Atomic Absorption Spectrophotometry

Abstract

Historically flame emission spectroscopy was developed first. Routine analysis showed the advantage of measuring absorption over emission for many metals. Instrumentation requirements are:—

1. Pressure regulators and flowmeters with the sources of gases;

2. Atomizer;

3. Burner;

4. Optical system;

5. Detector.

In addition, an energy source, e.g., a hollow cathode lamp, is needed.

Essentially the technique involved is the same as absorbance in a solution but taken into flame. The ground state atoms are expected to pick up the correct energy. A drop in intensity is measured.

Acetylene/air and acetylene/nitrous oxide are widely used now as flames. A complex theory of flames is discussed and conceptual difficulties are mentioned. Interference effects, e.g., compound formation and ionization are cited. Parameters affecting the signal are explored mathematically.

Sensitivities and detection limits are compared in respect of flame emission and atomic absorption for the elements of interest. Ways of improving performance are presented. Awareness of contamination problems is important.

For forensic applications and problems, absolute detection limits are lower with atomic absorption than in photometry for the majority of the listed elements. Among other advantages are specificity, several elements can be determined in one sample and one can operate on smaller samples than in some chemical methods, many elements can be surveyed with the one instrument, and there is freedom from time elapsing during colour development and drying of precipitates. All this is at relatively low cost, although sophisticated apparatus and elaborate accessories can be expensive.

More than 50 per cent of instruments bought have been applied to biochemical and agricultural projects. Versatility is evident from assays of the many trace elements present in all kinds of water.

Among disadvantages are failures with some lamps, too much trouble with the nebulizer, and the scarcity of information regarding the determination of major constituents. Suitable precision, adequate sensitivity, acceptable drift, allowable blank, and a satisfactory calibration curve are necessary for best accuracy.

Post-mortem urine samples were examined for a variety of elements and in the case of lead poisonings, blood and ‘wet oxidized’ tissue were analysed.

Reproducibility and difficulties of sampling and technique were compared using the three basic types of instrument available.

For maximum sensitivity and best detection limits the double-beam instrument is preferred. Lamp, detector, and electronic drift are eliminated and its stability is superior.