Abundance measurements

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) and simultaneously for the abundances of ions at any one m/z value. Once separated by m/z value, the ions must be detected (collected) and their numbers (abundances) measured for each m/z value. The resulting chart of m/z value versus abundance constitutes a mass spectrum. 

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

Reproducibility testing demonstrated that protein abundance measured using the spectral counting method exhibited a Pearson correlation R2 value >0.99 and a coefficient of variance of 14% (Fig. 20.6). 

We present here the results of abundance measurements of iron, calcium and nickel in four open clusters, from UVES spectra of solar type stars. A code developed by one of the authors (Francois) performs line recognization, equivalent width measurements and finally obtains the abundances by means of OSMARCS LTE model atmosphere [4]. Temperature, gravity and microturbulence velocity have to be input to the program. This is made in an automatic way for a grid of values chosen on photometric basis. Those that best reproduce excitation and ionization equilibria are selected and used, namely when no significant trend of the computed abundances is seen, neither versus the excitation potential of the line nor versus its equivalent width, and for which the abundances obtained with lines of different ionization stages of the same specie give equal results within the errors. This check is made with iron lines, we have in fact at least thirty Fe I lines in each star, and six Fell lines. 

In the central region of Sculptor we have high resolution spectra providing direct abundance measurements for numerous elements (Hill et al., in prep). In Fig. 3 we show the preliminary results for the a-elements (Ca, Mg Ti) compared with similar observations of stars in our Galaxy. It is clear that the overall distribution of [a/Fe] versus [Fe/H] in Scl does not match our Galaxy, except for a small number of the most metal poor stars in Scl which overlap with Galactic halo stars. 

Martin A, Mariotti A, Balesdent J, Lavelle P, Vuattoux R (1990) Estimate of organic matter turnover rate in savanna soil by 13C natural abundance measurements. Soil Biol Biochem 22 517-523... 

The earlier stable isotope dilution mass spectrographic work was accomplished with a thermal ion mass spectrometer which had been specifically designed for isotope abundance measurements. However, Leipziger [829] demonstrated that the spark source mass spectrometer could also be used satisfactorily for this purpose. Although it did not possess the excellent precision of the thermal unit, Paulsen and coworkers [830] pointed out that it did have a number of important advantages. 

For Solar-System actinides, given a set of production ratios / , / p calculated from the theory of the r-process, one can use meteoritic abundance measurements to derive the observed quantities... 

In this textbook many exciting topics in astrophysics and cosmology are covered, from abundance measurements in astronomical sources, to light element production by cosmic rays and the effects of galactic processes on the evolution of the elements. Simple derivations for key results are provided, together with problems and helpful solution hints, enabling the student to develop an understanding of results from numerical models and real observations. 

A. O. Nier. A Mass Spectrometer for Routine Isotope Abundance Measurements. Rev. Sci. Instrum., 11(1940) 212-216. 

Natural products such as wine, fruit juices, flavors, oils, and honey are prime targets for fraudulent adulteration because of their high prices. Sophisticated analytical methods (perhaps including isotope abundance measurements) are required to detect whether natural ingredients have been mixed with ones from cheaper synthetic sources. Isotope abundance is markedly different for natural vs. synthetic molecules and these differences can be exploited to detect adulteration. Several examples follow. 

In Inductively Coupled Plasma Mass Spectrometry. Montaser A (ed) Wiley-VCH, New York, p 83-264 Montaser A, Zhang H (1998) Mass spectrometry with mixed-gas and Helium ICPS. In Inductively Coupled Plasma Mass Spectrometry. Montaser A (ed), Wiley-VCH, New York, p 809-890 Nier AO (1940) A mass spectrometer for routine isotope abundances measurement. Rev Sci Instrum 11 212-216... 

Table 1. Natural abundances, measured isotope ratios, and enrichment factors used for Calcium isotopes.

Fletcher IR, Maggi AL, Rosman KJR, McNaughton NJ (1997b) Isotopic abundance measurements of K and Ca using a wide-dispersion multi-collector mass spectrometer and low-fractionation ionisation techniques. Int J Mass Spectrom Ion Proc 163(1-2) 1-17... 

Normal gas-source mass spectrometers do not allow meaningful abundance measurements of these very rare species. However, if some demands on high abundance sensitivity, high precision, and high mass resolving power are met, John EUer and his group (e.g., Eiler and Schauble 2004 Affek and Eiler 2006 EUer 2007) have reported precise (<0. l%c) measurements of CO2 with mass 47 (A47-values) with an especially modified, but normal gas-source mass spectrometer. A47-values are defined as %o difference between the measured abundance of all molecules with mass 47 relative to the abundance of 47, expected for the stochastic distribution. 

They carry the stamp of the Big Bang, whence their great interest for cosmology. Their lithium content in particular is a precious clue as to the nucleonic density of the Universe, combined with deuterium and helium abundances measured in extremely metal-poor media (see Appendix 1). 

Abundance measurements at high redshifts complement compositional studies of stars in our own Galaxy. These new measurements are like samples taken from different cosmic epochs. As we might have expected, they show an overall trend towards metal enrichment. However, the data are as yet fragmentary and fitful (see Chapter 8). 

One cannot overemphasise the importance of accurate abundance measurements and estimates of the errors involved in them. If suitable data are unavailable, or if their quality cannot be properly assessed, it is quite impossible to draw astrophysical conclusions. In this respect, systematic observation campaigns are preferable, collecting large and uniform samples of good quality. One might seek to achieve the aims on the following hst ... 

Table 1) and the aerosol element abundances measured in the 36 Individual vane samples. The distributions of crustal element abundances in aerosols were shown in Figures 6-11 along with the respective abundances of the elements in the source materials. 

Peak Shape and Profile Scan. In a typical benchtop LC-MS, abundance measurements are collected at 0.10-m z increments, as shown in Figure 13.2. When these data are presented in a mass spectrum, a single line can be shown. The height and position are derived from the profile scan. 

---