APPLICATION OF CHEMOMETRICS METHODS FOR THE EVALUATION OF DATA FROM THE STUDY OF LONG TERM STABILITY OF THE DETECTOR RESPONSE FOR TEST SOLUTIONS USED FOR CALIBRATION IN ION CHROMATOGRAPHY ^†

 

Janja Turšič, Marjana Novič, and Milko Novič

 

National Institute of Chemistry, Ljubljana, Hajdrihova 19, Slovenia

 

Abstract

A set of experiments performed to study long-term stability of the working standard solutions in ion chromatography (IC) was examined using different statistical methods. Random variations in the detector response using sufficiently stable and homogeneous samples were observed. Six anions were included in the study: Cl^-, NO₂^-, Br^-, NO₃^-, HPO₄^2- and SO₄^2- in concentration ranges 2 mgL^-1 (chloride, nitrite), 5 mgL^-1 (bromide, nitrate) and 10 mgL^-1 (phosphate and sulphate). Two duplicates were prepared from stock solution by dilution in regular time intervals of one week. One of the duplicates was stored in the refrigerator, while the other was kept in dark at room temperature. After separation, both conductivity and spectrophotometric detection (UV/Vis) were applied. The influence of analyst was minimized using completely automated and thermostated system. The duration of performed test was 3 months. It was shown with pair-wise comparisons of group means using t-test that there are significant fluctuations in the detector response in the whole time period of 3 months, no matter whether the measurements were performed on aged samples or on fresh samples prepared each week from the stock solution. The application of the ANOVA test also showed that groups of experiments performed on equally aged samples in some cases differ significantly. The frequent variation of the recorded detector signals does not origin mainly from aging of samples.