Aerogel volatiles concentrator and analyzer (AVCA) – Collection and concentration of trace volatile organics in aerogel for spectroscopic detection

Icarus 284 (2017) 150–156

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Aerogel volatiles concentrator and analyzer (AVCA) – Collection and concentration of trace volatile organics in aerogel for spectroscopic detection A. Tsapin a, S. Jones b,∗, M. Petkov b, D. Borchardt a, M. Anderson b a b

University California at Riverside, 900 University Ave., Riverside, CA 92521, USA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 125-109, Pasadena, California 91109, USA

a r t i c l e

i n f o

Article history: Received 11 July 2016 Revised 4 October 2016 Accepted 3 November 2016 Available online 9 November 2016 Keywords: Aerogel Spectroscopic analysis Volatile detection

a b s t r a c t A study was conducted to determine the efficacy of using silica aerogel to collect and concentrate ambient trace organics for spectroscopic analysis. Silica aerogel was exposed to atmospheres containing trace amounts of polycyclic aromatic and aliphatic hydrocarbons. The organics present were concentrated in the aerogels by factors varying from 10 to more than 10 0 0 over the levels found in the atmospheres, depending on the specific organic present. Since silica aerogel is transparent over a wide range of optical and near infrared wavelengths, UV-induced fluorescence, Raman and infrared spectroscopies were used to detect and identify the organics collected by the aerogel. Measurements were conducted to determine the sensitivity of these spectroscopic methods for determining organics concentrated by aerogels and the effectiveness of this method for identifying systems containing multiple organic species. Polycyclic aromatic hydrocarbons (PAHs) were added to simulated Mars regolith and then vaporized by modest heating in the presence of aerogel. The aerogels adsorbed and concentrated the PAHs, which were detected by induced fluorescence and Raman and FTIR spectroscopies. © 2016 Elsevier Inc. All rights reserved.

1. Introduction New approaches to conduct analyses to determine the distribution and composition of organic matter in the regoliths of planets and small bodies are needed to further NASA’s Planetary Program and specifically Astrobiology Program. Current spectrophotometers are capable of detecting and identifying organic molecules down to ppt or ppm levels, depending on the specific method. However, some organic moieties are probably present in extraterrestrial regoliths and atmospheres at concentrations below even these levels. One way of enhancing the spectroscopic signals available from volatile organics in extraterrestrial regoliths and atmospheres is to collect and concentrate these molecules in aerogels. Two spectroscopic based instruments that are currently being developed are VAPoR (Volatiles by Pyrolysis of Regolith) and SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals). VAPoR pyrolyzes regolith samples to release volatile organics that are then analyzed by

∗

Corresponding author. Tel.: +818 354 7805. E-mail address: [email protected] (S. Jones).

http://dx.doi.org/10.1016/j.icarus.2016.11.001 0019-1035/© 2016 Elsevier Inc. All rights reserved.

time-of-flight mass spectrometry (ten Kate et al., 2010; Malespin et al., 2012; Glavin et al., 2012). SHERLOC is being developed for the 2020 Mars Rover payload and would conduct analyses of organics and minerals by Raman and luminescent spectroscopies (Beegle et al., 2014). The advantages to the instrument being presented here (AVCA) are the following: (1) Passive collection by aerogel and minimal handling requirements of samples; (2) Trace molecular species are concentrated in aerogel by up to 3 orders of magnitude for subsequent analysis; (3) Combined analyses by induced fluorescence, Raman and Fourier Transform Infrared (FTIR) spectroscopies provide high sensitivity and specificity. Aerogels are materials composed of a very low density, high surface area porous network (Pierre, 2002; Livage, 1992; Hench, 1990; Brinker, 1990). The most common aerogels are made of silicon dioxide and are transparent to most optical wavelengths of light. Aerogels have been used in several NASA space missions and experiments, including the Orbital Debris Collection Experiment on the MIR Station, the Micro-particles Capturer in the International Space Station, the Stardust Mission, the 2003 Mars Exploration Rovers, and the Mars Science Laboratory (Jones, 2011; Horz et al., 1999; Baba, 2009). In these cases, aerogels were used to

