. The obtained H minimum values (1-3 mm) show that, within the optimal carrier gas speed, all columns had high efficiencies and representative to the flight ones

(measured with the dichloromethane for GC-4 and GC-5). The most efficient columns are GC-1 and GC-2 both bonded with polydimethylsiloxane (80 and 95% respectively) and phenyl (20 et 5%), followed by GC-4 and GC-5 (Table 3). Even if the GC-5 column is dedicated to the analysis of compounds with the same molecular weights as GC-1 and GC-2, the presence of cyanopropyle seemed to slightly limit its efficiency compared to the others. At high temperature, the optimal carrier gas velocity range is reduced for all columns (except GC-2), especially for the GC-4 and GC-5 columns. But within their optimal velocity range, their efficiencies are improved, with a broader dispersion for GC-4 (Table 3). The efficiency and optimal carrier gas velocity range of the GC-2 column depend on the nature of the chlorinated compounds considered. GC-2 seems indeed less efficient for the analysis of aromatic chlorohydrocarbons (e.g. chlorobenzene and 1,2,4-trichlorobenzene) and if used on SAM, this constraint will have to be considered.

The study of the SAM-GC columns efficiencies demonstrated that flight operating conditions were within or close to the optimal velocities range for the analysis and separation of aromatic and aliphatic chlorinated organics, and has a minor influence on the SAM analyses (Table 3). However, we note that columns efficiency would have been improved by using a higher carrier gas flow rate as initially planned. Moreover, the slight decrease of pressure in the SAM helium gas tank throughout the mission should not significantly influence the columns' efficiencies. U opt was equivalent to u SAM at low temperature, lower at high temperature for the GC-1 and GC-5 columns and slightly higher for GC-4. The current flight carrier gas velocity works within the optimal carrier gas velocities at low temperature since efficiency losses are below 10% for all the SAM-GC columns. This is not the case at the highest temperature and degradations are observed: ~15, 25, 14 and 46% for GC-1, GC-2, GC-4 and GC-5 respectively.

Thus, it is globally preferable to use the columns from the GC-1, 2 or 4 channels for the analysis of the chlorohydrocarbons, with GC-1 being the most efficient of all for each chlorohydrocarbons and at both SAM extreme temperatures. With time and the general decrease of the SAM mean carrier gas speed, efficiencies might significantly decrease except for the GC-4 column which should even have a better efficiency (Figure 3) and that should be the favorable column to use in future SAM analyses.

Thermodynamic study

Retention time, as mass spectrum, is a key parameter used for identifying the compounds detected with the SAM instrument. As the temperature of the columns changes during an analysis, it is of interest to follow the evolution of the retention times with the column temperature. This allows an assessment of the extent of evolution of the retention times as well as the possible modification of the elution order of the analytes. With this aim, we analyzed a few of the studied chlorohydrocarbons and measured their retention times obtained at different isothermal temperature programs in the 30-200°C range. Measuring the dead time for each analysis allowed us to calculate the capacity factor k', with ln(k')H/T, with H the enthalpy of exchange of the solute between the mobile and the stationary phase. We observe that the plots are linear as expected, allowing us to calculate the values of H for all the compounds. Figure 4 demonstrates the linear fits, and Table 4 summarizes the values of k' and H obtained with this study.

In Figure 4, we observe that the k' values are similar for all the WCOT columns, whereas the PLOT column gives higher k' values, representative of a higher retention power. We also observe that for WCOT columns, the curves do not cross except possibly at high temperature.

However, the technical constraints of the SAM chromatograph do not allow to reach such high temperatures. Therefore, a change in the order of elution of chlorohydrocarbons for these four columns is unlikely, regardless of the temperature of the columns in the SAM-GC. This property is of interest, as it makes the identification of the chlorohydrocarbons easier. For the GC-6 channel, some inversion of elution order can occur at the lowest temperatures. But for these temperatures, the retention of the tested species was too high for their analysis with SAM within the operating conditions of the instrument. Therefore, once again, the order of elution should remain constant within the SAM operating conditions.

Analytical ability of the SAM-GC to detect chlorohydrocarbons evaluated in

laboratory under simulated flight operating conditions

Goal and experimental strategy

The SAM-GCMS instrument has enabled the separation, detection and identification of organo-chlorinated compounds in different samples collected by Curiosity on Mars [START_REF] Freissinet | Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars[END_REF][START_REF] Glavin | Evidence for perchlorates and the origin of chlorinated hydrocarbons detected by SAM at the Rocknest aeolian deposit in Gale Crater[END_REF][START_REF] Szopa | Detection of indigenous organic matter at the surface of Mars by the SAM instrument onboard Curiosity[END_REF][START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF]. To support ongoing work, and to better anticipate flight data and their interpretation, an additional objective of this study was to build a database of retention times of the chlorinated compounds detected or potentially present in the chromatograms obtained with SAM on Mars, or those which could potentially outgas from samples in future analyses. Although SAM analyses are constrained by various parameters that influence the analytical conditions, we were able to reproduce the constraints related to the GC columns operation. The two major constraints in reproducing flight conditions in the laboratory were a limited time of analysis related to the thermal constraints of the SAM instrument components, and the carrier gas pressure mechanically and permanently set at 0.9 bar.

