Photodiode array detector design

An HPLC detector measures the concentration (or mass) of eluting analytes4 by monitoring one of their inherent properties, such as UV absorbance. A detector can be universal to all analytes or specific to particular classes of analytes. Common detectors and their attributes are listed in the Table 4.2. Early HPLC detectors are spectrometers equipped with small flow cells however, most modern units are compact and designed solely for HPLC. The ubiquitous UV/visible variable wavelength absorbance and the photodiode array detectors (PDA) are covered in more depth in this section. Note that mass spectrometers (MS) and nuclear magnetic resonance spectrometers (NMR) are discussed in the section on hyphenated systems. 

Photodiode array detectors are a class of UV detectors. The main difference of this type of detectors from the classical UV detector is that photodiode array detectors scan the entire spectra from UV all the way to visible light. This detector type is very beneficial in the sense that with well-designed software, it is possible to select the best wavelength for every component in the sample analyzed. As such, the analysis can be carried out more accurately. 

The optical detection systems used in MIPs are the same as those used for other atomic spectrometers and can be either single or multichannel. Fourier transform-based spectrometers have also been used. Conventional optical systems are best designed if the plasma is viewed from the exit of the discharge tube, as is possible with the TMqio type cavity, rather than through the walls of the discharge tube, which become etched. The commercially available AED uses a computer-controlled silicon photodiode array detector which has multielement detection capability over segments of spectra. In recent years, MIP sources have also been investigated as ion sources for mass spectrometry. 

Figure 1 Schematic of the principal components of a photodiode array detector. This diagram = shows the normal order for the optical components in most PDA detector designs. <...

HPLC is designed to separate compounds from complex mixtures based on polarity, solubility, and size properties of each compound. High-pressure liquid chromatography coupled with a photodiode-array detector (HPLC-DAD) provides extensive information on polyphenol stmctures [6, 110]. 

Most spectrometers use tungsten-halogen lamps with quartz windows. Cells for absorption measurements are usually quartz or fused silica transparent up to about 3000 nm. Cell lengths vary from 0,1 to 10 cm. Detectors range from PbS and PbSe photoconductors to InSb and InAs photodiodes. Array detectors, such as InGaAs detectors, have also become available for the region. Several commercial UV-visible spectrophotometers are designed to operate from 180 to 2500 nm and can thus be used to obtain NIR spectra. 

For many applications, diode array detection has become routine. A photodiode array was used for simultaneous detection of 100 capillaries in zone electrophoresis and micellar electrokinetic chromatography (MEKC).1516 Deflection of a laser beam by acoustic waves was reported as a means to scan six capillary channels on a microchip.17 The design of a low-noise amperometric detector for capillary electrophoresis has been reported.18... 

The older IR instruments were invariably dispersive double-beam designs. These were often of the double-beam-in-time variety shown in Figure 25-20c except that the location of the cell compartment with respect to the monochromator was reversed. In most UV/visible instruments, the cell is located between the monochromator and the detector in order to avoid photodecomposition of the sample, which may occur if samples are exposed to the full power of the source. Note that photodiode array instruments avoid this problem because of the short exposure time of the sample to the beam. Infrared radiation, in contrast, is not sufficiently energetic to bring about photodecomposition. Also, most samples are good emitters of IR radiation. Because of this, the cell compartment is usually located between the source and the monochromator in an IR instrument. 

Modern spectrophotometers are equipped with multichannel detecting devices that contain a large number of photodiodes (a photodiode array) and enable simultaneous detection over the whole range of the spectrum. Details of the design and the advantages of using such detectors in spectrophotometric measurements have been presented [17-20]. 

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