Identifying small molecules via high resolution mass spectrometry: communicating confidence.

T increased availability of high resolution mass spectrometry (HR-MS) in chemical analysis has dramatically improved the detection and identification of compounds in environmental (and other) samples. This has opened up new research opportunities in environmental sciences, demonstrated by over 200 research papers per year, increasing strongly (source: SCOPUS keywords “high resolution mass spectromet”, subject “envi”). The elucidation of small molecules such as emerging pollutants and their transformation products using HR-MSbased suspect and nontarget analysis is gaining in relevance, also in other fields (e.g., metabolomics, drug discovery, forensics). However, confidence in these HR-MS-based identifications varies between studies and substances, since it is not always possible or even meaningful to synthesize each substance or confirm them via complementary methods (e.g., nuclear magnetic resonance). These varying levels of confidence are very difficult to communicate to readers concisely and accurately. In Figure 1 we propose a level system, which arose from intense discussions within our department, to ease the communication of identification confidence and form the basis of further discussions on this topic. This level system is not intended to replace guidance documents (e.g., EU Guideline 2002/657/EG), but specifically covers the new possibilities in HR-MS-based analysis. Our discussion started with the levels published by the Metabolomics Standards Initiative (MSI), as we experienced many cases that fitted “in between” their proposed levels. While Jeon et al. first refined these levels, these were tailored to the specific investigation. The levels in Figure 1 reconcile differences in the two proposals, contain additional levels pertinent to screening methods and are clarified in the text below.