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either capture hypervelocity particles or as thermal insulation. For the detection of trace volatile organics, aerogels are used to collect and concentrate the organics and make them available in increased concentrations for detection and identification by spectrophotometers. Since molecules collected in aerogels remain adsorbed to the internal surface of the aerogel for extended periods of time, the analyses can be done over extended periods of time, then the aerogel can store the sample(s) for future analysis or the integration time of the instrument can be increased to increase the signal obtained. Studies were conducted to demonstrate that trace organic molecules could be collected and concentrated in silica aerogels and that they could then be detected and identified by infrared, Raman, or induced fluorescence spectroscopies. The advantages of collecting organics in aerogels for spectroscopic analysis are that this makes organics present at concentrations below detectable levels available for detection and it makes them available for detection by multiple spectroscopic methods. 2. Experimental 2.1. Aerogel production The aerogels used in this study were produced by a modified two-step method developed by Tillotson and Hrubesh (Tillotson, 1992). Tetraethyl orthosilicate (TEOS) was combined with ethanol, water and nitric acid. The mixture was refluxed, distilled and then diluted with acetonitrile to form the sol. The aerogel precursor was made by taking aliquots of the sol and combining them with acetonitrile, water and ammonium hydroxide. Once the wet gels had formed, they were dried by supercritical solvent extraction. The wet gels were pressurized to approximately 800 psig and then heated to 295°C. At this pressure and temperature the acetonitrile is a supercritical fluid and can be removed from the wet gel without collapsing the silica network. The pressure was gradually decreased to ambient pressure, while maintaining the same temperature. Once the dried gels were cooled, they were removed from the drying system. Since the ratio of the sol to the solvent determines the density of the final aerogel, a variety of different density aerogels can be produced. The densities produced and employed in this study ranged from 25 mg/cc to approximately 200 mg/cc. The surfaces of aerogels can be functionalized with different organic groups by adding silicon alkoxides with functional groups to the aerogel precursor mixture. The alkoxides used in this study were tetra ethoxy silane, 3-aminopropyltriethoxysilane, diphenyldimethylsilane, and tetrabutylortho silicate. These were chosen because they add distinct functional groups to the surface of the aerogels that might be beneficial in adsorbing organic molecules. 2.2. Volatile organics A list of candidate organic molecules that have been observed in space, meteorites and comets was compiled from literature articles (Cottin et al., 1999; Sephton, 2002; Bocktree-Morvan et al., 20 0 0; Tennyson, 20 03; Criukshank et al., 2014; Allamandola et al., 1987; Izawa et al., 2014; Swamy, 2005; Sandford, 2008). The list included aliphatic and aromatic compounds, as well as, some sulfur containing compounds. The list of compounds used as adsorbates in this study is given in Table 1. This list includes simple aliphatic organics with functional groups such as amine, aldehyde and cyano groups, as well as, organic aromatics. The aromatics are polycyclic aromatic hydrocarbons (PAHs) and are considered to be of particular importance in the synthesis and evolution of complex organic molecules in our solar system.

151 Table 1 List of organic molecules observed in asteroids, comets and space. Name

Formula

Methanol Formaldehyde Formic acid Formamide Methylamine Ethanol Acetonitrile Acetaldehyde Methyl formate Ethyl amine Acetic acid Acetamide Acetone Propanol Acrylonitrile Ethyl formate Ethyl acetate Ethylene glycol Benzene Naphthalene Fluorene Anthracene Phenanthrene Pyrene Carbon Disulfide Methyl Sulfoxide

CH3 OH HCHO HCOOH NH2 CHO H3 CNH2 CH3 CH2 OH CH3 CN CH3 CHO HCOOCH3 H 7 C2 N CH3 COOH CH3 CONH (CH3 )2 CO C3 H9 O CH2 CHCN C2 H5 OCHO CH3 COOCH2 CH3 (CH2 OH)2 C6 H 6 C10 H8 C13 H10 C14 H10 C14 H10 C16 H10 CS2 C2 H6 SO

