UV photodetectors

The detector itself consists of a small liquid flow cell through which the eluent from the column flows. UV light passes through the cell and hits the UV photodetector. The cell is usually made of quartz which is UV-transparent. To avoid band broadening, the flow cell volume is minimised about 10-15 pL in the case of a standard flowcell or 6-8 pL in the case of a semimicro flowcell, the choice of which depends, among other factors on the column and the application. 

Both SiC p-n diodes and Schottky barrier diodes are utilized as UV photodetectors operating in the 200 nm to 450 nm range. They can be used up to temperatures of at least 700 K and are expected to be radiation tolerant. 

Wu H, Sun Y, Lin D, Zhong R, Zhang C, Pan W (2009) GaN nanofibers based on electrospinning facile synthesis, controlled assembly, precise doping, and application as high performance UV photodetector. Adv Mater 21(2) 227-231. doi 10.1002/adma.200800529... 

ZnO-based SAW devices have been used for UV photodetection [206,207] and for gas [208-210] and biochemical [211] sensing. The SAW UV photodetector is based on the acousto-electric interaction. The photogenerated carriers increase the conductivity in the material, which in turn increases the attenuation and decreases the SAW velocity, and therefore, the SAW frequency. The change in the velocity Av, in a semiconductor/piezoelectric material bilayer due to acousto-electric interaction is given by [212]... 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

The developed prototype includes a source of ultraviolet (UV) radiation (1) with the wavelength of 350 nm, two photodiodes (2 and 3) based on a silicon monocrystal and placed at the angle of 20-25° relative to the plate with sNPS layer (4) and a photodiode (5) for detection of the incident UV light (Fig. 9.6). Upon adsorption of biomolecules the level of the sNPS photoluminescence and the output of the voltage of the consecutively connected photo detectors decrease. Use of two photodetectors of photoluminescence increases the biosensor sensitivity. 

The chemical species, whether they are charged positively or negatively, usually migrate towards the cathode (cf. 8.3.2). A detection system is placed near the end of the capillary. In the UV mode, for example, the capillary is inserted in the optical path between the source and the photodetector. This allows measurement of the absorbance of the solution while avoiding dead volumes. Electrochemical detection is conducted in a similar way microelectrodes are placed within the capillary. 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

Laboratory spectrophotometers and fluorimeters which allow monitoring of reactions that take place within a few seconds are fairly routine now, and time resolutions of tenths of a second are available detailed descriptions are available in texts on spectroscopy [26]. Solution cells may be glass or plastic for light in the visible range, but quartz cells are needed for UV work. In an absorption spectrophotometer, the light source and photodetector are in line. In a fluorimeter, the detector is at 90° to the incident light, so cells must have four optical faces. 

Photomultipliers and photodiodes are used in the UV-Vis and in the NIR-IR regions, respectively. In general, the optical system comprises different interchangeable boxes containing different photomultipliers or photodiodes so that a wide spectral range can be covered. The response time and the sensitivity of a photodetector can be varied by appropriate changes in the electronic circuits this can generate... 

Figure 6.2 Block diagram of a UV-visible absorbance detector. Light from a source, S, is focused into a flow cell, C, through which the HPLC eluent is passed, and the transmitted light intensity is measured at a photodetector, D. A wavelength selection element, W, is placed after the source. A second light path is produced by the beamsplitter, BS, passing through a reference flow cell, RC, to the reference photodetector, RD. L are lenses M is a mirror.

The main attraction of fluorescence detection for HPLC is that for strongly fluorescent molecules, it can offer limits of detection two or three orders of magnitude lower than UV absorbance methods. The reason for this lies in the difference in the nature of the measurements. In a UV absorbance detector the photodetector is constantly illuminated at a high... 

UV/vis absorbance detection is widely used in capillary electrophoresis. Absorptivity depends on the chromophore (light-absorbing part) of the solute, the wavelength of the incident light, and the pH and composition of the run buffer. A photodetector measures light intensities and the detector electronics convert this into absorbance [11]. 

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