Spectrometer sequential

Such double beam instruments can be controlled more easily in processes by relays or microprocessors. No mechanics for automatic exchange of sample and reference cells have to be included. The energetic efficiency of the light paths is lower. A double monochromator supplies higher quality photometry. The spectral resolution can be increased and the amount of stray light is drastically decreased. The slit in-between the two monochromator parts is essential. A high performance instrument is shown in Fig. 4.3. Such spectrometers are rather expensive but are very useful in the examination of complex photoreactions as well as in the measurement of problematic samples such as turbid solutions, viscous samples, or thin films. 

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Up to the present most Raman spectra were recorded with scanning instruments, sequential spectrometers, with excitation in the visible range. New developments are now changing this situation ... 

Dispersive spectrometers usually achieve this end by employing double or triple monochromators. Each monochromator distributes stray radiation of about 10 of the power of the exciting radiation over the entire spectrum. Consequently, sequential spectrometers for the scanning of Raman spectra employ multiple monochromators, i.e., double or even triple monochromators with additive dispersion (Fig. 3.5-1 a). 

For ICP-AES both sequential and simultaneous as well as combined instruments are used. In sequential spectrometers special attention is given to the speed of the wavelength access and in simultaneous spectrometers to the provision of background correction facilities. In combined instruments a number of frequently used channels are fixed and with a moving detector or an integrated monochromator... 

ICP/OES can be conducted either simultaneously or sequentially. Simultaneous instruments rely on a polychromator or direct-reading spectrometer to read up to 60 elements from the same sample excitation. Sequential analyses use a computer-controlled, scanning monochromator system. The light emitted by the sample in the plasma source is focused on the entrance slit of the monochromator and the spectrum is scanned through the region of interest. Typically, it is possible to determine several elements per minute in the sample in a sequential spectrometer. 

Single or double beam, normal optic sequential spectrometer... 

XRF. XRF (X-ray fluorescence) analysis was performed with a Philips PW1404 sequential spectrometer fitted with a scandium target x-ray tube and using UniQuant software (semi-quantitative, ppm to 100 weight percent sensitivity). A portion of the intact catalyst brick was removed from the inlet to the outlet and ground to a powder. The powder was then sandwiched between polypropylene films for analysis. 

Figure 8.14 Schematic of the optical path in a wavelength-dispersive sequential spectrometer, showing the positions of the collimators. [Courtesy of PANalytical, Inc., The Netherlands (www. panal)dical.com).]...

Figure 8.15 A sequential spectrometer with two tandem dectors, showing the placement of the collimators in the optical path. (From Jenkins et al., 1981, used with permission.)...

The analyzing crystal shown schematically in Fig. 8.14 has a flat surface. Flat crystals are used in scanning (sequential) spectrometers. Curved crystals, both natural and synthetic multilayers, are used in simultaneous spectrometers, electron microprobes, and for synchrotron X-ray spectrometry. The advantage to a curved crystal is that the X-rays are focused and the collimators replaced by slits, resulting in much higher intensities than with flat crystal geometry. This makes curved crystals excellent for analysis of very small samples. The use of a curved crystal and slits in a simultaneous spectrometer is illustrated schematically... 

Figure 115 A sequential spectrometer based on a Paschen-Range mount (Fison Instruments)...

The detection system of ICP-AES is of two types simultaneous and sequential spectrometers with radial viewed torches in Fig. 3. A simultaneous type, where many elements can be determined in a single scan, is displayed in Fig. 3a, and a sequential type where one element in a scan is determined is in Fig. 3b. 

There are three types of WDXRF instruments sequential spectrometers, which use a goniometer and sequentially measure the elements by scanning the wavelength simultaneous spectrometers, which use multiple channels, with each channel having its own crystal/detector combination optimized for a specific element or background measurement and hybrid spectrometers, which combine sequential goniometers or scanners with fixed channels as well as XRD channels and goniometers. Hybrid instruments will be discussed in Section 8.4 with XRD instruments. 

Multiple proportional counters are used in simultaneous X-ray spectrometers, described later, while one proportional counter is often used in tandem with an SC in a sequential system. It is for this reason that the detector has two windows as shown in Figure 8.38. X-ray photons pass through the proportional counter to the SC located behind it, as illustrated in Figure 8.31, and signals are obtained from both detectors. It should be noted that this tandem arrangement does not permit independent optimization of both detectors. There are sequential spectrometer systems available with independent proportional and scintillation detectors. 

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