Atomic double beam spectrophotometer

Strontium may be included in the diluent to be used as an internal standard with double-beam spectrophotometers. The signal from calcium is referenced to the simultaneously measured strontium signal, thereby reducing variability caused by fluctuations in the flame and atomization rate. 

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 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. 

Figure 28-18 Optical paths in a double-beam 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...

---