nai

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.

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.

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.