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Oxygen isotope analysis of phosphate: improved precision using TC/EA CF‐IRMS
Author(s) -
LaPorte D. F.,
Holmden C.,
Patterson W. P.,
Prokopiuk T.,
Eglington B. M.
Publication year - 2009
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/jms.1549
Subject(s) - chemistry , isotope ratio mass spectrometry , apatite , isotopes of oxygen , analytical chemistry (journal) , oxygen , fractionation , carbonate , mass spectrometry , isotope analysis , mineralogy , chromatography , nuclear chemistry , geology , oceanography , organic chemistry
Oxygen isotope values of biogenic apatite have long demonstrated considerable promise for paleothermometry potential because of the abundance of material in the fossil record and greater resistance of apatite to diagenesis compared to carbonate. Unfortunately, this promise has not been fully realized because of relatively poor precision of isotopic measurements, and exceedingly small size of some substrates for analysis. Building on previous work, we demonstrate that it is possible to improve precision of δ 18 O PO4 measurements using a ‘reverse‐plumbed’ thermal conversion elemental analyzer (TC/EA) coupled to a continuous flow isotope ratio mass spectrometer (CF‐IRMS) via a helium stream [Correction made here after initial online publication]. This modification to the flow of helium through the TC/EA, and careful location of the packing of glassy carbon fragments relative to the hot spot in the reactor, leads to narrower, more symmetrically distributed CO elution peaks with diminished tailing. In addition, we describe our apatite purification chemistry that uses nitric acid and cation exchange resin. Purification chemistry is optimized for processing small samples, minimizing isotopic fractionation of PO 4 −3 and permitting Ca, Sr and Nd to be eluted and purified further for the measurement of δ 44 Ca and 87 Sr/ 86 Sr in modern biogenic apatite and 143 Nd/ 144 Nd in fossil apatite. Our methodology yields an external precision of ± 0.15‰ (1σ) for δ 18 O PO4 . The uncertainty is related to the preparation of the Ag 3 PO 4 salt, conversion to CO gas in a reversed‐plumbed TC/EA, analysis of oxygen isotopes using a CF‐IRMS, and uncertainty in constructing calibration lines that convert raw δ 18 O data to the VSMOW scale. Matrix matching of samples and standards for the purpose of calibration to the VSMOW scale was determined to be unnecessary. Our method requires only slightly modified equipment that is widely available. This fact, and the demonstrated improvement in precision, should help to make apatite paleothermometry far more accessible to paleoclimate researchers. Copyright © 2009 John Wiley & Sons, Ltd.