Improvement of Precision Control in Optical Emission Spectrometry Quantifications for a Mineral Analysis Laboratory Using a Horwitz-Based Methodology

Abstract

Analytical methods used in mineral analysis laboratories are susceptible to significant sources of random error that define the level of intralaboratory precision. This requires a good quality control system to ensure that analytical performance is within statistical criteria established by institutional, national, or international bodies. The Horwitz equation and the Horwitz ratio (HorRat) are two related parameters derived from the historical reproducibility analysis that allow for predicting interlaboratory precision and establishing control criteria. The Horwitz ratio can be used not only to establish precision control methodologies between laboratories but also within them. This paper presents the development of a methodology based on the Horwitz ratio to enhance the existing precision quality system based on minimum difference tolerances through volumetric analysis. Data obtained from the quantification of iron, nickel, and cobalt by inductively coupled plasma optical emission spectrometry were analyzed for precision control verification. The HorRat values for iron ranged from 0.48 to 1.25 across concentrations ranging from 3.5 to 48.95%. For nickel, it ranged from 0.20 to 0.36 within the concentration interval of 0.2 to 4.99%. For cobalt, the HorRat ranged from 0.56 to 2.05 across concentrations from 0.01 to 0.499%. An acceptance criterion of HorRat < 1 was established, revealing problems in the established system resulting from the assumption of volumetric tolerances for a spectrometric method. The main deficiencies in the existing methodology were detected in the quantification of iron. The Horwitz-based methodology presented allowed for the improvement of intralaboratory precision and maintained better control over the process.