Compound-specific isotope analysis: extending toward [formula omitted] and [formula omitted

Abstract

Development of the technique of isotope-ratio-monitoring has been focused on implementing the measurement of 13 C 12 C of GC eluates (compound-specific isotope analysis). This technique has now been successfully extended to the measurement of 15 N 14 N in volatile nitrogen-bearing organic compounds and of 18 O 16 O in water. Precise and accurate measurements of 15 N 14 N (±0.4%) at natural abundance have been made on nanogram quantities of a variety of nitrogen-containing compounds. Nitrogen is quantitatively converted to N 2 in a novel post-GC interface which incorporates a micro-oxidation reactor (for oxidizing organic compounds eluting from the GC), a reduction reactor (for conversion of NO x to N 2) and CO 2 removal from the carrier gas stream; the N 2 is isotopically characterized in a high sensitivity gas isotope mass spectrometer (Finnigan MAT 252). For 18 O 16 O analysis, sub-microliter amounts of water were reacted with carbon to produce CO and H 2 (Unterzaucher reaction) in a high temperature micro-furnace. The effluent of the micro-furnace is introduced into a gas isotope mass spectrometer (Finnigan MAT Deltas-S) using an open split coupling. The isotope ratios are measured by simultaneous monitoring of the CO + ion currents at m/z = 28 and 30. Systematic experiments show that, while synthetic diamond is initially a better reaction substrate than either graphite or natural diamond, progressive graphitization of the diamond grains leads to a gradual increase in memory.