2.3. Spectroscopic methods 2.3.1. Spectroscopic analyses Spectroscopic methods were used to detect and identify volatile organic molecules concentrated by aerogels. Depending on the molecule, these techniques can be used individually or together for detection and identification. Fluorescence spectroscopy is especially useful in identifying PAHs since they fluoresce quite strongly, and have characteristic emission peaks. Aliphatic organics tend to have broad fluorescence peaks and many fluoresce in the same general region of the emission spectrum. FTIR is useful in identifying organic functional groups. While this does not necessarily lead to the identification of a specific molecule, it can detect the presence of groups of different molecules with common functional groups, e.g., nitriles that are individually in trace amounts that may be too small to be detected. Raman spectroscopy is used to detect individual species of molecules and can be used to identify both PAHs and aliphatic organics.

2.3.2. Infrared spectroscopy In Fourier Transform Infrared (FTIR) spectroscopic analysis, the absorption of specific wavelengths of infrared light to excite organic molecules is used to identify functional groups within molecules, and thus the molecules themselves. After conducting FTIR analyses in transmittance mode with a variety of aerogels, it was determined that 90 mg/cc aerogel discs that were approximately 2 mm thick were the optimal density and thickness for our analyses. A disc aerogel was placed in a sample chamber that contained approximately 1 mg of an organic compound. The chamber was heated (∼60 °C) to ensure that the organics were converted from a solid or liquid to the vapor phase. The FTIR data were collected on a Nicolet 6700 spectrophotometer operating in the transmission mode using Omnic software version 7.4. Spectra were collected with a scan number of 16 and a resolution of 4 cm−1 over the range 60 0 0 cm−1 to 375 cm−1 using air as background. All spectra were processed using default Omnic processing parameters. All infrared spectra were obtained un-

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der ambient conditions. For all infrared spectra shown here, the abscissa unit is cm−1 and the ordinate unit is relative intensity. 2.2.3. Raman spectroscopy The scattering of light can result in inelastic scattering, which is the basis of Raman spectroscopy. Raman spectra were collected with the focal point of the microscope set 50 μm below the aerogel surface. Since the objective also collects the Raman scattered photons the emitted light did not have to pass through the aerogel. Therefore, the dimensions of the aerogels used in the fluorescent spectroscopic analyses did not matter. 4 × 3 × 2 cm blocks of approximately 90 mg/cc silica aerogel were used for these analyses. One of the problems that can be encountered with Raman spectroscopy is the emission by contaminants, or in this case, fluorescence of the aerogel itself. However, the aerogels used imposed only weak backgrounds that did not interfere with the identification of the sample peaks. A Horiba LabRam HR Confocal Raman Spectrophotometer was used in this study. The excitation wavelength used was 532 nm with ∼15 mW irradiation power at the sample. A block of aerogel was placed in a sample chamber that contained approximately 1 mg of organic compound. The sample chamber was heated (∼60°C) for several hours to ensure that the organics were converted from a solid or liquid to the vapor phase. All Raman spectra were obtained under ambient conditions. For all Raman spectra shown here, the abscissa unit is cm−1 and the ordinate unit is relative intensity. 2.3.4. Induced fluorescence spectroscopy Since the detector in the fluorimeter used in this study was next to the excitation laser, the emitted light did not have to pass through the aerogel, the dimensions of the aerogels used in the fluorescent spectroscopic analyses did not matter. 4 × 3 × 2 cm blocks of approximately 90 mg/cc silica aerogel were used for these analyses. A UV fluorescent miniPL/Raman spectrophotometer made by Photon Systems was used in this study. The excitation wavelength used was 254 nm, and produced a small peak to the left in each spectrum. For the initial compound detection studies, a block of aerogel was placed in a sample chamber that contained approximately 1 mg of organic compound. The sample chamber was heated (∼60°C) for several hours to ensure that the organics were converted from a solid or liquid to the vapor phase. All fluorescence spectra were obtained under ambient conditions.

