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 focu sed on implementing the measurement of C-13/C-12 of GC eluates (compou nd-specific isotope analysis). This technique has now been successfull y extended to the measurement of N-15/N-14 in volatile nitrogen-bearin g organic compounds and of O-18/O-16 in water. Precise and accurate me asurements of N-15/N-14 (+/-0.4 parts per thousand) at natural abundan ce have been made on nanogram quantities of a variety of nitrogen-cont aining compounds. Nitrogen is quantitatively converted to N2 in a nove l 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 s tream; 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 mas s spectrometer (Finnigan MAT Deltas-S) using an open split coupling. T he isotope ratios are measured by simultaneous monitoring of the CO+ i on currents at m/z = 28 and 30. Systematic experiments show that, whil e synthetic diamond is initially a better reaction substrate than eith er graphite or natural diamond, progressive graphitization of the diam ond grains leads to a gradual increase in memory.