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Project Progress

Advancing Techniques for in situ Analysis of Complex Organics

Project Investigators: Dr. William B. Brinckerhoff

Note: Work prior to September 2007 was performed at the Applied Physics Laboratory/ Johns Hopkins University. Dr. Brinckerhoff re-located permanently to Goddard Space Flight Center in September 2007, and thereafter his research was performed at GSFC.

1. Objectives

Theme 4 work at JHU/APL and NASA/GSFC using laser time-of-flight mass spectrometry (TOF-MS) techniques continued in collaboration with the Goddard Center for Astrobiology (GCA) team and external partners. The specific objectives of the laboratory-based effort are to:

• Examine organics in standards, cometary analogs, and meteorites with LDMS;
• Develop an LDMS sensitivity-selectivity database for high mass, refractory organics in various matrices and optimize the method;
• Compare LD-based “prompt ionization” with LD + laser post-ionization (REMPI) and electron post-ionization (EPI) for use as organic chemical mapping technique; and

• Determine optimal combined LDMS and pyrolysis GCMS (and other) analyses of common samples which advise mission design.

2. Summary of Progress

2.1. Support of Technique/Sensor Development

With partial NAI support during this reporting period, laser TOF-MS systems were modified, tested, and optimized to increase sensitivity to organic compounds in complex samples. We also began designs of instrumental approaches to be attempted later this year and next. Activities included:

Upgrades to the Dual-Source TOF-MS (DS-TOF). The modification of the DS-TOF instrument to detect both positive and negative ions, initiated in a previous year, was completed after several rounds of performance testing. The final modification included addition of the ability to shift the voltages applied to the ions as they begin their flight through the sensor. Our initial, shifted set has the probe at ground for positive ions, compatible with a separate laser TOF-MS instrument used in this effort, and at -6 kV for negative ions. The ions are then accelerated to a common flight tube potential of -3 kV. The potential allows the detector to remain at a fixed voltage which is more stable than a floated design. Comparative spectra were obtained in both configurations and analysis of the results is ongoing. Our preliminary results confirm that higher voltages are desirable generally but are not critical for detection of lower mass ions up to about 1 kDa, because these ions do not require post-acceleration at the microchannel plate (MCP) detector.

Adaptation of 337 nm CB-TOF for NAI samples. A second instrument developed at APL (led by Timothy Cornish) was adapted for NAI use during this reporting period. Figure 1 shows the “chemical biological” (CB) TOF-MS which uses a reflectron geometry similar to that of the DS-TOF. For NAI samples, we fabricated a sample tray/holder that fits into the source vacuum lock of the CB-TOF, seen below the reflectron chamber on the right hand side of the figure. Samples loaded onto CB-TOF substrates must be localized to small and very flat spots owing to the tight optical and ion extraction constraints of the design. To accommodate samples of varying dimension and roughness, we developed a set of target holder plates that allow neat powder, intact chip, or droplet samples to be mounted. To assure that the sample surface was flush with the top of the plate, divots were machined in the mounting substrate. This modification also allowed the sample to be centered are the correct point within the instrument’s load-lock for laser desorption.

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Upgraded optics design. The spatial distribution of organics and elemental carbon in meteorites and on an in situ mission is of great interest. While ultimately the finest scale analyses (nm-μm) demand sample return, maps of high-mass species even at modest resolutions (μm-mm) would prove highly valuable. LDI with its 10-50 μm spot size may be used in an “organic imaging” mode. A similar technique, micro two-step laser desorption/ laser ionization MS (μL²MS), has been effectively applied in this mode (e.g., Kovalenko et al., 1992). By comparison, direct LDI is simpler to implement experimentally but is less selective and more prone to molecular fragmentation.

