Mass spectrometers, isotopic analyses

Special instruments (isotope ratio mass spectrometers) are used to determine isotope ratios, when needed, to better than about 3%. Such special instruments are described in Chapters 6, 7, and 48. The methods of ionization and analysis for such precise measurements are not described here. 

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 most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

Gillson GR, Douglas DJ, Fulford JE, Halligan KW, Tanner SD (1988) Nonspectroscopic interelement interferences in inductively coupled plasma mass spectrometry. Anal Chem 60 1472-1474 Gonfiantini R, Valkiers S, Taylor PDP, De Bievre P (1997) Adsorption in gas mass spectrometry. II Effects on the measurement of isotope amount ratios. Int J Mass Spectrom Ion Proc 171 231-242 Habfast K (1997) Advanced isotope ratio mass spectrometry. I magnetic isotope ratio mass spectrometers. In Modem Isotope Ratio Mass Spectrometry. Chemical Analysis Vol. 145. Platzner IT (ed). John Wiley and Sons, Chichester UK, p 11-82... 

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... 

Most isotope ratio measurements have been performed using sector mass spectrometers. Some work has been reported, notably by Heumann [35], in which a quadrupole-based system was used. Instruments used for measurement of isotope ratios are most often dedicated to that purpose. In most instances only a relatively small mass range needs to be monitored, just enough to encompass the isotopes of the analyte element. Without the ability to scan the entire elemental mass range [usually from mlz = 6 (Li) through mfc = 238 (U) for elemental analysis], mass spectrometers designed to measure isotope ratios cannot readily be adapted for other purposes. See Chapter 2 for a discussion of instrumentation required for elemental analysis of solid materials and Chapter 3 for a treatment of the in-strumenation needed for elemental analysis of solutions. 

Isotopic analysis separates isotopes on the basis of smaU but significant differences in mass (see Chapter 31 by Lipschultz, this volume). This can be done using an emission spectrometer or a mass spectrometer. With both types of instruments, the sample must be converted to a gas prior to analysis. Emission spectrometers operate based on the principle that a N2 gas molecule comprised of a and wiU fluoresce at a different frequency then a molecular comprised of two Ns or two Ns (reviewed in Preston, 1993). Isotope ratio mass spectrometers separate and quantify the different N2 gas molecules based on their behavior when they are accelerated through a magnetic field (reviewed in Mulvaney, 1993). Emission spectrometers seem to have faUen out of favor in the quest for bigger and better instruments. They do have advantages, however. Emission spectrometers are much cheaper to purchase and maintain, they are portable, they require smaUer masses of N for analysis than many mass spectrometers, and there is no possibility for carry over from sample to sample because each sample is sealed in an ampoule prior to analysis. 

Preston, T., and Owens, N. J. P. (1983). Interfacing an automatic elemental analyser with an isotope ratio mass spectrometer The potential for fully automated total nitrogen and nitrogen-15 analysis. Analyst 108, 971-977. 

The use of carbon isotopes to study DOC is becoming more prevalent due to technological advances in mass spectrometry. DOC generally occurs in natural waters in low concentrations, typically ranging between 0.5 ppm and 10 ppm carbon (Thurman, 1985 see Chapter 5.10). Thus, several liters to tens of liters of water were once necessary to extract enough DOC for conventional dual gas-inlet isotopic analysis. Today, automated total organic carbon analyzers (TOCs) are commercially available, and have been successfully interfaced with continuous flow isotope ratio mass spectrometers (CF-IRMS) for stable isotopic measurements of samples containing ppb concentrations of DOC (e.g., St-Jean, 2003). 

Fig, 6.13 Schematic diagram of a multifunctional GC-IRMS system. The device consists of a gas chromatograph coupled to an isotope ratio mass spectrometer via a pyrolysis or a combustion interface a, b and c for oxygen, hydrogen and carbon isotope analysis, respectively, and is additionally equipped with an elemental analyser (EA). After [205] with kind permission of the American Chemical Society... 