given volatile compound in the vapor phase versus the amount adsorbed by an aerogel. To determine the amount present in the vapor phase the Clausius–Clapeyron equation and the ideal gas law were used. The concentration factor is then given by the amount (by mass) in the aerogel divided by the amount (by mass) in the vapor phase surrounding the aerogel. Concentration factors were determined for several molecules to demonstrate the ability of aerogel to collect and concentrate molecules in an ambient atmosphere. The concentration factors were determined by calculating the amount of a given volatile compound in the vapor phase versus the amount adsorbed by an aerogel in that same environment containing a source of the volatile compound. To determine the amount present in the vapor phase, the Clausius–Clapeyron equation and the ideal gas law were used. From the Clausius–Clapeyron equation, the following equation was derived,

ln(P1 /P2 ) = (H/R )(1/T2 − 1/T1 ) where P1 is the pressure at temperature T1, P2 is the pressure at temperature T2, H is the heat of enthalpy and R is the gas constant (8.3145 J/mol K). This equation is solved for P2 , since P1 , T1 , T2 and H are known. For ethanol, P1 is 1 atm, T1 is 351.37 K, T2 is 296 K, and H is 3.86 × 104 J, so P2 is 8.5 × 10−2 atm. The Ideal Gas Law (PV = nRT) is then used to calculate the number of moles present in the atmosphere. This results in there being 3.9 mg of ethanol in a volume (V) of 24 cc. The concentration factor is then given by the amount (by mass) in the aerogel divided by the amount (by mass) in the vapor phase surrounding the aerogel. Concentration factors were determined for several molecules and were found to be as follows: ethanol 920, methanol 1330, acetonitrile 1140, naphthalene 1800. Since these experiments were not repeated many times, these values are taken to be only approximate. 3.2. Spectroscopic analysis The fluorescent spectra are relatively simple and the peaks associated with each molecule are easily identified. The significant absorption bands in the FTIR spectra have been marked with an arrow to indicate the bands associated with the sample molecule. The bands indicated with arrows have been identified in the scientific literature as being among those associated with the sample molecule, and are listed in the legend for that spectrum. The significant Raman peaks have been labeled with wavenumber values. These are peaks that have been identified in the scientific literature as being those associated with the sample molecule.

3. Results

3.3. Induced fluorescence spectroscopy

3.1. Aerogel optimization

Experiments were done to demonstrate that aerogels do concentrate molecules from ambient atmospheres where spectroscopic analysis of the molecules in the vapor phase is not possible. Fluorescence spectra were obtained of a sample chamber with no aerogel and no naphthalene, with no aerogel and 2 mg of naphthalene, and with aerogel and 2 mg of naphthalene. Fluorescence spectroscopy is quite sensitive to PAHs since they have strong and characteristic fluorescent emissions. Aliphatics such as alcohols and ketones also fluoresce, however they produce a broad emission band that varies slightly in peak wavelength for each molecule. While these spectra can be of limited use in detecting and identifying molecules, other spectrophotometric methods are considerably better at detecting these molecules. Figs. 1a and 1b show the induced fluorescence spectra of benzene, naphthalene, fluorene, anthracene, phenanthrene, and pyrene. These are the simplest PAHs with the carbon atoms arranged in from one to four conjoined aromatic ring systems. These spectra were obtained at concentrations of several hundred ppm, since

Considering that the aerogel network contributes a background signal to the optical measurements, the general tendency for optimizing these measurements is to reduce the aerogel density. However, adsorption studies demonstrated that higher density aerogels adsorbed greater amounts of the organic molecules of interest. An aerogel density of roughly 85 mg/cc proved to be a reliable material for handling, for organic volatiles collection and added acceptable spectroscopic backgrounds. Our initial studies of the adsorption of different aliphatic and aromatic molecules by functionalized aerogels demonstrated that silica aerogel without any added functionalities was the most adsorptive material for a broad range of adsorbates. Ethanol adsorption was used in the initial experiments to determine the concentration factor of the aerogel over the amount of a given molecule present in the ambient environment. The concentration factors were determined by calculating the amount of a

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14000 12000 10000 8000 Anthracene 8 ppm

6000 4000 2000 0 2400

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Fig. 3. Fluorescence of anthracene present in the chamber at a concentration 8 ppm. Fig. 1a. Fluorescence spectra of Naphthalene and Pyrene. 200000 150000 100000

sample NP + PHE + ANT

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Fig. 4. Fluorescence spectrum of naphthalene, phenanthrene and anthracene concentrated in aerogel. Naphthalene fluorescence emission peaks are at 320 0, 330 0, ˚ Anthracene fluorescence peaks are at 3750, 40 0 0, 420 0 A. ˚ Phenanthrene 3350 A. ˚ fluorescence peaks are at 3450, 3630, 3850 A. 100

