An article published on April 10, 2017 in the peer-reviewed journal Analytica Chimica Acta proposed a framework for semi-quantification of unknown substances and known substances for which an authentic reference standard is not available. The study was authored by Eelco Pieke and colleagues from the Research Group for Analytical Food Chemistry, National Food Institute, Technical University of Denmark.
Mass spectrometry-based chemical analytics often rely on the so-called targeted approaches. There, a set of analytes of interest are being identified and quantified against their respective authentic reference standards, and the methods used are a priori optimized specifically for these analytes. Such targeted methods leave “very little room for unknown substances,” the authors emphasized. However, chemical risk assessment often seeks to prioritize multiple compounds based on their concentration. Many of these compounds may lack authentic reference standards, and their chemical identity may even be completely unknown. In such a situation, screening and semi-quantification may be carried out more efficiently by using an untargeted method, such as the one proposed and evaluated in their study.
As a proof-of-concept, the authors analyzed the responses of 17 marker analytes with chemically diverse structures. Each of these analytes was semi-quantified using a different analyte as a reference marker, and the outcomes were assessed under various conditions in order to investigate the influence of various instrumentation parameters and analytical approaches on the method performance. For example, the authors found that selecting the quantification marker based on retention time differences provided for a better quantification compared to selections based on accurate mass differences. The predictions obtained with an optimized method had error range with a maximum of factor three.
The optimized method was then successfully applied to an extract of a paperboard food contact material, where it allowed to semi-quantify over 300 unknown substances. With this, the authors demonstrated that, even without substance identification and lacking any authentic standards, their method “was able to estimate the concentration of a virtually unlimited number of compounds thereby providing valuable data to prioritize compounds in risk assessment studies.”
Emma Davies (May 18, 2017). “Danish team estimates levels of unknown chemicals in complex mixtures.” Chemical Watch
Pieke, E., et al. (2017). “A framework to estimate concentrations of potentially unknown substances by semi-quantification in liquid chromatography electrospray ionization mass spectrometry.” Analytica Chimica Acta (published April 10, 2017).