Algorithm to determine matrix-effect crossover points for overcoming interferences in inductively coupled plasma-atomic emission spectrometry

Abstract

An algorithm is described for automated identification of matrix-effect-free crossover points, which is the observation height at which no apparent matrix effect exists, in ICP-AES. The first priority of the algorithm is to ensure that all effects from experimental artifacts are filtered out and that all analytical results reported by the algorithm are accurate. The backbone of the algorithm consists of applying segmented polynomial regressions to the vertical determined-concentrations profiles of the analyte; the crossover point is then found by solving the root of the algebraic difference between the two polynomials. Three additional features were incorporated into the algorithm to improve its accuracy. First, the residual of the segmented polynomial regression, which gauges the level of noise in the vertical determined-concentration profile, was limited to a threshold value of 0.01 (i.e., 1%); residuals due to the fitting process could be made negligible by an appropriate choice of polynomial order. Second, the total number of samples was increased from two to three (the three samples are the undiluted original sample and two diluted aliquots of the original sample with different dilution factors) for determination of the crossover point, which in turn results in three statistically equivalent crossover points. Third, the dispersion of the three determined crossover points provides an estimate of how accurate the final result is. The ability of the algorithm to avoid misidentification of the interference-free crossover points was evaluated under different experimental situations (e.g., noisy vertical emission profiles caused by low analyte concentrations, and the presence of non-plasma-related interferences). With this algorithm, it was found that the analyte needs to be present at a concentration roughly 100 to 200 times its detection limit for accurate results to be obtained.