Practical and theoretical considerations in the gas chromatography/combustion/isotope ratio mass spectrometry δ¹³C analysis of small polyfunctional compounds

Abstract

Carbohydrates and proteins are among the most abundant naturally occurring biomolecules and so suitable methods for their reliable stable isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) are required. Due to the non-volatile nature of these compounds they require hydrolytic cleavage to their lower molecular weight subunits and derivatisation prior to GC/C/IRMS analysis. The addition of carbon to the molecules and any kinetic isotopic fractionation associated with derivatisation must be accounted for in order to provide meaningful stable isotope values and estimates of propagated errors. To illustrate these points amino acid trifluoroacetate/isopropyl esters and alditol acetates were prepared from authentic amino acids and monosaccharides, respectively. As predicted from the derivatisation reaction mechanisms, a kinetic isotope effect was observed which precludes direct calculation of delta(13)C values of the amino acids and monosaccharides by simple mass balance equations. This study shows that the kinetic isotope effect associated with derivatisation is both reproducible and robust, thereby allowing the use of correction factors. We show how correction factors can be determined and accurately account for the addition of derivative carbon. As a consequence of the addition of a molar excess of carbon and the existence of a kinetic isotope effect during derivatisation, errors associated with determined delta(13)C values must be assessed. We illustrate how such errors can be quantified (for monosaccharides +/-1.3 per thousand and for amino acids between +/-0.8 per thousand and +/-1.4 per thousand). With the magnitude of the errors for a given delta(13)C value of a monosaccharide or amino acid quantified, it is possible to make reliable interpretations of delta(13)C values, thereby validating the determination of delta(13)C values of amino acids as TFA/IP esters and monosaccharides as alditol acetates.

Direct chemical evidence for widespread dairying in prehistoric Britain

“…Most importantly, a high abundance of sherds (78%) contained predominantly dairy fats. Furthermore, 14 of the sherds yielded triacylglycerol distributions containing low (C 40 to C 44 acyl carbon atoms) molecular weight components characteristic of degraded dairy fats (18), and all these sherds plotted within the reference dairy fat field, thereby corroborating the origin of the fatty acids based on their ␦ 13 C values. On the basis of the above findings and faunal evidence (summarized in Table 2), we are now in a position where we can compare the frequency of occurrence of dairy fats with herd structures.…”

“…The diethyl ether was dried over magnesium sulfate and then evaporated under a stream of nitrogen. Alditol acetates were dissolved in dichloromethane for analysis by GC-combustion-isotope ratio MS (18).…”

Cometary glycine detected in samples returned by Stardust

“…The precision of the calculated value also depends on the precision of the three measurements described above (i.e., derivatized sample, derivatized standard, underivatized standard) and can be calculated using Equation 2 (Docherty et al 2001):…”

CO2 uptake by a soil microcosm

“…The d values for fatty acids were calculated from triplicate samples and were corrected after IRMS analysis. This was done because after derivitisation to the corresponding FAME, 94% of the carbon isotope measured comes from the fatty acid itself and the remaining 6% as a result of the methoxy carbon (Docherty et al, 2001). This results in a very minor alteration in the d value, especially when dealing with largely enriched lipids as reported here.…”