title: Can gas chromatography combustion isotope ratio mass spectrometry be used to quantify organic compound abundance?
abstract: Quantifying the concentrations of organics such as phospholipid fatty acids (PLFAs) and n-alkanes and measuring their corresponding (13)C/(12)C isotope ratios often involves two separate analyses; (1) quantification by gas chromatography flame ionisation detection (GC-FID) or gas chromatography/mass spectrometry (GC/MS), and (2) (13) C-isotope abundance analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC-C-IRMS). This requirement for two separate analyses has obvious disadvantages in terms of cost and time. However, there is a history of using the data output of isotope ratio mass spectrometers to quantify various components; including the N and C concentrations of solid materials and CO(2) concentrations in gaseous samples. Here we explore the possibility of quantifying n-alkanes extracted from sheeps' faeces and fatty acid methyl esters (FAMEs) derivatised from PLFAs extracted from grassland soil, using GC-C-IRMS. The results were compared with those from GC-FID analysis of the same extracts. For GC-C-IRMS the combined area of the masses for all the ions (m/z 44, 45 and 46) was collected, referred to as 'area all', while for the GC-FID analysis the peak area data were collected. Following normalisation to a common value for added internal standards, the GC-C-IRMS 'area all' values and the GC-FID peak area data were directly compared. Strong linear relationships were found for both n-alkanes and FAMEs. For the n-alkanes the relationships were 1:1 while, for the FAMEs, GC-C-IRMS overestimated the areas relative to the GC-FID results. However, with suitable reference material 1:1 relationships were established. The output of a GC-C-IRMS system can form the basis for the quantification of certain organics including FAMEs and n-alkanes.
authors.affiliation: The James Hutton Institute, Craigiebuckler, Aberdeen, UK. barry.thornton@hutton.ac.uk
authors.forename: Barry
authors.initials: B
authors.lastname: Thornton
authors.forename: Zulin
authors.initials: Z
authors.lastname: Zhang
authors.forename: Robert W
authors.initials: RW
authors.lastname: Mayes
authors.forename: Mona N
authors.initials: MN
authors.lastname: Högberg
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Midwood
mesh_terms: D000473:Alkanes / Q000032:analysis* / Q000737:chemistry; D000818:Animals; D002247:Carbon Isotopes / Q000032:analysis*; D005227:Fatty Acids / Q000032:analysis* / Q000737:chemistry; D005243:Feces / Q000737:chemistry; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D016014:Linear Models; D010743:Phospholipids / Q000032:analysis* / Q000737:chemistry; D012756:Sheep
chemical_list: D000473:Alkanes; D002247:Carbon Isotopes; D005227:Fatty Acids; D010743:Phospholipids