Development and Optimization of an Analytical System for Volatile

Jul 15, 2014 - desorption kinetics of VOC (many hours) in the vacuum chamber during laboratory experiments, the low pressure under which they sublime ...
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Development and Optimization of an Analytical System for Volatile Organic Compound Analysis Coming from the Heating of Interstellar/Cometary Ice Analogues Ninette Abou Mrad, Fabrice Duvernay, Patrice Theulé, Thierry Chiavassa, and Grégoire Danger* Aix-Marseille Université, CNRS, PIIM, UMR 7345, 13013 Marseille, France S Supporting Information *

ABSTRACT: This contribution presents an original analytical system for studying volatile organic compounds (VOC) coming from the heating and/or irradiation of interstellar/cometary ice analogues (VAHIIA system) through laboratory experiments. The VAHIIA system brings solutions to three analytical constraints regarding chromatography analysis: the low desorption kinetics of VOC (many hours) in the vacuum chamber during laboratory experiments, the low pressure under which they sublime (10−9 mbar), and the presence of water in ice analogues. The VAHIIA system which we developed, calibrated, and optimized is composed of two units. The first is a preconcentration unit providing the VOC recovery. This unit is based on a cryogenic trapping which allows VOC preconcentration and provides an adequate pressure allowing their subsequent transfer to an injection unit. The latter is a gaseous injection unit allowing the direct injection into the GC-MS of the VOC previously transferred from the preconcentration unit. The feasibility of the online transfer through this interface is demonstrated. Nanomoles of VOC can be detected with the VAHIIA system, and the variability in replicate measurements is lower than 13%. The advantages of the GC-MS in comparison to infrared spectroscopy are pointed out, the GC-MS allowing an unambiguous identification of compounds coming from complex mixtures. Beyond the application to astrophysical subjects, these analytical developments can be used for all systems requiring vacuum/cryogenic environments.

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cometary ice may undergo in interplanetary environments, such as UV irradiations and/or cosmic rays. These processes activate the initial molecules, leading for example to radical recombination forming new and particularly more reactive molecules. While analogues are irradiated, the ice is warmed, simulating a comet becoming active when approaching the Sun. This warming allows an increase in the complexity of chemical reactivity, leading after desorption of volatile organic compounds (VOC) to organic residues. Such an evolution may also occur for interstellar ices; in this case, VOC are delivered in the inner envelope which surrounds the young star called Hot cores/Hot corinos. The composition of organic residues has been largely reported,6−13 while the characterization of VOC has been limited. Whole analyses of these residues have shown that a few thousands of molecules are present, while the initial ice composition includes only simple molecules such as H2O, CH3OH, and NH3. This demonstrates the complexity of the chemical reactivity that occurs during the process of ice heating and/or irradiation. For understanding this chemical evolution, the characterization of VOC sublimating during the ice warming is important, because they may take an important role in the residue formation.

tudying the formation and the chemical evolution of organic matter starting from molecular clouds to the early solar system and Earth is of crucial importance for understanding the origin and evolution of the solar nebula as well as at the end, the emergence of life on Earth.1,2 Among the different objects that are studied, comets are of prime interest. They have preserved the original material of the solar nebula which subsequently led to our solar system. The determination of the organics in comets is an important objective for understanding the chemical evolution occurring during the solar system formation. Remote sensing along with in situ investigations of comets throughout more than 20 missions during the last decades allowed the identification of numerous organic species such as methanol, formaldehyde, symmetric hydrocarbons lacking a permanent dipole moment, and sulfurand nitrogen-bearing molecules.3−5 However, probing such objects is a difficult task. For enhancing data interpretations obtained from cometary missions and understanding the chemical reactivity that occurs in cometary environments, laboratory experiments have been developed. They consist of recreating in the laboratory the evolution that a cometary material can undergo when a comet approaches the Sun. Analogues of cometary ice are formed inside a vacuum chamber (generally 10−9 mbar) by depositing a gas mixture relevant to cometary ice composition (H2O, NH3, CH3OH, CO2, etc.). These analogues are then subjected to physical processes that a © 2014 American Chemical Society

Received: May 28, 2014 Accepted: July 15, 2014 Published: July 15, 2014 8391

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Figure 1. Overall presentation of the VAHIIA analytical system: (A) the vacuum chamber where ice analogues are formed; (B) the developed preconcentration unit mainly composed of six pneumatic valves V1 to V6 and of a preconcentration loop; (C) the developed gas injection unit mainly composed of two sample injection loops branched on valves E1 and E2. We note that the path of compounds from the vacuum chamber to the preconcentration unit is represented with the black arrows in part (B), and the green valves such as B1, B2, and B3 isolate or connect two parts of the system.

