Comparison of Axially and Radially Viewed Inductively Coupled Plasmas for Multi-element Analysis: Effect of Sodium and Calcium

Abstract

Conventional figures of merit such as LODs, interferences due to Ca and Na, the Mg II 280.270 nm/Mg I 285.213 nm plasma robustness ratio, precision and accuracy were employed to compare the analytical performance of radially and axially viewed ICPs. The cool region in the plasma, where EIE interferences predominate, was symmetrically peeled away using an end-on stream of argon as opposed to a 90 ° shear gas flow, thus leaving the central axial zone free of the interfering sheath. LODs were enhanced by factors varying from about 2 to 20 and could further be improved by optimizing the nebulization pressure. Additional enhancements by factors of 2–16 were obtained using ultrasonic nebulization. It was observed that interferences due to Na and Ca were relatively small and were similar in the axial and conventional radial configurations. Interferences due to Ca were larger than those caused by Na, probably owing to the larger amount of energy required to dissociate the matrix. The extent of matrix effects, which did not exceed 10–20%, is attributed to the robust plasma as evidenced by Mg II 280.270/Mg I 285.213 nm ratios of 7 at 0.9 kW and ≊12 at 1.4 kW using normal aerosol flow rates. The use of internal standards such as Sc II 361.384 and Cd II 226.502 nm, measured sequentially, improved the accuracy of determination using conventional and ultrasonic nebulization. The major, minor and trace element contents of several geological CRMs, varying widely in composition, compared favorably with the recommended data. It was concluded that the ICP operating conditions and performance for quantitative multi-element analysis using the present axial configuration do not differ significantly from those of the conventional radial configuration.