25th International Meeting on Organic Geochemistry IMOG 2011

P-029
Mass spectrometric analysis of intact polar lipids: pros and cons
of Q-ToF, Ion Trap and Single Quadrupole detection
Lars Wörmer, Julius Lipp, Jan Schröder, Kai-Uwe Hinrichs
Org. Geochemistry Group, MARUM Center for Marine Environmental Sciences & Dept. of Geosciences,
University of Bremen, Bremen, Germany (corresponding author:lwoermer@marum.de)
Intact polar lipids (IPLs) have gained importance as
biomarkers, as demonstrated for example by the
inferred high quantity of archaeal biomass in
subseafloor sediments [1]. Thanks to soft ionization
techniques such as electrospray ionization, mass
spectrometry is a suitable method for the detection of
these lipids and is being successfully employed
widely. But to keep advancing the understanding of
the significance of these biomarkers, analytical
improvement is due. Goals to be fulfilled deal, among
others, with chemical characterization of novel lipids,
the accurate quantification of lipid concentration and
the reduction of detection limits in complex matrices.
In the current work we present the optimization of IPL
detection on a Bruker maXis Q-ToF and compare the
obtained results with analyses performed on a
Thermo Finnigan LCQ Deca XP Plus Ion Trap and an
Agilent Technologies 6130 Quadrupole.
Concerning proper identification of unknown lipids, Q-
ToF mass spectrometry shows the advantage of high
mass accuracy. Mass-to-charge (m/z) ratios showed
an error below 5 ppm, which is orders of magnitude
more accurate than the tested Ion Trap and Single
Quadrupole equipment. This is an important
advantage, as chemical composition may often be
directly inferred from the observed m/z ratio if proper
mass calibration is performed. Additionally, in the
case of archaeal GDGT lipids, fragmentation patterns
obtained from Ion Trap and Q-ToF differ clearly. The
latter one not only showed the loss of polar
headgroups, but also allowed the detection of single
biphytane chains, an observation which may be useful
for structural elucidation.
Concerning the quantification of lipids, calibration
curves were performed for eight compounds in a
range from 0.5 to 25 ng injected lipids (e.g. Figure 1).
Overall good linearity was observed for Q-ToF and
Single Quadrupole, but Ion Trap showed evident
signs of curve saturation at the higher concentrations.
Considering all eight compounds, flattening of the
response always began at the same intensity,
independently from actual analyte concentration.
Therefore, and because saturation is mainly observed
at the Ion Trap, we tentatively explain this
phenomenon with issues related to space-charge
effects. Dimer formation, which was observed in all
three systems, also appears to play an important role.
The strong linearity observed is important as it
confirms the robustness of relative abundances to be
measured in environmental samples.
The matrix effect due to the complexity of non-defined
compounds becomes evident when pure IPLs are
added to sediment extracts. Responses and signal-tonoise
ratios in these samples may be more than 10fold
decreased in comparison with pure compounds.
This decrease is system- and compound specific, and
best results were obtained with the Single
Quadrupole, where use of SIM mode provides great
additional sensitivity. Independent of this loss of
sensitivity, linearity appears to be largely maintained.
Still, matrix effects have to be dealt with in order to
guarantee optimal IPL detection. Therefore, further
methodological development is needed concerning
extract clean-up prior to MS detection. Our
presentation will highlight the advantages and
disadvantages of individual MS detection techniques
and provide recommendations for maximizing
performance when analyzing IPLs in highly complex
matrices.
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Figure 1: Calibration data for C21-PC (black circle), PEarchaeol
(white circle) and C16-PME (gray triangle)
measured at (from left to right) Ion Trap, Q-ToF and Single
Quadrupole. Data for each system are normalized to largest
peak
References
[1.]Lipp et al. 2009. Nature. 454: 991-994
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