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Project Progress

1. Background

Work on laser time-of-flight mass spectrometry (TOF-MS) techniques continues at JHU/APL in collaboration with the Goddard Center for Astrobiology (GCA) team and external partners. We seek to contribute to research on the following major questions within the GCA:

1. What are the fine scale morphology and composition of comet nuclei?
2. Do comets comprise an enabling inventory of complex pre-biotic organics?
3. How should we approach the characterization of this inventory?

Laser desorption (LD) methods are complementary to non-laser GCA work ongoing at GSFC; they sample distinct yet related sets of compounds from complex samples. Our work is also a link between analyses performed with similar facility instrumentation at other labs in that we can measure quantitatively the scientific consequences of miniaturization or other resource limitations. The specific objectives of the laboratory-based effort at APL are to:

1. Examine organics in standards, cometary analogs, and meteorites with LDMS;
2. Develop an LDMS sensitivity-selectivity database for high mass, refractory organics in various matrices and optimize the method;
3. Compare LD-based “prompt ionization” with LD + laser post-ionization (REMPI) for use as organic chemical mapping technique; and
4. Determine optimal combined LDMS and pyrolysis GCMS (and other) analyses of common samples which advise mission design.

2. Progress Summary

During 2006-2007 the areas of NAI effort included: (1) support of technique/sensor development; (2) analog sample-based calibration; and (3) studies of Titan tholin analogs.

2.1. Support of Technique/Sensor Development

NAI work at APL supports basic and applied scientific research that complements ongoing instrument development, research, and mission conceptualization efforts supported under various other programs. Mass spectrometers already in place or under development for multiple purposes have had primary support from PIDDP, ASTID, and MIDP grants. The NAI work seeks to investigate and optimize these techniques for application to analysis of organics in planetary samples (in meteorites, on missions, or in returned samples) related primarily to primitive bodies or Mars. The basic designs of the instruments used are fixed, however a great deal of analytical flexibility remains in the final configuration of certain platforms, such as with the choice of laser desorption/ionization methods, voltage modes, sample configurations, and sample manipulation approaches. Only modest hardware modifications are required by any particular choice, however the choices themselves must be made with great care so we can understand how the science results are affected by them.
Laser TOF-MS systems were modified with partial NAI support to maximize the sensitivity to low-concentration organic compounds in complex samples. Techniques included:

1. Lowering base vacuum pressure to reduce chemical background noise. This was done with a cleaner and less-frequent sample change-out plan, continuous dry nitrogen gas/moderate heating applied to chamber walls during/following sample probe modification periods to minimize H2O adsorption, and replacement of a diaphragm pump with a more powerful scroll pump to provide lower backing pressure for the turbopump.
2. Use of lower laser repetition rates during some sample analyses. In the analysis of organic compounds in “complex” samples such as Mars analogs and meteorites, in which the majority of desorbed species are inorganic neutrals (elements, oxide fragments, and clusters), it was previously assumed that averaging a large number of laser pulses was needed to obtain sufficient signal due to high background noise. This was typically done using a relatively high laser repetition rate, such as 20 Hz. However, it was observed that the signal often attenuated quickly during such analyses, which could not be simply attributed to the removal of a putative thin surface layer of organics. That is, the attenuation profile was not highly dependent upon the sample organic concentration or the total ion signal. A highly likely explanation for this behavior was inferred once the attenuation behavior reduced dramatically when the laser rep rate was lowered to 1-2 Hz. At higher rep rates, the neutral molecular cloud above the sample was not removed efficiently by vacuum pumping and constituted a dense dispersing medium for prompt ions after several seconds of laser pulsing. Consequently, a new series of sample averages (running at 1 Hz while exhibiting higher signal-to-noise and improved reproducibility) are being conducted on a set of analogs. In addition, a modification to the ion source components of the TOF-MS is being designed to improve the gas conductance.
3. Use of shorter shot-averages to maintain high resolution and/or sensitivity. Several factors play into the mass resolution of any mass spectrometer. In TOF-MS the width of a mass peak is a variation in the arrival times of ions at the detector due to variable start times and positions, dispersion of the flight paths of the ion population, and general scattering. In addition, during laser analysis of a heterogeneous sample, shape and mean TOF of a given peak can drift due to changing sampling conditions and ion flux. When averaged, the signal from low-level compounds can be spread in time so much that it is not evident above the noise baseline within the nominal integration period. By limiting raw averages to approximately 20-50 shots, sharp peaks of low amplitude can be preserved. Several such averages are then summed manually after small shifts in the TOF-to-m/z scale are corrected to line up peak centroids.
4. Selectable positive/negative ion mode laser TOF-MS. The dual source time-of-flight (DS-TOF) instrument, along with a similar instrument available at APL for comparative analyses, have recently been modified to permit a novel approach for detecting both positive and negative ions within a single instrument. Negative ions sample a complementary and often quite illuminating population of laser-desorbed molecules from complex solids. Ionization of such species is driven mainly by electronegativity rather than (cat)ionization potential, and as such depends somewhat more sensitively on the particulars of molecular bonding. As such negative ionization can be highly sensitive and selective for certain species such as C, O, N, OH, S, Cl, and a wide range of O- and C-bearing (including organic) compounds. Very encouraging preliminary results on some rock and meteorite samples using negative ionization in an APL instrument similar to DS-TOF led us to begin an upgrade plan. Typically, separate (opposing) paths and detectors are employed to permit such bi-polar capability. However, this approach would not result in the most compact instrument design. Using a single flight tube in a switchable positive/negative ion detector has been done previously by decoupling the MCP detector assembly from the read-out anode by inserting a photoconverter to pick up exiting electrons and convert them into a proportional current signal. However, this design is less desirable for very compact, coaxial, high-speed instruments such as LDMS. In the new design, the MCP assembly itself floats with the flight tube, which is switchable in polarity, with the relative voltage drop to the grounded anode is equal for positive and negative modes. This has been accomplished in the upgraded DS-TOF (Figure 1) by isolating the flight tube (now at some bias) from the chamber with a metal sleeve, and further removing the detector entry grid, since the base of the reflectron is now at the MCP entry voltage. There is no post-acceleration in this design, however, it should not be needed for the “lower” mass range (< 2 kDa) under focus here. The sample probe and detector are simply biased to any positive or negative voltage.

