Exobiology in the Solar System & The Search for Life on Mars - ESA

SP-1231
150
determ<strong>in</strong>ed <strong>in</strong> <strong>in</strong>dividual phases <strong>in</strong> situ by add<strong>in</strong>g an LA-ICP ion source to <strong>the</strong> MS.
This method is very fast and, if a separate MS can be dedicated to that task, it will
give by far <strong>the</strong> most data <strong>for</strong> analys<strong>in</strong>g <strong>the</strong> phase equilibria and <strong>for</strong>mation conditions
of:
• primary phase assemblages;
• secondary phases;
• atmosphere-related phases (coat<strong>in</strong>gs, efflorescences, etc);
• organic matter.
This <strong>in</strong>strument should also be capable of per<strong>for</strong>m<strong>in</strong>g isotopic abundance measurements
and thus could be very important <strong>in</strong> <strong>the</strong> search <strong>for</strong> life or life remnants.
II.5.9.6 O<strong>the</strong>r Techniques
O<strong>the</strong>r techniques can be envisaged <strong>for</strong> analys<strong>in</strong>g compounds of very low volatility or
non-volatile compounds (<strong>in</strong>organics, PAHs, am<strong>in</strong>o acids, pur<strong>in</strong>es, macromolecular
organics). <strong>The</strong>y <strong>in</strong>clude derivatisation system-GC-MS, High-Per<strong>for</strong>mance Liquid
Chromatography (HPLC) and Supercritical Fluid Chromatography (SFC).
Derivatisation processes are often used <strong>in</strong> <strong>the</strong> laboratory to analyse non-volatile
organics by GC or GC-MS. This is <strong>the</strong> case with am<strong>in</strong>o acids. A two-step chemical
derivatisation (esterification of <strong>the</strong> acidic group followed by trifluoroacetylation of
<strong>the</strong> am<strong>in</strong>o group) trans<strong>for</strong>ms <strong>the</strong> non-volatile am<strong>in</strong>o acids to volatile N-TFA esters,
which can be quantitatively analysed by GC or GC-MS. However, this has never been
used <strong>in</strong> space and an automatic derivatisation system compatible with space
constra<strong>in</strong>ts has yet to be developed.
HPLC is also a powerful technique <strong>for</strong> analys<strong>in</strong>g complex mixtures of high
molecular weight, low-volatility compounds. It uses liquid solvent (<strong>in</strong>stead of <strong>the</strong>
GC’s carrier gas), a requirement that might be very difficult to qualify <strong>for</strong> space
activity. However, if developed <strong>for</strong> space application, it would be an <strong>in</strong>strument of
tremendous importance <strong>for</strong> exobiological studies. Thus, <strong>the</strong>re is a clear need <strong>for</strong>
research and development of space HPLC and HPLC-MS systems.
A related method to HPLC is SFC. In this case, a very unreactive gas is compressed
to a temperature at which it becomes supercritical. Under such circumstances,
some species such as CO 2, can become an extremely powerful solvent, with
properties that can be varied extensively by <strong>the</strong> addition of small amounts of
modify<strong>in</strong>g agents. Compounds <strong>in</strong> solution <strong>in</strong> supercritical fluids can be separated
chromatographically. An <strong>in</strong>terest<strong>in</strong>g possibility <strong>for</strong> Mars might be utilis<strong>in</strong>g <strong>the</strong>
atmosphere itself as a supercritical reagent <strong>for</strong> carry<strong>in</strong>g out organic analysis. Land<strong>in</strong>g
spacecraft would <strong>the</strong>n not have to transport hazardous consumables <strong>for</strong> chromatographic
separations, only <strong>the</strong> hardware and an appropriate pump. SFC has also not
been used <strong>in</strong> space, but it might be somewhat easier to develop than HPLC.
European laboratories are study<strong>in</strong>g <strong>the</strong> feasibility of <strong>the</strong>se different concepts.
II.5.9.7 <strong>The</strong> Analysis of H 2O 2
<strong>The</strong> quantitative analysis of H 2O 2 will be difficult by GC, MS or GC-MS techniques
and, ow<strong>in</strong>g to its scientific importance, it will require a specific <strong>in</strong>strumentation.
Hydrogen peroxide is usually analysed <strong>in</strong> environmental sciences by complex
techniques based on chemilum<strong>in</strong>escence or UV fluorescence. <strong>The</strong>se techniques
<strong>in</strong>volve, after air sampl<strong>in</strong>g, chemical derivatisation, us<strong>in</strong>g liquid transfers, chemical
reactors, <strong>in</strong>jection valves and enzyme storage. Fur<strong>the</strong>rmore, <strong>the</strong>y require very
accurate calibration. Thus, although <strong>the</strong>ir sensitivity is very high (a few ppt – parts per
trillion, 10 12 !), <strong>the</strong>y do not seem adequate <strong>for</strong> space application.
Ano<strong>the</strong>r type of <strong>in</strong>strumentation can also be used to analyse H 2O 2 quantitatively,
although it is less sensitive (parts per million range). It <strong>in</strong>volves electrochemical
measurements with:
i. rotat<strong>in</strong>g electrode (amperometric titration),
ii. specific H 2O 2 electrode (oxidation or reduction on a plat<strong>in</strong>um disc electrode),