Furthermore, radiotelescopes or current space missions such as the Rosetta mission14,15 will analyze VOC sublimation from interstellar or cometary grains and will need information on the various VOC that can come from such grains for enhancing their data interpretation. However, to our knowledge, experiments dedicated to VOC characterizations from experimental simulations have not yet been developed. For these reasons, we were interested in developing an original experimental setup for a nonspecific and an online analysis of these VOC. Nevertheless, to conduct a screening of VOC sublimating from cometary ice analogues, specific analytical developments have to be performed to overcome constraints regarding analytical sensitivity and selectivity. VOC sublime in a high vacuum chamber around 10−9 mbar, and their desorption can take many hours depending on analytes and on experimental protocols. Mass spectrometry with low mass resolution (residual gas analyzer (RGA) using quadrupole mass spectrometer QMS) is often used for the analysis of simple chemical systems, in particular for corroborating infrared characterizations.16 However, RGA is not efficient for a nontargeted screening of a wide panel of compounds sublimating from complex ice mixtures because of the low resolution and the electron impact ionization used, which cannot distinguish compounds having the same m/z ratio in a mixture. Furthermore, the saturation of the detector induced by highly abundant compounds prevents the detection of lower abundance analytes. As an alternative, gas chromatography coupled to mass spectrometry (GC-MS) may be used. The gas chromatograph provides the separation of analytes using an adequate chromatographic column, while the mass

spectrometer enables the identification of the molecular species, and if necessary their structural analysis to confirm their identification. This technique has been largely used to analyze volatiles coming from atmospheric samples or from the heating of ice cores,17−19 to study photodegradated products of polyoxymethylene after the condensation of volatile species on a coldfinger and their subsequent warming,20 or to develop analytical systems21−25 for in situ analyses for the Rosetta mission. However, these analyses have not been performed under online conditions, and because gas chromatography analyses are commonly performed at atmospheric pressure and need fast injections for obtaining the highest efficiencies, three main problems have to be solved for an online analysis of VOC coming from interstellar or cometary ice analogues: the low pressure under which they are formed, the low sensitivities due to important desorption time, and the presence of water, the main constituent of ice analogues. For increasing the sensitivity, enrichment methods are classically used in air samples such as solid-phase microextraction (SPME)26,27 which enables excellent recoveries28 especially in the case of thermally unstable and polar VOC. However, it is limited by the fragility of the fiber and the low sorbent volume. Furthermore, the need to connect the SPME fiber to the vacuum chamber inhibits its use. Another widespread technique is adsorptive preconcentration using specific adsorbents to collect the gaseous phase,29,30 but it requires the use of a solvent to extract target compounds, which may lead to signal overlap between the solvent and analytes besides the fact that the whole analytical procedure is time- and solvent-consuming. Furthermore, depending on analytes, a 8392

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(conditions discussed further). The temperature was then increased at a rate of 15 °C min−1 up to 220 °C and kept isothermal for 3 min. The temperature of the transfer line from GC to MS was set at 250 °C. The ion trap mass spectrometer was used in the electron impact ionization mode (ionization energy of 70 eV). The ion source temperature was set to 250 °C and the maximum ion time in the trap was of 25 ms. The signal was collected with a full scan mode in the mass range between 15 and 300 u and a scan event time of 0.16 s. For the purpose of analytical quality control, a standard of methyl acetate was regularly injected from position B3 (Figure 1) during system calibration to test the sensitivity of the GC-MS part (exp#1 in Table S-1, Supporting Information), and multiple blanks were run within the sequence to detect residual contamination. For this study, the GC has been modified due to the addition of a gaseous sample injection unit mainly composed of sample loops detailed further on and in Figure 1C. In addition, both injectors have also been modified to allow gaseous sample injection. The first modification allows the carrier gas to flush the sample injection loop at the injector level, and the second modification consists of adding a stainless steel tube in the injector that brings the sample from the injection loop to the injector (Figure S-1, Supporting Information). The whole interest of having both of these modified SSL injectors in our instrument is to make possible the use of multiple columns for the analysis of molecular species having different physical and chemical properties. Furthermore, column swapping is facilitated by the use of a vent-free GC/MS adapter (Frontier Laboratories LTD) (i.d. 0.15 mm, length ca. 50 cm, column ultra-ALLOY metal capillary) in the interface between the GC and the MS which limits the air flow into the MS at values lower than 0.5 mL min−1 during the switching of columns. For our current developments, only the stabilwax column has been installed on the back GC injector (Figure S-1, Supporting Information). Chemicals. The developed system was calibrated and optimized using six compounds belonging to different chemical classes, representing chemical functions present or likely to be present in cometary environments based on experimental simulation and interstellar comparison. Selected compounds are polar, because it is assumed based on the initial overall composition of ice analogues (H2O, CH3OH, NH3, etc.) that sublimated compounds would be mainly polar. The first is diethyl ether, generated during experimental simulation,23 and its detection was highlighted.1 The second is acetaldehyde, first identified in comet 1P/Halley.3 The third is methyl acetate expected from laboratory simulations.23,25 The fourth is methanol which is one of the most abundant cometary volatiles, with abundances often beyond 1% but below 5%.5 The fifth is ethanol which has been detected in the molecular cloud by radioastronomy32 and also expected from laboratory simulations.23,25 The last is acetonitrile first detected in comet Hyakutake. For the laboratory experiments, the standards of diethyl ether (anhydrous, assay ≥99%, ACS reagent), acetaldehyde (puriss. p.a, anhydrous, assay ≥99.5% (GC), Fluka), methyl acetate (anhydrous, assay 99.5%), methanol (for pesticide residue analysis, Fluka Analytical), and acetonitrile (for pesticide residue analysis, Fluka Analytical) were purchased from Sigma-Aldrich, Saint Quentin Fallavier, France. Ethanol (assay 96%) was acquired from API Laboratory, Vailhauques, France.

