GC-C-IRMS

Figure 14.7 Principal derivatisation strategies for the GC C IRMS of lipids [trimethylsiylation (cholesterol) and esterification (fatty acids)] and amino acids (esterification acylaction)...

GC-C-IRMS instrumentation enables the compound-specific isotope analysis of individual organic compounds, for example, n-alkanes, fatty acids, sterols and amino acids, extracted and purified from bulk organic materials. The principle caveat of compound-specific work is the requirement for chemical modification, or derivatisation, of compounds containing polar functional groups primarily to enhance their volatility prior to introduction to the GC-C-IRMS instrument. Figure 14.7 summarises the most commonly employed procedures for derivatisation of polar, nonvolatile compounds for compound-specific stable isotope analysis using GC-C-IRMS. 

Figure 14.8 Partial m/z 44 and m/z 45/44 traces obtained by GC C IRMS analysis of commonly occurring amino acids (as their triflouroacetyl isopropyl ester derivatives) together with a range ofco injected internal standards...

GC-C-IRMS was first demonstrated by Matthews and Hayes (1978). However, it was somewhat later that Barrie and others (Barrie et al., 1984) coupled a GC, via a combustion interface, to a dual collector mass spectrometer to produce the forerunner of today s GC-C-IRMS instruments. Even so, true determinations of 815N values of individual compounds by GC-C-IRMS remained elusive until finally demonstrated by Hayes and co-workers (Merritt and Hayes, 1994). More recently the precision of GC-C-IRMS instruments has been improved further still with uncertainties in 813C values as small as 0.5 %o for samples containing 5 pmol C and 0.1 %o for 100 pmol samples having been demonstrated (Merritt and Hayes 1994). Instruments available commercially today, from several manufacturers, all conform to the same general principles of design. 

Figure 14.11 813C values of cholesterol recorded by GC C IRMS from a wide range of skeletal members and teeth confirming the consistency of the signals and potential utility for palaeo dietary reconstruction... 

Figure 14.13 8 13C values of the most abundant FAs and cholesterol in the Kwadgy Dan Ts inchj bone and skin samples (determined by GC C IRMS) compared with 813C values determined for the same compounds in individual s bone extracted from rats subjected to a pure C3 terrestrial diet, a 20% marine protein (tuna) diet and a 70% marine (tuna) diet (Jim, unpublished data)... 

GC-C-IRMS enables amino acids to be separated by GC, combusted on-line, for stable isotopic analysis thereby avoiding manual preparative steps (see Figures 14.5, 14.8 and 14.9). The 513C values of individual collagen amino acids are highly robust and by... 

Figure 14.14 Linear correlations between calculatedbased upon individual amino acids determined by GC C IRMS and measured, bulk collagen 813C values (dotted line represents x=y)...

Both Merritt and Hayes [639,640] and Merritt et al. [641] have investigated the statistical limits to attainable precision for GC-C-IRMS techniques. For carbon isotope ratio measurements with precision not limited by counting... 

Now, GC-IRMS can be used to measure the nitrogen isotopic composition of individual compounds [657]. Measurement of nitrogen isotope ratios was described by Merritt and Hayes [639], who modified a GC-C-IRMS system by including a reduction reactor (Cu wire) between the combustion furnace and the IRMS, for reduction of nitrogen oxides and removal of oxygen. Preston and Slater [658] have described a less complex approach which provides useful data at lower precision. Similar approaches have been described by Brand et al. [657] and Metges et al. [659]. More recently Macko et al. [660] have described a procedure, which permits GC-IRMS determination of 15N/14N ratios in nanomole quantities of amino acid enantiomers with precision of 0.3-0.4%o. A key step was optimization of the acylation step with minimal nitrogen isotope fractionation [660]. 

Although GC-C-IRMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study [614,677,678]. Measurement of chlorine stable isotope ratios in natural samples such as rocks and waters has become routine [626,679,680], but few measurements of chlorine isotopes in chlorinated aliphatic hydrocarbons have been reported [614]. A chlorine isotope effect was found in ferf-butyl chloride [681], demonstrating that 37Cl is more strongly bound to carbon than is 35Cl. Significant differences in the <5i7Cl values of some atmospheric chlorinated... 

The present section represents a brief introduction to modern GC-C-IRMS... 

Mansuy et al. [97] investigated the use of GC-C-IRMS as a complimentary correlation technique to GC and GC-MS, particularly for spilled crude oils and hydrocarbon samples that have undergone extensive weathering. In their study, a variety of oils and refined hydrocarbon products, weathered both artificially and naturally, were analyzed by GC, GC-MS, and GC-C-IRMS. The authors reported that in case of samples which have lost their more volatile n-alkanes as a result of weathering, the isotopic compositions of the individual compounds were not found to be extensively affected. Hence, GC-C-IRMS was shown to be useful for correlation of refined products dominated by n-alkanes in the C10-C20 region and containing none of the biomarkers more commonly used for source correlation purposes. For extensively weathered crude oils which have lost all of their n-alkanes,it has been demonstrated that isolation and pyrolysis of the asphaltenes followed by GC-C-IRMS of the individual pyrolysis products can be used for correlation purposes with their unaltered counterparts [97]. 

GC-C-IRMS Gas chromatography-combustion-isotope ratio mass spectrometry... 

The ocean has received far less attention than wetlands and soils as a source and sink of CH4. Studies of CH4 distributions in the Eastern Tropical North Pacific showed two CH4 maxima (Rurke et al., 1983) a surface maximum presumably associated with methanogensesis in fecal fellets (Karl and Tilbrook, 1994) and a deeper maximum. The mass spectrometric GC/C/IRMS technique discussed earlier allows measurement of fi CEU in small samples of seawater (Holmes et al., 2000). This technique was applied in the Eastern Tropical North Pacific (Sansone et al., 2001) to study methanogenesis and methane oxidation. The Sansone et al. (2001) results show a distinct difference in the fi CEU from the two CH4 maxima, leading to the suggestion that the deeper maximum may result from long distance offshore transport of CH4 with a shelf or seep source similar to that shown in Cynar and Yayanos (1993, figure 3). 

The isotopic characterisation of organic compounds by classical elemental analysis with isotope ratio mass spectrometry (EA-IRMS) demands milligram amounts of pure compounds, which can not always easily be provided. Most analytes in flavour characterisation are volatile, and therefore, after their extraction, coupling of (capillary) gas chromatography (cGC) with IRMS would be an ideal tool for the isotope ratio analysis of the individual substances. For C- ]127, 128] and N-analysis ]179, 180] this has been realised for a long time by combining GC to IRMS via a combustion (C) unit (GC-C-IRMS), even for polar substances after their derivatisation ]181, 182]. 

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