A laser GC-IRMS technique for in situ stable isotope analyses of carbonates and phosphates

Details A laser GC-IRMS technique for in situ stable isotope analyses of carbonates and phosphates A laser GC-IRMS technique for in situ stable isotope analyses of carbonates and phosphates

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.2909s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 60, Issue 15, pp.2909-2916

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Scientific paper

A technique is described whereby in situ carbon and oxygen isotope analyses of carbonates and organic phosphates can be made with the use of a CO 2 laser. The CO 2 gas generated by thermal decarbonation from the laser is entrained in a helium carrier gas, passes through a chromatographic column (GC), and is admitted directly into the isotope ratio mass spectrometer (IRMS). No vacuum systems, pumps, or cryogenic traps are used. All carbonates and biogenic phosphates can be analyzed, no special sample preparation is required and analyses can be made every 3 minutes. The use of a helium carrier gas allows for extremely small samples to be analyzed and the GC column effectively separates CO 2 from any other potential contaminating gases (e.g., SO 2 which is a particular problem in organic apatite). The average reproducibility of calcite, dolomite, magnesite, rhodochrosite, siderite, and smithsonite (ZnCO 3 ) is 0.29 for oxygen and 0.1 for carbon (1 ); the most "homogeneous" samples are reproducible to better than 0.1 for carbon and 0.2 for oxygen. The difference between the laser and conventional values for carbon isotope ratios [ 13 C (laser-conv)] is 0.05 ± 0.30 for all carbonates (excluding siderite). The 18 O(laser-conv) value varies from carbonate to carbonate and may be related to the electronegativities of the cations, grain size (or crystallinity), formation of CO and O 2 , and reaction with included organic matter. For calcite and rhodochrosite, the 18 O(laser-conv) value is 0.3 ± 0.4 ; for siderite, magnesite, and dolomite, the 18 O(laser-conv) value is 1.7 ± 0.3 . The 13 C values of tooth enamel are the same as those obtained by conventional acid digestion. The laser 18 O values are equal to the 18 O values of the phosphate, and approx. 7 lighter than the "carbonate" oxygen. The carbonate group in the apatite (equiv. 7.6% oxygen) exchanges with the (PO 4 = )-bound oxygen to produce CO 2 with a 18 O equal to the phosphate oxygen. The laser technique provides a rapid alternative to the difficult phosphate extraction technique for oxygen isotope measurements in tooth enamel.

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