Reconstructing bulk isotope ratios from compound-specific isotope ratios

Morrison, D.J., Cooper, K. and Preston, T. (2010) Reconstructing bulk isotope ratios from compound-specific isotope ratios. Rapid Communications in Mass Spectrometry, 24(12), pp. 1799-1804. (doi: 10.1002/rcm.4577)

Abstract

Carbon isotope analysis by bulk elemental analysis coupled with isotope ratio mass spectrometry has been the mainstay of δ13C analyses both at natural abundance and in tracer studies. More recently, compound-specific isotope analysis (CSIA) has become established, whereby organic constituents are separated online by gas or liquid chromatography before oxidation and analysis of CO₂ for constituent δ13C. Theoretically, there should be concordance between bulk δ13C measurements and carbon-weighted δ13C measurements of carbon-containing constituents. To test the concordance between the bulk and CSIA, fish oil was chosen because the majority of carbon in fish oil is in the triacylglycerol form and ∼95% of this carbon is amenable to CSIA in the form of fatty acids. Bulk isotope analysis was carried out on aliquots of oil extracted from 55 fish samples and δ13C values were obtained. Free fatty acids (FFAs) were produced from the oil samples by saponification and derivatised to fatty acid methyl esters (FAMEs) for CSIA by gas chromatography/combustion/isotope ratio mass spectrometry. A known amount of an internal standard (C15:0 FAME) was added to allow analyte quantitation. This internal standard was also isotopically calibrated in both its FFA (δ13C = -34.30‰) and FAME (δ13C = -34.94‰) form. This allowed reporting of FFA δ13C from measured FAME δ13C values. The bulk δ13C was reconstructed from CSIA data based on each FFA δ13C and the relative amount of CO2 produced by each analyte. The measured bulk mean δ13C (SD) was -23.75‰ (1.57‰) compared with the reconstructed bulk mean δ13C of -23.76 (1.44‰) from CSIA and was not significantly different. Further analysis of the data by the Bland-Altman method did not show particular bias in the data relative to the magnitude of the measurement. Good agreement between the methods was observed with the mean difference between methods (range) of 0.01‰ (-1.50 to 1.30).

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Preston, Professor Tom and Morrison, Professor Douglas
• Authors: Morrison, D.J., Cooper, K., and Preston, T.
• Subjects: Q Science > QD Chemistry
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Rapid Communications in Mass Spectrometry
• ISSN: 0951-4198
• ISSN (Online): 1097-0231
• Published Online: 17 May 2010