Isotopes in mass spectrometry

The isotopic distributions of several elements commonly found in organic compounds are listed in Table 14.2. 

Mass spectra can also show M-l-2 peaks due to 0 or from having two heavy isotopes in the same molecule (such as and H, or two Cs). These situations are unusual, though, so the M+2 peaks tend to be very small. The presence of a large M+2 peak is evidence of a compound containing either chlorine or bromine, because each of these elements has a high percentage of a naturally occurring isotope that is two units heavier than the most abundant isotope. 

From the natural abundance of the isotopes of chlorine and bromine in Table 14.2, we can conclude that if the M-E 2 peak is one-third the height of the molecular ion peak, then the compound contains a chlorine atom because the natural abundance of Cl is one-third that of Cl. If the M and M-E2 peaks are about the same height, then the compound contains a bromine atom because the natural abundances of Br and Br are about the same. 

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The atomic masses used in common chemical calculations are based on averages resulting from mixtures of isotopes. In mass spectrometry, the calculation is based on the mass of the predominant isotope of each element. As the isotopes are separated in the spectrometer, we always face several peaks with different masses, and with intensity ratios defined as described earlier. Thus, for example, dichloromethane has a classical molecular mass equal to 12.01 + 2 x 1.00 + 2 x 35.45 = 84.91 Da. The molecular mass in mass spectrometry is (if mass defects are neglected) 12 + 2 + 2 x 35 = 84 u. Several isotopic peaks are observed in the spectrum, the second most important being observed at m/z 86 with an intensity equal to 64.8 % of that of the m/z 84 peak. 

Isopropyl group (Section 2 13) The group (CH3)2CH— Isotactic polymer (Section 7 15) A stereoregular polymer in which the substituent at each successive chirality center is on the same side of the zigzag carbon chain Isotopic cluster (Section 13 22) In mass spectrometry a group of peaks that differ in m/z because they incorporate differ ent isotopes of their component elements lUPAC nomenclature (Section 2 11) The most widely used method of naming organic compounds It uses a set of rules proposed and periodically revised by the International Union of Pure and Applied Chemistry... 

Two other techniques that depend only on base SI units are coulometry and isotope-dilution mass spectrometry. Coulometry is discussed in Chapter 11. Isotope-dilution mass spectroscopy is beyond the scope of an introductory text, however, the list of suggested readings includes a useful reference. 

This accurate measurement of the ratio of abundances of isotopes is used for geological dating, estimation of the ages of antiquities, testing athletes for the use of banned steroids, examining fine details of chemical reaction pathways, and so on. These uses are discussed in this book under various headings concerned with isotope ratio mass spectrometry (see Chapters 7, 14, 15, 16, 17, 47, and 48). 

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

Isotopic cluster (Section 13.22) In mass spectrometry, a group of peaks that differ in mh because they incorporate different isotopes of their component elements. 

In recent years, together with enantioselective analysis, the determination of the natural abundance of stable isotopes by means of stable isotope ratio mass spectrometry (TRMS) can be very useful for the assignment of the origin of foods and food ingredients, and of authenticity evaluation (24). 

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 ratio of the analyte is measured versus a... 

The amounts of the standard isotopic species and the tracer isotopic species are represented by X and X for the sample and the reference material. The reference substance is chosen arbitrarily, but is a substance that is homogeneous, available in reasonably large amounts, and measurable using standard analytical techniques for measuring isotopes (generally mass spectrometry). For instance, a sample of ocean water known as Standard Mean Ocean Water (SMOW) is used as a reference for and 0. Calcium carbonate from the Peedee sedimentary formation in North Carolina, USA (PDB) is used for C. More information about using carbon isotopes is presented in Chapter 11. 

Raw foods were freeze-dried and analyzed for carbon isotopes using mass spectrometry. Cooked foods were prepared following historic recipes, then were freeze-dried prior to analysis. For the trace element analysis, foods (both raw and cooked) were wet ashed using nitric acid in Teflon lined pressure vessels and digested in a CEM Microwave oven. Analysis of Sr, Zn, Fe, Ca and Mg was performed using Atomic Absorption Spectrometry in the Department of Geology, University of Calgary. 

Coplen, T.B. (2011). Guidelines and recommended terms for expression of stableisotope-ratio and gas-ratio measurement results. Rapid Gommunications in Mass Spectrometry, 25, 2538-2560. DOl 10.1002/rcm.5129 Cormie, A.B., Luz, B., Schwarcz, H.P. (1994a). Relationship between the hydrogen and oxygen isotopes of deer bone and their use in the estimation of relative humidity. Geochimica et Cosmochimica Acta, 58, pp. 3439-49. 