Fig 1b. Fluorescence spectra of Benzene, Phenantrene, Anthracene, and Fluorene. 80 18000 16000

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Fig 2. Fluorescence of naphthalene at different concentrations.

they were done to demonstrate that fluorescence spectra can be obtained of PAHs concentrated in aerogels and that they are distinct enough to use to identify each molecule. In each case, the spectra match the spectra published in the literature from previous studies done by other researchers. To test the detection limit of this method to the presence of naphthalene, serial dilutions of naphthalene in ethanol were prepared to gradually reduce the concentration level of naphthalene in the samples. Fig. 2 shows the fluorescence spectra of naphthalene concentrated in aerogels from these diluted samples. The emission peaks for naphthalene can be observed to a level of 0.0032 mg, which is equivalent to a level of 2.5 ppm in the chamber. The spectrum labeled blank is that of an unexposed sample of aerogel. A similar dilution experiment was done with anthracene as the aromatic molecule. A series of aerogel samples were exposed to different amounts of anthracene and then observed by induced fluorescent spectroscopy. The spectrum obtained is shown in Fig. 3. In this case, anthracene was detected at the 8 ppm level. A study of the detection limits of fluorene and anthracene were also done and determined to be 7.5 ppm and 2.5 ppm, respectively.

Fig. 5. FTIR spectrum of acetone concentrated in aerogel.

Samples with two or three PAHs mixed together were prepared to demonstrate that the spectra obtained from them could be used to detect and identify them. Naphthalene is identified by the emis˚ Phenanthrene sion peaks at approximately 320 0, 330 0 and 3350 A. is identified by the emission peaks at approximately 340 0, 350 0, ˚ Anthracene can be identified by the emission peaks and 3840 A. ˚ Fig 4 is the spectrum of a at approximately 3770 and 3990 A. sample containing three PAHs; naphthalene, phenanthrene and anthracene. By identifying the peaks associated with each molecule, each PAH can be identified even when several species are present. 3.4. Fourier transform infrared spectroscopy Figs. 5 and 6 show spectra that were obtained for acetone and acetonitrile adsorbed by aerogel. FTIR spectra for acetic acid, acetaldehyde, ethyl acetate, methyl ethyl ketone, acrylonitrile, benzene and formic acid were also recorded. For each spectrum, FTIR peaks observed in previously published spectra were used to identify the compounds (Socrates, 2001). In Fig. 5, the band at 1749 cm−1 is associated with the CO group and the bands at 2926 and 2966 cm−1 are associated with the CH groups. In Fig. 6, the CN

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Fig. 9. Fluorescence spectra of samples of simulated Martian regolith with added different amounts of naphthalene (NP).

Fig. 6. FTIR spectrum of acetonitrile concentrated in aerogel.

2966

512 3009 1434

111 blank 115 aldehyde

484 1354

cm-1 364.… 531.… 697.… 863.… 1030… 1196… 1362… 1529… 1695… 1861… 2028… 2194… 2360… 2527… 2693… 2860 3026… 3192… 3359… 3525… 3691… 3858…

1800 1600 1400 1200 1000 800 600 400 200 0

Fig. 7. Raman spectrum of acetaldehyde concentrated in aerogel. 700 1156

600

(Socrates, 2001). These peaks are labeled on the figures. In Fig. 7 the peaks assignments are as follows; 3009 cm−1 υ (CH3 ), 2964 cm−1 υ (CH3 ), 1434 cm−1 def(CH3 ), 512 cm−1 σ (C–C=O) and 484 cm−1 σ (C–C=O). In Fig. 8 the peak assignments are as follows; 3160 cm−1 υ (NH2 ), 3130 cm−1 υ (CH), 1425 cm−1 σ (CH2 or CH3 ), 1156 cm−1 σ (N1 –H) and 954 cm−1 σ (R). A spectrum of carbon disulfide revealed no identifiable peaks. Despite the fact that Raman bands can be much weaker than FTIR bands, it proved useful in detecting and identifying volatile organic molecules concentrated by aerogel. Silica is a good medium for this purpose since silica has very weak Raman bands above 500 cm−1 , which is the region where the volatile organic molecules Raman peaks are located.