We have been working with a particular implementation of organic imaging based on laser focusing to a fixed point, while moving the sample along x, y, and z coordinates in vacuo. This approach provides the most reproducible LDI conditions from point to point, and assures approximately normal incidence on average, yielding high sensitivity. The analysis point on the sample is always directly beneath the ion extraction inlet, resulting in consistent peak shapes. However, mechanical complexity and the tight tolerances involved in combining laser and ion optical apertures suggest an alternative decoupled approach when looking toward miniature instrumentation. Moving the laser and microscope objectives off the TOF ion axis reduces working distance from 15-20 cm to ~2-3 cm, yielding a low-f-number geometry. The laser is thus not required to clear the internal diameter of the ion extraction lens, and the sample may be rastered with a laser XY-scanning mirror, outside the vacuum chamber. With this optical system modification, we are further developing protocols for organic imaging of surfaces.

Upgraded laser Electron Impact Quadrupole Mass Spectrometer (EI-QMS) design. The post-ionization of neutral molecules laser-sampled from analogs is a continuing area of investigation for this NAI effort as well as a complementary Exobiology project, for both analytical (desorption) and hypervelocity impact simulation (plasma ablation) objectives. In contrast to laser-induced ions, the neutral product of laser desorption includes species with higher ionization potentials such as lower-mass atomic and diatomic volatiles, which evolve over microsecond, rather than nanosecond, timescales. We have pursued various methods to demonstrate the potential of electron post-ionization for analysis of complex planetary materials, in which matrix effects and the presence of many isobaric compounds make the obvious advantages of the approach (such as high selectivity) more difficult to realize. A new instrument platform with promise for extending the analytical power of laser desorption of meteorites and analogs has been defined as part of the laboratory set up at GSFC, motivated by objectives of the NAI work but funded separately. This new platform uses a high-performance EI quadrupole mass and energy spectrometer, which will be used to analyze and calibrate the laser-induced neutral gas as well as prompt ions from our range of solid samples as well as a variety of other sources.

2.2. Analog Sample-Based Calibration

We have continued to analyze analog samples in this effort, using commercial (Bruker) and miniature instruments. Some of these are extended studies of organic and mineral composition of the sample set provided as part of the Astrobiology Sample Analysis Program (ASAP), and others are from the “core NAI” series that we have been developing over the past three years.

LDI-MS data for biomolecular analysis are most commonly collected in positive ion mode, owing primarily to the development of matrix assisted LDI (or MALDI), in which protonation of an analyte M gives the positive ion MH⁺. Negative ions are also used in MALDI; in certain cases these form less ambiguous parent and fragment peaks. In direct (non-matrix) LDI, negative ions form by electron attachment to desorbed neutrals, during which the supply of electrons is reduced. At lower laser intensities (ε?, negative ionization is electron-supply limited; the ionization probability is correlated with electron affinity. While isomers tend to fragment into common positive ions, negative fragment ions can be sensitive to the particular structure of the parent. This difference can be exploited to distinguish properties such as cyclic vs. chain structure by characterizing such biases over a range of ε, a possibility we are investigating.

We have observed that meteorites and analog spectra of both polarities are typically intense and distinct. An example where negative ions have proven quite valuable in correlating LDI with published data is the CM2 carbonaceous chondrite Murchison. Figure 2 compares TOF-MS data in both polarities for bulk Murchison powder. The cation spectrum contains a high background from saturating Na^+^ and K^+^ peaks. While complex organics can be seen in positive mode, the peak pattern is biased toward organics that readily ionize via Na^+^ or K^+^ attachment. The negative ion spectrum lacks the “salt” signals and shows strong evidence of the known hydrocarbon phase of this meteorite, with parent and alkylated PAH series separated by CH ₂ units.

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Negative ion spectra for ASAP samples from Rio Tinto and Svalbard Gypsum were collected in a separate series of analyses (following standard positive mode and MS/MS runs). All samples were ground to fine powder. The samples both exhibited distinct negative ion spectra with series of low molecular weight peaks associated with inorganic mineral fragments as well as organic compounds (Figures 3 and 4). These series are most readily distinguished by the neutral loss patterns, e.g. 14 Da for organics (CH₂) and 16 Da for oxide minerals (O). The negative spectra are not obscured by Na and K peaks found in positive ion spectra of geological samples.

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