For this part of the study, spare SAM GC columns were used (Table 2) in SAM-like operating conditions. To mimic the best SAM-flight carrier gas flow conditions, the flow restriction method described in Section 2.4 was used. The temperature programs used for each column and their maximum time of analysis allowed (measured in flight or induced by the constraints of the analytical channel considered) are presented Table 5.

Range of chlorohydrocarbons detectable under SAM flight operating conditions

To evaluate the elution properties of the SAM-GC columns and help the attribution of peaks observed in SAM chromatograms, the retention times of 26 organo-chlorinated compounds (Table 1) were measured in the laboratory using both GCMS described in Section 2.3. The retention times measured are presented in Table 6. Figure 5 shows chromatograms obtained after analyzing the same set of chlorohydrocarbons on all four WCOT columns studied. Figure 6 presents the analytical capabilities of the PLOT column used for the GC-6 channel of the SAM instrument.

The maximal times of analysis of each column (Table 5) are represented by a double line in Table 6 and by a dash-line of the color corresponding to each column in the chromatograms (Figure 5 and Figure 6). All the chlorohydrocarbons with retention times lower or equal to these maximum time of analyses are detectable by the SAM-GC if they are present in the gases released by the solid samples collected by Curiosity and submitted to thermal or chemical treatment in SAM. The compounds eluted beyond these limits are not detectable within the current flight operating conditions. These results show that the columns bonded with liquid stationary phases are able to elute most of the tested chlorohydrocarbons within the limited time of analysis of the instrument for the columns considered. Only Hexachlorobenzene cannot elute on time for any of the SAM-GC WCOT columns, and Pentachlorobenzene would only be eluted on time with the GC-1 and GC-2 columns, in the flight conditions simulated in laboratory. With the PLOT column used in the GC-6 channel, only a limited number of compounds, among those selected for this study, elutes within the given maximum time of analysis, chloroethane being the heavier one. Despite we were able to elute the 1,3,5-trichlorobenzene with a retention time of 251.10 minutes in the laboratory, such a high retention time combined to the thermal limitations of the GC-6 column makes this molecule non detectable with the SAM instrument on Mars.

By the end of the Curiosity mission on Mars, some margins could be used to push the instrument to its limit, or develop new strategies to analyze a sample could be imagined (e.g. two successive chromatograms of the same sample to elute the compounds remaining in the column after a first run) to increase the maximum time of analysis on critical chromatographic channels (GC-5 for instance). However, the operating constraints are so tight that it is not expected to significantly increase this time without damaging the chromatograph. The consequence of these limitations is that the columns MXT-20 (GC-1), MXT-5 (GC-2), and Chirasil-β Dex (GC-4) can elute and detect all the chlorohydrocarbons from chloromethane to pentachlorobenzene within the SAM instrument analytical conditions, and the MXT-CLP (GC-5) column is limited to the trichlorobenzene isomers. The Q-PLOT column (GC-6) is limited to the lightest gaseous chlorinated compounds such as the chloromethane and the chloroethane. However, its high retention power could be useful in the future to attempt quantifying the lightest chlorohydrocarbons which can be coeluted on the other columns.

To conclude, even if the limitation of the maximal time of analysis of the SAM-GC could result in the non-detection of chlorohydrocarbons because of their too late elution (e.g.

tetrachlorobenzene on the GC-5 channel ( 14) the instrument is capable to elute and detect a wide range of alkyl and monoaromatic chlorohydrocarbons that can be formed during the pyrolysis or the derivatization of a sample of Mars rocks. This capability is sufficient to give an overview of the main chlorohydrocarbons released by the sample preparation process from which information about the nature of the sample compounds and reactivity can be deduced.

Separation power of the SAM-GC columns under the flight operating conditions

Under the nominal SAM-flight operating conditions which are similar for all the columns, we observe that the order of elution of the chlorohydrocarbons selected for the study is quite comparable. For GC-1 and GC-2, this is true for almost all the compounds. This could be expected as the stationary phases of the corresponding columns are very similar (polydimethylsiloxane partly (PDMS) substituted by phenyl), with only a difference in the concentration in phenyl (20% and 5% respectively). This means that the modification in polarity of the stationary phase induced by the variation of phenyl concentration is not sufficient to significantly change the retention mechanisms of the chlorohydrocarbons analyzed. With both columns, the number of coelution observed is limited (<2) and the peaks shape is quite nominal.

