Once the mass spectrum is obtained, it is matched against the content of spectral databases of reference compounds in order to retrieve its structure. Various chemical databases provide spectra data of metabolites such as Human Metabolome Database, METLIN, MassBank and mzCloud . Certain databases are focused on the metabolites of specific organisms, such as the Human Metabolome Database, or on specific molecular classes, such as the LIPID MAPS Structure Database, while others have greater coverage such as METLIN. However, despite the intense ongoing efforts to map the metabolome of various organisms, existing databases cover only a small percentage of the actual metabolites that occur in organisms. Particularly for humans, it is estimated that less than 10% of metabolites have experimental reference mass spectra which means that a large percentage of the current practice cannot identify a large percentage of the molecules that are found in human tissues. It is estimated that in untargeted metabolomics studies less than 2% of the detected spectral features are identified .