The printed or digital output of a chromatographic instrument. A chromatogram is a plot of the response of the detector as a function of elution time as shown in the illustration. In the example shown on the next page, a sample containing three components was introduced into the chromatographic system. Component 1 emerged from the chromatographic column first (eluted first), and the detector recorded a response at 1.3 minutes. This time is assigned based on the top, or apex, of the peak and is called the retention time (tr). Component 2 had a retention time of 2.8 minutes and component 3, 4.6 minutes. The sizes of the peaks recorded on a chromatogram are proportional to relative concentrations, so here, the sample had the highest concentration of component 2 and the lowest concentration of component 3. Instruments used in forensic science that produce a chromatogram include gas chromatography (GC) and high performance liquid chromatography (HPLC).