1425 1320

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3.6. Spectra of organics concentrated from simulated regolith samples

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cm-1 357.73584 516.90564 676.0755 835.2453 994.4151 1153.585 1312.7548 1471.9246 1631.0944 1790.2642 1949.434 2108.6038 2267.7737 2426.9434 2586.1133 2745.283 2904.4529 3063.6226 3222.7925 3381.9622 3541.1321 3700.302 3859.4717

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Fig. 8. Raman spectrum of imidazole concentrated in aerogel.

group is identified by the stretching and bending bands located at 2629 cm−1 and 2944 cm−1 . Spectra from carbon disulfide, ethanol, methanol, imidazole, sulfamic acid and formalin did not exhibit any peaks that could definitively be identified as those associated with the sample compound. The absorption bands from the water present in the aerogel and of the silica that constitutes the aerogel network interfered with the observation of sample absorption bands in the wavenumbers between 30 0 0 and 350 0 cm−1 and below 110 0 cm−1 . Since there is typically useful infrared spectroscopic information in these regions, this proved to be problematic. However, FTIR is especially useful in identifying functional groups in molecules. By using aerogels as a concentrator, molecules with common functional groups could be identified, whereas without the aerogel these molecules might not be detected. Molecules containing nitrile, keto, aldehyde, and carboxylic acid groups are the most easily detected. 3.5. Raman spectroscopy Figs. 7 and 8 show the Raman spectra obtained for acetaldehyde and imidazole. Raman spectra for methyl ethyl ketone, naphthalene and phenanthrene adsorbed by aerogels were also obtained. For each spectrum, Raman peaks observed in previously published spectra were used to identify the compounds

JSC Mars 1-A Simulant from Orbital Technologies Corp. was used to simulate an extraterrestrial regolith in these studies. It is composed of a palgonite tephra from a Hawaiian volcanic cinder cone. PAHs from Table 1 were added to 5 g aliquots of simulated Mars regolith. The regolith with the added PAHs was then added to a sample chamber. A small glass dish holding disc of aerogel was also placed in the chamber, keeping the regolith and the aerogel separate. The chamber was heated (∼60°C) for approximately one hour. The amount of organic compound added ranged from 2 to 0.02 mg. Fig. 9 shows the fluorescent spectra of naphthalene that had sublimed from 5 g of simulated regolith containing naphthalene and was collected and concentrated by aerogel. The blank aerogel was placed in a chamber with regolith that did not contain naphthalene and was heated at ∼60°C. The amount of naphthalene introduced to the regolith varied from 1 mg to 0.04 mg. These amounts are equivalent to 200 ppm to 8 ppm of naphthalene in the 5 g of simulated regolith. However, it must be noted that no testing was done to determine the amount of naphthalene that may not have sublimated and was still present in the simulated regolith. 5 mg of acetaldehyde was added to 5 g of simulated Mars regolith. The regolith was added to a chamber containing a disc of aerogel and heated (∼60°C) for approximately one hour. The aerogel was then analyzed using FTIR spectroscopy. The material was dispersed in potassium bromide (KBr) powder (7 mg aerogel in 260 mg KBr) and analyzed using Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy in a Bruker Vertex 70v spectrophotometer. The decrease in the background observed with this method is due to the fact that there is less aerogel present in the KBr sample. The Bruker spectrophotometer, rather than the Nicolet, was used due to instrument availability. Fig 10 shows the spectrum obtained. The stretching bands associated with the hydrocarbons (3001 and 2846 cm−1 ) and the carbonyls (2733 cm−1 ) are seen in the acetaldehyde control spectrum (red) and the carbonyl concentrated from the regolith by the aerogel (green). Also,

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Fig. 10. FTIR spectrum of acetaldehyde from simulated Mars regolith concentrated in aerogel that has been concentrated in aerogel. Blue – aerogel. Red – aerogel exposed to acetaldehyde. Green – aerogel with acetaldehyde concentrated from simulated Mars regolith. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

seen are the hydrogen deformation bands around 1400 cm−1 of the acetaldehyde. This demonstrates that volatile organics can be detected by this method when they are present in a regolith at the ppt level after having been driven out of the regolith by modest heating.

