Photodiode array multichannel detection

Figure 7. A comparison of photodiode array multichannel detection to photomultiplier tube single channel detection for the upconverted emission spectrum of coumarin 520.

Post-column detection of chlorophyll derivatives is often accomplished by ultraviolet and visible spectroscopic techniques, which take advantage of the strong electronic absorption spectra of these pigments (units F4.3 F4.4). While these methods have enjoyed wide application (Schwartz et al., 1981 Khachik et al., 1986), a major advance was made with the introduction of photodiode array (PDA) detection. Multichannel photodiode array detection allows for simultaneous monitoring of multiple wavelengths, resulting in the generation of online electronic absorption spectra of a compound as it elutes from the HPLC column. Because of the uniqueness of electronic absorption spectra of individual chlorophyll derivatives, these techniques have enjoyed extensive application for tentative identification of components from complex mixtures and extracts... 

Milano et al. [153, 154] and Cook [34] introduced an approach to derivative spectra by substituting electronic wavelength modulation for the mechanical systems used in derivative spectrometers. This effect is achieved by superimposing a low-amplitude, periodic wave form on the horizontal sweep signal. In this way spectra were generated. Warner et al. [155] applied a vidicon detector for fast detection of fluorescence spectra and obtained derivatives of the stored data by digital computation. Cook et al. [156] also made use of a silicon vidicon detector for multichannel operations in rapid UV-VIS spectrophotometers with the possibility of first-order differentiation. For the same purpose Milano et al. [93, 157] used a multichannel linear photodiode array for detection of spectra in polychromator optics and stored data manipulations (d ). Technical explanations of the principles of diode array and vidicon devices cem be found in [158-161]. 

Alfredson, T. A. and Sheehan, T., Recent developments in multichannel, photodiode-array, optical LC detection, /. Chromatogr. Sci., 24, 473, 1986. 

Figure 11.11—Multichannel detection, a) Multichannel detection with a diode array located in the focal plane. The light beam is diffracted by the concave dispersive system after travelling through the sample. Note the absence of an exit slit b) spectrum of a 1 1 000 solution of benzene in methanol. This spectrum represents a typical spectrum without smoothing and is obtained with commercial photodiodes (note in contrast to mid IR spectroscopy, interferometry followed by Fourier transform has led to few commercial achievements in this area).

The advent of multichannel detection methods, such as photodiode array detection (DAD), for monitoring chromatographic processes has opened up new prospects for the resolution of overlapping peaks. 

Multichannel instruments These are equipped with a photodiode array detection system. The radiation from a tungsten or deuterium lamp is focused on the sample or solvent cell, and then passes to a diffracting grating. The scattered radiation arrives at the diode array, which simultaneously detects and analyzes various wavelengths. 

The optical multichannel photodiode array most used in Raman spectroscopy is the IPDA consisting of a one-dimensional array of amplified photodiodes. The mechanism behind the IPDA detection is that each photodiode converts photons to separated electron-hole pairs (semiconductor-amplification device). In some solid-state detectors (e.g.,... 

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]. 

Photodiode Array Versus Photomultiplier Detection. The advantages of photodiode array detection, PDA, as compared to photomultiplier tube, pmt, detection for emission spectroscopy are well known (19). These advantages are especially important for the specific examples we discuss here, namely, upconverting emission spectroscopy. This is dramatically demonstrated in Figure 7 where we compare single channel pmt versus multichannel PDA detection of a small portion of the upconverted fluorescence spectrum of coumarin 520 in ethanol solvent at room temperature. 

We have also studied the digital signal intensity versus light intensity behavior of the photodiode array detector used in method B. Non-linear effects have been observed for Vidicon multichannel analyzers when employed in measuring the intensity of picosecond duration light. Photodiode arrays, in contrast, have rarely been employed in picosecond spectrometer, and, we are not aware of a previous study of the linearity of these devices for the detection of high intensity picosecond duration light. 

Another example of the use of optical multichannel detection is the picosecond spectroscopic study of acridine, s-tetrazine, and rhodamine B by Barbara et. al.(26) In this type of study, a sample containing the compound of interest is placed in the path of an A,6-ps FWHM laser pulse. The laser pulse is focused onto the sample cell, and the emitted light is collected and directed, by an assembly of lenses, into a streak camera which is capable of time-resolving the emission. Processing and analysis of the streak camera data are accomplished by means of an assembly consisting of a two-dimensional photodiode array... 

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. 

Most multiparametric procedures are restricted to two active components, and the main challenge is in using different reagents and detection wavelengths. The use of photodiode array spectrophotometers and, more recently, multichannel fiber optic spectrophotometers facilitates this approach. Solenoid valves, to control the flow system, have proved to be very useful in the development of techniques for simultaneous determination of Zn, Fe, Cu, Ca, and Mg in pharmaceutical drugs. Derivative... 

Multichannel spectrometers which allow the simultaneous determination of a large number of elements, as in atomic emission spectrometry have not encountered a breakthrough in AAS yet. However, over a number of years, work with high-intensity continuous sources and high-resolution echelle spectrometers for multielement AAS determinations has aroused some interest [160]. Fourier transform spectrometry and multichannel detection with photodiode arrays has opened new perspectives for the simultaneous detection of... 

AAS determinations deserves attention and recently led to commercially available instrumentation (see, e.g., Ref. [286]). This was fostered by the fact that Fourier transform spectrometry and multichannel detection with photodiode arrays opens up new prospects for the simultaneous detection of a larger number of spectral lines and by the considerable improvements in high-intensity sources. 

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