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Hydrogen and carbon stable isotope standards for organic
compound-specific investigations
Compound-specific hydrogen isotope ratios for organic hydrogen are now
analytically accessible by a combination of GC, high-temperature
pyrolysis to elemental hydrogen, and subsequent on-line irm-MS (Sessions
et al., 1999, Org. Geochem. 30: 1193-1200; Scrimgeour et al., 1999,
Rapid Comm. Mass Spec. 13: 271-274; Hilkert et al., 1999, Rapid Comm.
Mass Spec. 13: 1226-1230; Tobias and Brenna, 1997, Anal. Chem. 69:
3148-3152). Complete irmGCMS-systems with interfaces are commercially
available, but there has been no isotopically defined set of organic
compounds for routine calibration and D/H quality control. Standards
that reliably establish or confirm isotopic calibrations must be in the
same form as the unknown analytes. In contrast, the use of intermittent
spikes of introduced elemental “standard” hydrogen gas is fraught with
potential problems because it does not take into account D/H
fractionations that may occur in the analytical train between injection
in the GC and the exit of the pyrolysis reactor.
The Biogeochemical Laboratories at Indiana University, in collaboration
with Woods Hole Oceanographic Institution (A. Sessions, J.M. Hayes),
first established the purity of n-alkanes (range from C-12 to C-50) and
n-alkanoic acid methyl esters (C-10, C-20, C-30 FAMEs) by GC-MS,
followed by the measurement of D/H and 12C/13C ratios for each compound.
Up to five replicate analyses for each compound were performed off-line,
via conventional combustion of milligram-amounts of individual compounds
in quartz ampules and cryogenic purification of combustion gases in a
vacuum line. Water was converted to elemental hydrogen in contact with
uranium, followed by collection of hydrogen gas using a Toepler pump.
Gas yields and elemental H/C ratios were routinely monitored
manometrically for quality control. Hydrogen and carbon isotopic ratios
were determined using MAT252 mass-spectrometers at Indiana University.
The hydrogen isotopic calibration employed the conventional
normalization to VSMOW (zero per mil) and SLAP (-428 per mil).
We currently have 3 types of standards available for the research
community:
1. Mixture “A” of fifteen n-alkanes (C-16 to C-30) containing ca. 100
nmol H2 per compound per microliter of solution. This is equivalent to
approximately 1200 ng alkane per microliter of solution. This solution
is ideal for establishing the precision and accuracy of an irmGCMS
instrument.
2. Mixture “B” of fifteen n-alkanes (C-16 to C-30) containing a 5-fold
range of concentrations (arranged in three pentads with rising
concentrations; see Table below), from 20 nmol H2 to 100 nmol H2 per
compound per microliter of solution. This mixture is designed
specifically to test the accuracy of H3+ correction in
hydrogen-isotope-IRMS (see Sessions et al., 1999). In addition, the
solution can be used to measure the H3+ factor under conditions closely
matching those experienced by analytes. This mixture continues to be
invaluable in the laboratory at WHOI and is used every day.
3. Individual n-alkanes (C-12 to C-50) and individual fatty acid methyl
esters (C-10, C-20, C-30). While peaks of “standard” H2 gas can be used
to calibrate the delta values of unknown compounds, a more robust
approach is to use co-injected compounds as the isotopic standards. We
therefore offer individual solutions of any or all of these compounds,
each containing ca.100 nmol H2 per microliter of solution. Custom
mixtures are available upon request.
As a public university, we will supply this reference standard material
as a service to the scientific community rather than as a commercial
product. Trial amounts sufficient for multiple injections are available
free of charge (international shipping charges may apply). To offset
our significant investment in analytical effort, we will ask for
compensation for each 0.5 mL aliquot of standard mixtures sealed under
argon in brown glass ampules, which will reflect a cost of a few cents
per injection after appropriate dilution of our product to match the
sensitivity of modern irmGCMS systems. Additional information, isotope
ratios, and a chromatogram with traces of masses 2 and 3 for mixture “B”
are available in the 'service' section of the following home page:
http://www.indiana.edu/~geosci/people/faculty/schimmelmann.html.
Our standards will be accompanied with documentation covering purity,
isotopic ratios, and concentrations. Although intended primarily for
D/H research, our standards are equally useful for irmGCMS work on
carbon isotope ratios. Our isotope standards are research materials
offered for use without guarantees and without acceptance of any
responsibilities for damages arising from its use or possible failure in
any application. It is not sold for profit and is distributed as a
service to those engaged in geochemical research.
--
Arndt Schimmelmann, Ph.D.
Biogeochemical Laboratories
Department of Geological Sciences
Geology 129, Indiana University
1005 East 10th Street
Bloomington, IN 47405-1405
Ph (812) 855-7645
home (812) 339-3708
FAX (812) 855-7961
e-mail [log in to unmask]
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