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/.