A system for identifying a compound from tandem mass spectrometry product ions without using any a priori precursor ion information, comprising: a tandem mass spectrometer configured to analyze a sample producing at least one measured product ion spectrum from a precursor mass-to-charge ratio range; and a processor in communication with the tandem mass spectrometer configured to: (a) receive the at least one measured product ion spectrum from the tandem mass spectrometer , (b) select a measured product ion from the at least one measured product ion spectrum, and retrieve a list of possible compounds that could generate the measured product ion from a dictionary of known compounds, wherein the dictionary is configured to store, for each known product ion of the known compounds, a list of all compounds that could produce the mass of the known product ion, (c) select another measured product ion from the at least one measured product ion spectrum and retrieve another list of known compounds that could generate the another measured product ion from the dictionary, (d) examine all retrieved lists of possible compounds and identify compounds common to all the retrieved lists of possible compounds, (e) repeat steps (c) and (d) until one compound is identified as being common to all retrieved lists of possible compounds and identify the one compound as a compound of the sample. a tandem mass spectrometer configured to analyze a sample producing at least one measured product ion spectrum from a precursor mass-to-charge ratio range; and a processor in communication with the tandem mass spectrometer configured to: (a) receive the at least one measured product ion spectrum from the tandem mass spectrometer , (b) select a measured product ion from the at least one measured product ion spectrum, and retrieve a list of possible compounds that could generate the measured product ion from a dictionary of known compounds, wherein the dictionary is configured to store, for each known product ion of the known compounds, a list of all compounds that could produce the mass of the known product ion, (c) select another measured product ion from the at least one measured product ion spectrum and retrieve another list of known compounds that could generate the another measured product ion from the dictionary, (d) examine all retrieved lists of possible compounds and identify compounds common to all the retrieved lists of possible compounds, (e) repeat steps (c) and (d) until one compound is identified as being common to all retrieved lists of possible compounds and identify the one compound as a compound of the sample. (a) receive the at least one measured product ion spectrum from the tandem mass spectrometer , (b) select a measured product ion from the at least one measured product ion spectrum, and retrieve a list of possible compounds that could generate the measured product ion from a dictionary of known compounds, wherein the dictionary is configured to store, for each known product ion of the known compounds, a list of all compounds that could produce the mass of the known product ion, (c) select another measured product ion from the at least one measured product ion spectrum and retrieve another list of known compounds that could generate the another measured product ion from the dictionary, (d) examine all retrieved lists of possible compounds and identify compounds common to all the retrieved lists of possible compounds, (e) repeat steps (c) and (d) until one compound is identified as being common to all retrieved lists of possible compounds and identify the one compound as a compound of the sample. <EOS>