Sample preparation IRMs

CF-IRMS provides reliable data on micromoles or even nanomoles of sample without the need for cryogenic concentration because more of the sample enters the ion source than in DI-IRMS. CF-IRMS instruments accept solid, liquid, or gaseous samples such as leaves, soil, algae, or soil gas, and process 100-125 samples per day. Automated sample preparation and analysis takes 3-10 min per sample. The performance of CF-IRMS systems is largely determined by the sample preparation technology. A variety of inlet and preparation systems is available, including GC combustion (GC/C), elemental analyzer, trace gas pre-concentrator and other. The novel... 

As for all trace-level analyses, sample preparation and handling are of crucial importance. In addition to all the usual problems of GC-MS, measurements of isotope ratios must ensure that none of these steps introduce any isotope discrimination. Any chemical reactions, including conversion of the organic sample molecules to the simple gases which are those actually analyzed, must be quantitative (100% conversion) to avoid kinetic isotope effects [627]. Until relatively recently, all gas IRMS experiments employed a dual-inlet system to permit switching between sample and standard C02 contained in two bellows containers. The pressures in the two bellows are adjusted to be equal and,... 

An important criticism of the use of combustion trains is that combustion is not site specific, that is all atoms in the analyte end up in the gas transferred to the IRMS. For studies of carbon isotope effects this is invariably C02. The question is especially important for carbon isotope analysis because analyte molecules of interest usually contain several different kinds of carbon atoms and therefore combustion methods average or dilute the IE s of interest. Should site specific isotope ratios be required another method of sample preparation (usually much more tedious) is necessary. Combustion methods, however, are frequently used to study nitrogen and sulfur IE s because many organic molecules are singly substituted with these atoms. Obviously, oxygen isotope effects cannot be determined using combustion trains because external oxygen is employed. Rather some type of pyrolytic sample preparation is required. 

Authenticity evaluation has recently received increased attention in a number of industries. The complex mixtures involved often require very high resolution analyses and, in the case of determining the authenticity of natural products, very accurate determination of enantiomeric purity. Juchelka et al. have described a method for the authenticity determination of natural products which uses a combination of enantioselective multidimensional gas chromatography with isotope ratio mass spectrometry (28). In isotope ratio mass spectrometry, combustion analysis is combined with mass spectrometry, and the 13C/12C ratio of the analyte is measured versus a C02 reference standard. A special interface, employing the necessary oxidation and reduction reaction chambers and a water separator, was used employed. For standards of 5-nonanone, menthol and (R)-y-decalactone, they were able to determine the correct 12C/13C ratios, with relatively little sample preparation. The technical details of multidimensional GC-isotope ratio MS have been described fully by Nitz et al. (29). A MDGC-IRMS separation of a natural ds-3-hexen-l-ol fraction is... 

D. Sample Preparation for Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS)... 

Both ICP-MS and LA-ICP-MS are increasingly replacing other MS techniques that have been dominant analytical methods for precise isotope ratio measurements for many decades. For all types of inorganic IRMS, however, precision and accuracy are primarily limited by sample preparation, introduction, and analytical methodology. 

The CF-IRMS sample introduction technique consists of a helium carrier gas that carries the analyte gas into the ion source of the IRMS. This technique is used to connect an IRMS to a range of automated sample preparation devices, rather than performing offline conversions, as in the case of the dual inlet technique. Two of the preparation techniques are ... 

While the dual inlet technique is generally the most precise method for stable isotope ratio measurements, the continuous-flow technique offers online sample preparation, smaller sample size requirements, faster and simpler analyses, increased cost-effectiveness, and the possibility of interfacing with other preparation techniques, including elemental analysis, gas chromatography (GC), and liquid chromatography (LC). For these reasons, CF-IRMS is the subject of the majority of research currently being conducted in the field of forensic science [1],... 

BSIA The BSIA technique is based on the use of an automated sample preparation instrument for the conversion of the sample of interest into simple gases for IRMS analysis. This instrument is called an elemental analyzer (EA), which is coupled to the IRMS (elemental analyzer-isotope ratio mass spectrometer, EA-IRMS). The isotopic values obtained from analysis utilizing EA-IRMS represent the isotopic composition of the entire sample (i.e., if the sample consists of a complex mixture, then the isotopic composition will be of the combined mixture as opposed to the isotopic composition of the individual components in the mixture). 