Huelsemann F., Flenker, U., Koehler, K, Schaenzer, W. (2009). Effect of a controlled dietary change on carbon and nitrogen stable isotope ratios of human hair. Rapid Communications in Mass Spectrometry Vol 23, pp. 2448-2454. 

Walter, W.D. Leslie, Jr., D.M. (2009). Stable isotope ratio analysi to differentiate temporal diets of a free-ranging herbivore. Rapid Communications in Mass Spectrometry, Vol. 23, pp. 2190-2194. (http //dx.doi.org/10.1002/rcm.4135)... 

CANFIELD w K (1993). Study of beta-carotene metabolism in humans using 13C-beta-carotene and high precision isotope ratio mass spectrometry. Annals of the New York Academy of Sciences 691 86-95. 

Dizdaroglu, M. (1993). Quantitative determination of oxidative base damage in DNA by stable isotope-dilution mass spectrometry. FEBS Lett. 315, 1-6. 

Separation and detection methods The common methods used to separate the Cr(III)/(VI) species are solvent extraction, chromatography and coprecipitation. In case of Cr(VI) from welding fumes trapped on a filter, a suitable leaching of the Cr(VI) from the sample matrix is needed, without reducing the Cr(VI) species. The most used detection methods for chromium are graphite furnace AAS, chemiluminescence, electrochemical methods, ICP-MS, thermal ionization isotope dilution mass spectrometry and spectrophotometry (Vercoutere and Cornelis 1995)- The separation of the two species is the most delicate part of the procedure. 

Ellerbe PM, Sniegoski LT, and Welch MJ (1995) Isotope dilution mass spectrometry as a candidate definitive method for determining total glycerides and triglycerides in serum. Clin Chem 41 397-404. 

Kingston HMS, Huo D, Lu Y, and Chalk S (1998) Accuracy in spedes analysis spedated isotope dilution mass spectrometry (SIDMS) exemplified by the evaluation of chromium species. Spectrochim Acta 536 299-309. 

Kessler A, Siekmann L (1999) Measurement of urea in human serum by isotope dilution mass spectrometry. A reference procedure. Clin Chem 45 1523-1529. 

SlEKMANN L (1979) Determination of steroid hormones by the use of isotope dilution mass spectrometry a definitive method in clinical chemistry. J Steroid Biochem 11 117-123. 

Calcium exists in the human body as Ca(II) protein-bound and free Ca (II) ions (Dilana et al. 1994). For total extracellular Ca in plasma, serum and urine a definitive isotope dilution-mass spectrometry (ID-MS) method exist. Free Ca(II) in plasma/serum can be determined with PISE, but no definitive and reference methods exist. For Ca in faeces, tissue and blood flame atomic absorption (FAAS) is used widely. 

Gunther D, Heimich CA (1999) Enhanced sensitivity in laser ablation-ICP mass spectrometry using helium-argon mixtures as aerosol carrier. J Anal At Spectrom 14 1363-1368 Habfast K (1998) Fractionation correction and multiple collectors in thermal ionization isotope ratio mass spectrometry. Inti J Mass Spectrom 176 133-148... 

Richter S, Goldberg SA, Mason PB, Traina AJ, Schwieters JB (2001) Linearity tests for secondary electron multipliers used in isotope ratio mass spectrometry. Inti J Mass Spectrom 206 105-127 Rihs S, Condomines M, Sigmarsson O (2000) U, Ra, and Ba incorporation dining precipitation of hydrothermal carbonates imphcations for Ra-Ba dating of impure travertines. Geochim Cosmochim Acta 64 661-671... 

Figure 5. Histogram Th/U for clinopyroxenes in peridotites and pyroxenites from the Ronda peridotite massif Concentrations were measured by isotope dilution mass spectrometry in acid-leached clinopyroxenes. This histogram shows that pyroxenites do not have larger Th/U ratios than peridotites. Thus, the correlation found between ( °Th/ U) and Th/U cannot be explained by mixing of peridotite and pyroxenite melts as advocated in Sigmarsson et al. (1998). Data from Hauri et al. (1994) and Bourdon and Zindler (unpublished). It can be shown with a simple Student t-test that the two populations are indistinguishable.

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