For GC-4, more co-elutions are observed (>3) compared to GC-1 and GC-2. This difference is mainly due to a peak broadening generally observed with this type of stationary phase, and a modification in the retention of the compounds that lead to longer retention times for all the analytes, as it was previously described. In term of selectivity, even if not systematically, it seems that the multi-chlorinated alkyl chlorohydrocarbons are rather more retained with GC-4, leading to significant elution order changes, such as for 1,4-dichlorobutane. The source for this change of selectivity is difficult to determine as it can be due to the absence of phenyl in the stationary phase of the column, to the presence of cyclodextrin diluted in the PDMS, or both effects. Regarding GC-5, we notice that the separation pattern is very similar to the GC-2 one.

The only exceptions are the multi-chlorinated methanes which are much less retained with GC5.

This means that the stationary phases used in these columns are quite similar in term of polarity, at least for chlorohydrocarbons. Even if we have no details about the composition of the GC-5 stationary phase, our result show that it should be made of PDMS contain phenyl substituents at concentrations equivalent to the GC-2 stationary phase, and an additional substituent (CNPP) explaining the few differences in retention observed. It is possible that the difference in selectivity between those two columns would be more important for other chemical families and would be a key parameter to select either GC-2 or GC-5 for an analysis.

Depending on the objective given for the analysis of a sample, it is possible to select the more appropriate column among the five ones integrated in the SAM-GC for analysing organic molecules. This is obvious that for a broad screening of chlorohydrocarbons, the four WCOT columns are more appropriate to be used, but we showed that GC-1, GC-2 and GC-5 channels are preferable as they achieve a reasonable separation of all the tested compounds, regardless of their mass. The last WCOT column, GC-4, shows a better separation of the lightest chlorohydrocarbons but more heavier compounds are coeluted. For this reason, GC-4 should be preferred to enhance the separation of the lightest chlorohydrocarbons if necessary. Finally, the PLOT column should be used to specifically resolve chloromethane from chloroethane, for quantification requirements for instance. It is interesting to underline that SAM-GC can work with two columns working in parallel. Also, if the separation of both light and heavier chlorohydrocarbons is required in one unique run, an analytical strategy can consist in selecting the appropriate complementary columns to meet this objective, e.g. GC-5 and GC-6. Finally, we must keep in mind that the chlorohydrocarbons analysed with the SAM instrument on Mars are not the only chemical compounds released by the sample. Also, even if the detection benefits from the mass spectrometric measurement, additional coelution with non chlorohydrocarbons organic compounds should be limited to prevent any ambiguity in the compounds identification [START_REF] Mahaffy | The Sample Analysis at Mars Investigation and Instrument Suite[END_REF][START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF].

To conclude, the study of the separation achieved with the SAM chromatograph under flight-like conditions shows that the combination of columns selected are able to separate all the alkylated and mono-aromatic compounds tested within the limit of time imposed by the flight instrument technical constraints, even if the instrument design did not originally consider the presence of these compounds among the chemical species released by the soil and rock samples.

Conclusion

Whereas the detection of chlorohydrocarbons was not originally planned in the development of the SAM instrument, we showed that the set of five columns selected for achieving the chromatographic analysis of organic compounds is suited for the separation and detection of a wide range of aliphatic and monoaromatic chlorinated organics within the constrained operating conditions used on Mars. These operating conditions should allow a high level of efficiency of all the columns to be maintained throughout the duration of the mission.

While the qualitative study of the columns used in the GC-5 channel already allowed to identify and quantify several chlorine bearing organic molecules [START_REF] Freissinet | Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars[END_REF][START_REF] Szopa | Detection of indigenous organic matter at the surface of Mars by the SAM instrument onboard Curiosity[END_REF][START_REF] Millan | In situ analysis of martian regolith with the SAM experiment during the first mars year of the MSL mission: Identification of organic molecules by gas chromatography from laboratory measurements[END_REF], the work presented in this paper demonstrates the complementarity of the chromatographic columns bonded with different stationary phases and selected for the SAM instrument to achieve the separation and identification of alkyl chlorohydrocarbons up to 1-Chlrooctane, and monoaromatic chlorohydrocarbons up to Pentachlorobenzene. The conservation of the order of elution of the chlorohydrocarbons for all the columns regardless of the operating temperature used will also simplify the identification of the detected molecules throughout the Curiosity mission. This analytical capability will be precious in order to extract the most interesting information about the composition and reactivity of the Mars samples collected by the rover and analyzed with SAM, thus allowing to make great progress in the characterization of the Gale crater environment and its potential for past habitability.