reactivity with the solvent cannot be ruled out. To avoid solvents, thermodesorption of the adsorbent may be used to collect analytes, but the major disadvantages of this technique are low peak resolution and the possible instability of certain analytes at high temperature. Furthermore, the large presence of water can rapidly saturate the sorbent. Cryogenic preconcentration remains one of the typical VOC enrichment methods for air samples31 has the advantage of not leading to water saturation problems, not causing matrix effects, sorbent bleeding, carryover phenomena, and not requiring solvents, and mainly it is a nonselective technique that concentrates compounds having different physicochemical properties. However, commercial cryogenic traps coupled to GC cannot be used in our context because they cannot be coupled to high vacuum systems, and their storage capacity can be limited. Nevertheless and considering the advantages of preconcentration with cryogenic techniques, we developed a specific preconcentration unit based on a cryogenic trapping of VOC, allowing the coupling between the cryogenic vacuum chamber and GC-MS. Furthermore, for the GC injection, we developed a specific gas sample injection unit for allowing online gaseous analysis with the GC. The two units constitute the VAHIIA system (volatile analysis from the heating of interstellar/ cometary ice analogues). In this contribution, we present the development of the cryogenic trap unit and of the gas injection one, their calibration, their optimization, and their validation for transferring VOC from the vacuum chamber to the GC-MS for their analysis.

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EXPERIMENTAL SECTION Experimental Setup. The basic equipment and techniques employed for the formation and the followup of ice analogues have been previously reported.16 A brief description of the overall system is provided herein (Figure 1). All experiments described were performed in a high vacuum chamber presenting a pressure of 5 × 10−9 mbar at 295 K and 10−9 mbar at 20 K. The gaseous mixture was deposited at a rate of 6 × 10−1 mol min−1 on a copper-plated surface kept at 20 K with a closed helium cycle model 21 CTI cold head (Figure 1A). Under these conditions, an ice analogue is formed on the sample holder. Control of sample is provided by the recording of infrared spectra in reflection mode between 4000 and 500 cm−1 using a Nicolet Magna 750 FTIR spectrometer with an MCT detector. Each spectrum was averaged over 100 scans with a 1 cm−1 resolution. The warming of the ice mixture to room temperature was performed by stopping the cryogenic system. For the VOC analysis, the chamber is connected to a GC-MS (GC Trace 1310 and MS ion trap ITQ 900 from Thermofisher) with stainless steel tubes as represented in Figure 1. The GC split/splitless (SSL) injector was maintained at 250 °C, and for the majority of tests (exp#1 until exp#10 in Table S-1, Supporting Information), a split mode injection with a ratio of 10 or 25 was conducted which ensures a good compromise between a high sensitivity while avoiding column saturation. Separation of target analytes was achieved using a Stabilwax column purchased from Restek (Crossbond Carbowax Polyethylene glycol stationary phase, 30 × 0.25 mm i.d. × 0.25 μm d.f.) and using helium (alpha gaz 2) as the carrier gas with 1 mL min−1 constant flow rate. This Stabilwax column has been selected mainly for its high tolerance to waterrich matrixes such as cometary ice analogues. The temperature program started at 35 °C and was held for 1 or 2 min 8393

