Sometimes two different molecules can also have a similar pattern of ionized fragments in a mass spectrometer (mass spectrum). Combining the two processes reduces the possibility of error. It is extremely unlikely that two different molecules will behave in the same way in both a gas chromatograph and a mass spectrometer. Therefore, if a mass spectrum matches the analyte of interest, the retention time of that spectrum can be checked against a characteristic GC retention time to increased confidence that the analyte is in the sample.