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Downstream from the column, the chromatograph has one last device: the detector, which provides a signal with the passage of each analyte. All of the signals will form the chromatogram. This detector uses variations in physical amplitude related to the presence of a solute, in addition to that of the carrier gas. Some detectors are universal; that is they are sensitive to practically every solute, while others are selective detectors that are sensitive only to specific compounds (Figure 2.14). The most effective rely on the adaptation of spectral methods, or on mass spectrometry, which not only helps generate the conventional chromatogram but also helps identify each solute (see the Chapter 16 on Mass Spectrometry). Nevertheless, this is expensive, and we always find a number of conventional detectors in laboratories.