Analytically useful spectral lines

Any difference in the behaviour of the analyte atoms in the sample and in the standard implies an interference. AAS using a line source for excitation suffers little spectral interference. Background interference in AAS is more important. This nonspecific absorption is caused by ... 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

The set of energy levels associated with a particular substance is a unique characteristic of that substance and determines the frequencies at which electromagnetic radiation can be absorbed or emitted. Qualitative information regarding the composition and structure of a sample is obtained through a study of the positions and relative intensities of spectral lines or bands. Quantitative analysis is possible because of the direct proportionality between the intensity of a particular line or band and the number of atoms or molecules undergoing the transition. The various spectrometric techniques commonly used for analytical purposes and the type of information they provide are given in Table 7.1. 

Segal, I., Kloner, A., and Brenner, I. B. (1994). Multielement analysis of archaeological bronze objects using inductively coupled plasma-atomic emission spectrometry -aspects of sample preparation and spectral-line selection. Journal of Analytical Atomic Spectrometry 9 737-744. 

As mentioned in item 5 of Section 9.1, the light sources used in atomic absorption instruments are sources that emit spectral lines. Specifically, the spectral lines used are the lines in the line spectrum of the analyte being measured. These lines are preferred because they represent the precise wavelengths that are needed for the absorption in the flame, since the flame contains this analyte. Spectral line sources emit these wavelengths because they themselves contain the analyte to be measured, and when the lamp is on, these internal atoms are raised to the excited state and emit their line spectrum when they return... 

Also known as specific susceptibility. mas sa.sep-ta bil-ad-e mass-to-charge ratio analychem In analysis by mass spectroscopy, the measurement of the sample mass as a ratio to its ionic charge. mas to charj. ra sho ) matrix analy chem The analyte as considered in terms of its being an assemblage of constituents, each with its own properties. ma-triks matrix effects analy chem 1. The enhancement or suppression of minor element spectral lines from metallic oxides during emission spectroscopy by the matrix element (such as graphite) used to hold the sample. 2. The combined effect exerted by the various constituents of the matrix on the measurements of the analysis. ma triks i.feks )... 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Spectral interferences. These interferences result from the inability of an instrument to separate a spectral line emitted by a specific analyte from light emitted by other neutral atoms or ions. These interferences are particularly serious in ICP-OES where atomic spectra are complex because of the high temperatures of the ICP. Complex spectra are most troublesome when produced by the major constituents of a sample. This is because spectral lines from other analytes tend to be overlapped by lines from the major elements. Examples of elements that produce complex line spectra are Fe, Ti, Mn, U, the lanthanides and noble metals. To some extent, spectral complexity can be overcome by the use of high-resolution spectrometers. However, in some cases the only choice is to select alternative spectral lines from the analyte or use correction procedures. 

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

As indicated previously, NMR may be used simply as an analytical technique for monitoring the decomposition of a reactant or formation of a product. In addition, NMR and ESR merit a special mention due to their importance in studying the dynamics of systems at equilibrium these so-called equilibrium methods do not alter the dynamic equilibrium of the chemical process under study. They have been used to study, for example, -transfer reactions, valence isomerisations, conformational interconversions, heteronuclear isotopic exchange processes (NMR) and electron-transfer reactions (ESR). These techniques can be applied to the study of fast or very fast reactions by analysis of spectral line broadening [16,39],... 

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

To reduce the detrimental effects of spectral interferences on element quantitation, laboratories select the spectral lines that are least affected by the background, and use the background compensation and interelement correction routines as part of the analytical procedure. The instrument software uses equations to compensate for overlapping spectral lines the effectiveness of these equations in eliminating spectral interferences must be confirmed at the time of sample analysis. That is why laboratories analyze a daily interelement correction standard (a mixture of all elements at a concentration of 100mg/l) to verify that the overlapping lines do not cause the detection of elements at concentrations above the MDLs. 

The complication in analysing the spectrum is that the energy levels, and hence the spectral lines, can no longer be fitted by a power series in the quantum numbers that converges sufficiently rapidly to form a simple pattern of energy levels and spectral lines, so that simple analytical formulae cannot be used, and assignment becomes a problem. Usually the spectrum can only be analysed in conjunction with an appropriate theoretical treatment of the hamiltonian for the interacting levels. 

The problem is to prepare solutions suitable for the desired analytical results. Once a homogeneous analytical solution is available, the differences between the techniques used with dissolved metals are no longer great since the same nebuliser systems can be employed. For the determination of major components it is preferable to use less sensitive spectral lines of the same elements rather than use greater dilutions. 

The function of the spectrometer is to accept as much light from the source as possible and to isolate the required spectral lines. This may be impossible where there is a continuous spectrum in the same region as the analytical line for example, the magnesium line of 286.2 nm coincides with a hydroxyl band. In direct reading instruments, electronic devices may be used to supplement the resolution of the spectrometer by modulating the intensity of the analytical signal. In absorption and fluorescence the light source is modulated in emission the spectral line is scanned (816) or the sample flow modulated (M23). 

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