Flame lines, analytical importance

The general construction of an atomic absorption spectrometer, which need not be at all complicated, is shown schematically in Fig. 1. The most important components are the light source (A), which emits the characteristic narrow-line spectrum of the element of interest an absorption cell or atom reservoir in which the atoms of the sample to be analysed are formed by thermal molecular dissociation, most commonly by a flame (B) a monochromator (C) for the spectral dispersion of the light into its component wavelengths with an exit slit of variable width to permit selection and isolation of the analytical wavelength a photomultiplier detector (D) whose function it is to convert photons of light into an electrical signal which may be amplified (E) and eventually displayed to the operator on the instruments readout, (F). 

When major components of plating solutions are determined, large dilutions may be required (e.g. a factor of 5000) to bring the sample into the normal working range for flame analysis. Such dilutions will, however, minimise any interference effects and viscosity effects from additives, and are thus to be preferred to the use of less sensitive lines or burner rotation. The above interference effects may be important in the determination of trace metal levels and attempts should be made to match the standards for major component levels, or to use the method of standard additions. Solvent extraction to remove the analyte from the matrix may be necessary. 

Because of the wide analytical range already accessible with second harmonic generation, many elements routinely determined by conventional AAS in analytical flames or furnaces can also be determined by AAS with diode lasers. The availa-blility of laser diodes with lower wavelengths will only make the approach cheaper, as then second harmonic generation will become superfluous. The elements now accessible with X > 630 nm with resonance lines are already manifold Li, Na, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Cr, Ba, La, Hf, Ta, W, Re, Ir, Pt, Tl, Pb, Nd, Sm, Eu, Gd, Ho, Tm, Yb and Lu. Also U and some of the actinides can be determined. Important elements such as Be, Mg, As and Hg with diodes emitting in the blue region will eventually become accessible. 

Spectral interferences of analyte lines with other atomic spectral lines are of minor importance as compared with atomic emission work. Indeed, it is unlikely that resonance lines emitted by the hollow cathode lamp coincide with an absorption line of another element present in the atom reservoir. However, it may be that several emission lines of the hollow cathode are within the spectral bandwidth or that flame emission of bands or a continuum occur. Both contribute to the non-absorbed radiation, by which the linear dynamic range decreases. Also, the nonelement specific absorption (see Section 4.6) is a spectral interference. 

In on-line precipitation-dissolution systems with flame AAS detection, the volume of solvent used to dissolve the collected precipitate should be as small as possible in order to increase the concentration of the analyte in the eluent. On the other hand, the flow-rate of the dissolution solvent should be reasonably close to the free uptake rate of the nebulizer, so as not to starve the nebulizer excessively. Failing to do so will result in a decrease in sensitivity and possibly deterioration in precision due to the formation of air bubbles in the solvent. Strong solvents have to be used in order to satisfy these requirements. However, the form of the precipitate is also important for achieving fast dissolution. Gelatinous and curdy precipitates have larger surface areas than crystalline precipitates, and thus are more readily dissolved using the same solvent. 

Atomic absorption spectrometry (AAS) is nowadays one of the most important instrumental techniques for quantitative analysis of metals (and some few metalloids) in various types of samples and matrices. The history of atomic absorption spectrometry dates back to the discovery of dark lines in the continuous emission spectrum of the sun by WoUaston in 1802. The lines are caused by the absorption of the elements in the atmosphere of the sun. His work was taken up and further pursued by Fraunhofer in 1814. In 1860, Kirchhoff and Bunsen demonstrated that the yellow hne emitted by sodium salts when introduced into a flame is identical with the so-caUed D-Hne in the emission spectrum of the sun. However, it took nearly one century before this important discovery was transferred into a viable analytical technique. In 1955, Alan Walsh published the first paper on atomic absorption spectroscopy [4]. At the same time, and independently of Walsh, AUce-made and Wilatz pubhshed the results of their fundamental AAS experiments [5, 6]. But it was the vision of Walsh and his indefatigable efforts that eventually led to the general acceptance and commercialisation of AAS instrumentation in the mid-1960s. Further instrumental achievements, such as the introduction of the graphite furnace and the hydride generation technique, in the second half of the 1960s further promoted the popularity and applicability of the technique. 

The half width of elemental lines is of the order of 0.002 nm when observed by emission spectroscopy with flame or electrothermal atomisation. A number of reasons can cause broadening of the linewidth, of which the most important and best understood are natural, pressure, resonance, and Doppler broadening. If a stable and sensitive detection is to be achieved, the linewidth of the excitation radiation must be narrower than the full width at half maximum (FWHM) of the analyte line. Under these conditions, the entire radiant energy produced by the excitation source will be available for absorption by the analyte. The typical line sources used for atomic absorption are element specific excitation sources such as the hollow cathode lamp or the electrodeless discharge lamp. But even continuum sources can be used with appropriate instrumental designs. 

To realize these important advantages, it is necessary that the output of the source be free of contaminating lines from other elements in addition, the atomizer should emit no significant background radiation. In some instances with electrothermal atomizers, background radiation is minimal, but certainly, it is not with typical flames. To overcome this problem, filters, located between the source and detector, have often been used to remove most of the background radiation. Alternatively, solar-blind photomultipliers, which respond only to radiation of wavelengths shorter than 320 nm. have been applied. For these devices to be used effectively, analyte emission must be below 320 nm. , ... 

A AS and AFS. When part of the light coming from the lamp is scattered by small particles in the atomiser e.g. droplets or refractory solid particles) or absorbed non-specifically e.g. by undissociated molecules existing in the flame), important analytical errors will be derived if no adequate corrections are made. Scattered and dispersed radiation decrease the lamp intensity and create false analytical signals. Fortunately, both sources of false signals can be easily distinguished from the specific analyte signals which do occur at the analytical line only (and not outside it in the spectrum), and this basic differential feature can be used for correction. 

The strongest lines of potassium are the doublet 7660/7690 A which is almost always used for analytical purposes, although the blue line at 4044 A can be useful also. Self-absorption is of little importance but ionisation is quite a serious problem if a hot flame is used. 

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