Flame Atomic Absorption Spectrometers

At present, however, the usual flame emission method is obtained by simply operating a flame atomic absorption spectrometer in the emission mode (see Fig. 21.3). 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

A major breakthrough came in Australia when Alan Walsh1,2 realized that light sources were available for many elements which emitted atomic spectral lines at the same wavelengths as those at which absorption occurred. By selecting appropriate sources, the emission line widths could be even narrower than the absorption line widths (Figure 2). Thus the sensitivity problem was solved more or less at a stroke, and the modern flame atomic absorption spectrometer was bom. 

Figure 3.7. Continuous USASD of milk powder and infant formula prior to the FAAS determination of iron and zinc. C — coil, D — digestant, DC — digestion chamber, FAAS — flame atomic absorption spectrometer, IV— injection valve, PP—peristaltic pump, S / — switching valve, UB — ultrasonic bath and W — waste. (Reproduced with permission of Elsevier, Ref [11].)...

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

Spectral interferences are uncommon in AAS owing to the selectivity of the technique. However, some interferences may occur, e.g. the resonance line of Cu occurs at 324.754 nm and has a line coincidence from Eu at 324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely to cause any problems for Cu determination. In addition to atomic spectral overlap, molecular band absorption can cause problems, e.g. calcium hydroxide has an absorption band on the Ba wavelength of 553.55 nm while Pb at 217.0nm has molecular absorption from NaCl. Molecular band absorption can be corrected for using background correction techniques (see p. 174). The operation of a flame atomic absorption spectrometer is described in Box 27.6. 

Chemistry, and in particular physical and analytical chemistry, often requires a numerical or statistical approach. Not only is mathematical modelling an important aid to understanding, but computations are often needed to turn raw data into meaningful information or to compare them with other data sets. Moreover, calculations are part of laboratory routine, perhaps required for making up solutions of known concentration (see p. 170 and below) or for the calibration of an analytical instrument (see p. 171). In research, trial calculations can reveal what input data are required and where errors in their measurement might be amplified in the final result, e.g. flame atomic absorption spectrometer (see Chapter 27). Table 39.7 Sets of numbers and operations ... 

The liquid sample introduction system most commonly used on an ICP-MS is very similar to that used on a flame Atomic Absorption Spectrometer or an ICP-OES. Liquid samples can be directly injected using a pneumatic nebulizer and a spray chamber. 

Why are monochromators of a higher resolution found in ICP atomic emission spectrometers than in flame atomic absorption spectrometers ... 

Instruments/ equipment ICR-8000 flame atomic absorption spectrometer (Aurora Biomed Inc., Vancouver, BC, Canada), Poly-D-lysine-coated 96-well assay plates (Becton Dickinson, Billerica, MA). 

Fig. 77. Flame atomic absorption spectrometer. A. Single beam and B dual-beam system, (a) hollow cathode lamp (b) flame (c) monochromator (d) rotating mirror ...

Sample digestion and flame atomic absorption spectrometer analysis of samples... 

Atomization of the sample is usually facilitated by the same flame aspiration technique that is used in flame emission spectrometry, and thus most flame atomic absorption spectrometers also have the capability to perform emission analysis. The previous discussion of flame chemistry with regard to emission spectroscopy applies to absorption spectroscopy as well. Flames present problems for the analysis of several elements due to the formation of refractory oxides within the flame, which lead to nonlinearity and low limits of detection. Such problems occur in the determination of calcium, aluminum, vanadium, molybdenum, and others. A high-temperature acetylene/nitrous oxide flame is useful in atomizing these elements. A few elements, such as phosphorous, boron, uranium, and zirconium, are quite refractory even at high temperatures and are best determined by nonflame techniques (Table 2). 

Figure 2.45 Schematic diagram of a single-beam flame atomic absorption spectrometer (While the components of an atomic fluorescence spectrometer are similar, the emitted radiation is monitored perpendicular to the incident radiation).

Warm up the flame photometer or flame atomic absorption spectrometer in emission mode, following manufacturer s directions. Using an air-acetylene burner, set the flow rates of air acetylene to (a) oxidizing flame, (b) stoichiometric flame, and (c) reducing flame, following manufacturer s directions. 

The flame atomic absorption spectrometer is inherently a flow-through detector, with which the sample solutions are continuously fed into the nebulizer-burner system through suction. Despite the relatively large volume of the spray chamber (usually about 1(X) ml) in comparison to the spectrometric flow-cell, the detector was shown to have very little contribution to the dispersion of the injected sample in comparison to other components of the FI system [11]. With careful optimization, as little as 50-80 //I sample may be injected to achieve 80-95% of the steady state signal obtained by conventional sample introduction (see Fig. 2.14). 

FigJ.14 Schemai.% diagram of a FI manifold for liquid-liquid extraction flame atomic absorption spectrometry. P, pump RG. reagent S, aqueous sample SG, phase segmentor, D, displacement bottle E, extraction coil PS, membrane phase separator R, restrictor or impedance coil W, waste V, injector valve AAS, flame atomic absorption spectrometer fl4]. 

Instead of flames, atomic absorption spectrometers sometimes employ graphite furnaces or (relatively speaking) cold quartz tubes as atomisers these devices are not normally required for the purpose of qualitative analysis (unless the volume of sample available is very small), and will not be discussed here. 

FIA-FAAS. Figure 94 shows the principle of the FIA-flame atomic absorption spectrometer. The carrier stream (usually, deionized water or... 

ESMS electrospray mass spectrometry FAAS flame atomic absorption spectrometer... 

Figure 30. Flame atomic absorption spectrometer A) Single beam B) Dual-beam system...

In Section 3.1B, we describe Ihe methods and instrumentation for flow injection analysis (FIA). FIA methodology serves as an excellent means of introducing samples into a flame atomic absorption spectrometer. Alternatively, wc may think of an atomic absorption spectrometer as a useful detector for an FIA system. From any perspective, the peristaltic pump and valve arrangements of FIA described in Chapter. 3.3 are a convenient means to sample analyte solutions reproducibly and efficiently, especially when it is important to conserve sample. The carrier stream of the FIA system consisting of deionized water or dilute electrolyte provides continuous flushing of the flame... 

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