3.1. Compound-specific 34S analysis using GC–ICPMS
The sulfur isotopic composition of individual GC-amenable
organic compounds from all fractions was measured using multicollector
inductively-coupled plasma mass spectrometry (MCICPMS;
Thermo Neptune+
) an Agilent 6890 GC instrument for
gas-phase sample introduction and separation following the
approach described by Amrani et al. (2009). This GC instrument
was operated with the same column type and operating conditions
as the Varian instrument but differed in that it had a split/splitless
injector. Accuracy was established by repeated analysis of external
standard mixtures containing three to six OSCs with known d34S
values. Mean d34S values for the combined data for each standard
were within 1.4‰ of published EA-IRMS values (Amrani et al.,
2009) and had a root-mean-squared variance of 1.5‰. Due to
improvements in chromatography and baseline standardization,
the 2012 data are all accurate to within 1.0‰ and have a rootmean-squared
variance of 0.9‰. Many of the polar fractions (F3,
see below) of the extracts were analyzed in triplicate to assess
any additional uncertainty due to the complexity of the chromatograms.
Triplicate analyses for most of the major peaks in F3
achieved a d34S standard deviation of 1.3–1.8‰, although several
peaks with less well resolved baselines returned a wider range of
values. F2 chromatograms were much less crowded and d34S values
for duplicate F2 runs varied by a maximum of 0.8‰. This performance
is comparable with that reported by