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DEVELOPMENT OF THE INTERFACE BETWEEN THE VACUUM CHAMBER AND THE GC As described previously, volatile organic compounds are formed in a vacuum chamber (10−9 mbar) and desorb during a long period. A direct injection of these VOC in a GC is impossible because GC works at atmospheric pressure and requires fast injections. For obtaining an online injection, we developed a specific interface linking the vacuum chamber and the GC, the VAHIIA interface (Figure 1). This interface consists of two different units. The first corresponds to a preconcentration unit which is directly connected to the chamber (Figure 1B). The second corresponds to a gaseous injection unit developed in collaboration with Interscience (Figure 1C) that connects the preconcentration unit to the GC injector. The developed preconcentration unit has two mains objectives: first, preconcentrating analytes prior to the GC analysis; second, reaching a pressure sufficient to provide a GC analysis. The injection unit allows a direct injection of the gaseous species into the GC, avoiding any contamination and sample loss and improving the robustness of the analysis. In this section, we present the characteristics of each unit as well as their optimization and calibration. Preconcentration Unit. Considering the need for preconcentrating samples prior to their analysis and transferring them from the low pressure chamber to atmospheric pressure, and taking into account the advantages of cryogenic trapping already discussed, we have developed a preconcentration unit between the vacuum chamber and the GC. This unit is composed of a preconcentration loop and six pneumatic valves (CMDA series, Les Automatismes Appliqués) (Figure 1B): V1 is connected to helium gas via a thermal mass flow meter/ controller (Bronkhorst EL-Flow Select Series), V2 is connected to the vacuum chamber, V3 and V4 are connected to a preconcentration loop, V5 is connected to the injection unit of the GC, and V6 is connected to the turbomolecular pump of the vacuum chamber. For recovering VOC sublimating from the chamber, the upper part of the chamber is isolated from the turbomolecular pump. The valve B1 is opened, allowing the connection of the preconcentration unit to the vacuum chamber. Valves V2, V3, V4, and V6 are then opened, allowing a differential pumping of VOC (besides the cryogenic one) through the preconcentration loop, because the V6 is connected to the turbomolecular pump. During VOC trapping, the preconcentration loop is pumped and cooled in liquid nitrogen (77 K). All compounds having sublimation temperature higher than 77 K can be trapped in the loop, and species having sublimation temperature lower than 77 K are not recovered (see Table S-2 in Supporting Information for vapor pressure of compounds at 77 K). For the calibration and the optimization of the preconcentration unit, its performance for transferring online analytes to the GC injection unit was investigated. Series of tests were conducted by isolating the preconcentration unit from the cryogenic chamber (B1 is closed). Individual analytes or analytes in mixtures were introduced in the preconcentration loop directly from a 50 mL homemade glass bulb balloon (with an O-ring and “mecaverre” valve D-type, 180° outlet) connected at position B2 (Figure 1). Precisely, the balloon was filled with standard compounds at a determined partial pressure in a glass line of 1 L (which has been previously pumped to a pressure around 10−3 mbar) and then closed and connected at position B2. The

quantity of analytes in the balloon is calculated using the ideal gas law

PV = nRT

(1)

and noted in Table S-1 (Supporting Information) for each test, where P (Pa) is the absolute pressure of the gas, V (m3) is the volume of the glass line, n (mol) is the amount of analyte, R (8.31 J mol−1 K−1) is the ideal gas constant, and T (293 K) is the absolute temperature of the gas. During calibration experiments when compounds are introduced at position B2, the preconcentration period is of 2 min, while this period is optimized when species are pumped from the chamber (details further on). In both cases, when the estimated time for compound recovery is reached, all valves are closed. The loop is then warmed to 70 °C for obtaining a flash sublimation of stacked compounds. Subsequently, helium can be introduced in the loop by opening valves V1 and V3 via the flow meter/ controller to increase sample pressure and to facilitate analyte transfer to the injection unit of the GC by opening valves V4 and V5. A summary of valve positions following the corresponding procedural stage of the experiment is presented in Table S-3, Supporting Information. Transfer of Analytes from the Preconcentration Unit to the Injection Unit of the GC. Compounds transfer from the preconcentration unit to the injection unit is obtained by opening valves V4 and V5. This transfer is quite difficult, considering that after the loop warming, the total pressure is only of few millibars (20 mbar) in the preconcentration loop, a pressure that is near the one present in the injection unit (around 5 mbars). Therefore, the equilibration time of analytes between the two units is rather long. Tests have shown that without helium addition (exp#3 in Table S-1, Supporting Information), the sensitivity is very low, suggesting that only a fraction of the compound is transferred to the injection unit (Figure 2A). For enhancing this analyte transfer, an over-

Figure 2. Various helium additions for compounds transfer from the preconcentration unit to the injection unit: the case of an individual component (3.5 × 103 nmol methyl acetate, split ratio 25) (A) and component mixture (0.4 × 103 nmol of each component, split ratio 10) (B).