The combination of gas chromatography and an isotope ratio measurement mass spectrometer (ir/n-GC-MS) was used to study the detoxification and degradation pathways of tropane alkaloids involving iV-demethylation to the respective nor-compounds. Molinie et al. developed protocols for sample preparation, including liquid-hquid and SPE extraction, and analysis of the kinetic isotope effects (KlEs) with irm-GC-MS and ir/M-elemental analysis (EA)-MS [100]. Degradation experiments were performed with a Pseudomonas strain that uses tropane alkaloids as sole source of carbon and nitrogen. The authors found that liquid-liquid extraction from the aqueous medium in combination with ir/n-GC-MS was the most convenient method for measurement of the ratio. Later, Kosieradzka... 

Method B Accelerator mass spectrometry (AMS) and isotope ratio mass spectrometry (IRMS) techniques to quantify the biobased content of a given product. Sample preparation methods are identical to Method A. Stored CO2 is sent to an AMS facility for final processing and analysis. The maximum error is 1-2% for AMS and 0.1-0.5% for IRMS. 

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. 

Gas chromatography-mass spectrometry has also been demonstrated to be a powerful tool in both the qualitative and quantitative analyses of essential oil components. Literature on capillary and enantioselective capillary GC on-line coupled to isotope-ratio MS (IRMS) has been reviewed by Mosandl [1]. These techniques, which are valuable tools in control of authenticity of flavors and essential oils, are discussed in relation to various aspects, such as sample preparation and cleanup, chromatographic behavior of enantiomers, detection systems, and the use of internal isotope standards for quantitation. 

Equilibrium isotope effects usually are associated with phase transition processes such as evaporation, diffusion or dissociation reactions. When incomplete phase transition processes occur during sample preparation, they lead to severe alteration of the initial isotope ratio. In fact, and that is of highest importance for all sample preparation steps in IRMS, only complete conversion reactions are acceptable to maintain the integrity of the original isotope ratio of the sample. 

The irm system consists of three components the sample preparation device, isotope ratio mass spectrometer and the gas and electronic interface between the two. Depending on the nature of the... 

Figure 13 Comparison of isotope ratio monitoring (irm) sample preparation and introduction for bulk versus compound specific sample analysis. In bulk sample analysis, the GC follows the combustion step whereas in the oompound specific irm system CO2 and N2 from a single peak both enter the mass spectrometer at the same time. In both cases the amount of gas flowing to the ion souroe is 00m parable. The difference in sensitivity comes from the differenoe in oarrier gas flow rate (about 100 ml min- versus -1.5 ml min- ).

The development of this technique has proceeded along several independent paths with two principal lines being elemental analyzer-IRMS and capillary gas chromatography-IRMS. In elemental analyzers, samples are combusted to CO2, N2, SO2, and H2O, which are either chemically trapped or separated on GC columns. The advantages of these techniques are an automated preparation with low costs per sample and a large sample through-put. 

CE-IRMS consists of an IRMS coupled on-line to a device that produces the CO2 to be analyzed. In the case of sediment organic matter, the on-line connection is to an elemental analyzer. The coupled elemental analyzer-IRMS has greatly simplified isotopic measurements key improvements over conventional off-line preparations include significant reduction of sample size and analysis time. Some loss in precision occurs, although the reproducibility of (5 C values is commonly 0. l%c. 

Although spectroscopic methods are far less precise than IRMS for isotope ratio measurements, they can have advantages in terms of costs and ease of operation. These techniques also remove the laborious chemical preparation required for conventional IRMS. They are most commonly applied to the analysis of samples that are artificially enriched in the heavier isotopes of nitrogen, carbon, and oxygen. 

Figure 15.6 contains a diagram displaying the principal sections of a dual inlet instrument [1,28]. Once prepared, the pure sample gas is injected into the IRMS via a variable volume gas reservoir, referred to as a bellow. The reference gas is also injected into the mass spec-... 

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