Liquid Chromatography Detectors - The UV Detectors > The Diode Array Detector > Page 52

If the peak was pure, the ratio of the adsorption at the two wavelengths (those selected were 225 and 245 nm) would remain constant throughout the elution of the entire peak. The upper diagram in figure 31 shows this ratio plotted on the same time scale and it is seen that a clean rectangular peak is observed which unambiguously confirms the purity of the peak for chlorthalidone. The wavelength chosen to provide the confirming ratio will depend on the UV adsorption characteristics of the substance concerned, relative to those of the most likely impurities to be present, consequently the wavelengths must be chosen with some circumspection. Another interesting example of the use of the diode array detector to confirm the integrity of an eluted peak is afforded by the separation of the series of hydrocarbons shown in figure 32.

The separation was carried out on a column 3 cm long, 4.6 mm in diameter and packed with a C18 reversed phase on particles 3 m in diameter.

Courtesy of the Perkin Elmer Corporation

Figure 32. The Separation of Some Aromatic Hydrocarbons

The separation appears to be satisfactory and all the peaks appear to represent individual solutes; without further evidence, it would be reasonable to assume that all the peaks were pure. However, by plotting the adsorption ratio, , for the anthracene peak it becomes apparent that the peak tail contains an impurity as the clean rectangular shape of the peak top is not shown. The absorption ratio peaks are shown in figure 33.