Archive for August, 2007

Matrix Effects in ICP-OES Analysis

Friday, August 17th, 2007

Matrix effects in ICP-AES analysis

Ralph Obenauf and Vanaja Sivakumar

SPEX CertiPrep, Metuchen, NJ

The sample “matrix” is the bulk composition of the sample such as water, organic compounds, acids, dissolved solids and salts, etc. Matrix effects can influence the ability of an analytical method, to qualitatively identify and quantitatively measure target compounds in environmental and other samples, by indirectly affecting the intensity and resolution of observed signals. To obtain defensible results the analyst must account for all matrix effects. In ICP-OES analysis the ionic-to-atomic line intensity ratio can be used as an indicator for determining plasma related matrix effects. Elements such as sodium and calcium, which are ubiquitous in nature, have low ionization potentials and as a consequence are some of the most easily ionized elements. In this study the influence of sodium as well as the acid concentration on the ionic-to-atomic intensity ratio of chromium, cadmium and lead has been investigated.

Effect of Sodium concentration:

We observed a considerable change in the recovery of cadmium and chromium, which exhibits a profound influence on these analytes by varying the concentration of Sodium. The relative intensity of chromium and cadmium atom lines were much higher than the respective ionic lines for these analytes. The effect is significant even at 50 ppm sodium. The lead concentration is affected, relatively, less by the change in the sodium concentration.

Effect of acid concentration:

The matrix effect due to changes in the acid concentration was very significant for cadmium and lead in nitric acid, While the chromium atomic line was unaffected, a decrease was observed in the in intensity for the chromium ionic line. The higher the acid concentration, the lower the intensity for lead and cadmium, and therefore resulting in poorer detection limits. We observed much smaller changes in analyte recoveries for increases in hydrochloric acid concentrations when compared with comparable nitric acid concentration changes.Matrix effects have a profound effect on the accuracy of trace elemental analysis by ICP. Excess acid from acid digestion can be a source of error in the analysis. Because of the different behavior of the atomic and ionic lines, at least two internal standards are needed to compensate for matrix effects. It is absolutely essential to match the sample matrix to the standard to assure accurate and defensible data.