Isotope abundance and mass

Table 7.75 Isotopic Abundances and Masses of Selected Elements 7.124...

TABLE lO.I encountered Isotopic abundances and masses of elements in peptides and other biological molecules. ... 

Due to the relative uniformity of ion formation by the RF spark (although its timing is erratic), the most widely used method of quantitation in SSMS is to assume equal sensitivity for all elements and to compare the signal for an individual element with that of the total number of ions recorded on the beam monitor. By empirically calibratii the number of ions necessary to produce a certain blackness on the plate detector, one can estimate the concentration. The signal detected must be corrected for isotopic abundance and the known mass response of the ion-sensitive plate. By this procedure to accuracies within a factor of 3 of the true value can be obtained without standards. 

The mass spectral assignments for Au SG (1-9) were confirmed by comparing the spectra with that calculated using the isotopic abundance and those of Au S(h-G) clusters with the same mobility (10-14). Clusters 1-9 can be assigned to Auio(SG)io, Aui5(SG)i3, Auis(SG)i4,... 

Isotope abundance and elemental information is exhibited in cold El mass spectra, due to the enhanced molecular ion and the ionisation of isolated molecules without Cl-type reactions. Supersonic LC-MS provides a linear response, unlike LC-PB-MS. LC-SMB-MS is expected to compete with APPI, APCI and PB LC-MS modes (Scheme 7.8 and Figure 6.4). 

Table 2.5 Natural isotopic abundances and monoisotopic masses of common elements...

The thermal ion mass spectrometer was specifically developed for the measurement of isotope abundances and is capable of excellent precision. Although the spark source mass spectrometer used in this work lacks some of this precision, it has proved very useful in stable isotope dilution work. It has a number of advantages, including greater versatility, relatively uniform sensitivity, and better applicability to a wide range of elements. 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

Mass spectrometric methods are, by far, the most effective means of measuring isotope abundances. A mass spectrometer separates charged atoms and molecules on the basis of their masses and motions in magnetic and/or electrical fields. The design... 

An outstanding feature of inorganic mass spectrometry is its determination of precise and accurate isotopic abundances and isotope ratios. Isotopes of the same element (of the same number of protons or atomic number of element, Z) are, by definition, nuclides with different mass m and mass number A (A = Z + N) due to the different number of neutrons (N) in the nucleus. Isotope analyses are of special interest for characterizing the composition of samples with respect to stable and unstable isotopes in quite different concentration ranges - from the analysis of matrix elements down to the trace and ultratrace concentration level.1-9 Of 1700 isotopes, nearly 16 % (264 isotopes) are stable. The chemical elements Tc, Pm, Th, U and the transuranic elements do not possess stable isotopes. 

Thus chemists need to know an element s isotopic abundance and the mass of each isotope to calculate the average atomic mass. How do chemists determine the isotopic abundance associated with each element How do they find the mass of each isotope They use a mass spectrometer, a powerful instrument that generates a magnetic field to obtain data about the mass and abundance of atoms and molecules. You will learn more about the mass spectrometer in Tools Techniques on page 166. You can use the data obtained with a mass spectrometer to calculate the average atomic mass given in the periodic table. 

Lead occurs naturally as four isotopes. These isotopes (with their isotopic abundances and atomic masses) are 2° Pb (1.37%,... 

Q O Naturally occurring magnesium exists as a mixture of three isotopes. These isotopes (with their isotopic abundances and atomic masses) are Mg-24 (78.70%, 23.985 u), Mg-25 (10.13%, 24.985 u), and Mg-26 (11.17%, 25.983 u). Calculate the average atomic mass of magnesium. 

O O You know that silver exists as two isotopes silver-107 and silver-109. However, radioisotopes of silver, such as silver-105, silver-106, silver-108, and silver-110 to silver-117 are known. Why do you not use the abundance and mass of these isotopes when you calculate the average atomic mass of silver Suggest two reasons. 

In section 5.1, you learned how to use isotopic abundances and isotopic masses to find the average atomic mass of an element. You can use the average atomic mass, found in the periodic table, to describe the average mass of an atom in a large sample. 

Explain how chemists use a mass spectrometer to determine isotopic abundance and the masses of different isotopes. 

Table of Isotopic Abundances, Atomic Masses and Ionization Energies of Elements... 

TABLE 7-4. Natural Isotope Abundance and Exact Masses of Common Elements... 

The measurement of isotopic abundances began early this century following the discovery of neon isotopes by J.J. Thompson in 1912. F.W. Aston developed the mass spectrometer into a quantitative instrument for measuring isotopic abundances and by 1935 the isotopic composition of most elements was known. The first International Table of Stable Isotopes was drawn up in 1936, while the latest table of Isotopic Compositions of the Elements appeared recently (4). Lead is an element for which there was early evidence of natural variations in its isotopic composition (5) these were ultimately used to measure the age of the Earth (6). Natural variations have been reported in 43 other elements although many relate to exceptional samples. There are 18 elements in which variations are not uncommon, although most of these elements have relatively light atoms with atomic numbers less than 16 (4). 

Three isotopes of magnesium occur in nature. Their abundances and masses, determined by mass spectrometry, are hsted in the following table. Use this information to calculate the atomic weight of magnesium. 

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