Practice and Principles of Isotopic Measurements in Organic Geochemistry John M Hayes Department of Geology and Geophysics Woods Hole Oceanographic Institution Woods Hole MA 02543 JHayes WHOI edu edited by Alex L Sessions Revision 2 August 2002 Practice and Principles J M Hayes Preface These notes have been compiled from teaching materials developed by John Hayes between 1980 and 1995 Since their goals are to introduce basic concepts and practices they are not grossly outdated in spite of their antiquity Where necessary or desirable minimal revisions indicate more recent data and practices The sections include Practice and Principles of Isotopic Measurements in Organic Geochemistry This review originally appeared as pp 5 1 to 5 31 in W G Meinschein ed Organic Geochemistry of Contemporaneous and Ancient Sediments a volume published in 1983 by the Great Lakes Section of the Society of Economic Paleontologists and Mineralogists That book was intended to provide the notes for a short course offered in connection with the 1983 National Meeting of the Geological Society of America but the short course drew inadequate enrollment and was cancelled The original document included the introduction and sections 1 and 2 reprinted here as well as an earlier version of section 3 Primary Standards of Stable Isotopic Abundances Presented here as section 3 this material was revised and expanded frequently in the years after 1983 notably for use in a short course sponsored by Finnigan MAT in Beijing in 1986 and for frequent use in subsequent classes at Indiana University Limits on the Precision of Mass Spectrometric Measurements of Isotope Ratios Originally prepared for class use some time in the 1980s this text Section 4 explains how shot noise or ion beam noise or ion statistics fundamentally limit the precision that can be attained in any measurement based on integration of an electrical current page 2 Uncertainties in Blank Corrected Isotopic Analyses These notes Section 5 were prepared for use by workers in the Biogeochemical Laboratories at Indiana University during the 1980 s accordingly their presentation is far from publishable They deal with the common problem in which an analytical result reflects the composition of a sample and in addition the magnitude and isotopic composition of an analytical blank Further Notes July 2002 Section 6 elaborates on notes inserted in earlier sections J M Hayes Practice and Principles Introduction The modern era of precise isotopic measurements and exploration of natural variations in isotopic abundances began with the work of Nier and Gulbransen 1939 who observed that the relative abundance of carbon 13 was greater in inorganic material carbonates than in organic material Later Harold Urey and his students at the University of Chicago greatly extended the precision and scope of these measurements The measurement of oxygen isotopic abundances in carbonates was introduced for paleotemperature studies Urey et al 1951 Epstein et al 1951 and Harmon Craig 1953 published his landmark study of the biogeochemistry of the stable isotopes of carbon Wickman 1956 provided an early estimate of the division of sedimentary carbon between organic and inorganic materials Much earlier West 1945 published the first report of carbon isotopic studies in petroleum geochemistry an accomplishment that has received too little recognition In view of the growing importance of isotopic studies in organic geochemistry for major reviews see Degens 1969 and Deines 1980 it is the goal of this review to provide the reader with an introduction to the practical and theoretical bases of isotopic geochemistry 1 Mass Spectrometric Analyses and Sample Handling 1 1 The Nature of the Problem Because the equilibrium or kinetic characteristics of one isotopic species usually differ from those of another by only a few percent the variations in isotopic abundances imposed by those differences are small Highly precise analytical techniques are required and it has been found convenient to compare samples measuring isotopic differences rather than to attempt absolute measurements of isotopic abundances Obtaining short term high precision is not a problem but getting long term stability can be extremely difficult More graphically when absolute measurements are employed a single sample may yield isotope ratios of 0 0100886 0 0000007 today and 0 0100251 0 0000008 tomorrow In contrast if that sample is compared to an arbitrary reference sample on both days the differences observed between the sample and the reference will probably be constant This stability will be observed because the same instrumental fluctuations that have affected the absolute ratio of the unknown sample will also have affected the absolute ratio of the reference sample Provided everything is compared to the same reference either directly or indirectly very small differences between isotope ratios can be accurately measured This technique of differential comparison has been a cornerstone of precise isotopic analysis since it was introduced around 1950 in Harold Urey s laboratories at the University of Chicago McKinney et al 1950 Sample Preparation Differential measurement of isotopic abundances requires that each sample be compared to a standard i e a reference point which differs from the sample if at all only in its isotopic composition This is often convenient and simple For example precise comparisons of isotopic compositions of carbonates can be made by comparing mass spectra of samples of carbon dioxide prepared from each material The mechanism of comparison is neither so obvious nor so simple when isotopic compositions of organic materials are of interest though there is nothing fundamentally wrong with comparing isotopic compositions of organic materials directly For example the difference in carbon isotopic compositions of two samples of benzene C6H6 molecular weight 78 might be measured by comparison of the mass 79 mass 78 ion current ratios and if both samples were not available at the same time each might be satisfactorily compared to an intermediate standard The impracticality of this simple approach becomes clear however when it is recognized that i the measurement would be invalid if the hydrogen isotopic compositions of the samples differed significantly ii an isotopic standard would be required for each compound of interest if indirect comparisons were to be made iii comparisons between different compounds would be