25th International Meeting on Organic Geochemistry IMOG 2011

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Enhanced biomarker detection in microbial mats using ToF-
SIMS
Tim Leefmann 1 , Christine Heim 1 , Alexander Kaever 2 , Peter Meinicke 2 , Jukka Lausmaa 3 ,
Peter Sjövall 3 , Volker Thiel 1
1 Geobiology Group, Geoscience Centre, Georg-August-University of Göttingen, Göttingen, Germany,
2 Department of Bioinformatics, Institute of Microbiology and Genetics, Georg-August-University Göttingen,
Göttingen, Germany, 3 Department of Chemistry and Materials Technology, SP Technical Research Institute
of Sweden, Borås, Sweden (corresponding author:Tim.Leefmann@geo.uni-goettingen.de)
During the last years, Time-of-Flight Secondary Ion
Mass Spectrometry (ToF-SIMS) has increasingly
attracted the interest of organic geochemists by
offering a new way of studying biomarkers at the
microscopic level, and without extraction, thus largely
maintaining the physical integrity of the sample. Major
drawbacks, however, of ToF-SIMS as a new organic
geochemical technique are (i) a lack of reference
spectra, as electron impact mass spectra are not
necessarily similar to SIMS spectra [1], and (ii)
insufficient knowledge on the influence of matrix
effects on the SIMS spectra.
In this study these problems are addressed by the
analyses of seven standard substances that represent
commonly targeted compound classes in biomarker
studies (chlorophyll a, β,β-carotene, ergosta-5,7,22trien-3β-ol,
5α-cholestane, 5α-cholesta-3-one, L-αphosphatidylcholine,1,2-di-O-hexadecyl-sn-glycero-3phosphoethanolamine).
These compounds were
analysed on silicon wafers, on agarose films, on an
embedding agent for cryosections, and on slices of
marble, thus simulating the molecular environments of
typical matrices present in biomarker analyses by
ToF-SIMS. An auto-peak-search routine was used to
generate peak lists from the mass spectra.
Compound specific peaks were extracted from these
peak lists by Wilcoxon rank-sum test and translated
into compound-specific, matrix-independent mass
spectra. Using these reference spectra it was possible
to identify the target biomarkers in extracts from
cyanobacterial mats and, in a second step, image
their lateral distribution in corresponding microbial mat
cryosections with ToF-SIMS. Summing of all peaks
assigned to a specific compound led to a strong
signal enhancement, e.g. the signal intensity of β,βcarotene
in both, extract and mat cryosections,
increased by ~2 orders of magnitude compared to the
molecular ion peak. Similar amplification factors were
found for chlorophyll a in the same samples.
These results underpin the careful interpretation and
statistical evaluation of reference spectra as essential
prerequisites for the successful use of ToF-SIMS in
the analysis of trace amounts of biomarkers in
complex environmental samples.
References:
[1] Spool, A.M., Interpretation of static secondary ion
spectra. Surface and Interface Analysis, 2004. 36(3):
p. 264-274.
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