Selectivity wavelength filters

Flame Photometric Detector3 With the flame photometric detector (FPD), as with the FID, the sample effluent is burned in a hydrogen/air flame. By using optical filters to select wavelengths specific to sulfur and phosphorus and a photomultiplier tube, sulfur or phosphorus compounds can be selectively detected. 

Fluorophores can be visualized in fluorescence microscopy using special filter blocks that are composed of the excitation filter, dichroic mirror and emission filter. The excitation filter must select wavelengths of light from a light source that fall in the maximum absorption region of the fluorophore. The emission filter must pass the fluorescent wavelengths but not the excitation wavelengths. The dichroic mirror... 

Excitation filter selects wavelengths of light from a light source that fall in the maximum absorption region of a specific fluorophore. 

The Model 835 multiwavelength filter photometer (Fig.3.44) provides energy at 254 nm with a low-pressure mercury lamp and at 280,313,334 and 365 nm with a medium-pressure mercury source. Selected wavelengths between 380 and 650 nm are also available with a quartz-iodine light source. Absorbance ranges of 0.01-2.56 AUFS are provided. Short-term noise levels are 5 X 10-s AU with the low-pressure mercury source and 1 X 10 4 AU with the other lamps. The design and dimensions of the cell are the same as for Model 840. A 24-jtzl cell is standard with the medium-pressure mercury lamp and the quartz—iodine lamp. 

A multicolour focal plane array is presented in US-A-4956555. A first and a second group of filters each includes a dielectric/thin metal/dielectric layer combination. The filters of the two groups differ in that the thickness of the dielectric layers are adjusted to selected wavelength bands. 

When the pollutant of interest is present in the stack, the energy content of the reference path is unaffected (because the absorption is already complete at the selected wavelength). On the other hand, the IR energy reaching the detector through the neutral filter is reduced (due to the absorption of the pollutant gas), and the ratio between the beams reflects the pollutant concentration at the level of concern in the stack. 

The basic components of spectrophotometers are a light source, wavelength selector, absorption cell (cuvette), and photodetector. Colorimeters or absorptiometers commonly use nondispersive wavelength selection (a filter with bandwidth 4 -40 nm) and solid state or simple phototube detectors, while spectrophotometers employ a prism or grating monochromator (with bandwidth down to 0.2 nm) and a photomultiplier. Colorimeters are inexpensive and most appropriate for repetitive measurements of absorption at a fixed wavelength. The more expensive spectrophotometer can also fulfill this function, but its main purpose, by virtue of its accurate and variable wavelength control, is the measurement of absorption spectra. 

Fixed-wavelength detector using mercury, zinc, or cadmium lamps with wavelength selection by filters... 

Variable-wavelength detector using a deuterium or tungsten lamp with wavelength selection by a monochromator Filter photometer using a deuterium lamp with wavelength selection by filters Scanning ultraviolet (UV) detector Photodiode array detector... 

Grating-grating fluorometers are convenient for method development, because they permit selection of any excitation or emission wavelength. Filter-filter instruments, on the other hand, are simpler, easier in use, less expensive, more sensitive, and better suited for transferring an HPLC method between laboratories. 

The monochromatization using a P-filter employs the presence of the K absorption edge to selectively absorb Kp radiation and transmit the Ka and Kai parts of the x-ray spectrum, as shown schematically in Figure 2.12. Thus, a properly selected P-filter material has its K absorption edge below the wavelength of the Kaj characteristic line and just above the wavelength of the Kp line. ... 

Figure 2.12. Left - the schematic of the x-ray emission spectrum shown as the solid line overlapped with the schematic of the u(X) function of the properly selected P-filter material (dotted line). Right - the resultant distribution of intensity after filtering as a function of the wavelength.

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