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2013 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  SEP 2012 – AUG 2013

Advancing Techniques for in Situ Analysis of Complex Organics: Laser Mass Spectrometry of Planetary Materials

Project Summary

This line of work within the Goddard Center for Astrobiology (GCA) seeks to connect key science objectives related to understanding organics in our solar system to specific techniques and protocols that may enable us to achieve those objectives with in situ investigations. In particular, laser mass spectrometry (MS) techniques are being developed for analysis of complex, nonvolatile organic molecules, such as those that might be found at Mars, Titan, comets, and other planetary bodies, with limited chemical sample manipulation, preparation, and processing (as may be required by flight missions). The GCA laser MS effort is complementary to both (i) instrument development work supported by NASA programs such as ASTID, PIDDP, and MatISSE, to forward the design and testing of new prototype spaceflight hardware, and (ii) ongoing research and development within Theme 4 of the GCA, concerning analytical chemical sample analysis as well as across GCA (particularly with Theme 3) to define combined analysis techniques that may affect future mission design. There are additionally aspects of this effort that relate to understanding synthetic pathways for certain complex organics in planetary environments. Areas of activity with GCA support during this period included: * Comparative study of prompt and two-step laser desorption MS (LDMS) analyses * Development of protocols for induced molecular dissociation and tandem mass spectrometry (MS/MS) * Mars analog analyses using laser TOF-MS, ion trap MS, and SAM-like protocols

4 Institutions
3 Teams
3 Publications
0 Field Sites
Field Sites

Project Progress

In collaboration with ongoing ASTID (Brinckerhoff et al.) and PIDDP (Getty et al.) projects in our lab, the GCA-supported effort has focused on the demonstration of a two-step laser MS (L2MS) protocol and its comparison with one-step, or prompt, LDMS. In L2MS, analytes are desorbed as neutrals using a ~3 micron wavelength IR laser, followed within microseconds by an ionization pulse from a 266 nm UV laser. The L2MS approach has the advantage of less organic fragmentation (with lower desorption intensities) and high selectivity for aromatic species, such as polycyclic aromatic hydrocarbons (PAHs), with the chosen ionization wavelength. We have recently reported (Getty et al. 2012) the demonstration of this important capability, well-known from the astrobiology work of collaborator R. Zare and coworkers, on the Goddard miniature laser TOF-MS. Figure 1 shows an example comparing LDMS and L2MS example spectra from the reference compound dihydroxybenzoic acid (DHB) which represents an important structural class of organic for Mars or cometary exploration missions. Using a desorption wavelength of 2.9 microns, selected with an tunable optical parametric scillator (OPO) laser, the DHB parent ion is detected at high signal-to-noise, with minimal fragmentation as compared with the corresponding prompt UV LDMS spectrum.

Comparison of one-step (upper panel) and two-step (lower panel) miniature laser TOF-MS analyses of dihydroxybenzoic acid demonstrating the possibility of using of two-step techniques on an in situ mission, where aromatic species can be isolated from a complex matrix.

Induced molecular dissociation is the deliberate fragmentation of gas-phase molecules under conditions that allow focused structural analysis with tandem mass spectrometry (MS/MS). We have explored the application of both photo-induced dissociation (PID), which uses a pulsed laser to fragment the species of interest, and collision-induced dissociation (CID), in which a highly-localized collision gas is introduced into the instrument prior to mass analysis. While CID is extremely challenging to achieve effectively in a miniature mass spectrometer, given limited pumping capacity and potential for arcing, we have recently succeeded in demonstrating He-gas CID of a simple test compound – pyrene – on an existing 12 cm length reflectron TOF-MS with a valved, micron-scale orifice into a modified collision cell (Figure 2). With the collision cell active, the pattern of fragment ions formed matches that seen in the literature, which is encouraging given the potential for significant mass biases in post-cell analysis. The mass resolution is somewhat degraded in this mode due to the unoptimized ion optics of this preliminary test setup.

Comparison of prompt laser desorption/ionization time-of-flight (TOF) mass spectra of pyrene with collision-induced dissociation mode on (upper panel) and off (lower panel) indicating the introduction of a tiny quantity of localized He gas in the field-free flight tube of the instrument produces efficient fragmentation of the time-selected compound, enabling tandem mass spectrometry (MS/MS).

In collaboration with the Mars Science Laboratory’s Sample Analysis at Mars (SAM) investigation and with the Mars Organic Molecule Analyzer (MOMA) mass spectrometer project, under development at Goddard for the 2018 ExoMars rover, our team has initiated a series of Mars analog test campaigns, comparing laser TOF-MS, laser ion trap mass spectrometer (ITMS), used on MOMA (Brinckerhoff et al. 2013), and GCMS, used on SAM and MOMA. The MOMA ITMS uses Mars-ambient laser desorption, followed by ingestion of prompt ions through a capillary into the ion trap, using a fast aperture valve. In contrast, laser TOF-MS is a strictly high-vacuum technique. Each approach has its pros and cons, and its peculiar biases for organic and inorganic compound analyses with “unprepared” solid samples. GCA-supported analyses of Mars-analog sediment samples from Lake Hoare, Antarctica (Bishop et al., 2013) have enabled us to probe the variation of total and compound-specific organic limits of detection (LODs) in different mineral matrices, in particular where varying levels of oxidation produce different spectral interferences, including the production of cluster ions in LDMS of metal-bearing minerals and high-C/H ratio organics. These results are being folded into data reduction tools as well as instrument operational protocols (such as laser energy scanning) under development for the highly resource-limited MOMA experiment.