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3.1. Analog Sample-Based Calibration

As part of our plan to compare spectra from the miniature LDMS instruments with those obtained on the same samples by facility instruments, we have begun systematically analyzing some of our chemical standards, analogs, and meteorite samples on a new Bruker Autoflex LDMS available at APL (Figure 2). This instrument operates much like the miniature Tower TOF and DS-TOF instruments, although it has a much higher voltage capability and longer flight tube, leading to higher mass resolution. The laser is a 337 nm N2 system, common on commercial MALDI-TOF instruments. This system also has negative ion detection and some tandem capability, permitting deliberate fragment analysis of chosen m/z values.

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Samples are mounted on a custom Al slide that fits into the Bruker backing plate, which in turn sits on an in-vacuum motion stage, analogous to Tower TOF. Analysis points are selected with a black and white video microscope with a few-mm FOV.
Bruker spectral data are shown in Figures 3 and 4 for two Mars-analog samples: Mauna Kea jarositic tephra and Columbia River basalt, respectively. In both cases, the first few low-energy laser pulses on a fresh surface desorbed a large quantity of organics, with characteristic CH₂ repeat units and molecular weights as high as 1 kDa. It could not be determined whether these compounds were present in the field or were introduced subsequently during handling. The data shown here are averages recorded after this initial transient had disappeared. The “bulk” material contained very low organic concentrations. The salt-containing tephra tended to yield higher-mass positive ions through potassium-addition, as K has a low ionization potential. These peaks are likely associated with potassium sulfite clusters with varying O atom number. By contrast, this route is not available for negative ions, and as such these data include some oxide clusters as well as small organic compounds. Negative ionization may be the best approach for such samples.
The basalt sample proved a good absorber of near-UV laser light and did not require high pulse energy or long averages to obtain both elemental and small-organic composition within one scan. The inset of Figure 4 shows the complete major and minor cation composition between m/z 20 and m/z 70. At higher masses a few oxide peaks are evident, along with several small organics such as PAHs and alkyl-PAHs.
Other analogs, such as the albite 1481 BANAB1, included mainly individual mm-scale mineral grains. These typically exhibited a sharp laser intensity desorption threshold. In the Bruker

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LDMS, specific points on each grain were targeted for analysis. While most of each grain’s surface did not yield organics beyond the first few shots, some albite grains had inclusions or other features that appeared to incorporate extensive organics at depth. As mineral grains might be capable of preserving water-borne organics in such features over long timescales (e.g., on Mars), the need for micro-analysis of intact sample surfaces in situ warrants further attention and investigation.

A number of analyses were conducted to understand further the sensitivities of various laser TOF-MS approaches to detection of high mass organics in neat samples, and to compare those with parallel studies at GSFC with GCMS, LCMS, and other techniques. In support of Objective 2 (establishing an LDMS organics database), we continued to: (i) analyze systematically a sequence of both procedural blanks and standard reference materials; (ii) record new spectra from geostandard and meteorite samples under identical conditions as the blanks and references; and (iii) evaluate the accuracy of high sensitivity elements such as Li that may be tracers of aqueous alteration on parent bodies of carbonaceous meteorites and of past liquid transport on Mars.