Figure captions:

Figure 1. Comparison of the temperature programs applied to the three GC columns that have been used in flight so far. The measured flight temperature program is plotted in blue and the temperature programs applied in the laboratory are represented with a red dash-line. The times to which the injection traps of the GC-4 and GC-5 channels are activated are represented on the abscissa axis by black stars and are called « IT flash ». As the GC-1 channel is not equipped with an injection trap, no black star is present in the graph. Figure 2. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the GC-6 column doted of a solid stationary phase. Points correspond to the experimental data and the lines to the fits from the Van-Deemter-Golay equation. The black line, represented at 12.82 cm.s -1 , corresponds to the mean linear carrier gas velocity with the inlet column theoretical pressure on the SAM-flight instrument. Table 3. Summary of the values relative to the efficiency of the SAM-GC columns with liquid stationary phases: H range (minimum and maximum), the H SAM estimated when flight data were available, the optimal carrier gas velocity compared to the carrier gas velocities measured in SAM for each corresponding column or theoretically calculated and the estimated loss of efficiency in percentage compared to their laboratory capabilities for each column that has been used in flight so far.

Table 4.

Values of k' and-ΔH for the 5 SAM-GC columns studied. K' is given for each compound at 100°C. The -ΔH values were calculated from the slopes of the straight line ln (k') as a function of 1000/T from Figure 4. 5. Temperature programs applied in laboratory to the SAM spare columns. The temperature programs used for the channels GC-4 and 5 are identical to those used on the SAMflight instrument. The program temperature of the GC-1 channel is identical to the flight one except that no plateau at the initial temperature was used. Those used for the columns of the GC-2 and GC-6 channels are predictive since the related columns have not been used on Mars yet.

Values relative to efficiency

Chromatographic column

  value allowing us to obtain the deadtime representative of the flight model (~1.3 bar, as described in the results section).

Figure 3 .

 3 Figure 1.Comparison of the temperature programs applied to the three GC columns that have been used in flight so far. The measured flight temperature program is plotted in blue and the temperature programs applied in the laboratory are represented with a red dash-line. The times to which the injection traps of the GC-4 and GC-5 channels are activated are represented on the abscissa axis by black stars and are called « IT flash ». As the GC-1 channel is not equipped with an injection trap, no black star is present in the graph. Figure2. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the GC-6 column doted of a solid stationary phase. Points correspond to the experimental data and the lines to the fits from the Van-Deemter-Golay equation. The black line, represented at 12.82 cm.s -1 , corresponds to the mean linear carrier gas velocity with the inlet column theoretical pressure on the SAM-flight instrument. Figure3. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the SAM-GC columns doted of liquid stationary phases. H (mm) is plotted in function of in cm.s -1 . Points correspond to the experimental data and the lines to the fits plotted from the Van-Deemter Golay equation. The carrier gas velocities, corresponding to pressures measured in flight, were represented, when possible, by a black line, or theoretically evaluated. Crosses correspond to the H calculated from the flight data of dichloromethane, a chlorohydrocarbon systematically detected and present in all flight chromatograms. Figure4. Evolution of ln (k') in function of 1000/T for the chlorinated compounds on the SAM-GC columns from channels GC-1, GC-2, GC-4, GC-5 and GC-6. Experimental data were obtained for a temperature range between 40 and 200°C corresponding to the extreme temperatures used for SAM. A linear regression has been fitted to the experimental data points on the range of the figure. Figure 5. Chromatograms of the chlorohydrocarbons obtained for the channels GC-1 (MXT-20), GC-2 (MXT-5), GC-4 (Chirasil-β Dex) and GC-5 (MXT-CLP) in SAM-flight operating conditions listed in Table 4. Compounds: 1: 2-chloropropane, 2: dichloromethane, 3: 1-chloro-2-methyl-1-propene, 4: trichloromethane, 5: 3-chloro-2-methyl-1propene, 6: 1-chlorobutane, 7: tetrachloromethane, 8: 1,2-dichloropropane, 9: 1,2-dichlorobutane, 10 : chlorobenzene, 11: 1-chlorohexane, 12: 1,4-dichlorobutane, 13: 2-chlorotoluene, 14: 1,2-dichlorobenzene, 15: 1chlorooctane, 16: 1,2,4-trichlorobenzene, 17: 1,3,5-trichlorobenzene, 18: 1,2,4,5-tétrachlorobenzene, 19: pentachlorobenzene, 20: 1-chloroheptane. Compounds marked with a star * are co-eluted.Figure 6. Chromatogram of the chlorohydrocarbons obtained for the channel GC-6 (Q-Plot) in SAM-flight operating conditions listed in Table 4. Compounds: 1: Non-retained compound (Air), 2: chloromethane, 3: chloroethane, 4: dichloromethane, 5: 2-chloropropane, 6: trichloromethane, 7: 1-chloro-2-methyl-1-propene, 8: 3chloro-2-methyl-1-propene, 9: 1-chlorobutane, 10: tetrachloromethane, 11: 1,2-dichloropropane, 12: 1,2dichlorobutane, 13: Chlorobenzene, 14: 1-chlorohexane, 15: 1,4-dichlorobutane, 16: 2-chlorotoluene.

