Development and Application of Liquid Chromatographic Retention Time Indices in HRMS-Based Suspect and Nontarget Screening

There is an increasing need for comparable and
harmonized retention times (tR) in liquid chromatography (LC) among
different laboratories, to provide supplementary evidence for the identity
of compounds in high-resolution mass spectrometry (HRMS)-based
suspect and nontarget screening investigations. In this study, a rigorously
tested, flexible, and less system-dependent unified retention time index
(RTI) approach for LC is presented, based on the calibration of the
elution pattern. Two sets of 18 calibrants were selected for each of ESI+
and ESI-based on the maximum overlap with the retention times and
chemical similarity indices from a total set of 2123 compounds. The
resulting calibration set, with RTI set to range between 1 and 1000, was
proposed as the most appropriate RTI system after rigorous evaluation,
coordinated by the NORMAN network. The validation of the proposed
RTI system was done externally on different instrumentation and LC conditions. The RTI can also be used to check the
reproducibility and quality of LC conditions. Two quantitative structure−retention relationship (QSRR)-based models were built
based on the developed RTI systems, which assist in the removal of false-positive annotations. The applicability domains of the
QSRR models allowed completing the identification process with higher confidence for substances within the domain, while
indicating those substances for which results should be treated with caution. The proposed RTI system was used to improve
confidence in suspect and nontarget screening and increase the comparability between laboratories as demonstrated for two
examples. All RTI-related calculations can be performed online at http://rti.chem.uoa.gr/.