COMPOUND-SPECIFIC ISOTOPE ANALYSIS - EXTENDING TOWARD N-15 N-14 AND O-18 O-16

Development of the technique of isotope-ratio-monitoring has been focused on implementing the measurement of C-13/C-12 of GC eluates (compound-specific isotope analysis). This technique has now been successfully extended to the measurement of N-15/N-14 in volatile nitrogen-bearing organic compounds and of O-18/O-16 in water. Precise and accurate measurements of N-15/N-14 (+/-0.4 parts per thousand) at natural abundance have been made on nanogram quantities of a variety of nitrogen-containing compounds. Nitrogen is quantitatively converted to N2 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 N2) and CO2 removal from the carrier gas stream; the N, is isotopically characterized in a high sensitivity gas isotope mass spectrometer (Finnigan MAT 252). For O-18/O-16 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.