In an untargeted metabolomic approach, thousands of metabolites are studied in one sample; however, the chemical identity of the compound is not known as in targeted approach. The data about the compound are acquired during the study or after the completion of the study, and this information is used to annotate or determine the metabolites.
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In order to maximize the interpretation of the sample and its impact, robust compound identification is performed in non-targeted approach. With robust identification or annotation, biological data interpretation is done for a single research study and the data from varied research groups can be compared.
Metabolite annotation and identification standards
The Chemical Analysis Working Group (CAWG) of the Metabolomics Standards Initiative (MSI) and Core Information for Metabolomics Reporting (CIMR) has categorized metabolites into four categories. They are:
- Level 1: Identified metabolites
- Level 2: putatively annotated compounds
- Level 3: putatively characterized compound classes
- Level 4: unknown compounds
Researchers need to clearly distinguish and identify the level while reporting the metabolites.
Most of the metabolites that have already been identified, characterized, and reported are of non-novel nature. Metabolites that were not recognized during the first time are recognized depending on co-characterization with reliable samples. A minimum standard of reporting for Level 1 non-novel compounds that are identified is made available.
Metabolites that are not available commercially are identified as putatively annotated compounds (Level 2). Chemical reference standards are not available for these metabolites; however, they are identified based on the physicochemical properties that are in line with spectral databases.
In the spectral and chemical databases, no structure is proposed for metabolites identified as putatively characterized compound classes (Level 3). These compounds are identified depending on the distinctive physicochemical similarities or spectral properties of a given chemical class of compounds.
Compounds are termed as unknown compounds (Level 4) when they are not identified or classified, yet can be distinguished and computed depending on the available spectral data.
CAWG supports the traditional principles for identifying novel metabolites and recommends researchers to name the compound in line with International Union of Pure and Applied…