Flame OES

Flame OES can be performed using most modem atomic absorption spectrometers (discussed in Chapter 6). No external lamp is needed since the flame serves as both the atomization source and the excitation source. A schematic diagram of a flame emission spectrometer based on a single-beam atomic absorption spectrometer is shown in Fig. 7.2. For measurement of the alkali metals in clinical samples such as serum or urine, only a low-resolution filter photometer is needed because of the simplicity of the spectra. The filter photometer is discussed in Section 7.1.1.2. Both instmments require a burner assembly, a flame, a wavelength selection device, and a detector. 

The central component of a flame emission spectrometer is the burner assembly. The assembly has a device to nebulize the sample and then introduces the sample aerosol into the flame. In the flame, free atoms are formed and then excited, which causes them to emit radiant energy. For analytical purposes, it is essential that the emission intensity be steady over reasonable periods of time (1-2 min). The Lundegardh or premix burner is the most commonly used and is depicted in Fig. 6.8(a). 

In the premix burner, the sample, in solution form, is first aspirated into a nebulizer where it forms an aerosol or spray. An impact bead or flow spoiler is used to break the droplets from the nebulizer into even smaller droplets. Larger droplets coalesce on the sides of the spray chamber and drain away. Smaller droplets and vapor are swept into the base of the flame in the form of a cloud. An important feature of this burner is that only a small portion (about 5%) of the aspirated sample reaches the flame. The droplets that reach the flame are, however, very small and easily decomposed. This results in an efficient atomization of the sample in the flame. The high atomization efficiency leads 

Oxidation of — Formation of excited atoms atoms and emission of radiation 

The common nebulizers used in flame emission spectroscopy are the pneumatic nebulizer and the cross-flow nebulizer, just as in AAS. 

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Another type of interference that can arise in the atomiser is called ionisation interferences . Particularly when using hot atomisers, the loss of an electron from the neutral atom in metals with low ionisation energy may occur, thus reducing the free atom population (hence the sensitivity of the analyte determination, for which an atomic line is used, is reduced). These interferences can be suppressed in flames by adding a so-called ionisation suppressor to the sample solution. This consists in adding another element which provides a great excess of electrons in the flame (he. another easily ionisable element). In this way, the ionisation equilibrium is forced to the recombination of the ion with the electron to form the metal atom. Well-known examples of such buffering compounds are salts of Cs and La widely used in the determination of Na, K and Ca by FAAS or flame OES. 

Element Atomic absorption sensitivity (ppm/1% absorption) Spectrophotometric sensitivity of Th-Arsenazo III colour reaction (pg/cm2) Minimum determinable concentration (ppm in original sample) Flame OES OES emission (photoelectric) (photographic) ... 

Two wavelength selectors are used in flame OES, monochromators and filters. 

The radiation intensity measured may not represent the concentration of analyte in the sample accurately because of the presence of interferences. Interferences fall into two categories spectral and nonspectral. Three principal sources of interference are encountered in flame OES they are the same interferences that occur in EAAS. 

There are two types of spectral interferences in flame OES (1) background radiation and... 

Flame OES can be used to determine the concentrations of elements in samples. The sample usually must be in solution form. Generally, one element is determined at a time if using an AAS system in emission mode. Multichannel instruments are available for the simultaneous determination of two or more elements. Detection limits can be very low as seen in Appendix 7.1, Table Al. Detection limits for the alkali metals are in the ppt concentration range when ionization suppression is used. One part per trillion in an aqueous solution is 1 pg of analyte per mL of solution or 1 x g/mL. Most elements have detection limits in the high ppb to low ppm range. 

An important factor in quantitative analysis by flame OES is the solvent used for the samples and standards. When water is the solvent, the process of atomization is endothermic and relatively slow. If the solvent is organic, the reactions in the flame are exothermic and atomization is rapid. Other things being equal, more free atoms are liberated and... 

Table 7.5 Blank Corrected Calibration Curve for Li by Flame OES...

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