3.2. Studies of Titan Tholin Analogs

The term tholin was assigned by Carl Sagan to laboratory analogs of complex heteropolymers in Titan’s aerosols. Various analytical techniques have been used to elucidate the structure and composition of tholin samples. While Titan tholins have been examined extensively with gas chromatography mass spectrometry based on various preparative methods followed by bulk sample heating, laser desorption has not been typically applied to their analysis. However as tholins are thought to be at least partially made of refractory polymer material, LDMS is potentially well-suited and complementary to existing protocols for tholins. Pulsed laser desorption can form gas phase ions directly from solid sample surfaces, limiting analytical contamination. We used both positive and negative mode LD-TOF-MS to detect a polymeric phase in Titan tholins with molecular weights to several kDa. The results reported below are summarized from a presentation by Ganesan et al. at the 2007 LPSC [1]. (In summer 2004, Anita Ganesan was an Intern in the GCA, working with Theme 3 investigators J. Nuth, N. Johnson, and J. Dworkin at GSFC. She subsequently spent 1.5 years working at APL, and in Sept. 2007 she entered a graduate program at MIT.)

The tholins used were prepared at the University of Paris, LISA courtesy of GCA collaborators Patrice Coll, Francois Raulin, and Cyril Szopa. A mixture of N2:CH4 (98:2) at 4 mbar was subjected to electrical discharge, at room temperature and at Titan atmospheric temperature (~100-150 K). For convenience, these samples are referred to as “hot” and “cold” samples. Samples were synthesized without exposure to air or moisture and stored in Eppendorf vials; no oxygen is expected in their composition.

Samples were analyzed by LDMS, using a Bruker Autoflex TOF/TOF. Desorption and ionization occurred by means of a 337 nm N2 laser with laser energy of up to 100 _J and spot diameter of ~100 _m. Samples were affixed to double-sided tape and mounted on aluminum sample holders. Double-sided tape added no significant background signal. Further analyses were performed by tandem mass spectrometry, to allow for the deliberate fragmentation of selected mass/charge (m/z) values. A similar analysis was performed on the miniature LDMS instrument developed at APL. This instrument operated in a mode analogous to the Autoflex, however the laser used was a 355 nm Nd:YAG and the mass analyzer used a lower extraction voltage to compensate for a much shorter flight tube.

The Autoflex spectra reveal a polymeric structure to the tholins and this is evident in both positive and negative ions (Figure 5). Peaks occur in repetitive cluster units. There is no significant difference between the hot and cold sample spectra, in respective polarities. The peak clusters of hot and cold sample spectra occur at essentially the same m/z values over a wide range, with a slight difference only in amplitude. The overall oligomer size distribution is consistent for both samples. The individual and cluster peak profiles in positive and negative ion spectra differ: negative ion peaks have a sharp onset followed by a high-time tail, in contrast to positive ion peaks which exhibit a more symmetric shape. This difference may result from negative ions having a broader distribution of formation times, potentially due to the additional [M-H]- route available for prompt negative ionization in laser desorption. From a coarse viewpoint the positive ion clusters appear to be spaced at 13 Da intervals. However, the intervals are also consistent with alternating 14 Da and 12 Da units. These data are highly suggestive of a conjugated polymeric structure, such as polyacetylene. The formation of stable polyacetylene-based oligomers may occur via an alternating CH₂ and -C addition scheme. It is also possible that there are multiple polymeric structures that result in what appears to be a single structure spaced by 13 Da. Some studies of other tholin samples indicate the presence of a regular 14 Da cluster spacing corresponding to the addition of a methylene unit. Negative ion clusters exhibit a bimodal spacing, alternating between 13 Da and 12 Da. This pattern is also consistent with negative ions forming as either M or [M-H]-. As such, the positive and negative spectra are very likely two views of a common parent molecular structure.

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The results of tandem mass spectrometry (or MS/MS) performed on the positive parent ion at m/z 149, reveal losses of small molecules including NH3, HCN, and CH3CN (Figure 6). This indicates the presence of nitrogen-containing functional groups, such as nitriles and amines, likely as oligomer terminating units.

A polymeric structure similar to that obtained with the Autoflex is evident in the positive ion spectra of the miniature instrument (Figure 7). Clusters can be clearly discerned, however, the distribution peaks at a lower mass than in the analogous Autoflex spectrum. Such a difference could be explained by differing mass dependence of ion transmission in the two instruments.

In accordance with the Autoflex data, including the results from MS/MS, it is possible that these tholin samples contain polyacetylene with terminal nitrile and amine groups. It has been hypothesized that polyacetylene is a component of the Titan haze and these results are consistent

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with that claim. Polyacetylene would preferentially form in the upper atmosphere of Titan (>500 km). It is not apparent that tholins synthesized at different temperatures exhibit a difference in LDMS-detected composition. This may be consistent with previous work by Hodyss et al. However, other studies indicate that initial gas concentration and pressure do play a significant role. LDMS is very sensitive to nonvolatile polymeric compounds and future analyses may assist with the resolution of these issues. LDMS instrumentation shows promise as a key enabling element of a future in situ mission to Titan.

Meeting Abstract:

1. A. L. Ganesan, W. B. Brinckerhoff, P. Coll, M-J. Nguyen, F. Raulin, T. J. Cornish, S. A. Ecelberger (2007), “Analysis of Titan tholins by laser desorption mass spectrometry.”, Lunar and Planetary Science 38, Abstract 1948 (CD-ROM).