Beam Atomic Absorption Spectrophotometer

It is pertinent to mention here that a double-beam atomic absorption spectrophotometer is absolutely independent of (a) lamp drift, (b) sensitivity of detector with time. 

The light hollow-cathode-lamp source (A) passes through the slit S, and strikes at mirrors M, and M2. The Mirrors M3 splits chopped beam from the source into two parts one passes through the mirror M4-slit 

52- flame (B)-mirror M8 and strikes at mirror M9 to reach mirror M10, and the second strikes at mirror Mg-slit 

53- mirror M , M8 and M9 respectively to reach the mirror M10. The mirror M8 and M9 serve as a beam recombination zone (BR). The recombined beam gets reflected by mirrors M10 passes through the field lens (C), slit S4, strikes at Mn, passes through the grating (D), to the mirror M12 and ultimately passes out through the exit (S5) and the monochromator assembly (MA) into the detector (E) and finally to the read-out device 

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A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

The samples were analysed by injecting 25 pi aliquots into an HGA 2000 Perkin-Elmer graphite furnace attached to a Jarrell-Ash 82-800 double beam atomic absorption spectrophotometer. Graphite tubes in the furnace were replaced after 75-100 analyses. Metal concentrations were determined by comparing the peak heights of the samples to the standard curve established by the determination of at least five known standards. The detection Emits of this technique for 1% absorption were 0.9 pmol/1 (Fe), and 0.2 pmol/1 (Mn). The coefficient of variation was 11% at 6.5 pmol/1 for iron and +12% at 11.8 pmol/1 for manganese. 

FIGURE 9.14 An illustration of the light path for a single-beam atomic absorption spectrophotometer with a... 

Figure 26.1 Schematic Diagram of a Single-Beam Atomic Absorption Spectrophotometer.

The optical path of a double-beam atomic absorption spectrophotometer is depicted in Figure 26.2. The various essential components comprising the optical arrangement in Figure 26.2 are enumerated after the figure. 

Figure 26.2 Optical path of a Double-beam Atomic Absorption Spectrophotometer.

Double-beam atomic absorption spectrophotometers are designed to control variations which may occur in the radiation source but they are not as effective as double-beam molecular absorption instruments in reducing variation because there is no blank sample in flame techniques. 

Figure 14.4—The diverse components of a single beam atomic absorption spectrophotometer. Model IL 157, built in the 1980s. 1, source 2, burner 3, monochromator 4, detector (design according to Thermo Jarrell Ash Corp.).

Figure 28-18 Optical paths in a double-beam atomic absorption spectrophotometer.

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figures 10.25 and 10.26). There are, however, several important differences that are considered in this section. 

The optical scheme of an atomic absorption spectrophotometer is illustrated in Fig. 14.4, which shows a basic single beam instrument. 

The apparatus is set up as shown in Figure 1-3, p. 9, with the cell placed in the beam path of the atomic absorption spectrophotometer. 

Detection limits for the double beam Model 303 Atomic Absorption Spectrophotometer are for elements in water solution. Organic solvents, such as MIBK, improve detection limits by factors of 2 to 4. 

Most commercially available atomic absorption spectrophotometers have damping built in. Single beam instruments can usefully employ damping time constants up to about 20 seconds. Double beam instruments, with their greater freedom from drift, can benefit from damping time constants as high as two minutes. 

Fig. 28. Design of an atomic absorption spectrophotometer (double-beam alternating light device) 1) = Source of radiation 2) = Flame 3) = Monochromator 4) = Detector 5) = Amplifier 6) = Indicating instrument...

The formed free atoms absorb the light at a characteristic wavelength from a hollow cathode lamp that is positioned on one side of the flame. A spectrophotometer with a grating monochromator located on the other side of the flame measures the intensity of the light beam. Because absorption is proportional to the number of free atoms that are produced in the flame, the light energy absorbed by the flame is a measure of the element s concentration. The FLAA technique is relatively free of interelement spectral interferences, but it has the sensitivity that is inferior to ICP-AES or GFAA. 

Flameless atomic absorption spectrophotometry is essentially very simple. A substrate upon which the sample matrix can be deposited is placed in or immediately adjacent to the spectrophotometer light beam, and a means of heating this substrate rapidly to 800°-3500°C is provided. Electrical resistance is usually the heating method used. The substrate... 

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