In situ instrument to detect prebiotic compounds in planetary ices

Understanding the distribution of organic molecules in solar system materials is a high priority goal of NASA's Astrobiology Program. To this end, our team is now developing a compact instrument prototype—the Organics Analyzer for Sampling Icy Surfaces (OASIS)—that will be compatible with a flight mission to the surface of an icy planetary body. The technique of liquid chromatography-mass spectrometry (LC-MS) has given us an important new understanding in Earth-based investigations of the inventory of prebiotic organic compounds in extraterrestrial materials. Here, we demonstrate that LC-MS is compatible with instrument miniaturization (see Figure 1) via the use of microfluidic components and a compact time-of-flight mass spectrometer (TOF-MS).^1–3

Figure 1. A breadboard prototype of the Organics Analyzer for Sampling Icy Surfaces liquid chromatography-mass spectrometry (top) that includes fluid-handling components, a temperature-controlled liquid chromatograph, and the time-of-flight mass spectrometer. The mission targets (below) include planetary bodies with an abundance of water or water ice that may signal a reservoir of prebiotic organic molecules.

LC-MS is a liquid-phase technique that is well suited to analyzing small, non-volatile organic molecules. It works by entraining a sample in a solvent (the mobile phase) and passing the mixture through an analytical column that contains a high-surface-area resin (the stationary phase), which separates the solutes (analytes) according to their structure and chemistry. The separation is determined by the physicochemical interactions between the three components: the stationary phase, mobile phase, and targeted molecular species. In reversed-phase, high-performance liquid chromatography (HPLC), which uses high pressure to assist in separating compounds, the mobile phase is a polar solvent (such as water). As different compounds will interact with the phases with varying strengths, a spatial and temporal separation of a series of compounds is achieved as the analyte solution passes through the column. The time it takes for a given compound to traverse the column is known as the retention time. Once isolated, the individual compounds can be analyzed using TOF-MS to acquire a mass spectrum. In this way, both dimensions of the measurement—chromatographic and mass spectral—can increase confidence in identifying sample compounds.

Figure 2. (a) The microscale high-performance liquid chromatography (HPLC) component is fabricated using silicon and glass micromachining techniques at the 4" wafer scale. (b) On-chip HPLC columns were fabricated with varying column lengths ranging from 40mm to 100mm to demonstrate stationary phase column-filling techniques and proof-of-concept chiral separation of amino acid mixtures.

Figure 3. The reflectron time-of-flight mass spectrometer operates at low power and enables high mass range detection of prebiotic organic compounds. The sample gases are ionized and accelerated in the ion source and transmitted to the single-bounce reflectron, which acts like an ion mirror. The reflectron effectively doubles the ion path length for improved temporal (and, therefore, mass) resolution between the ion packets arriving at the detector.

To obtain the mass spectrum, the elution products are passed through the MS-TOF instrument, which is an ion spray technique that releases the separated analytes into the gas phase. Ion spray techniques are also considered soft ionization methods because fragmentation effects that can obscure intrinsic molecular structures are minimized. The masses of the gas-phase ions are determined by the time they take to reach the detector (thus, time-of-flight). The time-of-flight technique offers the dual advantage of high performance with a large mass range that is compatible with complex compounds and low power requirements compatible with the resource constraints of future mission payloads.

We constructed a liquid chromatograph of inert materials using micromachining and microfabrication techniques (see Figure 2). We have taken measures to limit retention-time peak broadening through the effects of turbulence by forming a smooth, circular channel (75μm in diameter) in a pyrex-silicon chip stack. The analytical column is formed by filling this channel with a slurry of stationary phase resin, for example, beads with tailored surface chemistry to target separations of amino acids (including determining enantiomeric ratios). We introduced the analyte solution into the microscale channel using a commercially available nanoferrule connector. This chip configuration has been shown to sustain back pressures up to 5000psi, which is compatible with HPLC operational conditions.

The ion spray nozzle is compatible with an edge outlet configuration, as was previously reported.⁴ Pulsed orthogonal extraction is used to extract the ions into the TOF-MS analyzer (see Figure 3), which has a rectangular symmetry with a wide acceptance aperture for transmission of ions into the flight region. The ion source consists of a series of elongated acceleration, focusing, and steering electrodes. We constructed the flight region from resistive glass that was patterned with conductive traces to define the desired electric field profile in the dual-stage reflectron. A rectangular microchannel plate detector was used to detect the arrival of ion packets by recording the time of flight for each mass packet as a function of time using an analog-to-digital time-of-flight board.

Our OASIS efforts aim to develop an in situ capability for liquid chromatographic and mass spectrometric analyses of planetary surfaces that are considered to be high priority to NASA's Astrobiology Program. LC-MS is a highly capable and sensitive analytical tool in terrestrial laboratories, and the implementation of the technique as a part of a future NASA planetary mission would advance our understanding of the distribution, abundances, and enantiomeric ratios of key prebiotic compounds on icy surfaces. The use of microfabrication methods and a compact mass spectrometer prototype will address future mission science needs while minimizing the mass and power requirements of the instrument system.

In future work, we will assemble a breadboard instrument that will address key measurements and performance characterization towards a future in situ instrument for icy planetary environments.

We acknowledge support from the NASA Astrobiology Science and Technology for Instrument Development Program.