Practical field-portable atomic absorption analyser

Abstract

A new compact atomic absorption spectrophotometer has been developed. Designated the Scintrex AAZ-2, the system used flameless atomization and Zeeman modulation for maximum sensitivity and selectivity respectively. The most important factor of AA instrumentation is the method used for vapourizing the sample. The atomizer furnace in the AAZ-2 is a tungsten ribbon, rather than the conventional carbon furnace, to avoid the need for high electrical power input and consequent water cooling. The ribbon is U-shaped to fit between the pole pieces of a powerful electromagnet which provides the magnetic field required for Zeeman modulation of the absorption signal. Zeeman modulation refers to the spectral displacement of atomic absorption lines in the presence of a magnetic field. By pulsing the field, the emission from the source lamp alternately does, or does not, coincide with the analyte absorption. A synchronous detection system then allows the extraction of the modulated signal from any unmodulated absorption due to nonspecific absorbers such as particulate scattering or some molecular species. Zeeman modulation provides a method of baseline correction that is both more efficient and simpler than the commonly used deuterium are correction and requires no moving components. The AAZ instrument has already been characterized for a number of metals, in particular, those of exploration interest such as Au, Ag, Cu, Pb, Ni, Co. Zinc has been determined but difficulties in avoiding high blank concentrations have been found. Detection limits are typically 1 ng/ml or better (e.g. Au — 0.5 ng/ml, Ag — 0.06 ng/ml). The metals usually requiring conversion to a gaseous hydride form before analyses have also been studied. Tin in the sub-ppm range has been determined after a simple extraction procedure. Work on arsenic, selenium, antimony and bismuth is planned. The entire machine weighs 26 kg and consumes 0.5 kW making it suitable for use in remote areas using portable power generating equipment. Argon at low pressure and flow rate is the only gas needed for operation. When used for analysis of samples in a base camp, close to the scene of a geochemical drilling program, the AAZ-2 can provide quick turnaround of the results and therefore on-the-spot guidance of these programs without loss of sensitivity or data quality. In addition, the possibility of contamination by high-level samples is somewhat less in a field camp than in a long established laboratory.