Evaluating an Alternate Method for Analysis of Meteoric Amino Acids
Pia
1University
1
Sen ,
Jason P.
2
Dworkin
Assessment and Optimization of GC-MS Method
▪ Carbonaceous chondrites are primitive stony meteorites that
contain a diverse suite of soluble organic compounds, among
them a variety of amino acids.
▪ More than 80 amino acids have been discovered
in these meteorites
Murchison
Meteorite
Jamie E.
2
Elsila ,
of Texas at Austin, Austin, TX 78712, USA. 2Catholic University of America, Washington, DC 20064, USA.
3NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
Introduction
β
José C.
2,3
Aponte ,
Existing methods for GC-MS analysis of amino acids have
drawbacks and require tradeoffs in reaction time, stability, ability
to separate enantiomers, and number of carbon atoms added.
Table 1. Comparison of derivatization methods for GC-MS analysis of amino acids.
Derivatization
Method
Chlorofomate
ester
α
α-Amino Acid
Reaction
time
10 s
Stability
(approx. time)
> 72 h
Chiral
separation
Y
Water
stable
Y
2h
5 min
1h
1h
<8h
> 24 h
<6h
<2h
Y
Y
N
N
N
N
N
N
α-Helix
▪ Many amino acids are chiral, meaning they can exist in two
non-superimposable mirror image enantiomer forms. Abiotic
processes create racemic mixtures (equal concentration of the
two enantiomers), while creation of an enantiomeric excess
requires the presence of a chiral force. Molecules are built
using one enantiomer. Life on Earth uses primarily the Lenantiomer of amino acids.
Abiotic processes
produce racemic
mixtures (L:D=1:1)
Enantiomers: mirror image
Evaluation of meteoritic amino acids is important because…
▪ L-Enantiomeric excesses of a few amino acids (e.g. isovaline)
have been found in some carbonaceous chondrites.
▪ It is possible that there is a link between the delivery of
enantiomerically enriched amino acids by meteorites and the
origins of homochirality on Earth. Understanding the
enantiomeric distribution of amino acids in extraterrestrial
samples can help evaluate this link.
Why use GC-MS for analysis?
▪ Amino acids are not volatile and
derivatization is needed for analysis
by
gas
chromatography-mass
spectrometry (GC-MS),
▪ After derivatization, amino acids in
meteorite extracts can be identified
by retention time and mass
spectrum, using standards as a
comparison,
▪ Can be coupled to isotope ratio mass spectrometers to
determine compound-specific isotopic composition which can
be used to determine origin of organic compounds.
Limitations of Method
TFAA/IPA
DMF/DMA
MTBSTFA
TMS
Effectiveness of the chloroformate method is
challenged by the formation of byproducts that
add complexity to the chromatogram, and by the
low reactivity of hindered compounds
TFAA/IPA: Trifluoroacetic anhydride/isopropanol. DMF/DMA: N,N-dimethylformamide
dimethylacetal. MTBSTFA: N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide. TMS:
Trimethylsilyl.
We tested and optimized the chloroformate ester method
using 35 amino acid standards, being able to separate 19
compounds (14 chiral, 5 achiral), at concentrations ranging
between 20-36 nM.
▪ Meteorite extracts can
be analyzed on either a
non-chiral (does not
separate enantiomers)
or chiral (separates
enantiomers) GC
column.
▪ Chiral analysis provides
more information, but
has more potential for
co-elutions.
Conclusions
▪ We have evaluated a GC-MS method capable of
resolving 19 meteoritic amino acids.
The
chloroformate ester derivatization has the following
advantages:
▪
Quick procedure (10 sec. reaction)
▪
Doesn’t need heat (room temperature)
▪
Stable products (>72 hours at room temp.)
▪
High compound resolution possible
▪
Efficient at low concentrations
▪ Future work:
▪
Test chloroformate ester derivatives to
determine suitability for liquid
chromatographic analyses
▪
δ13C isotopic analysis of chloroformate-esterderivatized meteoritic amino acids
Acknowledgements and References
This work was supported by the NASA Astrobiology Institute
and the Goddard Center for Astrobiology.
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