Qualitative atomic spectroscopy

The quasi-classical theory of spectral shape is justified for sufficiently high pressures, when the rotational structure is not resolved. For isotropic Raman spectra the corresponding criterion is given by inequality (3.2). At lower pressures the well-resolved rotational components are related to the quantum number j of quantized angular momentum. At very low pressure each of the components may be considered separately and its broadening is qualitatively the same as of any other isolated line in molecular or atomic spectroscopy. 

Most analytical problems require some of the constituents of a sample to be identified (qualitative analysis) or their concentrations to be determined (quantitative analysis). Quantitative analysis assumes that the measurands, usually concentrations of the constituents of interest in a sample, are related to the quantities (signals) measured using the technique with which the sample was analysed. In atomic spectroscopy, typical measured signals are absorbance and intensity of emission. These are used to predict the quantities of interest in new unknown samples using a validated mathematical model. The term "unknown sample is used here to designate a sample to be analysed, not considered at the calibration stage. 

Chemical Analysis. The presence of silicones in a sample can be ascertained qualitatively by burning a small amount of the sample on the tip of a spatula. Silicones bum with a characteristic spaddy flame and emit a white sooty smoke on combustion. A white ashen residue is often deposited as well. If this residue dissolves and becomes volatile when heated with hydrofluoric acid, it is most likely a siliceous residue (437). Quantitative measurement of total silicon in a sample is often accomplished indirectly, by converting the species to silica or silicate, followed by determination of the heteropoly blue siUcomolybdate, which absorbs at 800 nm, using atomic spectroscopy or uv spectroscopy (438—443). Pyrolysis gc followed by mass spectroscopic detection of the pyrolysate is a particularly sensitive tool for identifying silicones (442,443). This techmque relies on the pyrolytic conversion of silicones to cyclics, predominantly to [541-05-9] which is readily detected and quantified (eq. 37). 

The viewing region of the plasma can achieve a temperature of 5000-6000°C and is reasonably stable. The sample solution is aspirated into the core area between the two arms of the Y where it is atomised, excited and viewed. This technique keeps with the atomic spectroscopy theory in that the measurements are obtained by emission from the valence electrons of the atoms that are excited, and the emitted radiation consists of short well-defined lines. All these lines fall in the UV or VIS region of the spectrum and identification of these lines permits qualitative/quantitative detection of elements. 

Atomic spectroscopy is the oldest instrumental elemental analysis principle, the origins of which go back to the work of Bunsen and Kirchhoff in the mid-19th century [1], Their work showed how the optical radiation emitted from flames is characteristic of the elements present in the flame gases or introduced into the burning flame by various means. It had also already been observed that the intensities of the element-specific features in the spectra, namely the atomic spectral lines, changed with the amount of elemental species present. Thus the basis for both qualitative and quantitative analysis with atomic emission spectrometry was discovered. These discoveries were made possible by the availability of dispersing media such as prisms, which allowed the radiation to be spectrally resolved and the line spectra of the elements to be produced. 

Atomic spectroscopy is employed for the qualitative and quantitative determination of around 70 elements primarily for the analysis of a wide range of metals (often for trace analysis). Atomic spectroscopy can provide information regarding the identity and concentration of atoms in a sample irrespective of how these atoms are combined. In contrast, molecular spectroscopy gives qualitative and quantitative information about the molecules (or particular functional groups present in molecules) in a sample. 

Among the various types of atomic spectroscopy, only two, flame emission spectroscopy and atomic absorption spectroscopy, are widely used and accepted for quantitative pharmaceutical analysis. By far the majority of literature regarding pharmaceutical atomic spectroscopy is concerned with these two methods. However, the older method of arc emission spectroscopy is still a valuable tool for the qualitative detection of trace-metal impurities. The two most recently developed methods, furnace atomic absorption spectroscopy and inductively coupled plasma (ICP) emission spectroscopy, promise to become prominent in pharmaceutical analysis. The former is the most sensitive technique available to the analyst, while the latter offers simultaneous, multielemental analysis with the high sensitivity and precision of flame atomic absorption. 

The analytical measurement of elemental concentrations is important for the analysis of the major and minor constituents of pharmaceutical products. The use of atomic spectroscopy in this regard has been the subject of several reviews (2,3,35,36). Metals are major constituents of several pharmaceuticals such as dialysis solutions, lithium carbonate tablets, antacids, and multivitamin-mineral tablets. For these substances, spectroscopic analysis is an important tool. It is indispensable for the determination of trace-metal impurities in pharmaceutical products and the qualitative and quantitative analysis of metals, essential and toxic, in biological fluids and tissues (37). Beyond this, several drugs which do... 

Traditionally, analytical atomic spectroscopy implied the use of electromagnetic radiation in the ultraviolet ( 200-350nm) and visible ( 350-800nm) region of the spectra for qualitative and quantitative analysis. With the use of some of the same sources to produce ions for detection using mass spectrometry, the term often encompasses the area of elemental mass spectroscopy. In this section the focus will remain on optical techniques. 

Atomic x-ray fluorescence spectroscopy (17,18), referred to simply as x-ray fluorescence spectroscopy (XRF), is a technique for qualitatively or quantitatively determining elemental composition by measurement of the wavelength and intensities of the electron transitions from the outer to the inner energy levels of the atom. The energy associated with these transitions (—0.6-60 keV) is significantly greater than that associated with the transitions in optical atomic spectroscopy (approximately a few electron volts). The emitted x-ray spectrum does not depend on the way in which the atomic excitation is produced, but in XRF, a beam of energetic x-rays is used. 

The use of mg instead of jU, [i.e., Eq. (4-5) instead of (4-3)] has no effect on the qualitative nature of the solutions. However, it does produce small errors in eigenvalues— errors that are significant in the very precise measurements and calculations of atomic spectroscopy (Problem 4-1). In what follows we shall use yt, but for purposes of discussion we will pretend that the nucleus and center of mass coincide. 

Atomic spectroscopy in various forms, is used for the qualitative and quantitative analysis of about 70 elements. First, we revise the principles behind the technique. 

Atomic absorption, along with atomic emission, was first used by Guystav Kirch-hoff and Robert Bunsen in 1859 and 1860, as a means for the qualitative identification of atoms. Although atomic emission continued to develop as an analytical technique, progress in atomic absorption languished for almost a century. Modern atomic absorption spectroscopy was introduced in 1955 as a result of the independent work of A. Walsh and C. T. J. Alkemade. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident. 

The focus of this section is the emission of ultraviolet and visible radiation following thermal or electrical excitation of atoms. Atomic emission spectroscopy has a long history. Qualitative applications based on the color of flames were used in the smelting of ores as early as 1550 and were more fully developed around 1830 with the observation of atomic spectra generated by flame emission and spark emission.Quantitative applications based on the atomic emission from electrical sparks were developed by Norman Lockyer (1836-1920) in the early 1870s, and quantitative applications based on flame emission were pioneered by IT. G. Lunde-gardh in 1930. Atomic emission based on emission from a plasma was introduced in 1964. 

Metal impurities can be determined qualitatively and quantitatively by atomic absorption spectroscopy and the required purification procedures can be formulated. Metal impurities in organic compounds are usually in the form of ionic salts or complexes with organic compounds and very rarely in the form of free metal. If they are present in the latter form then they can be removed by crystallising the organic compound (whereby the insoluble metal can be removed by filtration), or by distillation in which case the metal remains behind with the residue in the distilling flask. If the impurities are in the ionic or complex forms, then extraction of the organic compound in a suitable organic solvent with aqueous acidic or alkaline solutions will reduce their concentration to acceptable levels. 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

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