A mathematical model for improving the ion chromatography method by applying external adjustment standards

Ion chromatography (IC) is included in the list of very efficient and fast methods for obtaining results without losses and with the minimization of the human factor. The aim of this paper is to demonstrate the mathematical model for adjusting anion concentrations (NO2- nitrite, NO3- nitrate, Cl- chloride and SO42- sulphate) for water samples by using external standards. The method uses an interpolation and requires a certain number of injections with an external standard (if a single calibration curve is used) or with several external standards (if many calibration curves are used for the same anion). The importance of interpolation is to obtain a theoretical value of the adjustment standard for any sample by reference to the real readings of this external standard. In this study, interpolation works by determining an equation of degree n, covering n interpolations, using the read values of some adjustment standards (eg. A(R20-X) and A(R100-X)). The obtained equation can be used later for any injection X , characteristic of a real sample N-X, in order to obtain the adjusted areas A(R20-X*) and A(R100-X*). Thus, it becomes possible to calculate the factor f that serves to adjust the areas of the real samples. This strategy demonstrates that the calibration curve can be done quite rarely, and that the results obtained should not be affected by the periodicity of the calibration curve, in compliance with the performance parameters of the method (eg according to Shewhart-type control diagrams and Proficiency Testing (PT) schemes). Even if the results obtained without interpolation follow the recommendations given by US EPA Method 300.0:1993 (the peak area of the adjustment standard does not exceed +/- 10% of the area of the adjustment standard read on calibration) and by ISO 10304-1:2007 (the retention time for the anion peaks does not exceed +/- 10% in a batch and the minimal resolution for any two investigated peaks of the recorded chromatograms (R) was > 1.3), the method performs a continuous adjustment of the sample concentrations to the used calibration curves, providing more relevant results.