Figure 4 .

 4 Figure 1.Comparison of the temperature programs applied to the three GC columns that have been used in flight so far. The measured flight temperature program is plotted in blue and the temperature programs applied in the laboratory are represented with a red dash-line. The times to which the injection traps of the GC-4 and GC-5 channels are activated are represented on the abscissa axis by black stars and are called « IT flash ». As the GC-1 channel is not equipped with an injection trap, no black star is present in the graph. Figure2. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the GC-6 column doted of a solid stationary phase. Points correspond to the experimental data and the lines to the fits from the Van-Deemter-Golay equation. The black line, represented at 12.82 cm.s -1 , corresponds to the mean linear carrier gas velocity with the inlet column theoretical pressure on the SAM-flight instrument. Figure3. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the SAM-GC columns doted of liquid stationary phases. H (mm) is plotted in function of in cm.s -1 . Points correspond to the experimental data and the lines to the fits plotted from the Van-Deemter Golay equation. The carrier gas velocities, corresponding to pressures measured in flight, were represented, when possible, by a black line, or theoretically evaluated. Crosses correspond to the H calculated from the flight data of dichloromethane, a chlorohydrocarbon systematically detected and present in all flight chromatograms. Figure4. Evolution of ln (k') in function of 1000/T for the chlorinated compounds on the SAM-GC columns from channels GC-1, GC-2, GC-4, GC-5 and GC-6. Experimental data were obtained for a temperature range between 40 and 200°C corresponding to the extreme temperatures used for SAM. A linear regression has been fitted to the experimental data points on the range of the figure. Figure 5. Chromatograms of the chlorohydrocarbons obtained for the channels GC-1 (MXT-20), GC-2 (MXT-5), GC-4 (Chirasil-β Dex) and GC-5 (MXT-CLP) in SAM-flight operating conditions listed in Table 4. Compounds: 1: 2-chloropropane, 2: dichloromethane, 3: 1-chloro-2-methyl-1-propene, 4: trichloromethane, 5: 3-chloro-2-methyl-1propene, 6: 1-chlorobutane, 7: tetrachloromethane, 8: 1,2-dichloropropane, 9: 1,2-dichlorobutane, 10 : chlorobenzene, 11: 1-chlorohexane, 12: 1,4-dichlorobutane, 13: 2-chlorotoluene, 14: 1,2-dichlorobenzene, 15: 1chlorooctane, 16: 1,2,4-trichlorobenzene, 17: 1,3,5-trichlorobenzene, 18: 1,2,4,5-tétrachlorobenzene, 19: pentachlorobenzene, 20: 1-chloroheptane. Compounds marked with a star * are co-eluted.Figure 6. Chromatogram of the chlorohydrocarbons obtained for the channel GC-6 (Q-Plot) in SAM-flight operating conditions listed in Table 4. Compounds: 1: Non-retained compound (Air), 2: chloromethane, 3: chloroethane, 4: dichloromethane, 5: 2-chloropropane, 6: trichloromethane, 7: 1-chloro-2-methyl-1-propene, 8: 3chloro-2-methyl-1-propene, 9: 1-chlorobutane, 10: tetrachloromethane, 11: 1,2-dichloropropane, 12: 1,2dichlorobutane, 13: Chlorobenzene, 14: 1-chlorohexane, 15: 1,4-dichlorobutane, 16: 2-chlorotoluene.

Figure 5 .Figure 6 .

