Transient atomisation by cathodic sputtering in a glow discharge for atomic absorption spectrometry

Abstract

Analytical applications of cathodic sputtering in a glow discharge have almost wholly involved producing a steady-state atomic vapour, which suffers from relatively poor sensitivity and limit of detection common to the steady-state mode of atomisation. The formation of atoms in and loss of atoms from an analysis volume (defined by the geometry of the incident radiation from the hollow-cathode lamp) can be described by consecutive, first-order reactions, [graphic omitted], where D is the pre-atomisation species, B is the analyte atom that absorbs, C is the analyte atom that is lost and k₁ and k₂ are first-order rate constants. For k₂≫k₁, d[B]/dt= 0 (t is time in seconds) and the system is said to have reached a steady state. At the steady state, [B]=[D]₀k₁/k₂, which makes [B] much less than [D]₀, the initial concentration of the analyte atoms. Hence, the steady-state mode of atomisation gives low sensitivity.

The objective of transient atomisation is to maximise [B] by making k₁≫k₂. For k₁≫k₂, [B]≈[D]₀, and [B] then reaches its maximum possible value, giving the maximum absorbance. This paper presents the results of a preliminary study on transient atomisation with an Atomsource, used in conjunction with a Perkin-Elmer Model 5000 atomic absorption spectrometer and a Perkin-Elmer Model 7500 professional computer. Aqueous solutions of salts of Al, Cd, Co, Cu, Fe, Mn, Mo, Ni and V, one element in one solution, were used. The rate constants, k₁ and k₂, were evaluated from kinetic profiles and yielded k₁/k₂≫ 1, which satisfied the condition required for making [B]≈[D]₀, yielding high peak-height sensitivity and an excellent detection limit for the above elements. Kinetic analysis revealed that the rate constant for atom formation in the early stages of the reaction (after a transition period), attained a much larger value (to be called k₃) at later times. This rate constant for atom formation, k₃, obtained from later times of atomisation, rather than k₁ obtained from earlier times of atomisation, was used (i.e., k₃≫k₂) as the criterion for the peak-height sensitivity.