Atomic Mass of Selected Elements

Elemental fluorine, which is a member of the halogen family, is a pale yellow-green, irritating gas with a sharp odour and atomic mass of 18.998. Fluorine is chemically most reactive of all the elements and does not therefore occur naturally in the free state. In combination it comprises 0.065% of the earth s crust, being the 13th element in abundance [1], and is an inevitable part of the biosphere and human life. The term fluorine is, in this report, used to denote the element in any of its forms and fluoride to denote free inorganic fluoride to which a fluoride ion-selective electrode (ISE) responds. 

The 1981 report on atomic weights includes a complete review of the natural isotopic composition of the elements and also tabulates the relative atomic masses for selected radioisotopes. This information is required for the conversion of spectroscopic data from that corresponding to specific isotopes to the naturally occurring isotopic abundance. 

The way in which Avogadro s law was applied by Cannizzaro in 1858 for the selection of the correct approximate atomic masses of elements was essentially the following. Let us accept as the molecular mass of a substance the mass in grams of 22.4 liters of the gaseous substance reduced to standard conditions. (Any other volume could be used —this would correspond to the selection of a different base for the atomic mass scale.) Then it is probable that, of a large number of compounds of a particular element, at least one compound will have only one atom of the element per molecule the mass of the element in the standard gas volume of this compound is its atomic mass. 

Figure 22-11 Variation in nuclear binding energy per nucleon with atomic mass.This plot shows the relative stability of the most stable isotopes of selected elements. The most stable nucleus is lFe, with a binding energy of 8.80 MeV per nucleon...

Table 4.8 lists the naturally occurring isotopes of selected elements and their atomic mass, along with their most prevalent isotopes. 

The upper part of the figure illustrates why the small difference in mass between an ion and its neutral molecule is ignored for the purposes of mass spectrometry. In mass measurement, has been assigned arbitrarily to have a mass of 12.00000, All other atomic masses are referred to this standard. In the lower part of the figure, there is a small selection of elements with their naturally occurring isotopes and their natural abundances. At one extreme, xenon has nine naturally occurring isotopes, whereas, at the other, some elements such as fluorine have only one. 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

Since all of technetium s isotopes are produced artificially, the element s atomic weight (atomic mass units) is determined by which isotopes are selected for the calculation. 

Imaging atom-probes, although severely limited in mass resolution, are very useful where one seeks information about the spatial distribution of chemical species on the emitter surface as well as in the bulk. They find many applications in studies of metallurgical problems,55 in studies of chemisorptions and surface reactions56 and oxidation of metals,57 etc., as will be discussed in later chapters. In using imaging atom-probes, it is important to select the system carefully and intelligently so that mass overlap of different elements can be avoided. 

Furthermore, isotope analysis is relevant for determining the atomic weight (Ar(E)) of elements. The Ar(E) is the average of all masses of all naturally occurring stable isotopes (taking into account the abundances of isotopes) of a chemical element (see Appendix I10). By consideration of the masses of isotopes (/ ,) and the known relative abundances of all stable isotopes (Xi) with i = 1 to n of a selected chemical element, the average atomic weight (Ar(E)) of this element can be calculated ... 

Eluate from a chromatography column can be passed through a plasma to atomize and ionize its components and measure selected elements by atomic emission spectroscopy or mass spectrometry. An atomic emission detector directs eluate through a helium plasma in a microwave cavity. Every element of the periodic table produces characteristic emission that can be detected by a photodiode array polychromator (Figure 20-14). Sensitivity for sulfur can be 10 times better than the sensitivity of a flame photometric detector. 

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