 56 Figure 1.Comparison of the temperature programs applied to the three GC columns that have been used in flight so far. The measured flight temperature program is plotted in blue and the temperature programs applied in the laboratory are represented with a red dash-line. The times to which the injection traps of the GC-4 and GC-5 channels are activated are represented on the abscissa axis by black stars and are called « IT flash ». As the GC-1 channel is not equipped with an injection trap, no black star is present in the graph. Figure2. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the GC-6 column doted of a solid stationary phase. Points correspond to the experimental data and the lines to the fits from the Van-Deemter-Golay equation. The black line, represented at 12.82 cm.s -1 , corresponds to the mean linear carrier gas velocity with the inlet column theoretical pressure on the SAM-flight instrument. Figure3. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the SAM-GC columns doted of liquid stationary phases. H (mm) is plotted in function of in cm.s -1 . Points correspond to the experimental data and the lines to the fits plotted from the Van-Deemter Golay equation. The carrier gas velocities, corresponding to pressures measured in flight, were represented, when possible, by a black line, or theoretically evaluated. Crosses correspond to the H calculated from the flight data of dichloromethane, a chlorohydrocarbon systematically detected and present in all flight chromatograms. Figure4. Evolution of ln (k') in function of 1000/T for the chlorinated compounds on the SAM-GC columns from channels GC-1, GC-2, GC-4, GC-5 and GC-6. Experimental data were obtained for a temperature range between 40 and 200°C corresponding to the extreme temperatures used for SAM. A linear regression has been fitted to the experimental data points on the range of the figure. Figure 5. Chromatograms of the chlorohydrocarbons obtained for the channels GC-1 (MXT-20), GC-2 (MXT-5), GC-4 (Chirasil-β Dex) and GC-5 (MXT-CLP) in SAM-flight operating conditions listed in Table 4. Compounds: 1: 2-chloropropane, 2: dichloromethane, 3: 1-chloro-2-methyl-1-propene, 4: trichloromethane, 5: 3-chloro-2-methyl-1propene, 6: 1-chlorobutane, 7: tetrachloromethane, 8: 1,2-dichloropropane, 9: 1,2-dichlorobutane, 10 : chlorobenzene, 11: 1-chlorohexane, 12: 1,4-dichlorobutane, 13: 2-chlorotoluene, 14: 1,2-dichlorobenzene, 15: 1chlorooctane, 16: 1,2,4-trichlorobenzene, 17: 1,3,5-trichlorobenzene, 18: 1,2,4,5-tétrachlorobenzene, 19: pentachlorobenzene, 20: 1-chloroheptane. Compounds marked with a star * are co-eluted.Figure 6. Chromatogram of the chlorohydrocarbons obtained for the channel GC-6 (Q-Plot) in SAM-flight operating conditions listed in Table 4. Compounds: 1: Non-retained compound (Air), 2: chloromethane, 3: chloroethane, 4: dichloromethane, 5: 2-chloropropane, 6: trichloromethane, 7: 1-chloro-2-methyl-1-propene, 8: 3chloro-2-methyl-1-propene, 9: 1-chlorobutane, 10: tetrachloromethane, 11: 1,2-dichloropropane, 12: 1,2dichlorobutane, 13: Chlorobenzene, 14: 1-chlorohexane, 15: 1,4-dichlorobutane, 16: 2-chlorotoluene.

Figure 1 .

 1 Figure 1. Comparison of the temperature programs applied to the three GC columns that have been used in flight so far. The measured flight temperature program is plotted in blue and the temperature programs applied in the laboratory are represented with a red dash-line. The times to which the injection traps of the GC-4 and GC-5 channels are activated are represented on the abscissa axis by black stars and are called « IT flash ». As the GC-1 channel is not equipped with an injection trap, no black star is present in the graph.

Figure 2 .

 2 Figure2. Evolution of the HEPT (mm) as a function of the mean linear carrier gas velocity (cm.s -1 ) for the GC-6 column bonded with a solid stationary phase. Points correspond to the experimental data and the lines to the fits from the Van-Deemter-Golay equation. The black line, represented at 12.82 cm.s -1 , corresponds to the mean linear carrier gas velocity with the inlet column theoretical pressure on the SAM-flight instrument.

Figure 2 Figure 3 .

 23 Figure 2

Figure 4 .

 4 Figure 4. Evolution of ln (k') in function of 1000/T for the chlorinated compounds on the SAM-GC columns from channels GC-1, GC-2, GC-4, GC-5 and GC-6. Experimental data were obtained for a temperature range between 30 and 200°C corresponding to the extreme temperatures used for SAM. A linear regression has been fitted to the experimental data points on the range of the figure.

Figure 4 Figure 5 .

 45 Figure 4

Figure 6 .

 6 Figure 6. Chromatogram of the chlorohydrocarbons obtained for the channel GC-6 (Q-Plot) in SAM-flight operating conditions listed in Table 4. Compounds: 1: Non-retained compound (Air), 2: chloromethane, 3: chloroethane, 4: dichloromethane, 5: 2-chloropropane, 6: trichloromethane, 7: 1-chloro-2-methyl-1-propene, 8: 3-chloro-2-methyl-1-propene, 9: 1-chlorobutane, 10: tetrachloromethane, 11: 1,2-dichloropropane, 12: 1,2dichlorobutane, 13: Chlorobenzene, 14: 1-chlorohexane, 15: 1,4-dichlorobutane, 16: 2-chlorotoluene.

