These two machines, used together, allow a much finer precision of substance identification than either unit used separately. It is not possible to make an accurate identification of a particular molecule by gas chromatography or mass spectrometry alone. The mass spectrometry process normally requires a very pure sample. In the past, gas chromatography use other detectors such as a Flame Ionization Detector. These detectors cannot separate different molecules that happen to take the same amount of time to travel through the column. (When two different molecules have the same retention time they are said to "co-elute".) The co-eluting molecules will confuse the computer programs that are reading a single mass spectrum for both molecules.