Application of inductively coupled plasma emission spectrometry in the analysis of environmental samples

Abstract

A discussion of the theory of inductively coupled plasma (ICP) emission spectrometry, with particular attention being paid to the question of detection limit, leads on to an overview of the analytical application of the ICP emission spectrometric technique in environmental sample analysis. Among the aspects which may influence accuracy are: (i) The sample matrix, in particular the acid matrix effect. (ii) The effect of sample uptake rate on excitation temperature and degree of ionization. Temperature was estimated by use of the two-line method. Inconsistencies in calculated temperatures illustrate the departure from local thermodynamic equilibrium in the ICP source. (iii) The effect of mains supply voltage changes on analyte emission intensity; with the effect on the radio frequency generator and spectrometer being considered separately. (iv) The effect of aerosol argon flow rate change on analyte emission intensity, where a correlation with excitation potential is shown to exist. Thereafter follows a study on the calibration of a twenty-eight channel ICP polychromator and the use of the software approach for interference correction. Second order polynomial interference correction coefficients were determined for interference from Ca, Mg, Fe and Mn. The importance of correction for drift in the concentration readout of a blank sample in analysis at trace concentrations is illustrated. An investigation into the memory-effect in the sample transport system of the polychromator showed that the dead volume displacement time is followed by a biphasic washout curve with both the fast and slow phases having a linear double logarithmic plot. The use of a weighted internal standardization method to correct for sensitivity change consequent to aerosol argon flow rate change was investigated as a possible means of improving accuracy and avoiding frequent recalibration. An investigation is also undertaken into the use of a scanning monochromator for quantitative analysis in the profile scan mode of measurement, where interference correction is carried out by background subtraction. A study is included on the use of an inverse Gaussian transformation as a means of estimating precision from a profile scan, and a discussion on the establishment and the influence of background structure on detection limits. An example is given on the determination of Pb and Cr in a sediment digest. The study is concluded with a discussion on quality control.