Figure 6

 6 Figure 6

  -15°C.min -1 ramp up to 175°C then 10° C.min -1 rate up to Tf = 250°C 30 min MXT-5 (GC-2) Ti = 40°C for 5 min -then 10° C.min -1 rate up to Tf = 190°C 30 min Chirasil-β Dex (GC-4) Ti = 40°C for 5.8 min then 10° C.min -1 rate up to Tf = 190°C 29 min MXT-CLP (GC-5) Ti = 35°C for 6.3 min then 10° C.min -1 rate up to Tf = 185°C 21 min MXT PLOT-Q (GC-6) Ti = 40°C for 5 min then 10° C.min -1 rate up to Tf = 190°C

Table 1

 1 Chemical and physical properties of the 26 chlorohydrocarbons selected for this study.

	Tables
	Chlorohydrocarbon name	Raw chemical Formula	Molar mass [g.mol -1 ]	Boiling point [°C]	Vapor [kPa] pressure (at 20°C)	N° chlorine	Aromatic	Isomerism Constitution Chiral
	Chloromethane	CH 3 Cl	50.5	-23.8	506.1	1	-	-	-
	Dichloromethane	CH 2 Cl 2	84.9	39.6	47.0	2	-	-	-
	Trichloromethane	CHCl 3	119.3	61.2	21.1	3	-	-	-
	Tetrachloromethane	CCl 4	153.8	76,7	11.9	4	-	-	-
	Chloroethane	C 2 H 5 Cl	64.5	12.5	133.3	1	-	-	-
	1-chloropropane	C 3 H 7 Cl	76.5	35.7	59.2	1	-	X	-
	2-chloropropane	C 3 H 7 Cl	76.5	35.7	59.2	1	-	X	-
	1-chlorobutane	C 4 H 9 Cl	92.5	78.0	10.6	1	-	X	-
	2-chlorobutane	C 4 H 9 Cl	92.5	78.0	10.6	1	-	X	X
	1-chloro-2-methylpropene	C 4 H 7 Cl	90.5	68.0	13.5	1	-	X	-
	3-chloro-2-methylpropene	C 4 H 7 Cl	90.5	71.0-72.0	13.5	1	-	X	-
	2-chlorotoluene	C 7 H 7 Cl	126.6	157.0-159.0	0.4	1	X	X	-
	chlorobenzene	C 6 H 5 Cl	112.5	132.0	1.2	1	X	-	-
	1,2-dichlorobenzene	C 6 H 4 Cl 2	147.0	180.5	0.2	2	X	X	-
	1,4-dichlorobenzene	C 6 H 4 Cl 2	147.0	174.0	0.2	2	X	X	-
	1,2,4-trichlorobenzene C 6 H 3 Cl 3	181.4	213.0	0.1	3	X	X	-
	1,3,5-trichlorobenzene C 6 H 3 Cl 3	181.4	208.0	0.1	3	X	X	-
	1,2,4,5-tetrachlorobenzene	C 6 H 2 Cl 4	215.9	240.0-246.0	6.7.10 -3 (25°C)	4	X	X	-
	Pentachlorobenzene	C 6 HCl 5	250.3	275.0-277.0	2.0.10 -3 (25°C)	5	X	-	-
	Hexachlorobenzene	C 6 Cl 6	284.8	323.0-326.0	1.4.10 -6	6	X	-	-
	1,2-dichloropropane	C 3 H 6 Cl 2	112.9	96.0	5.3	2	-	X	X
	1,2-dichlorobutane	C 4 H 8 Cl 2	127.0	125.0	3.7 (32°C)	2	-	X	-
	1,4-dichlorobutane	C 4 H 8 Cl 2	127.0	161.0-163.0	0.5	2	-	X	-
	1-chlorohexane	C 6 H 13 Cl	120.6	134.0	NA	1	-	X	-
	1-chloroheptane	C 7 H 15 Cl	134.6	159.0-161.0	0.7 (34°C)	1	-	X	-
	1-chlorooctane	C 8 H 17 Cl	148.6	183.0	13.6 (25°C)	1	-	X	-

Table 2

 2 Spare column specifications used for laboratory experiments.

	SAM Channel number Column name	Column type	Stationary phases	Target molecules
	GC1 MXT-20	WCOT	80% polydimethylsolixane with 20% phenyl	Medium weight (C 5 -C 15 )
	GC2 MXT-5	WCOT	95% polydimethylsiloxane with 5% phenyl	High weight > C 15
	GC4 Chirasil-β dex	WCOT	Polydimethylsiloxane with β cyclodextrin	Medium weight and enantiomers (C 5 -C 15 )
	GC5 MXT-CLP	WCOT	Polydimethylsiloxane with phenyl and cyanopropyle	Medium weight (C 5 -C 15 )
	GC6 MXT-Q	PLOT	Divinylbenzene	Light weight (C 1 -C 4 )

