MSL - SAM and CHEMIN, Discussion of the science/results from these instruments Options

[marsophile]

The cited paper allows for the possibility that the organic material that contributed to the chlorinated hydrocarbons was contamination from Earth (as did an MSL press briefing).

If one assumes that the supply of any organic contamination from Earth is finite and limited, then repeating the measurement a number of times should provide some insight into this issue: If the quantity of chlorinated hydrocarbon decreases steadily, that would tend to indicate contamination. A residual quantity might indicate an indigenous source.

[Don1]

Yes there is news from SAM and it ain't good. The leak from the wet chemistry cells is far more serious than they let on and as a result the data on organics from Rocknest is useless. They are seeing 100 times as much chloromethanes as Viking did, but the trouble is that all or most of it is coming from the reaction between the wet chem reagent and the perchlorates in the soil. Any Martian signal is completely drowned out. To quote the scientists: "Thus, at this time, while MTBSTFA contamination can explain all of the chlorohydrocarbons observed, we cannot exclude the possibility that traces of martian organics contributed to the chloromethanes measured by SAM."

The MTBSTFA leak looks like a serious problem which is going to reduce SAM's capability to detect Martian organics.

Data from here.(pdf)

[stone]

The cited paper allows for the possibility that the organic material that contributed to the chlorinated hydrocarbons was contamination from Earth (as did an MSL press briefing).

If one assumes that the supply of any organic contamination from Earth is finite and limited, then repeating the measurement a number of times should provide some insight into this issue: If the quantity of chlorinated hydrocarbon decreases steadily, that would tend to indicate contamination. A residual quantity might indicate an indigenous source.

The derivatization reagent MTBSTFA leaks slowly and will not go away in the sealed enviroment of SAM. SAM is a box with nearly no openings. The article of Arno Buch is describing the reactions of the MTBSTFA and they are not aproblem in its own, as long as the perchlorate is not everywhere. The perchlorates will make it very difficult to determin where the organic carbon is coming from. The chlorinated methanes can come from nearly every source.

[marsophile]

Couldn't they simply use up the supply of the derivatization reagent MTBSTFA by using it for the purpose for which it was intended?

[stone]

The MTBSTFA is used to search for amino acids and other not tha volatile compounds. The detection limit is a lot increased if you convert them to the sililylated compounds. This will be done when a measurement with the normal ovens shows at least some organics. SAM does not have a lot of those derivatisations ovens and in the end they could lose a significant science return (mostly related to a field of research forbidden to mention here) if they do not have the MTBSTFA anymore.

One further point is that the compound is not very volatile and if it is everywhere in the SAM box already using the rest might not improve the situation a lot.

ADMIN NOTE: The astrobiology rule is well understood on the Forum and therefore there is no need to mention that it should not be mentioned.

[marsophile]

Then it seems there is a catch-22 situation: They can't detect native organics because of the MTBSTFA contamination, and they won't use the MTBSTFA until they detect native organics.

If the contamination is worsening with time, then maybe it should be jettisoned as soon as possible since it seems it won't be used for its intended purpose anyway?

[stone]

Can we discuss SAM's stereochemistry capabilities?

SAM's stereochemistry capabilities lie within the GC. The mass spectrometer is blind for chirality, because it can only measure weight. The enantiomeres have to be separated within the GC. To do that the stationary phase in the columns has to be a chiral substance too. Only chiral compounds interact with each other in a way that there is a difference between the enantiomeres. One of the columns in sam is a GC4-Chirasil-β Dex CB . The column material is a β-cyclodextrin a cyclic sugar The β-cyclodextrin rings contain several glucose molecules all of them contain several chiral carbon atoms.

