Although the primary purpose of chromatography is the separation of a complex mixture into its component parts, as outlined here, a chromatographic separation also provides qualitative and quantitative information about our samples. More detailed examples of qualitative and quantitative applications are found in the chapters that follow. Qualitative Analysis As we learned in Section 26.2, solutes migrate through a chromatographic system at rate that is a function of the properties of the mobile phase and the stationary phase. This means that a particular solute will elute with a consistent retention time. If we expect a solute to elute with a retention time of 5.0 min, the presence of a peak at 5.0 min is suggestive of, but not definitive evidence of the solute's presence in our sample; however, the absence of a peak at 5.0 min is strong evidence that the solute is not present in our sample. For a complex mixture, this sort of screening technique is a useful qualitative application of chromatography. As we will see in the chapters that follow, the type of detector used to monitor a chromatographic separation may provide useful qualitative information. Quantitative Analysis A chromatographic separation yields a sequence of peaks that, ideally, represent a single solute. These peak's are characterized by an area that is proportional to the amount of analyte injected into the mobile phase. By injecting a series of standards, a calibration curve of peak area as a function of the analyte's concentration provides a way to determine the analyte's concentration in a sample. Any of the calibration strategies discussed in Chapter 1.5—external standards, standard additions, and internal standards—find use in a quantitative chromatographic analysis. Determining a solute's peak area is relatively straight-forward when using a computer-interfaced instrument with appropriate software. Alternatively, peak height, which is easier to measure, can be used as a substitute, although care must be taken to ensure that the peaks are symmetrical and that peak widths are consistent for the standards and the samples.