Some say the Mass Spectrometer is a detector for the Gas Chromatograph and others say the the Gas Chromatograph is an inlet for the Mass Spectrometer, either way, these two instruments compliment each other very well. The GC enhances the MS by providing realtively pure components while the MS compliments the GC by providing another means of compound identification beyond GC retention times. In general, the MS bombards a compound with electrons. The compounds ionize into fragments and the MS produces a spectra.