Scanning monochromator based instrument

The first commercial instrument dedicated to NIR was developed by Dickey-John and presented in 1971 at the Illinois State Fair (Table 2). The technology was licensed by Technicon and introduced soon thereafter. Technicon then developed its own scanning monochromator-based instrument. This was followed by instruments from Pacific-Scientific (now FOSS-NIR Systems) and LT Industries, to name a few companies. 

There are several measurement techniques that can be considered for use in IR-based instrumentation in the NIR and the mid-IR spectral regions, and these are summarized in Table 4.1. Some of these techniques are classical, as in the case of optical filter-based instruments and scanning monochromators. Others, such as acousto-optically tunable filter (AOTF), have been considered for 15+ years, but are mainly applied to NIR... 

No mulhplex or multichannel technique was available for the measurement of Raman spectra hence, the instruments were based on scanning monochromators with a single PMT detector. 

There are four basically different instrument designs, based on how incident energy is selected (McClure, 1994). These are grating instruments (monochromators), Fourier transform instruments, filter instruments and Diode Array-based instruments. Figure4 illustrates the basic design of a scanning NIR spectrophotometer. 

In light of the fact that Fourier transform instrumentation was largely responsible for expanding Raman spectroscopy into the analytical laboratory, it is perhaps interesting to consider why Raman spectroscopy is so popular today but Fourier transform Raman does not play the dominant role. After a discussion of the poor sensitivity of NIR Raman spectrometry using a scanning monochromator with PMT detection in Section 18.1, it was stated To improve this situation, either a multichannel or multiplex measurement was needed and the multiplex measurement came first. Multichannel measurements came very shortly afterward, however, and instruments based on polychromators with silicon-based charge-coupled-device (CCD) array detectors have become more popular than FT-Raman spectrometers. In this section we compare the performance of FT- and CCD-Raman spectrometers. 

It is important to scan at the wavelength of maximum absorption of the solute in the sorbent, where the absorption maximum for a compound adsorbed on silica gel particles is typically not the same as that for the same compound in an alcohol solution. Precise and sensitive determination requires that the absorption maximum be determined in situ on the plate. This is possible with an instrument equipped with sources of variable wavelength or monochromators. Narrow-bandpass filters are available for instruments based instead on filters. 

The older, conventional instruments are known as dispersive spectrometers, where the infrared radiation is divided into frequency elements by the use of a monochromator and slit system. Although these instruments are still in use today, the recent introduction of Fourier transform infrared (FT-IR) spectrometers has revitalized the field (4). The FT-IR system is based on the Michelson interferometer. The total spectral information is contained in an interferogram from a single scan of a movable mirror. There are no slits, and the amount of infrared energy falling on the detector is greatly enhanced. Together with the use of modem computer techniques, an entirely new breed of instrument has been created. 

To determine the structural parameters of these M-plane MQWs by HRXRD, we follow the same procedure as employed in Ref. 67. Symmetric X-ray o)-20 scans were taken with a Philips X Pert PRO triple-axis diffractometer with a GuXai source, a Ge(220) hybrid monochromator, and a Ge(220) three-bounce analyzer crystal. We first analyzed the angular positions of the satellite peaks kinematically based on the nominal growth times to obtain the structural parameters of the sample, implicitly assuming that segregation does not occur. Next, we employed simulations based on the dynamical diffraction theory [67] and varied the kinematically obtained parameters, until the intensity of the satellites matched the experiment in addition to their position. The simulations were performed for perfect interfaces and are convoluted with the instrumental resolution (25 ) only. As an example. Figure 6.15 shows (o-20 scans for sample MQW-A across the (a) GaN(nOO) and (h) GaN(2200) reflection. 

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