pressure is created in the preconcentration loop by adding helium. Preliminary tests confirmed that helium does not interact with compounds because, for a constant quantity of methyl acetate introduced from position B3 into the GC, no significant effects on sensitivity are observed when 100, 200, 300, 500, and 1000 mbar of helium are added (exp#2 in Table S-1, Supporting Information). Methyl acetate is selected to represent the six compounds tested in the calibration of the 8394

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internal diameter is not too small, which avoids plug formation during preconcentration. Figure 3 displays the results obtained

VAHIIA interface due to its intermediate physicochemical properties. The variability in peak areas in the pressure range used is below 10%, confirming that helium is a convenient carrier gas for enabling compounds transfer to the GC. When helium is added in the loop (exp#3 in Table S-1, Supporting Information), the differential in pressure obtained provides a considerable optimization of the analyte transfer between the two units. Various helium pressures were tested (exp#4 in Table S-1, Supporting Information) using a 2.5 mL preconcentration loop with 0.22 cm of internal diameter and a length of 68 cm (Figure 2A). The total pressure in the loop corresponds then to the sum of the partial pressure of each analyte and that of helium added. For example, for a 100 mbar pressure in the loop, the real amount of helium corresponds to 100 − x mbar, where x represents the partial pressure of analytes. The total pressure varied from 100 mbar (the lowest total pressure in the loop that can be reached with excellent precision and reproducibility) to 1000 mbar (atmospheric pressure). As shown in Figure 2A, when for the same amount of methyl acetate injected individually (3.5 × 103 nmol, split ratio 25) the amount of helium is increased, the signal response of methyl acetate decreases. These results are confirmed by tests conducted with a lower quantity of methyl acetate (0.4 × 103 nmol, data not shown). For individual analyses, the best sensitivity is obtained with a total pressure of 100 mbar. This is confirmed with other standards which present a decrease in sensitivity by a factor ranging from 1.3 to 2.9 when the total pressure varies from 100 to 1000 mbar (data not shown). The effect of helium addition was also verified with a mixture of compounds (exp#4 in Table S-1, Supporting Information). In this case, each compound is preconcentrated individually in the preconcentration loop at 77 K. As shown in Figure 2B, a total pressure of 100 mbar ensures the best sensitivity for all analytes (0.4 × 103 nmol for each compound, split ratio 10). However, when considering a more concentrated mixture (3.5 × 103 nmol of each compound, data not shown), a higher pressure (i.e., 300 mbar) may be needed for some compounds with an optimum pressure still comprised between 100 and 300 mbar. The addition of helium to the preconcentration loop highly increases the transfer to the injection unit with optimum sensitivities obtained for a total pressure ranging from 100 mbar to 300 mbar, depending on analytes and their concentrations. It should be highlighted that in the case of mixtures, possible analyte interactions in the preconcentration loop are considered and discarded in this case. Signal responses of methyl acetate injected alone and in the presence of other compounds (exp#5 in Table S-1, Supporting Information) are indeed not significantly different. Similar calibration curves are further obtained with increasing quantities of methyl acetate in both cases (Table S-4, Supporting Information). Volume Optimization of the Preconcentration Loop. As discussed earlier, and considering the low desorption kinetics of VOC from the sample holder (up to many hours), and the important volume of the chamber (approximately 400 L), compounds have to be preconcentrated before GC injection. The volume of the loop must be also optimized for obtaining the highest sensitivity. For this purpose, the performances of two different volumes of the preconcentration loop (2.5 and 5 mL, both with 0.22 cm of internal diameter) have been tested using the same quantity of methyl acetate (3.5 × 103 nmol) as detailed in exp#6 (Table S-1, Supporting Information). These loops were selected because they are long enough to be well submerged in liquid nitrogen and their

Figure 3. Methyl acetate (3.5 × 103 nmol) preconcentrated alone in two different loop volumes and transferred to the GC-MS with various helium pressures. For the 2.5 mL loop, some replicate injections (n = 3) were conducted.

