2009 Annual Science Report
University of Wisconsin Reporting | JUL 2008 – AUG 2009
Qualitative Analysis of Soils Samples Using Solid Phase Microextraction (SPME) and Gas Chromatography/mass Spectrometry (GC/MS)
The investigation of the physical and chemical properties of Mars soil analogues collected in arid deserts provide limits to exobiological models, evidence on the effects of subsurface mineral matrices, support current and planned space missions, and address planetary protection issues. We have collected samples in the Atacama desert and applied Solid Phase Micro-Extraction (SPME) to optimize the extraction of Polycyclic Aromatic Hydrocarbons (PAHs). PAHs are among the most abundant molecules found in various space environments in the solar system and beyond. SPME is a solvent-free extraction method invented and applied in a variety of sampling-detection scenarios. The aim of this study is to use SPME for fast screening and determination of PAHs in soil samples. This method minimizes sample handling and preserves chemical integrity of the sample. When compared to traditional extraction methods SPME may provide better analyte recoveries, less opportunity for rearrangement and decomposition of analytes, and faster analysis. This study and further optimization of this extraction technique provides important data for the calibration and performance of future Mars instrumentation that specializes on the detection of organic molecules.
A headspace SPME mode where the fiber is exposed to the headspace above the sample matrix is preferred for complex aqueous and solid matrices and was therefore chosen for this study. Analytes are absorbed into a thin polymer phase, which is deposited onto a silica fiber housed in a syringe needle. Sampling is accomplished by penetrating a septum of a reaction vessel with the syringe needle, depressing the plunger and exposing the fiber to the target mixture. The fiber is allowed to adsorb the analytes for a specified time (typically 5 to 60 minutes), after which the needle containing the fiber is withdrawn and then inserted into a hot GC injector, desorbing the analytes in splitless mode for GC/MS analysis. As such, SPME circumvents the use of solvent extraction. Headspace analysis is a suitable method to determine volatile/semivolatile substances directly without chemical pretreatment of samples. The analytes are allowed to partition into the coating for a predetermined time, and then the fiber is retracted back into the needle.
A preliminary analysis of PAHs abundances from Martian analog soils are shown in Figure 1. The amounts of PAHs in the samples were estimated relatively to the amounts calculated for the standard soil sample BCR-524 using the area count counts of each compound relatively to the respective area counts in the BCR-524 standard. Using our sampling parameters we were able to detect all 17 target compounds in the BCR-524 standard soil. PAHs amounts vary from 0.1 to 0.6 µg/g soil in desert samples (P, HOBO) while they range from 0.1 to 1 µg/g soil with phenantherene at 2.5 µg/g level in the Salten Skov sediment. Naphthalene and fluorine seem to be the most abundant PAHs in desert samples except for one (P-5) in which fluorine was not detected. Four ring PAHs, eg., fluoranthene and pyrene at low levels (10-40 ng/g) were detected in 5 desert soil samples (P-1, 10, 13, 14, and HOBO). Currently we proceed with the calibration and optimization of sampling parameters for headspace-SPME sampling.