Double beam atomic absorption

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. 

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 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. 

Double-beam atomic absorption instrumentation a = rotating half-silvered mirror b = front surface mirror. 

Figure 2.2 Diagram of a double beam atomic absorption spectrometer.

Figure 11.1 (a) Optics Perkin-Elmer Model 2280 single beam atomic absorption spectrometer (b) Optics Perkin-Elmer Model 2380 double beam atomic absorption spectrometer... 

How does a single-beam atomic absorption instrument differ from a double-beam instrument What advantages does one offer over the other ... 

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. 

A large number of commercial instruments are now available and are based either on a single- or double-beam design. Important instrumental features of a modem atomic absorption instrument include the following facilities. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

Figure 1.6 Configurations of instruments for atomic absorption spectrometry, (a) Single-beam spectrometer (b) double-beam spectrometer.

An atomic absorption instrument contains the same basic components as an instrument designed for molecular absorption measurements, as shown in Figure 28-16 for a single-beam system. Both single- and double-beam instruments are offered by numerous manufacturers. The range of sophistication and the cost (upward from a few thousand dollars) are both substantial. 

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