Most of the compounds here on earth necessary for biochemistry like sugars, amino acids and nucleo base are a real horror in GC, they are instable at high temperatures and contain a lot of O-H and N-H bonds making them capable to build up strong hydrogen bonds to each other. This hydrogen bonds lead to high boiling points and therefore high temperatures to get them into the gas phase to get them through the gas chromatograph. To overcome this derivatization reagents are used to substitute the hydrogen at the OH and NH groups by sillyl or methyl groups. MTBSTFA and DMFDMA and TMAH are possible compounds for this. In SAM this derivatization is done in the wet chemistry cells. All the resulting derivatized material is then put onto the chiral column and into the GC.

In the end you see in the GC plot two peaks very close to each other with exactly the same patern in the MS. With the peak area you can determine if one of the enantiomeres is more abundant than the other. One problem with this is the fact that racemesation, the chemical reaction of converting one pure enantiomere into a 50/50 mixture is fast for a lot of the compounds you are looking for. Even normal garden soil shows sometimes not the 100% R or S what you would expect from the text book knowledge.

[Mr Valiant]

I had no idea that the GC was so, so...definative. I read the Wiki article 'Isotopic labeling', a good primer.
Hope the leak issues can be resolved, an amazing instrument.

[marsophile]

Drilling is probably the best bet for getting a perchlorate-free sample into the ovens.

Best of all might be drilling into a gypsum vein. A gypsum deposit precipitated from water would be unlikely to contain perchlorate (since perchlorate is much more soluble than calcium sulfate) and might have trapped some material with organic chemicals.

In future missions, the simplest method of removing perchlorate from soil samples might be to wash them in sterilized pure water before heating them.

[stone]

If the perchlorates are ancient the vein should be free of perchlorates.

My little horror story would be:

The formation of the perchlorates by radical process involving water, inorganic chlorides and UV radiation would make them evenly distributed in the Martian dust since the beginning of Mars. Magnesium and calcium perchlorate are hygroscopic and a little bit of moisture creates a brine. So the first perchlorates rained down 3 -4 Gyears ago and from time to time the axis tilts and the polar caps move and every point on Mars gets a little moisture over 3-4 Gyears and the perchlorates are transformed into a brine which soaks into the small cracks of all surface rocks up to a depth were ground water is possible. With that the perchlorates every where except at points which are enclosed from any water intrusion.

I hope for a perchlorate free sample but the 5cm subsurface could be a few meters to few to get a perchlorate free sample.

[rickyjames]

Could somebody please elaborate on the statement made several weeks ago that the SAM unit has a serious leak in its stored reagents? Where is a link or reference to what has been stated publically about this issue by the mission team?

[stone]

"Although none of them have been punctured yet for the actual wet chemistry experiment, it is suspected that one or more may be releasing some of its reactant. "http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1249.pdf FROM BACKGROUND TO SIGNAL: CHALLENGES OF A SOLID SAMPLE ANALYSIS USING SAM GC-MS

[marsophile]

If the SAM and CHEMIN instruments performed measurements on a random mudstone on Earth, what would be the probability that they would detect organics?

[stone]

The heating like done by SAM is not done often on rock samples here on earth. So there is limited data most of it would have been in the SAM team. The mud stones especially if it is a layered structure have been shown to preserve organics but also bind the organics very well in the layered structures. If you chose the random mudstone well then SAM might not find anything. In most mudstones SAM would detect organics. For Amino acids and nucleo base the whole thing is very complicated. Only the SAM team knows to what extent the derivatisation will improve the situation. The silicates with large surface area also tend to be catalytic for numerous chemical reactions.

If CHEMIN will detect organics something very strange has happened. Crystals of organic material must be the main compound of the modstone. Carbon is a light element and does not give good x-ray diffraction, heavy elements work much better.Wikipedia gives a few examples of organic crystalline minerals. Carpathite is one of them. If this is the main compound of the sample SAM would be in deep trouble by the organic overload.

[jmknapp]

Check this report of SAM tests on Earth:

Mars Science Laboratory Shakedown in the High Arctic

...where SAM detected organics in a couple different deposits, not mudstone exactly though if I read it right, unless dolomite is considered a mudstone.