4. Conclusions Adsorption experiments were done that demonstrated that silica aerogels without any added groups collect and concentrate volatile organic molecules. Concentration factors of up to three orders of magnitude were observed. The adsorption of volatile organic molecules is dependent on the density of the aerogel, and a density of roughly 90 mg/cc was determined to be optimal for the work being done in this study. Silica aerogels that were approximately three centimeters thick were used for induced fluorescence and Raman spectroscopy, whereas FTIR spectroscopy used aerogels that were only 2 mm thick. FTIR required thinner aerogels because the signal had to travel through the aerogel. These thinner aerogels could also be used for fluorescence and Raman spectroscopy since these techniques only probe a thin layer at the surface of the aerogel, and therefore the thickness of the aerogel is not an issue. Induced fluorescence spectroscopy was used to detect polycyclic aromatic hydrocarbons concentrated by aerogels. The fluorescent emissions from these molecules provided an unambiguous signal for the identification of the species present in the aerogels. Induced fluorescence spectroscopy was able to detect the presence of multiple PAHs present in an aerogel and detected PAHs down to ppm levels. The presence of single PAHs and multiple PAHs were also detected in aerogels after the PAHs had been desorbed from a simulated Mars regolith sample and subsequently adsorbed by aerogels. Fourier transform infrared spectroscopy was used to identify functional groups present in volatile organic molecules and in some cases the specific molecule itself. For some molecules, such as acetonitrile, the presence of the CN (nitrile group) was readily apparent. For some molecules, the identification of certain functional groups was not possible due to the presence of absorption bands from the aerogel silica network and the water associated with the silica network. Functional groups in adsorbed molecules were detected down to ppt levels of concentrations.

Raman spectroscopy was used to identify volatile organic molecules adsorbed by the aerogels. Since silica produces only weak Raman absorption bands it does not interfere with the detection of adsorbed organic molecules. Various aliphatic organic molecules were identified by Raman spectroscopy after adsorption by aerogels. Molecular moieties were detected down to ppt levels of concentrations. By collecting and concentrating organics that had been released from simulated regolith, it has been demonstrated that the concept presented here is feasible. This experiment mimicked the transfer of volatiles from an extraterrestrial material, e.g., regolith, to the aerogel, where the volatiles were then detected. This concept compares favorably with the sensitivities of the other instruments, such as SHERLOC, which can detect organics by fluorescence in the ppm range and by Raman in the ppt range. However, this instrument concept has the additional advantage of employing three types of spectroscopy to better detect and identify molecular species. Fluorescent, Raman and infrared spectrophotometers for space flight missions have already been developed. The further development of this concept would require primarily the refinement of the sample and aerogel handling procedures. With continued work, it is believed that this technique could be very affective in detecting and identifying trace volatile organic molecules that might otherwise go undetected. Acknowledgment This work was performed by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. References Allamandole, L.J., Sandford, S.A., Wopenka, B., 1987. Inetrstellar polycyclic aromatic hydrocarbons and carbon in inetrplanetary dust particles and metorites. Science 237, 56–59. Baba, N., Kimoto, Y., 2009. Contamination groeth observed on the micro-particles capturer and sapce environment exposure device. J. Spacecraft Rockets 46 (1), 33–38. Beegle, L.W., Bhartia, R., DeFlores, L., Darrach, M., Kidd, R., Abbey, W., Asher, S., Burton, A., Clegg, S., Conrad, P.G., Edgett, K., Ehlmann, B., Langehorst, F., Fries, M., Hug, W., Nealson, K., Popp, J., Soborn, P., Steele, A., Wiens, R., Willford, K., 2014. SHERLOC: scanning habitable environments with raman and luminescence for organics and chemicals, an investigation for 2020. 11th International GeoRaman Conference Abstract #5101.

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