Setting Identification Criteria of Trace Levels of Compounds in Complex Matrices by GC-MS with 96 % True-Positive and 0.00001 % False-Positive Results Rates

Ricardo Bettencourt da Silva, Chemistry and Biochemisty Department, Faculty of Sciences of the University of Lisbon, Lisboa, Portugal

The identification of trace levels of compounds in complex matrices by conventional low-resolution gas chromatography hyphenated with mass spectrometry (GC-MS) is based in the comparison of retention times (RT) and abundance ratios (AR) of characteristic mass spectrum fragments of analyte peaks from calibrators with sample peaks. Frequently criteria set for the agreement between calibrators and samples signals, by laboratories or regulators, produce high rates of false positive and/or false negative results since are not based in adequate signals distribution models. Although retention times are approximately normally distributed being adequately studies using normal statistics, relative retention times (RRT) and AR are not normally distributed since variables are correlated and combined in a ratio.

This work presents a user-friendly software where mass spectrum ions abundances from analyte of calibrators are recorded and acceptance intervals of AR observed in samples peaks are defined based on ion abundances dispersion and correlation, and respective AR simulation using Monte Carlo Method.

This tool defines the true-positive results rate (TP) and estimates the false-positive results rate (FP) required to determine the identification uncertainty as the likelihood ratio (LR=TP/FP). Models of background noise from complex matrices are used to estimate FP.

The developed computational tools can be used to identify compounds from two ratios of three correlated mass spectrum ion abundances.

User friendly MS-Excel files are made available to allow the easy application of developed methodology in routine and research laboratories. The developed approach was successfully applied to the identification of chlorpyrifos-methyl and malathion in QuEChERS method extracts of vegetables with high water content for which the estimated Limit of Examination (i.e. the minimum mass fraction identified with adequate uncertainty), is 0.14 mg kg^-1 and 0.23 mg kg^-1 respectively.

R. J. N. Bettencourt da Silva, Talanta 150 (2016) 553-567.