3. Isotope Measurements
Methane was extracted from the stainless-steel storage cylinders on a
vacuum system at UCLA and purified by gas chromatography using
procedures described previously by Young et al. (2017). Gas aliquots of
a given sample were opened to a silica gel-filled stainless-steel U-trap
cooled with liquid nitrogen. Helium carrier gas was used to flush the
sample to the gas chromatograph. Separation was accomplished with a 3m
long, 1/8 in. OD stainless steel column packed with 5 Å molecular sieve,
followed in series by a 2m long ⋅ 1/8 in. OD stainless steel column
packed with HayeSep D porous polymer. Peaks from the gas chromatograph
were detected with an in-line thermal conductivity detector (TCD).
Because total pressures were > 1 bar in the reaction
vessels, and because samples were dominated by H2,
purification on the vacuum line had to be performed in sequence of
multiple aliquots. Once collection of a purified aliquot of methane was
complete, this methane was transferred to a glass vial filled with
silica-gel at liquid nitrogen temperature. Another sample aliquot would
then be purified and added to the same glass vial, until a given
reaction vessel had been entirely processed. For the cylinders to be
emptied, 6 to 8 aliquots were processed in this way on the vacuum line.
Samples were then transferred to the inlet of the mass spectrometer
where they were warmed and expanded into the mass spectrometer using a
silica-gel filled cold finger.
Mass spectrometry measurements were done on a Nu Instruments Panorama
with a mass resolving power that allows the simultaneous measurement of
ion currents for resolved12CH4+,13CH4+,12CH3D+,13CH3D+ and12CH2D2+ion beams, as described in Young et al., (2016). The analyte peaks are
in most cases baseline resolved from surrounding interferences, and
where there is the potential for a tail effect (in the case of13CH5+ tailing into12CH2D2+),
previous work shows that the effects on our final results are less than
quoted uncertainties (Young et al., 2016). The measured ratios of these
ion currents yield values for both
Δ13CH3D and
Δ12CH2D2 as well as
for bulk 13C/12C and D/H. The
methane quantities available for isotopic measurements were relatively
low. For standards and samples available in quantities > 80
μmol, relative abundances of13CH3D+ and12CH2D2+are measured for up to 10 and 20 hours, respectively. In these cases,
typical propagated internal uncertainties for the calculation of
Δ13CH3D and
Δ12CH2D2 are generally
±0.1‰ and ±0.5‰, respectively. In our study, often only 35 to 60 μmol of
CH4 were available (Table 1), and uncertainties are
larger, reflecting less favorable counting statistics. The internal
uncertainties range from 0.2‰ and 0.5‰ for
Δ13CH3D, and between 1.1‰ and 3.2‰ for
Δ12CH2D2.