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Abstract:
A manual "cryoflow' system is described for measuring C, N and S isotopic compositions of small gas samples in the 10–1000 nmol range. Using a continuous flow system with helium as the carrier gas, sample gases are injected or produced via combustion in an elemental analyzer, purified, separated and collected in separate traps with cryogens, then degassed into a slowly flowing helium stream (1–4 mL/min) for measurement with an isotope rato mass spectrometer. Blanks associated with combustion of organics in an elemental analyzer were 5–20 nmol for N and C, and not detectable for S, so that samples of >50 nmol C, N or S could be routinely analyzed with a precision of better than 0.3‰ after blank correction. In some cases, slow degassing of sample gases from traps over several minutes gave broad, low-amplitude peaks that yielded high-precision measurements (e.g., <0.05‰ δ13C differences between replicates). Work with this manual prototype showed that it is possible to routinely obtain high precision isotope measurements for C, N and S isotopes from the same small sample.
