Atom counting spectrometry

Principal characteristics of small sample liquid scintillation counting (lsc), gas proportional low-level counting (11c) and atom counting by accelerator mass spectrometry (AMS) are summarized in Table 1, and systems we have used are shown in figure 1. The most important differences (apart from cost and availability)... 

Figure 1.2 shows the basic instrumentation for atomic mass spectrometry. The component where the ions are produced and sampled from is the ion source. Unlike optical spectroscopy, the ion sampling interface is in intimate contact with the ion source because the ions must be extracted into the vacuum conditions of the mass spectrometer. The ions are separated with respect to mass by the mass analyser, usually a quadrupole, and literally counted by means of an electron multiplier detector. The ion signal for each... 

In atomic mass spectrometry, the rate of production of ions is measured directly. This is proportional to the concentration of ions, and hence atoms. A plot of ion count rate against atom concentration will therefore yield a straight line. 

The contribution of flow analysis to improving the performance of atomic spectrometry is especially interesting in the field of standardisation. FIA can provide a faster and reliable method to relate the absorbance, emission or counts (at a specific mass number) to the concentration of the elements to be determined. In fact, flow analysis presents specific advantages to solving problems related to the sometimes short dynamic concentration ranges in atomic absorption spectrometry, by means of on-line dilution. The coupling of FI techniques to atomic spectrometric detectors also offers tremendous possibilities to carry out standard additions or internal standardisation. 

Accelerator mass spectrometry (AMS) extends the capabilities of atom-counting using conventional mass spectrometry, by removing whole-mass molecular interferences without the need for a mass resolution very much better than the mass difference between the atom and its molecular isobar. This technique has been used with great success for the routine measurement of C, Be, " Al, C1 and, recently, (see Table 5.15). Analysis of " C by AMS can, for example, generate dates with a precision that is at least equal to the best conventional beta-particle-counting facility. In many cases, where small sample analysis is required, the AMS method has proved superior (Benkens, 1990). A complete description of AMS can be found in review articles (Litherland et al., 1987 Elmore and Philips, 1978) or recent conference publications. The application of AMS to measurement has been discussed in detail in Kilins et al. (1992). 

The yield for a low-mass sample, e.g., 1 mg or less for alpha-particle measurement, can be determined with nonisotopic carrier in an aliquot taken before preparing the counting source. The analytical technique can be instrumental, such as colorimetry or atomic absorption spectrometry. Subsequent source preparation, by precipitation, evaporation, or electrodeposition, must be quantitative or highly reproducible so that a reliable yield value for this final step can be included in the total yield. 

AAS, atomic absorption spectrometry (including graphite fur-liquid scintillation counting PG, polarography GC, gas chro-... 

Accelerator mass spectrometry (Fig. 21) is de-.signed for the most precise atom counting of cos-... 

Representative spectra from a lanthanide and an actinide are shown in figs. 21 and 22. The most abundant analyte peaks are from monatomic ions (M" ), and these are observed at sensitivities of 10 -10 count s per mg in solution. Ion count rates as low as 2counts s can be distinguished from the background, so the detection limits for most elements are of the order of 10-100 ng/ . At present, these powers of detection are superior to those obtainable with any other common multi-element technique. Atomic absorption spectrometry with electrothermal vaporization does provide detection limits in a similar range but is generally used only for single-element determinations. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

The information derived from 13C NMR spectroscopy is extraordinarily useful foT structure determination. Not only can we count the number of nonequivalent carbon atoms in a molecule, we can also get information about the electronic environment of each carbon and can even find how many protons each is attached to. As a result, we can answer many structural questions that go unanswered by TR spectroscopy or mass spectrometry. 

To tell someone what we mean by l mol, we could give them 12 g of carbon-12 and invite them to count the atoms (Fig. E.l). Because counting atoms directly is impractical, we use an indirect route based on the mass of one atom. The mass of a carbon-12 atom has been found by mass spectrometry to be 1.992 65 X 10 23 g. It follows that the number of atoms in exactly 12 g of carbon-12 is... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

S. D.-H. Shi, C. L. Hendrickson, and A. G. Marshall. Counting Individual Sulfur Atoms in a Protein by Ultrahighresolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Experimental Resolution of Isotopic Fine Structure in Proteins. Proc. Natl. Acad. Sci. U.S.A., 95(1998) 11532-11537. 

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