A manual 'cryoflow' system is d d b e d for measuring C, N and S isotopic compositions of small gas samples in the 10-1000 nmd range. Using a continuous flow system with helium as the carrier gas, sample gases are ix&&d 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 5-24 nmol for N and C, and not detectable for S, so that samples of S O nmol C, N or S muld be routinely analyzd 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%0 S' C 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.In the last five years, great advances have been made in the ease of measurement of stable isotopic compositions by coupling elemental analyzers and gas chromatographs to isotope ratio mass spectrometers. Sample gases are carried into the mass spectrometer as discrete peaks in a helium stream and isotopic compositions calculated from massweighted, integrated signals after subtracting baseline noise.Operation with GC interfaces gives peaks that are typically 2-10 seconds wide, and this time compression allows even very small amounts of material, down to nanomole and picomole levels, to be analyzed.' Elemental analyzers have also been coupled to mass spectrometers? and sequential measurement of N2 and CO, is now available in commercial instruments. Measurement of low nanomole and picomole samples is theoretically possible from elemental analyzers, but current measurement limits are about 1000 nmol for C or N for precisions that are acceptable for natural abundance measurements (i.e. 0.5%~ or bette9). This large sample demand, relative to GC applications, is governed by the high flow rates employed in elemental analyzers, and the broad nature of the eluted peaks. In this paper, a strategy is described to amplify signals gained from such small samples, by means of trapping in liquid nitrogen followed by slowly flushing a large fraction of the sample into the mass spectrometer. This strategy is based on conventional cryogenic techniques for collecting samples, rather than any chromatographic separation, and uses inexpensive apparatus available in most laboratories. This paper shows four applications of this cryofocus technique for C, N and S isotopic measurements.
EXPERIMENTAL SECTIONInterface design A commercially available design was used to couple the elemental analyzer (CHN-0 Rapid Type VT, Heraeus GmbH, Hanaul, Germany) to the mass spectrometer (Finni- gan Copration, Bremen, Germany, Delta-S mass spectrometer). This was achieved by adding traps and plumbi...
