Preliminary statistical investigation of anomaly detection in non-target environmental monitoring by comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry

Abstract

The notable challenges facing non-target environmental monitoring are the improvement of the reproducibility of extraction rates and the selection of internal standards for concentration correction. In the present study, a rapid and comprehensive analytical method, which we had developed in a previous study, greatly reduces the need for pretreatment. The method was applied to the actual measurement of river water. The water samples were divided into five sub-samples and analyzed by sorptive extraction using a magnetic stirrer coated with polydimethyl siloxane. Direct and whole sample extracts were determined by thermal desorption/comprehensive two-dimensional gas chromatography/ time-of-flight mass spectrometry. Approximately 2,000 of the components were detected, and 80 of these components were selected for statistical evaluation in order to investigate the stability of the method and the ability to detect differences among samples. The effectiveness of this technique was confirmed by statistical methods, including the Kruskal-Wallis test, which is used to examine nonparametric multigroup comparisons and with which once can detect differences by comparing raw data obtained as precise mass measurements without the need to identify the substance itself. In brief, we show that changes in signal intensity of any unknown substance can be detected. We note that variations in data (retention time, mass spectrum, and signal intensity) affect the ability to detect differences. The accuracy of each therefore had to be improved to enable sensitive and precise detection.