Similarly in gas chromatography the following procedure is used: 1) The chromatogram of the unknown is recorded 2) a known amount of the analyte(s) of interest is added 3) the sample is analyzed again under the same conditions and the chromatogram is recorded. From the increase in the peak area (or peak height), the original concentration can be computed by extrapolation. The detector response must be a linear function of analyte concentration and yield no signal (other than background) at zero concentration of the analyte.