GC-P-IRMS

The online determination of S Hv-smow values using GC-pyrolysis-IRMS (GC-P-IRMS) was developed recently [55] and has proved to be a powerful tool to define the authenticity of natural compounds [56-61]. However, as fruit fiavour extracts are rather complex, and the sample amount for hydrogen measurement has to be rather high owing to the low abundance of deuterium... 

Bilke S, Mosandl A (2002) Authenticity assessment of lavender oil i ing GC-P-IRMS H/ H-ratios of linalool and linalyl acetate. Eur Food Res Technol 214 532... 

Several procedures and systems for the on-line determination of the 8 H- and 8 0-values of water and organic compounds have been investigated in the last few years [183-194] and problems arising from isotope effects on the GC-separations of analytes and their pyrolysis (P) products have been discussed [195-202]. Today most of the complications have been overcome and the technical prerequisites for the on-line GC-C-IRMS analysis of carbon and the GC-P-IRMS analysis of hydrogen and oxygen isotopes in volatile organic compounds have been optimised and become available for routine application [203-205]. 

S. Bilke, A. Mosandl (2002 b) Authenticity assessment of lavender oils using GC-P-IRMS H/ H isotope ratios of linalool and linalylacetate. Eur Eood Res Technol 214. 532-535... 

ISO standard 3216 shows character and data for the Chinese type oil. Main component is trans-cinnamaldehyde. Synthetic cinnamaldehyde as well as coumarin is used for adulteration. This oil is often used for the adulteration of cinnamon bark oil. That is easy to detect, as coumarin is not a component of that oil. If o-methoxy cinnamaldehyde is found in cinnamon bark oil, it is a sign for adulteration with cassia oil. The naturality of the cinnamaldehyde can be detected by the combination of GC-combustion IRMS (GC-CTRMS) and GC-P-IRMS. 

ISO standard 9776 shows character and data for this oil. The main components are cumin aldehyde, p-mentha-1.3 dien 7al, p-mentha 1,4 dien 7al, y-terpinene, and P pinene. Blending is done by orange terpenes, p cymene, and piperitone. Detection is done by GC-MS and by the combination of GC-C-IRMS and GC P-IRMS as assessment of synthetic and natural cuminaldehyde is possible. 

GC-C-IRMS or GC-P-IRMS can be used for G and N, and for and 0, respectively. The first generation GC-C-IRMS, introduced around 1988, enabled combustion allowing C analysis. The second generation GC-C-IRMS instrumentation (1994) added a capillary reduction oven behind the combustion oven, allowing on-line isotope ratio measurements. Deuterated ( H) substrates... 

The variation in isotope abundance measurement in GC-C-IRMS and GC-P-IRMS is much less than in GC-MS. Therefore, molar enrichments of 0.01% can still be quantified. For this reason, IRMS techniques can replace MS techniques at low enrichment levels or can be added to the MS technique in order to extend an isotopic decay curve to enable multicompartment analysis. However, the latter application may be unnecessary since linearity of GC-C-IRMS is usually excellent. Linearity has been shown from 0.01 to 25% molar enrichment for valine... 

Of clinical interest also is the measurement of energy expenditure and body composition. Among other techniques, 2H2 0, and H20 are used for this purpose and isotope enrichments are measured in body water. So far, the technique required sample preparation steps converting water into H2 and exchanging oxygen between water and CO2. Possibly GC-P-IRMS will soon replace these procedures and offer rapid on-line measurement of and directly in body water. 

GC-C-IRMS GC combustion IRMS GC-P-IRMS GC pyrolysis IRMS... 

Kahle K, Preston C, Richling E, Heckel F, Schreier P (2005) On-line gas chromatography combustion/pyrolysis isotope ratio mass spectrometry (HRGC-C/P-lRMS) of major volatiles from pear fruit (Pyrus communis) and pear products. Food Chem 91 449 Tamura H, Appel M, Richling E, Schreier P (2005) Authenticity assessment of y- and 5-deca-lactone from Prunus fruits by gas chromatography combustion/pyrolysis isotope ratio mass spectrometry (GC-C/P-IRMS). J Agric Food Chem 53 5397... 

Recently published results obtained by very detailed studies of vanillin by H-NMR and C-GC-C-IRMS including the application of minor components as intrinsic standards (e.g. p-hydroxybenzaldehyde) together with compositional analyses should enable analysts to even detect sophisticated adulterations of vanilla flavour [276-278[. 