for both loops. The same trend of signal response in the function of the total pressure is observed. However, the best sensitivity is obtained with the 2.5 mL loop and particularly with total pressures ranging from 100 mbar to 200 mbar. It has to be noted that the sensitivity increase is lower than an expected factor 2 because the volume in eq 1 is the total volume of the unit (Vloop + Vinjection unit). Hence, the partial pressure of the compound in the 5 mL loop is lower than half of its value in the 2.5 mL, explaining the results in Figure 3. Considering these results, the 2.5 mL loop is selected for the rest of this study. Gas Injection Unit. For allowing the injection of gaseous samples coming from the vacuum chamber to the GC, the injection part of the GC has been modified in collaboration with Interscience Belgium. The modification of the device consisted mainly of adding a rheodine six-port valve before each GC injector (Figure 1C, and Figure S-1, Supporting Information). A sample loop is connected to each of these valves (E1 before the front GC injector and E2 before the back GC injector) (Figure S-1, Supporting Information). These sample injection loops are pumped (residual pressure of few mbars) and then filled with the gaseous sample transferred from the preconcentration loop due to the pressure differential between the preconcentration loop and the sample injection loop. Through valve commutation, their content is then discharged into the GC injector (Figure S-1A, Supporting Information). In the unloading position, the carrier gas flushes the sample loop, which pushes its content to the GC injector. Afterward, the valve commutes back again to the loading position (Figure S-1B, Supporting Information), which allows the cleaning of the loops by pumping out their content before subsequent injections. A detailed representation of this unit is presented in Figure S-1, Supporting Information, and valve positions are noted in Table S-3, Supporting Information. Sample Loop Volume and Its Influence on Sensitivity, Efficiency, and Resolution. The volume of sample loops influences the sensitivity and can be increased to improve it. Six injection loops were tested (25, 125, 250, 500, 750, and 1000 μL) (exp#8 in Table S-1, Supporting Information) with the same mixture of compounds: diethyl ether (20 nmol), acetaldehyde (102 nmol), methyl acetate (61 nmol), methanol 8395

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Information). On the basis of these results, the increase of the volume of the sample loop until 1000 μL still ensures an excellent sensitivity while maintaining a good efficiency, because chromatographic resolutions and column efficiencies are not degraded. Therefore, depending on the initial composition and concentration of samples, different sample loops are available to ensure a good compromise between sensitivity and efficiency. For the rest of this work, the 25 μL loop is used. It is to be noted that 0.15 s is necessary according to manufacturer instructions to unload a 25 μL loop content into the GC under a 1 mL min−1 helium flow. This period has been optimized for the other loop volumes prior to their use (exp#7 in Table S-1, Supporting Information). A value of 0.45 min was selected for loops of 250 μL and of 125 μL. A value of 1.8 min was selected for the 500, 700, and 1000 μL loops. For the 1.8 min unloading period, the initial temperature of the GC oven is held for 2 min instead of 1 min used for other loops. Repeatability Assessment. During the system optimization, many tests have been run in replicates to evaluate the repeatability of the method during the calibration of the VAHIIA system (exp#4, 6, and 8 in Table S-1, Supporting Information). Some of the experiments were conducted in the presence of individual compounds as well as in the presence of compound mixtures. The comparison between both cases can be considered as a measurement of the reproducibility and a good indicator of the system and method robustness (exp#5 in Table S-1, Supporting Information). Variation coefficients (n = 3 measurements) do not exceed 12% for the six target analytes when they are preconcentrated either alone or in a mixture, confirming the repeatability of the method. Replicated injections with the different injection loops also show a repeatability with variation coefficients below 13% (n = 3). These results are crucial and highlight the performance of the method and the robustness of the system because an excellent repeatability is obtained even though multiple steps are needed for sample preconcentration and injection (valve opening, loop warming, loop discharge, transfer, etc.).

(820 nmol), ethanol (820 nmol), and acetonitrile (61 nmol). As shown in Figure 4, the signal responses increase propor-

Figure 4. Mixture of 20 nmol of diethyl ether, 102 nmol of acetaldehyde, 61 nmol of methyl acetate, 820 nmol of methanol, 820 nmol of ethanol, and 61 nmol of acetonitrile preconcentrated in the 2.5 mL loop and injected using various injection loop volumes: 125, 250, 500, 750, and 1000 μL (split ratio 10).

tionally with the increase of the loop volume, and R2 is comprised between 0.9589 and 0.9898. At the exception of ethanol, the signal response is for instance 5 to 6 times higher with the 125 μL loop, or 25 to 29 times higher with the 1000 μL loop, than with the 25 μL loop, which highlights the sensitivity gain. Detection limits (LOD) of compounds were estimated using S (2) N where S/N is the chromatographic signal-to-noise ratio related to n, which is the quantity of substance introduced in the preconcentration loop (nmol). For instance, the detection limits of methyl acetate were 8.8 nmol with the 25 μL loop and of 0.6 nmol with the 1000 μL loop. Detection limits of all analytes are shown in Table S-5, Supporting Information. This increase in sensitivity with the increase in loop volume is of a key importance for our project. However, a possible drawback is that high volumes injected into the GC can overload the GC column and can affect peak efficiencies and chromatographic resolution. Efficiencies are calculated according to LOD = 3 ×

