MSL - SAM and CHEMIN, Discussion of the science/results from these instruments |
MSL - SAM and CHEMIN, Discussion of the science/results from these instruments |
Nov 21 2012, 11:46 AM
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Senior Member Group: Admin Posts: 3108 Joined: 21-December 05 From: Canberra, Australia Member No.: 615 |
ADMIN NOTE: Hi All, a new topic for the discussion of the science from the SAM and Chemin instruments.
There has been a very important amendment to Rule 1.3 which is explained here. Please remember Rule 1.3 at all times when discussing matters in this section. |
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Nov 23 2012, 03:08 PM
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#2
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Junior Member Group: Members Posts: 42 Joined: 19-October 12 Member No.: 6719 |
Can we discuss SAM's stereochemistry capabilities? I believe that SAM can sort out chirality, but I have seen very little written about this. As we all know, stereochemistry would allow us to help us determine, ah I hesitate to say this, the genesis of some organics. As far as I can tell, SAM can determine if a peak is chiral, but I am not sure if it can tell if there is a preponderance of L- versus D- for a particular organic chemical. |
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Feb 4 2013, 07:53 AM
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#3
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Junior Member Group: Members Posts: 93 Joined: 21-January 13 Member No.: 6845 |
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. |
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