Relative abundances, ions

Figure 7. Absolute abundances of ethanol contracted spectrum reduced so that no ions are greater than the lowest relative abundance ion in contracted spectrum of mixture. Contaminated ions are identified as those ions of the mixture that exceed the relative intensities of the attenuated ethanol pattern (plus the prescribed window tolerance).

J = 1 kg m2 s-2 Appendix II see Table 6-2). ATP is an extremely useful cellular energy currency because of its large free energy release upon hydrolysis — about 40 to 50 kJ mol-1 (10-12 kcal mol-1) —and because of its convenient form as a relatively abundant ion. 

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

Compounds that contain chlorine, bromine, sulfur, or silicon are usually apparent from prominent peaks at masses 2, 4, 6, and so on, units larger than the nominal mass of the parent or fragment ion. Eor example, when one chlorine atom is present, the P + 2 mass peak will be about one-third the intensity of the parent peak. When one bromine atom is present, the P + 2 mass peak will be about the same intensity as the parent peak. The abundance of heavy isotopes is treated in terms of the binomial expansion (a -I- h) , where a is the relative abundance of the light isotope, b is the relative abundance of the heavy isotope, and m is the number of atoms of the particular element present in the molecule. If two bromine atoms are present, the binomial expansion is... 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

The positive-ion electron-ionization spectra of BFB and DFTPP must exhibit molecular and specified fragment ions, the relative abundances of which must fall within a predefined range. Ion abundance criteria for BFB and DFTPP are shown in Table 41.1. 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear... 

Values quoted are ion (m/2) followed by relative abundance in parentheses. After m/2 109 and 80, the other ions may vary in abundance order dependent upon conditions employed (10). 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

Only the knowledge of relative useful ion yields and isotopic abundances is required to calculate elemental composition from the relative ion current measurements. The useful ion yield is the number of ions x detected relative to the number of atoms of element xsputtered. The measured relative ion current of two isotopes is... 

The structures shown for ions, such as in Equations 7 and 8, are entirely speculative since the mass spectrum gives only relative abundance of the various m/e-groups formed. Chemical reasoning, meta-stable-ion peaks, peak shifts in the deuterated analogs, and published mass spectra of similar compounds are the bases for the proposed modes of fragmentation and for the postulated structures.)... 

The mass spectrum of a compound is typically presented as a bar graph with masses (m/z values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base peak, and the peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion (M+). Figure 12.2 shows the mass spectrum of propane. 

Look for characteristic isotopic abundances that show the presence of bromine, chlorine, sulfur, silicon, and so on. If the deduced molecular ion is of sufficient intensity, the probable molecular formula may be determined using the observed isotopic abundances of the molecular ion region. Set the deduced molecular ion, M, at 100% abundance, and then calculate the relative abundances of M + 1 and M + 2 either manually or using the data system. 

Molecular ion Molecular ions of cyclic ketones are relatively intense. Characteristic fragment ions of cyclic ketones occur ai m/z 28, 29, 41, and 55. Cyclic ketones also lose CO and/or C>H (m/z 28) from the molecular ion (Q and higher). Low-abundance ions corresponding to loss of H20 are frequently observed. Keto-steroids are a special class of cyclic ketones and have abundani molecular ions. 

The o-nitrotoluene isomer is easy to identify because of the loss of OH from the molecular ion. All the nitrotoluenes lose 30 Daltons from their molecular ions. The m- and p-isomers can be distinguished from each other by the relative abundances of the m/z 65 ion versus the molecular ion, particularly if both isomers are present (see Figure 22.1). 

Fragmentation The esters of cinnamic acid follow the fragmentation of benzoates except there is a relatively abundant M - 1 peak. Prominent ions arc as follows ... 

Increasing the sample size or decreasing the repeller voltage may increase the relative abundance of the M + 1 ion. If the sample pressure is very high, dimers also may be produced. 

Efremov and coworkers investigated the mass spectra of 18 methyl-substituted diphenyl (63)40 and substituted phenyl mesityl sulfones (64)41. The mass spectra of practically all the compounds showed by the rearrangement ions [M — OH]+, [M —H20]+ and [M — (H20 + OH)]+, the relative abundances of which depend on the position of the substituent in the phenyl moiety (ortho effect). It was also evident that in 63 the introduction of the first methyl substituent clearly decreases the contribution of the sulfone sulfinate isomerization (equation 30) to their fragmentation whereas the further substitution had little or no effect on the isomerization process in both 6340 and 6441. 

In Table II the state of the ion and the recombination energies in electron volts (computed from (65)) are given. Some very uncertain information is included in the right hand column as to the relative abundances of the metastable states of the ions when produced by electron impact with 100-e.v. electrons from the indicated compounds. 

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