Ultraviolet detectors chromatograms from

Figure 6-2 Chromatogram from an HPLC reversed-phase separation of tricyclic antidepressants with the use of a UV photometer detector set at 2l5nm. Signal is displayed at 0.1 AUFS, HPLC high-performance liquid chromatography UV, ultraviolet AUFS, absorbance units full scale. (Courtesy Vydac/The Separations Group, Hesperia, Calif.)...

Because of refractive-index effects, an unretained solvent used to dissolve the sample— if different from the chromatographic mobile phase—often deflects the base-line when passing through an ultraviolet detector cell. This indicates the void volume or the void time. Consider the chromatogram in Figure 21.19. (a) Determine the capacity factors for each nitroaniline isomer, (b) Determine the selectivity factor for the m- and p-substituted isomers relative to the o-nitro-aniline. 

Analytes are detected at the outlet by their ability to absorb ultraviolet radiation from the lamp in Figure 0-4a. The graph of detector response versus time in Figure 0-5 is called a chromatogram. Theobromine and caffeine are the major peaks in the chromatogram. Small peaks arise from other substances extracted from the chocolate. 

Figure 8.2. Chromatograms of a turnip green sample using series ultraviolet and electrochemical detection 40.04 ng of methyl parathion and 44.52 ng of ethyl parathion injected mobile phase 64% acetonitrile, 36% 0.05 M ammonium acetate pH 5.0, flowrate, 1 ml/min, UV detection at 270 nm EC detector at -0.97 V vs. Ag/AgCl. From [79]...

The components in a mixture separate in the column and exit from the column at different times (retention times). As they exit, the detector registers the event and causes the event to be recorded as a peak on the chromatogram. A wide range of detector types are available and include ultraviolet adsorption, refractive index, thermal conductivity, flame ionization, fluorescence, electrochemical, electron capture, thermal energy analyzer, nitrogen-phosphorus. Other less common detectors include infrared, mass spectrometry, nuclear magnetic resonance, atomic absorption, plasma emission. 

When one of the constituents, A or B of a copolymer A-B, has an ultraviolet (UV) absorption and the other does not, a UV detector-refractive index (RI) combined detector system can be used for the determination of chemical composition or heterogeneity of the copolymer. A point-to-point composition, with respect to retention volume, is calculated from two chromatograms and a variation of composition is plotted as a function of molecular weight. The response factors of... 

Figure 4.25 A typical gel permeation chromatogram. The lower trace with short vertical lines is the differential refractive index while the upper curve is an absorption plot at a fixed ultraviolet frequency. The short vertical lines are syphon dumps numbered consecutively from the time of injection of the sample. The units of the ordinate depend on the detector, while those of the abscissa can be in terms of syphon volumes (counts) or volume of solvent.

The systems reported above all rely on detection by ultraviolet absorbance, and require some separate means of identifying the desired product among the various components (and thus collected fractions) of the reaction mixture. Efficiency and throughput can be considerably enhanced by real-time detection and identification of products. Two groups have now reported preparative HPLC systems where fractionation decisions are based upon output from a mass spectrometer detector. The first preliminary report of such a system came from a collaboration between CombiChem and Sciex [21]. This system bases fractionation decisions on the output of a single ion mass chromatogram for the predicted molecular ion of the desired product. Reverse phase preparative HPLC is used in conjunction with electrospray ionization mass detection. A full report of this work has appeared in early 1998 [22]. The second report of such a system came from a collaboration between Pfizer and Micromass [23]. This system uses a flexible combination of UV and/or ion chromatograms to control fractionation. Unlike other systems, fractionation parameters are set by the mass spectrometer control software. Variants of both of the above systems will probably become commercially available in late 1997 [24]. 

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