⎛ t ⎞2 N = 16⎜ R ⎟ ⎝w⎠

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TRANSFER OF VOC FROM THE VACUUM CHAMBER TO THE GC-MS After the evaluation of the performances of the developed preconcentration unit and of the gaseous injection unit, we tested the feasibility of the online transfer of VOC from the vacuum chamber to the GC-MS. However, and considering that species sublime very slowly and from an important volume (volume of the chamber 400 L), the 2 min period classically used in the calibration experiments for preconcentrating analytes is not sufficient. For this reason, a determination of an optimal period for ensuring the recovery of ongoing sublimated products is necessary. In this context, 4.1 × 103 nmol of diethyl ether are deposited inside the vacuum chamber on the sample holder cooled at 20 K (according to previously described procedure), forming an ice of diethyl ether. The cryogenic system is then stopped, allowing the slow warming of the coldfinger and its sample holder. When the coldfinger reaches a temperature near 135 K, diethyl ether starts to sublimate and is dynamically pumped out of the vacuum chamber (V6 opened) through the preconcentration loop cooled at 77 K. The compound is then transferred to the GCMS following the procedure described in Table S-3, Supporting Information. Different preconcentration periods were tested: 1, 2, 5, and 14 h (exp#9 in Table S-1, Supporting Information). Chromatographic areas do not present significant differences

(3)

where N is the number of theoretical plates, tR is the retention time of the compound (min), and w is the width at the peak base (min). N values are higher than 2000 for almost all compounds in all tested sample loops and on average they are only 2 times higher in the 25 μL loop than in the 1000 μL one (Table S-6, Supporting Information). Chromatographic resolution (RS) values are calculated according to t R(B) − t R(A) Rs = 2 × wB + wA (4) where RS is the resolution, tR(B) and tR(A) are the retention times of compounds B and A, respectively (min), wB and wA are the widths at the peak base of compounds B and A, respectively (min). RS values are higher than 1.5 with almost all injection loops, with 1.5 being a threshold value beyond which analyte separation is considered efficient (Table S-6, Supporting 8396

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However, this analytical method is inefficient for identifying each constituent individually. After deposition and before ice warming, the vacuum chamber is isolated from the turbomolecular pump by closing the gate valve separating the chamber from the pump and by opening the valve B1 which directly connects the vacuum chamber to the preconcentration unit. By this means, all species sublimating from the ice in the chamber are pumped through the preconcentration loop, because this latter is connected to the turbomolecular pump through valve V6. Species are then preconcentrated during 14 h and analyzed with the GC-MS according to the procedure already described. The “low-quantity” standard mixture (exp#10 in Table S-1, Supporting Information) is injected with a splitless injection instead of the split mode to evaluate the sensitivity gain obtained in the transfer. The GC-MS analysis shows the ability of the stabilwax column to separate the constituents of the mixture (Figure 5B). From this separation, each compound can be unambiguously identified based on its retention time and mass spectrum. Detection limits of compounds transferred from the vacuum chamber and analyzed in the splitless mode (“low-quantity” standard mixture) varied between 79 and 1477 nmol. These detection limits are sufficient to detect trace VOC sublimating from ice analogues. We should note that these values have to be considered as upper detection limits, because during gas deposition on the sample holder, a fraction is pumped out from the vacuum chamber. Detection limits should thus be even lower than those reported. Furthermore, an improvement in sensitivity can be expected by a factor of 12 when using the 1000 μL injection loop (Figure 4). It is interesting to note that detection limits calculated for diethyl ether deposited alone (exp#9 in Table S-1, Supporting Information) and deposited in a mixture (exp#10 in Table S-1, Supporting Information) are the same (39 nmol in the mixture and 41 nmol individually using the 25 μL loop), underlining the reproducibility and the robustness of the compound transfer from the vacuum chamber to the GC-MS. Compounds are detected with the MS thus demonstrating the feasibility of the online transfer of VOC from the vacuum chamber to the GC-MS. To evaluate the efficiency of the compound recovery, quantities of analytes deposited in the vacuum chamber are compared to those recovered and analyzed by GC-MS. The quantities of analytes introduced in the chamber are calculated using eq 1, while those analyzed by GC-MS are calculated based on the calibration curves obtained from exp#5 in Table S-1 and which parameters are reported in Table S-4, Supporting Information. Calculated quantities are presented in Table S-7, Supporting Information. Results show that diethyl ether quantified by GC is 2.8 times lower than the quantity initially introduced when it is injected alone (exp#9 in Table S-1, Supporting Information) and 2.3 times lower when it is present in an ice mixture (exp#10 in Table S-1, Supporting Information). The difference between both cases is small, highlighting again the reproducibility of the transfer of VOC from the chamber to the GC-MS. When considering the other compounds (at the exception of ethanol) in the mixture of exp#10, the loss factor is of 3.1 for acetaldehyde and methyl acetate, of 3.0 for methanol and of 3.8 for acetonitrile. The loss may be due to the pumping of a part of the compounds by the turbomolecular pump during their deposition on the sample holder, to the adsorption of some compounds on the tubing that connects the chamber to the GC, and/or to a partial recovery during the 14 h of preconcentration. Determining the