Mass spectrometric analysis of isotope ratios is in principle very accurate (isotope ratio mass spectrometers were developed for geochemical investigations), but the analytical problem comes from possible fractionation of isotopes in the workup procedure. Traditionally, isotope ratio mass spectrometry measurements of kinetic isotope effects have been confined to reactions which give a small molecule product (such as CO2), which can be directly introduced to the mass spectrometer. 

As shown in Fig. 2, in CSIA, organic compounds are separated with gas chromatography and on-line fed to a combustion imit where they are totally converted to simple gases such as CO2 or H2 (for isotope analysis of carbon and hydrogen, respectively). The combustion gases are transferred to an isotope-ratio mass spectrometer (IRMS) that measures precisely the isotopic composition of these gases. For example, in the case of carbon, the element by far most frequently determined today, the ratio of masses 44 ( C02) and 45 ( C02) is measured. 

ZnS or CdS precipitates from vent waters and any sulfur-bearing phases from the solid samples are analyzed for 5 a using an automated elemental analyzer interfaced to an isotope ratio mass spectrometer. Laser probe microanalysis has been used to study zonation in sulfide chimney walls (Crowe and Valley 1992 Shanks et al. 1998). 5 " S error is estimated to be 0.2%o. analysis of silicates, sulfates, and whole rock samples were carried out using the BrFs method of Clayton and... 

Fessenden, J. E., Cook, C. S., Lott, M. J., and Ehleringer, J. R., Rapid 0 analysis of small water and CO2 samples using a continuous-flow isotope ratio mass spectrometer, Rapid Commun. Mass Spectrom., 16, 1257-1260, 2002. 

Recent advances in isotope analysis include the ability to analyse isotopically smaller proportions of individual compounds by the use of more sensitive online isotope ratio mass spectrometers (cf. Merritt Hayes 1994 Merritt et al. 1994) and stable carbon isotope determination of individual n-alkanes, isoprenoids and biomarkers in petroleums is now a standard tool. Despite early work on collection of chromato-graphically separated individual n-alkanes followed by combustion and isotope determination (Welte 1969) it was for a number of years impossible to analyse isotopically long chained alkanes with the same comfort and ease as the... 

Operation of a SIMS instrument resembles both that of an isotope ratio mass spectrometer and an electron microscope. Most SIMS instruments include an optical microscope so that the sample can be directly viewed during analysis, which allows for accurate positioning of the area of interest on the sample. Data can be in the standard mode used for other types of mass spectrometers in which ions are produced and the mass spectrum is analyzed by scanning or peak-hopping. This mode is sometimes called the microprobe mode in SIMS. Another application for SIMS is the acquisition of ion-images. This mode is called the microscope mode because the SIMS is operated as an ion microscope. 

Off-line preparation A method that prepares a sample for isotopic analysis that uses a vacuum system in which the gaseous forms of the elements of interest are produced, purified, and put into vials for subsequent analysis in an isotope ratio mass spectrometer. 

Carbon stable isotope ratio (CSIR) analysis of sugar utilizing an isotope ratio mass spectrometer (IRMS) is routinely used to detect the addition of cane sugar and corn syrup to most juices with the exception of pineapple. The inability of CSIR analysis to detect beet sugar in orange and apple juices because of metabolic similarities between adulterant and authentic products has required alternative methods, e.g., combination of IRMS and nuclear magnetic resonance (NMR) spectroscopy. The method requires the determination of five isotope ratios after fermentation of the juice the H/ H and iSq/ISo ratios of the fermentation water plus the total C/ C and H/ H ratios on the methyl and methylenic groups of the alcoholic distillate. 

Stable isotope ratio mass spectrometers that interface directly to a gas chromatograph are commercially available. This type of analysis is known variously as gas chromatography-isotope ratio mass spectrometry (GC-IRMS), gas chromatography-combustion isotope ratio mass spectrometry, or compound-specific isotope analysis. The same principle of operation has also been used for more general isotope ratio analyses, where the components of interest are combusted as bulk samples. This type of arrangement, known as continuous flow-isotope ratio mass spectrometry, allows rapid automated analyses of and... 

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