d13C-values were measured by a GC-C-IRMS, consisting of the
autosampler unit AS 2000, the Trace GC 2000 by ThermoQuest, the
combustion unit Combustion Interface III and the isotope-ratio
mass spectrometer DeltaPlus (all units from Thermo Fisher, Bremen,
Germany). Volumes of 1.5 ml were injected into a liner (Type
TQ(CE) 3mmID TAPER) at a injector block temperature of 250 C in
splitless mode (splitless time: 1 min). Gas chromatography was
performed with a combination of two capillary columns: a 30 m
DB5-MS and a 15mDB1-MS (both: internal diameter 0.25 mm, film
thickness 0.25 mm; Agilent), a constant He-flux (99.996% pure) of
2 ml min1 and the temperature program presented in
Supplementary Table 2. CO2 reference gas (99.995% pure) was injected for 20 s into the detector several times throughout the
measurement to identify any drift of the d13C-values. The d13C of the
second reference gas peak was defined as 40‰ and all other d13C
values were calculated by comparison.
The chromatograms were evaluated with ISODAT NT 2.0.
Linear regressions were calculated from CO2 reference gas
(99.995% pure) peaks to correct for any drift during measurements.
To correct for amount-dependent 13C isotopic fractionation
during measurements (Schmitt et al., 2003), and for the addition of
C during derivatization, linear and logarithmic regressions of the
external standards d13C-values to their area were calculated. If both
regressions were significant, that with the higher significance was
applied. As the d13C-value for the derivatizing agentswas unknown,
the correction was performed according to Glaser and Amelung
(2002) (Eq. (3)).