Table

			-1	GC-2		GC-4		GC-5
		35°C	185°C	35°C	185°C	35°C	185°C	35°C	185°C
	H (mm)	0.5-1.0	0.8-1.4 1.0-1.3 1.8-3.0 1.2-2.8	0.9-3.5	1.6-2.3 0.5-0.8
	H SAM (when applicable)	NA	NA	NA	NA	1.5	NA	2.65	NA
	opt (cm.s -1 )	15-25	15-20	15-20	11-19	13-20	10-13	13-19	24
	SAM or theoritical (cm.s -1 )	16.66	12.82	16.66	12.82	15.7 (40°C)	13.7	16.66	12.8
	Estimated loss of efficiency (~%)	8	15	4-8	25	0-8	14		4	46
		GC-1	GC-2	GC-4	GC-5	GC-6
	Compound	k' at 100°C	-ΔH (kcal. mol -1 )	k' at 100°C	-ΔH (kcal. mol -1 )	k' at 100°C	-ΔH (kcal. mol -1 )	k' at 100°C	-ΔH (kcal. mol -1 )	k' at 170°C	-ΔH (kcal. mol -1 )
	Dichloromethane	0.08	3.57	0.08	2.90	0.11	3.37	0.06	2.64	1.11	5.26
	1,2-dichloropropane	0.27	4.22	0.24	3.32	0.56	4.77	0.20	3.04	5.43	6.71
	Chlorobenzene	0.79	4.46	0.64	3.95	1.13	5.80	0.46	3.94	15.28	7.15
	2-chlorotoluene	2.76	5.30	1.31	3.78	2.19	5.38	0.86	3.83	27.53	5.47
	1,2,4-trichlorobenzene	7.00	4.15	5.25	5.05	9.78	6.56	3.36	4.58	-	-

Table 6 .

 6 Retention times (RT) of the chlorohydrocarbons detected with the SAM instrument, or potentially present in the gases released by future martian solid samples analysed with the instrument. They are given in minutes in SAM-flight operating conditions for each SAM spare GC column. For each column, the double-line indicates the current maximal time of analysis for each SAM-GC column. ND: Non-detected.

	Compound	RT (min) GC-1	RT GC-2	RT GC-4	RT GC-5	RT GC-6
	Non retained compound (Air: t 0 )	3.00	2.98	3.07	2.98	3.00
	Chloromethane	3.20	3.15	3.24	3.12	8.74
	Chloroethane	3.42	3.25	3.61	3.37	13.70
	2-Chloropropane	3.63	3.62	4.19	3.76	20.60
	Dichloromethane	3.89	3.87	4.97	3.83	21.00
	1-chloropropane	3.90	4.06	5.10	4.09	21.92
	2-chlorobutane (chiral)	4.34	4.90	7.50-7.58	4.66	25.30
	1-Chloro-2-Methyl-1-Propene	4.44	5.01	7.55	5.00	25.09
	Trichloromethane	4.55	4.99	9.65	4.68	24.50
	3-Chloro-2-Methyl-1-Propene	4.56	5.22	8.65	5.21	25.70
	1,2-dichloroethane	4.76	5.84	8.50	5.30	26.25
	1-Chlorobutane	4.89	5.81	9.06	5.82	26.84
	Tetrachloromethane	5.02	6.04	10.85	5.33	27.36
	1,2-Dichloropropane (chiral)	5.52	6.95	11.77-11.87	7.39	30.74
	1,2-Dichlorobutane	6.85	9.90	14.72	10.85	43.13
	Chlorobenzene	7.29	11.13	14.66	11.78	47.83
	1-Chlorohexane	7.53	11.34	14.38	12.27	51.14
	1,4-Dichlorobutane	8.53	12.68	17.99	14.27	60.05
	2-Chlorotoluene	8.62	13.76	16.29	14.50	77.64
	1-Chloroheptane	8.97	14.15	16.55	14.85	84.21
	1,4-Dichlorobenzene	9.85	15.34	18.15	15.85	101.60
	1,2-Dichlorobenzene	9.90	15.57	19.62	16.58	106.30
	1-Chlorooctane	10.27	15.91	19.82	16.86	154.32
	1,2,4-Trichlorobenzene	11.34	17.89	20.43	18.18	241.33
	1,3,5-Trichlorobenzene	12.12	18.28	21.36	19.12	251.13
	1,2,4,5-Tetrachlorobenzene	14.13	21.26	24.16	21.53	ND
	Pentachlorobenzene	16.74	24.25	29.32	24.55	ND
	Hexachlorobenzene	ND	ND	ND	ND	ND

Acknowledgments

The authors acknowledge the financial support of the French national space agency, the Centre National d'Etudes Spatiales (CNES) (SAM-GC grant).