Mass Spectrometry and Isotopic Abundance

Magnetic field strength. Stronger fields deflect a given beam more than weaker fields. 

Masses of the particles. Because of their inertia, heavier particles are deflected less than lighter particles that carry the same charge. 

Charges on the particles. Particles with higher charges interact more strongly with magnetic fields and are thus deflected more than particles of equal mass with smaller charges. 

Unless otheiwise noted, all content on this page is Cengage Learning. 

The distribution of isotopic masses, although nearly constant, does vary somewhat depending on the source of the element. For example, the abundance of in atmospheric CO2 is slightly different from that in seashells. The chemical history of a sample can often be inferred from small differences in isotope ratios. See the Chemistry in Use essay on Stable Isotope Ratio Analysis. 

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Mass Number and Isotopes 4-8 Mass Spectrometry and Isotopic Abundance... 

For marble provenance studies, the most successful technique seems to be the measurement, through mass spectrometry, of the abundance ratios of the stable isotopes of carbon and oxygen (116). However, no single technique appears to provide unequivocal results, especially in cases such as the different Mediterranean sources, and a combination is often necessary to arrive at an approximate place of origin (117). 

Mass spectrometry measures the abundance of ions versus their m/z ratio, and it is common practice to use the ratio /mH//mD = kn/ku as a direct measure of the isotope effect. The typical procedure for determining isotope effects from intensity ratios... 

With permission from J.H. Beynon, Mass Spectrometry and its Application to Organic Chemistry, Amsterdam, 1960. The columns headed FM contain the formula masses based on the exact mass of the most abundant isotope of each element these masses are based on the most abundant isotope of carbon having a mass of 12.0000. Note that the table includes only C, H, N, and O. 

The introductory chapter is brief but provides an ample introduction to mass spectrometry and leaves one comfortable as he/she moves on to the historical and instrumentation chapters that follow. A few of the basic equations are given as part of the review of basic concepts. In these few pages Dr Becker clearly introduces the concepts of atomic mass units relative to carbon, isotopes and isotope abundance. Figures 1.1 and 1.2 go hand in hand in providing the reader with the three major parts of a mass spectrometer (source, ion separation, detection) and show various alternatives for each of these. The subtle use of color in these and subsequent figures adds an attractive benefit for the reader. 

Keshishian H, Addona T, Burgess M, Kuhn E, Carr SA (2007) Quantitative, multiplexed assays for low abundance proteins in plasma by targeted mass spectrometry and stable isotope dilution. Mol Cell Proteomics 6 2212-2229... 

With thermal systems either in the gas phase or in solution, it is in ter -molecular isotope effects which are more commonly studied. Intramolecular isotope effects involve distinguishing and measuring two, or more, chemically identical but isotopically different products produced in the same reaction vessel from the same reactant. The situation is different in mass spectrometry. Intramolecular isotope effects are conveniently studied, because the chemically identical products are naturally separated according to their masses. Intermolecular isotope effects on ion abundances are also easily measured, but, as regards kinetics and mechanism of reaction, their value is limited. Whereas an intramolecular isotope effect (on ion abundances) reflects kinetic isotope effects, an intermolecular isotope effect (on ion abundances) reflects kinetic isotope effects, isotope effects on the internal energy distribution, P(E), and other factors as well and the effects cannot be easily separated (vide infra). 

Mercury has a relatively even distribution of its seven stable isotopes (196, 0.15% 198,10.0% 199, 16.7% 200,23.2% 201,13.2% 202,29.8% 204,6.8% Friedlander et al., 1981 Lauretta et al., 2001). This pattern presented cosmochemists with a formidable task when mercury isotopic distributions in meteorites were examined (e.g., Jovanovic and Reed, 1976 Thakur and Goel, 1989). Analytical difficulties apparently resulted in inaccurate determinations of the bulk abundance and isotopic composition of some meteorites, leading to the so-called mercury problem examined meteorites did not show the same bulk abundance and isotopic distribution as terrestrial material (Grevesse, 1970 Lauretta et al., 1999). Subsequent advances in mass spectrometry, and especially the development of multi-collectors. 

To test for the possible oxidation of the sulfur in the methionine side-chains, the molecular masses of the multiple methionine proteins were determined by electrospray mass spectrometry and were found to agree to within 5 Da with the theoretical values expected on the basis of natural isotopic abundance (7-Met variant expected 18694.82 Da, observed 18698.75 Da 10-Met variant expected 18762.82 Da, observed 18767.54 Da). This indicates little if any oxidation of the introduced methionines. 

Isotopes can be separated by mass spectrometry and Figure 1.1a shows the isotopic distribution in naturally occurring Ru. Compare this plot (in which the most abundant... 

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