GC-IRMS

FTICR. Fourier-transform ion cyclotron resonance GC/IRMS. gas chromatography isotope ratio mass spectrometry... 

Figure 10.4 Schematic representation of the multidimensional GC-IRMS system developed by Nitz et al. (27) PRl and PR2, pressure regulators SV1-SV4, solenoid valves NV— and NV-I-, needle valves FID1-FID3, flame-ionization detectors. Reprinted from Journal of High Resolution Chromatography, 15, S. Nitz et al, Multidimensional gas cliro-matography-isotope ratio mass specti ometiy, (MDGC-IRMS). Pait A system description and teclinical requirements , pp. 387-391, 1992, with permission from Wiley-VCFI.

Conventional IRMS requires relatively large sample volumes in a purified gaseous form. Recently, an on-line GC-IRMS system has been developed which combines the high purification effect of GC with the utmost precision of IRMS. Sometimes this system may not be Sufficient to determine characteristic minor components from complex matrices, and therefore MDGC-IRMS systems have been developed for the analysis of complex plant extracts and flavour components (25-27). 

Figure 10.4 shows a schematic representation of the multidimensional GC-IRMS System developed by Nitz et al. (27). The performance of this system is demonstrated with an application from the field of flavour analysis. A Siemens SiChromat 2-8 double-oven gas chromatograph equipped with two FIDs, a live-T switching device and two capillary columns was coupled on-line with a triple-collector (masses 44,45 and 46) isotope ratio mass spectrometer via a high efficiency combustion furnace. The column eluate could be directed either to FID3 or to the MS by means of a modified Deans switching system . 

The authors demonstrated the importance that correct use of the MDGC-IRMS System is essential for the achievement of precise and accurate measurements. Table 10.4 reports the GC-IRMS measurements of some standard reference materials, obtained with different cut conditions. As can be seen from this table, premature cuts result in 8 C values which are significantly higher than the true values, while delayed cuts give lower 8 C values. This fact indicates that the beginning of the peak is enriched in C, while the end is depleted. 

Table 10.4 GC-IRMS measurements of standard reference materials obtained under different cut conditions (25)...

Sharp, Z.D. and Cerling, T.E. 1995 A laser GC-IRMS method for in-situ carbon and oxygen isotope analyses of carbonates and phosphates. Geological Society of America, Abstracts Program 27 A255. 

The principal advantage of conventional GC-IRMS in this context is its ease of use and lower sample size requirements, while the IRMS approach provides superior accuracy and precision, particularly at lower enrichments. IRMS data are usually expressed using the 6yX notation, given as... 

Since IRMS is a single-parameter chromatographic detector incapable of speciation, GC-IRMS places severe demands on the chromatographic resolution to guarantee peak purity [631,632]. This is sometimes checked by splitting the effluent between the IRMS and a conventional El mass spectrometer [633]. 

Compound-specific isotope analysis (CSIA) by GC-IRMS became possible in 1978 due to work of Mathews and Hayes [634], based on earlier low-precision work of Sano et al. [635]. The key innovation was the development of a catalytic combustion furnace based on Pt with CuO as oxygen source, placed between the GC exit and the mass spectrometer. The high pressure of helium (99.999% purity or better) ensures that all gas flows are viscous. After being dried in special traps avoiding formation of HC02 (i. e., interferes with 13C02) by ion-molecule reactions in the ion source, the C02 is transmitted to a device that regulates pressure and flow and then into the ion source [604]. 

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]. 

This new generation of mass spectrometers is often combined with chromatographic techniques. The sample size required for an isotope measurement has been drastically reduced to the nano- or even pico-molar range (Merritt and Hayes 1994). Important features of the GC-IRMS technique are (Brand 2002) ... 

GC, GC-MS, GC-IRMS GC-MS, GC-IRMS GC-MS, GC-IRMS GC-MS, GC-IRMS HPLC-MS, HPLC-IRMS Py-GC-MS GC-MS, Py-GC-MS, GC-IRMS, NMR, X-ray... 

Aliphatic alcohols, acids, hydrocarbons Abiotic GC, GC-MS, GC-IRMS... 

Table 17.5 Specifications for capillary GC-IRMS coupling techniques i ing DELTA pliK XL, Thermo Electron, Bremen, Germany [52]...

Isotope ratios are given as deviations, in relation to a defined primary standard (zero point). The polyethylene foils CH 7 and NBS 22-oil are commercially available secondary standards, certificated and managed by the International Atomic Energy Agency. However, GC-lRMS systems cannot be calibrated without the aid of alternative peripheries like an elemental analyser (EA) or a dual inlet, owing to the lack of commonly accepted reference materials applicable in GC-IRMS techniques (Fig. 17.11). 

In the course of a feasibility study, sponsored by the European Union, the components of the GC separation efficiency test, according to K. Grob, were tested for their usability as certificated tertiary standards. Seven compounds are now available as a ready-to-use mixture for testing the accuracy of the GC-IRMS measurements, and furthermore simultaneously provide important information about the actual quality status of the GC column system used [53]. 

Compound specific stable isotope analysis using gas chromatography combined with an isotope ratio mass spectrometer - GC-IRMS (see also Chapter 7) - is now a mature analytical technique in environmental science and technology, especially in the area of contaminant source attribution and in assessing the biodegradation of contaminants.108 Several studies have focused on 13C/12C, 180/160 and 170/160 isotope ratio measurements for volatile organic and metalor-ganic compounds to study isotope fractionation effects and to identify contamination in the environment.109... 

Frank, C., Dietrich, A., Kremer, U. and Mosandl, A. (1 995) GC-IRMS in the authenticity control of the essential oil of Coriandrum sativum L. Journal of Agricultural and Food Chemistry 43(6), 1634-1 637. 

However, as mentioned previously, methods have been developed which can measure these isotopic ratios in individual compounds and at specific sites within those compounds. The measurement of C isotope ratios in individual compounds can be achieved using gas chromatography (GC) IRMS, but compound-specific information for other isotopes is usually obtained by first isolating the individual components, for example, remove the sugar fraction, and measure its lsO leO ratio [29]. 

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

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