A method was developed to employ National Institute of Standards and Technology (NIST) 2008 retention index database information for molecular retention matching via constructing a set of empirical distribution functions (DFs) of the absolute retention index deviation to its mean value. The effects of different experimental parameters on the molecules' retention indices were first assessed. The column class, the column type, and the data type have significant effects on the retention index values acquired on capillary columns. However, the normal alkane retention index (Inorm) with the ramp condition is similar to the linear retention index (IT), while the Inorm with the isothermal condition is similar to the Kováts retention index (I). As for the Inorm with the complex condition, these data should be treated as an additional group, because the mean Inorm value of the polar column is significantly different from the IT. Based on this analysis, nine DFs were generated from the grouped retention index data. The DF information was further implemented into a software program called iMatch. The performance of iMatch was evaluated using experimental data of a mixture of standards and metabolite extract of rat plasma with spiked-in standards. About 19% of the molecules identified by ChromaTOF were filtered out by iMatch from the identification list of electron ionization (EI) mass spectral matching, while all of the spiked-in standards were preserved. The analysis results demonstrate that using the retention index values, via constructing a set of DFs, can improve the spectral matching-based identifications by reducing a significant portion of false-positives.
Bibliographical noteFunding Information:
The authors thank Drs. Steve Stein and Edward White of the National Institute of Standards and Technology (NIST) for their help with the interpretation of the NIST08 retention index database. This work was supported by National Institute of Health (NIH) grant 1RO1GM087735 through the National Institute of General Medical Sciences (NIGMS) and 1RC2AA019385 through National Institute on Alcohol Abuse and Alcoholism , respectively.
- Empirical distribution function
- Retention index