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.