Introduction of curvature index and improvement of calibration curves used in ion chromatography (IC), atomic absorption spectrometry (AAS), UV-Vis spectrophotometry and catalytic combustion

Abstract

Background

The calibration curve represents the relationship between known CRM and the response of the instrument (e.g absorbance). Most of the time, in analytical chemistry standards, the linear function (of the 1st degree) is used to characterize the calibration curves. This, at first glance, has some major advantages: simple equation, simple linearity estimation criterion, and easy way of calculation. However, from a quality point of view, the grade 1 calibration curve does not characterize a working domain very well, because it does not take into account the curvature and has larger residuals.

Results

Here we show that, most of the time, the non-linear secondary function fits the calibration curves much better (argument valid for UV-Vis spectrophotometry, atomic absorption spectrometry (FAAS and ETAAS), catalytic combustion and ion chromatography (IC)), having lower residuals, but also a higher coefficient of determination, $$. The limitation of the secondary non-linear function use is given by the insufficiency of data related to its qualitative characterization. Through this work, we have introduced new characterization criteria for non-linear secondary functions such as curvature angle and curvature index. Although in the specialized literature, the term linearity of the calibration curve has existed for a long time, in reality it has not been fully interpreted. The curvature index developed in this work allows the controlled implementation of non-linear curves, but also with increasing the precision of the results. (135)

Significance

The curvature index characterizes the degree of curvature of the 1^st or 2^nd degree calibration functions. Using the 2nd degree calibration function allows the improvement of the obtained results, and curvature parameters (curvature angle and curvature index) allow qualitative characterization of many data sets used for calibration. The degree of curvature of a calibration curve is a new term not found in the literature, which will allow their characterization from a qualitative point of view. (75)