Smith-Hieftje system

In the previous section it has been shown that the measured sample absorbance may be higher than the true absorbance signal of the analyte to be determined. This elevated absorbance value can occur by molecular absorption or by light scattering. There are three techniques that can be used for background correction the deuterium arc the Zeeman effect and the Smith-Hieftje system. 

All instruments should be equipped with a background correction facility. Virtually all instruments now have a deuterium arc background correction. The Zeeman system is also available in instruments marketed by the Perkin-Elmer Corporation and the Smith-Hieftje system by Thermo Electron Ltd. 

In many modern atomic absorption spectrometers, this correction may be done automatically and simultaneously, time resolution of a few milliseconds being used to separate the two signals. Careful co-alignment of the two source beams is very important. To overcome this need, two other background correction systems have come into use over recent years, the Smith-Hieftje system and the Zeeman system. 

In the Smith-Hieftje system, the lamp power is subjected to short pulses of high current.17 This causes momentary bursts of high atom concentration in the hollow cathode. The emission line profile is broadened as an atom cloud forms just outside the cathode, which causes absorption at the centre of the emitted line profile, as shown in Figure 7. Effectively the single narrow emission line is split into a pair of lines immediately adjacent to the original line centre. Thus, in the normal mode, atomic and molecular absorption are measured, but in the pulsed mode, only molecular absorbance is monitored. The difference between the two signals provides a corrected atomic absorbance signal. 

Another type of background correction system that has found some use is that developed by Smith and Hieftje. The Smith-Hieftje background correction technique is of especial use when there is strong molecular interference, such as that observed by phosphate on selenium or arsenic determinations. If the hollow-cathode lamp is run at its normal operating... 

Other background correction systems include the Zeeman effect and the Smith-Hieftje background correction. A detailed description of the operational principles of these methods is beyond the scope of this chapter and the required information can be found in the relevant literature.7,13 The advantages of these methods over deuterium lamps are that high background signals (up to 2.0 units) and structured backgrounds can easily be corrected for. 

Figure 7 Schematic representation of how the Smith-Hieftje background correction system works. On the left, the source emits a simple, sharp line at low current, and both atomic and molecular absorption would be measured. On the right, this simple line has effectively been split by a pulse of high lamp current into a pair of lines at either side of the atomic absorption profile, and only molecular absorption or scatter would be detected...

Samples with high iron content may give too bw results when applying a deuterium background correction system (Van der Lee et al., 1987). In this case Smith-Hieftje background correction is necesserary. 

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