Common molecular ions table

The molecular ion overlaps due to plasma and solvent species are most severe below mass/charge 82. A high-resolution, double-focusing mass spectrometer was used to identify molecular ions observed in ICP-MS (Table 3.3) [3]. Common molecular ions that produce intense signals from plasma and solvent species include ArO+, ArOH+, ArH+, ArN+, Ar+, Ar2H+, 0+, N2+, NO+, and 0+. Other molecular ions become a problem at lower analyte concentrations. These include CO, CO H+, NOt, ArO+ ions with minor isotopes of Ar or O, ArC+, ArN+, and minor isotopes of Ar as ArJ. 

Peaks for and Br in Figure 21-15 are a characteristic isotope pattern. Informa-tion on the composition of organic compounds is obtained from the relative intensities at M + 1 and where M + 1 is one mass unit above the molecular ion. Table 21-1 tells us that C is the common isotope of carbon and has a natural abundance of 1.1%. Other common elements in organic compounds, H, O, and N, each have just one major isotope with little of the next-higher-mass isotope. Therefore the compound has a ratio of intensities of the molecular ion given by... 

Interference. ICP-MS suffers from two principal types of interference [301]. Spectroscopic interferences arise when an interfering species has the same nominal miz as the analyte of interest. The interfering species can be either an isotope of another clement (which are well documented and hence easily accounted for), or a molecular ion formed between elements in the sample matrix, plasma gas, water, and entrained atmospheric gases. The molecular ions are less easy to correct for since they will vary depending on the nature of the sample matrix. Some common molecular ion interferences are shown in Table 7. Many of these interferences can be overcome by choosing an alternative isotope of the analyte which is free from interference, though a sacrifice in sensitivity may result. If a clean" isotope is not available then one recourse is to separate the analyte from the matrix before the analysis using... 

There are many different polyatomic anions, including several that are abundant in nature. Each is a stable chemical species that maintains its stmcture in the solid state and in aqueous solution. Polyatomic anions are treated as distinct units when writing chemical formulas, naming compounds, or drawing molecular pictures. The names, formulas, and charges of the more common polyatomic anions are listed in Table 3-4. You should memorize the common polyatomic ions because they appear regularly throughout this textbook. 

Table 6.12. Commonly observed neutral losses from molecular ions...

The molecular ions of some of the alkaloids isolated more recently have been obtained and have been useful indicators of the substituents present on a common nucleus, particularly for the Buchenavia alkaloids (6). All spectra so far reported have been obtained using electron impact fragmentation. Table III lists the fragmentation patterns reported. Thus far no studies have been made on the breakdown mechanisms, but the cleavage of the flavonoid and nitrogenous rings is obviously an important process. 

Electron ionization (El) was the primary ionization source for mass analysis until the 1980s, limiting the chemist to the analysis of small molecules well below the mass range of common bioorganic compounds. This limitation motivated the development of the techniques commonly known as ESI, 1 MALDI, 2 and fast atom bombardment (FAB) 3,4 (Table 1). These ion sources allow for rapid and easy peptide analyses that previously required laborious sample preparation or were not possible with electron ionization. The mechanism of ionization these ion sources employ, which is somewhat responsible for their ability to generate stable molecular ions, is protonation and/or deprotonation. 

Table 5.1 Common spectroscopic interferences caused by molecular ions, and the resolution that would be necessary to sej the analyte and interference peaks in the mass spectrum.

Table 5.3 Some commonly occurring spectroscopic interferences caused by molecular ions derived from plasma gases, air and water and the sample matrix.

The electrical conductivity.—E. Klein10 showed that if there is a difference between the conductivity of a mixture of salts in soln. and the mean conductivities of the separate constituents, a double salt is probably formed. The molecular conductivity of a salt, and if possible of its components at different dilutions, has been employed to determine the number of component ions in a soln. it was used, for example, by A. Werner (1893-1901) with the cobalt, chromium, platinum, and other ammines.11 In moderately cone. soln. the double salts are but little ionized, and the difference between the conductivities of eq. soln. of potassium zinc chloride, ZnCl2.2KCl, and of the sum of the constituents amounts to nearly 36 per cent., a value which is greatly in excess of that whieh would be due to the mutual influence of salts with a common ion. Tables of the molecular conductivities of salts show that with very few exceptions, at a dilution of 1024 litres and 25°, most salts have conductivities approximating those indicated in Table XIX. 

Table A4.3 Some common losses from molecular ions 1437...

Whether or not a high-resolution mass spectrometer is available, molecular ion peaks often provide information about the molecular formula. Most elements do not consist of a single isotope, but contain heavier isotopes in varying amounts. These heavier isotopes give rise to small peaks at higher mass numbers than the major M+ molecular ion peak. A peak that is one mass unit heavier than the M+ peak is called the M+l peak two units heavier, the M+2 peak and so on. Table 12-4 gives the isotopic compositions of some common elements, showing how they contribute to M+l and M+2 peaks. 

Hard ionisation techniques commonly fragment molecular ions, leading to the loss of neutral species and the formation of fragmentation ions. Some common species lost in mass spectra, and possible chemical inferences that can be drawn from this information, are shown in Table 13.10. In contrast, examples of common fragment ions that are formed are listed in Table 13.11. 

High-resolution mass spectrometers measure miz ratios to four (or more) decimal places. This is valuable because except for carbon-12, whose mass is defined as 12.0000, the masses of all other nuclei are very close to—but not exactly—whole numbers. Table 13.1 lists the exact mass values of a few common nuclei. Using these values it is possible to determine the single molecular formula that gives rise to a molecular ion. 

Mass spectra of arsilidene complexes, [Cp Mn(CO)2]2AsR, 17-24 (Table 3), contain molecular ion peaks. The most common fragment ions correspond to elimination of 2CO, R and Cp Mn(CO)2. The abundances of these ions depend strongly on the R substituent at the As atom (Table 4). Substituents R such as F, I, OCS, NCS and N3 are easily eliminated directly from the molecular ion. Loss of Ph, however, occurs only after elimination of all carbonyl ligands, and R = H, R = c-Hex are not lost at all. Only compounds with R = Ph, H and c-Hex have mass spectra which display peaks corresponding to (M—2CO) and (M-4CO) ". The formation of (M — R — nCO) (n = 2,4) ions is characteristic of complexes containing R = I, OCS, NCS and N3. The complex having R = F is intermedi-... 

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