Photodiode Array PDA Detectors

A PDA detector provides UV spectra of eluting peaks in addition to monitoring the absorbance of the HPLC eluent like the UVA is absorbance detector. It is the preferred detector for testing impurities and for method development. PDA facilitates peak identification during methods development and peak purity evaluation during method validation. Detector sensitivity was an issue in earlier models but has improved significantly (more than ten-fold) in recent years.  

FIGURE 14 Waters Millennium screen showing contour map, chromatogram at 278nm and spectra of an active ingredient and a degradant of an impurity test seunple. 

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FIGURE 13.22 Sur ctants on the 6 column set, using a photodiode array (PDA) detector. The Span and Tween 80 can be distinguished from each other very nicely in a mix. THF was the solvent used at 50°C. (A restrictor after the detector minimizes bubbles.)... 

Identification (ID) tests in Category IV require only specificity for their validation. Identification by HPLC usually involves comparison of the retention time (%) or relative retention time (RRT) of a sample and standard injection. The increasing use of photodiode array (PDA) detectors in HPLC methods also allows identification by comparison of UV spectra for standards and samples, in addition to retention characteristics. The information required for either ID test by HPLC can be gathered while performing any other HPLC method for a given sample. Identification tests are often incorporated into the assay method and the satisfactory completion of specificity for the assay will meet the requirements for ID as well. 

The following guidelines are recommended for UV/Vis absorbance or photodiode array (PDA) detectors ... 

A system is typically comprised of multiple instrument components. Therefore, there is usually an individual IQ for each of these instruments and for any corresponding instrument control/data-handling software. The typical instrument components making up an HPLC system include a binary or quaternary HPLC pump, an autosampler supporting multiple vials or microtiter plates (autosamplers often include cooled Peltier trays for sample stability), a column oven, and a UV-Vis or photodiode array (PDA) detector. 

The emergence of sensitive and affordable array detectors in 1980s and 1990s has also improved measurement capability in the UV-vis. Commercially available UV-vis instrumentation with photodiode-array (PDA) detectors appeared in the mid-1980s. This made it possible to produce a UV-vis spectrophotometer... 

Figure 4 presents an example of rapid pKa measurement using a pressure-assisted system in combination with a photodiode array (PDA) detector. The migration time of DMSO (EOF marker) was measured at 220 nm, whereas the migration time of the analyte, naphazoline, was measured at 270 nm. The CE run time as well as data analysis time was drastically reduced. Consequently, this system allows the analysis of more than 96 compounds in one day. The limitation of this method is the application to drugs without UV chromophore at more than 250 nm. In some cases, it was effective to remove DMSO by evaporation under vacuum followed by the addition of methanol or acetonitrile as a neutral marker. 

Photodiode array (PDA) detectors are becoming more popular as their ability to use both retention times and UV spectra to aid in peak identification becomes more widely recognized. With most PDA software packages, libraries of standard compounds can be created by the analyst. When samples are run, the spectra and/or the run times of the peaks in the sample can be compared with those stored in the libraries. Because the shapes of UV spectra are dependent on the experimental conditions under which they are obtained, however, it is unlikely that standard libraries will become commercially available. 

Figure 4.8. A schematic of a UV-Vis absorbance detector (a) and a photodiode array (PDA) detector (b). Diagrams adapted from and reprinted with permission from Academy Savant.

Figure 12.7. Schematic drawing of the optical switch used in multiplex photodiode array (PDA) detector.

For LC, reversed-phase conditions are widely used for the separation of dyes, but in the case of basic dyes (cationic dyes used in acrylics), a silica column is utilized. Mobile phases also depend on the chemical properties of the dyes and ion-pair chromatography is widely used. Detection is achieved by measuring the absorbance in the visible region as well as at 254 nm. A photodiode array (PDA) detector makes possible simultaneous detection at several wavelengths and spectral analysis of the chromatographic peaks. If the absorption spectrum for the peak for each separated dye component is available, the color of the dyes can be assessed objectively and the combination of retention times and peak ratios obtained from the chromatogram the spectral data make it possible to conduct an accurate and detailed comparison of the dyes. 

Numerous variations and enhancements are available for many of the system components. Solvent delivery systems that operate with multiple solvents to produce programmable changes in composition during the course of the analysis are used often. This enables the system to also perform FfPLC analysis of additives or oligomeric resins such as phenolics and epoxies. For these applications it is also necessary to use additional detectors, such as the photodiode array (PDA) detector indicated in the middle... 

A more recent development in photometric detection is the photodiode array (PDA) detector which allows on-line recording of absorption spectra (e.g., Sims, 1993 Liang et al., 1993). As only a few standards are available, the identification of unknown pigments thus becomes much easier. It should, however, be noted that slight shifts in the absorption maxima may occur, as published spectra are usually obtained in pure solvents. Mantoura and Llewellyn (1983) and Latasa et al. (1996) have provided extensive lists of molar absorptiv-ities for a wide range of pigments. 

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