Matrix interferences in the analysis of organic solutions by inductively coupled plasma-atomic emission spectrometry

Abstract

Matrix interferences are observed during the analysis of organic solutions by inductively coupled plasma-atomic emission spectrometry (ICP-AES) when the composition and/or physical properties of the samples and calibration standards are not matched closely. Analysis of widely different petroleum samples at different dilutions can be prohibitively time consuming if error due to matrix interferences is rigorously avoided. Use of one or more internal standard elements helps to minimize mismatch error. However, one would like to analyze a variety of oils at different dilutions using a single solvent without the need for precise matrix matching or addition of internal standards. This study focused on the relative impact of varying organic solution composition and properties such as viscosity, surface tension, density and volatility on the magnitude of matrix interferences observed during their analysis by ICP-AES. Solutions of toluene, xylene, tetralin, 75 neutral lube base oil and bright stock were prepared in o-xylene and tetralin containing 2–20 wt % solute and 1 μg g of each of 21 elements. The physical properties of each solution were measured and the solutions were analyzed by ICP-AES using calibration standards consisting of essentially pure xylene or tetralin. Hydrogen emission intensity measurements also were acquired during analysis to aid in interpretation of the results. A 5–25 % suppression effect was observed for elements contained in a 20 % solution of lube base stock in xylene. Analysis of the corresponding tetralin solution revealed only a slight suppression effect (0–3 %). Increasing amounts of heavy oils in xylene produced a significant plasma cooling effect suppressing the emission intensities of “hard” lines to a greater extent than “soft” lines. No plasma cooling effect was observed while aspirating heavy oils in tetralin. These observations may be explained in terms of the organic aerosol/vapor loading of the ICP plasma combined with the thermochemistry of the atomization process. Organic solutes/solvents having higher H/C ratios require higher atomization energies. Atomization and heating of a xylene aerosol/vapor stream may consume 10–20 % of the total plasma power under typical operating conditions. The reduction of matrix interferences realized using tetralin as solvent for the analysis of wear metals in lube oil and for heavy fuel oil is illustrated with the analysis of NBS standards.