Table 6.8 S C-values ([%o]y p g) of monoterpene flavouring compounds in lemon oils of different origins. Adapted from [325] with kind permission. Copyright [1993] Springer Verlag. All identified compounds are characterised by their relative amounts (concentration c [area %]) as calculated after the peak heights of the GC detector, and by their S C-values. The S C-values are the average of three measurements obtained by GC-C-IRMS...

Intermolecular isotopic correlations are thus indicative for the authenticity of natural flavour mixtures. The method for their assessment is GC-C/P-IRMS (combustion/ reductive pyrolysis). As an early example for intermolecular isotope correlations, the result of a GC-C-IRMS analysis of the essential oil from Coriandrum sativum is given in Fig. 6.19 [327]. Further examples are corresponding analyses of oils from Artemisia vulgaris [327], Coriandrum sativum [337] and various lemon oils (Table 6.8) ]325, 338, 339]. In any of these cases typical correlations are found between the compounds of the same origin, even when their average 8-values may differ between... 

Most flavourings are complex mixtures of many compounds. As IRMS makes only sense with pure analytes, a strict purification of individual substances is indispensable. Therefore GC-IRMS has been further developed and optimised to multi-compound isotope ratio analysis by its coupling IRMS to capillary (c) and multidimensional (MD) gas chromatography (see 6.2.2.2.2). This methodology demands a strict intrinsic control and standardisation [340] apart from the international standards (see Table 6.3) also secondary standards like the polyethylene foil IAEA-CH7 or the NBS22 oil are available from the IAEA in Vieima. However, as these substances are also not suitable for the direct standardisation of data from a coupled GC system for flavour isotope analysis, certificated tertiary laboratory standards for hydrogen have been developed by parallel analysis of flavour compounds by TC/EA-IRMS and MDGC-P-IRMS [210]. 

Without any doubt the most valuable development in mass spectrometry has been multi-compound/multi-isotope analysis. This implies the application of GC-C/P-IRMS to the on-line analysis, not only of carbon but also of other isotopes, preferably of hydrogen and oxygen, in the individual components of a mixture, and the use of the metabolic and isotopic correlations obtained from such an analysis. In the course of this chapter, the potential of (positional) oxygen isotope analysis has been emphasised several times and this will still be a challenge of the future. The advantage of GC-C/ P-IRMS is its speed in performance and the very moderate demand on sample size and purity, and also its implication for automation. The information available can easily be correlated to that of other (classic) analyses. However, a disadvantage will be always that the data concern a global mean value for the whole molecule in question. 

S. Faulhaber (1997) GC/IRMS analysis of mandarin essential oils. 1. 5 Cppg and 5 N, p values of methyl N-methylanthranilate. J. Agric. Food Chem. 4 2579-2583... 

Gaschnitz, R., Krooss, B. M., Gerling, P., Faber, E., and Littke, R., On-line pyrolysis-GC-IRMS isotope fractionation of thermally generated gases from coals. Fuel, 80, 2139-2153, 2001. 

Meier-Augenstein, W. (2004) Laboratory set-up for GC-MS and continuous-flow IRMS. In Handbook of Stable Isotope Analytical Techniques, Vo. 1, edited by de Groot, P.A. Amsterdam, The Netherlands Elsevier B.V, pp. 1038-1042. 

O Malley V. P., Burke R. A., and Schlotzhauer W. S. (1997) Using GC-MS/Combustion/IRMS to determine the ratios of individual hydrocarbons produced from the combustion of biomass materials appUcation to biomass burning. Org. Geochem. 27, 567 581. 

While P T, providing the lowest MDLs for VOCs, is a routinely used extraction method for the trace level quantification, the on-line coupling with irm-GC-MS has rarely been reported (Schmidt et al, 2004). 

Figure 4.24 Reproducibility and accuracy of P T-irm-GC-MS. Plotted are the differences from the pure liquid standards measured by EA-IRMS and the horizontal bars correspond...

Figure 4.25 Chromatogram of an online P T-irm-GC-MS analysis, nie concentrations of the different analytes were adjusted to achieve similar signal intensities (1.7 ng/L (toluene) -32 ig/L (CCI4)). The three first peaks correspond to the reference COj gas pulses.

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