between 1 and 2 h of preconcentration. A slight response increase is observed for the 5 h period, while the one at 14 h led to about 3 times higher sensitivity (in comparison to the 1 h period). Indeed, with our experimental setup a period of 14 h of sample recovery is at least necessary, because the coldfinger inside the vacuum chamber has to be completely warmed for obtaining a complete recovery of sample. A large part of the sample can indeed be adsorbed on the upper part of the coldfinger when the lower part is already warmed. A longer trapping period has not been tested, because the sensitivity obtained is sufficient for this test and is a compromise between a potential increase of sensitivity and a time-consuming experiment. The last test (exp#10 in Table S-1, Supporting Information) is conducted with two mixtures of compounds (diethyl ether, acetaldehyde, methyl acetate, methanol, ethanol, and acetonitrile) having different quantities. One with 4.1 × 103 nmol of each standard (“high-quantity” standard mixture) and the other one with 10 times lower quantities of each compound (0.41 × 103 to 0.73 × 103 nmol) (“low-quantity” standard mixture). In both cases, the six compounds are deposited at 20 K and the corresponding deposit monitored using the FT-IR spectrometer. The corresponding spectra are displayed in Figure 5A. The six molecules introduced are not supposed to react together during the warming; therefore, product formation is not expected. The infrared analysis gives an insight on the chemical composition of the ice, because chemical functional groups of alcohols, esters, aldehydes, ethers, and nitrile can be identified.

Figure 5. Infrared spectra of two mixtures of diethyl ether, acetaldehyde, methyl acetate, methanol, ethanol, and acetonitrile deposited on the coldfinger in the vacuum chamber (A). GC-MS analysis of the same ice mixtures after their sublimation in the vacuum chamber and after their recovery (14 h) using the transfer unit (B). The diluted mixture is analyzed with splitless injection mode, while the concentrated mixture is analyzed with the split mode (split ratio 25). 8397

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Analytical Chemistry

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supported by the ANR project VAHIIA (grant ANR-12-JS080001-01) of the French Agence Nationale de la Recherche. The authors also thank the department IS-X expert center of Interscience Belgium for technical assistance in the development of the injection unit of the VAHIIA interface.

loss factor is crucial in applying corrective measurements in the quantitative approach.

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CONCLUSION Laboratory experiments and developments are of prime importance for the understanding of data coming from space missions. In this contribution, we present the development, the calibration, and the first optimization steps of an original analytical system for the analysis of nontargeted volatile organic compounds coming from the heating of interstellar/cometary ice analogues (the VAHIIA system). This latter consists of coupling a cryogenic vacuum chamber, where ice analogues are formed, to a developed preconcentration unit for concentrating these volatile organic compounds before their GC-MS analysis. A preconcentration loop of 2.5 mL gives the best sensitivity, and helium has to be added to this loop after sample warming for obtaining an optimal transfer of compounds from this unit to the injection unit of the GC-MS. The preconcentration unit is directly linked to this gaseous injection unit which is coupled to the GC-MS for an online injection of the preconcentrated gaseous species. The volume of sample loop can vary between 25 and 1000 μL without significantly affecting efficiencies and chromatographic resolutions. Detection limits are on the order of nanomoles using the 25 μL injection loop, and an improvement in sensitivity by a factor of 12 is expected using the 1000 μL loop. These detection limits are very satisfactory to detect trace levels of VOC. System and method performance have also been evaluated through the assessment of the repeatability of analyses, which does not exceed 13% for the majority of compounds. All conducted tests either at the preconcentration or injection stages highlight the system robustness. The developed system has successfully ensured the online transfer of VOC from the vacuum chamber to the GC-MS, the latter allowing the unambiguous identification of mixture of constituents. Further work for optimizing GC-MS parameters to gain in sensitivity and selectivity is under preparation. This new analytical system can be used for all subjects needing an online analysis of volatile compounds coming from low or high vacuum environments in relation to environmental or astrophysical conditions. It can also be applied to investigate many open questions in cometary research such as the following: Do chemical reactions occur in the cometary coma? Are extended sources involved in the production of cometary volatiles?

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by the French national programs “Programme National de Planétologie” (P.N.P, INSU), “Programme de Physique et Chimie du Milieu Interstellaire” (P.C.M.I, INSU), and the “Centre National d’Etudes Spatiales” (C.N.E.S) from its exobiology program. This work was further 8398

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