Instrumentation monochromators

Dispersive Instruments. In dispersive instruments monochromators are employed for selection of the wavelength. When a line-like radiation source is employed, a monochromator of low resolution is adequate, but for a continuum radiation source a high resolution monochromator is required. In dispersive equipment the exit slit width is narrower than that in non-dispersive equipment. In this way, thermal background emission and stray light originating from the atomizer can be considerably decreased, but at the same time the optical transmission also decreases. The schematic construction of a dispersive AFS instrument is shown in Figure 144. 

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. 

Set a scan rate of 2 nm s if the instrument monochromators are not controlled by stepping motors. In the case where the monochromators are controlled by stepping motors set the step size to 0.5 nm and the integration time to 0.2 s. This step size will be such that most sample fluorescence spectra can be corrected. 

In addition to geometric requirements, AS/NZ 4399 describes the effect of fluorescent samples on diffuse transmittance measurements for two types of spectrophotometer designs. The discussion is specific to whether the instrument monochromator is placed before or after the sample in other words, whether the sample illumination is monochromatic or polychromatic. 

In addition, the GH and NH outlier tests should have the same sensitivity on the host instruments as they do on the master instrument. Our experience over the past 20 years with standardization of NIRS instruments, monochromators manufactured by others, filter instruments, and now the Infractec instrument has shown that this goal can be achieved using the methods described in this chapter. They have worked for ground and unground samples, dry and high moisture samples, solids, liquids and mixtures of the two, filter and monochromator instruments, and in reflectance and transmission. 

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

An instrument for measuring absorbance that uses a monochromator to select the wavelength. 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

Ultrasensitive Equipment In recent years all components of Raman equipment (laser, sampling optics, filtering, monochromator, and detector) have been clearly improved. This has led to an enormous increase in sensitivity and has enabled direct observation of adsorbed molecules with carefully optimized instruments without the need for further enhancement or resonance effects. 

Photomultipliers are appreciably more sensitive sensors than the eye in their response to line or continuum sources. Monochromators are fitted to the light beam in order to be able to operate as substance-speciflcally as possible [5]. Additional filter combinations (monochromatic and cut-off filters) are needed for the measurement of fluorescence. Appropriate instruments are not only suitable for the qualitative detection of separated substances (scanning absorption or fluorescence along the chromatogram) but also for characterization of the substance (recording of spectra in addition to hR and for quantitative determinations. 

When recording excitation and fluorescence spectra it must be ensured that monochromatic light falls on the detector This can best be verified in instruments built up on the kit principle or in those equipped with two monochromators (spectrofluonmeters) The majority of scanners commercially available at the moment do not allow of such an optical train, which was realized in the KM3 chromatogram spectrometer (Zeiss) So such units are not able to generate direct absorption or fluorescence spectra for the charactenzation of fluorescent components... 

In these instruments the monochromated beam of radiation, from tungsten and deuterium lamp sources, is divided into two identical beams, one of which passes through the reference cell and the other through the sample cell. The signal for the absorption of the contents of the reference cell is automatically subtracted from that from the sample cell giving a net signal corresponding to the absorption for the components in the sample solution. 

The first successful application of the continuous wave (CW) He-Ne gas laser as a Raman excitation source by Kogelnik and Porto (14) was reported in 1963. Since that time, significant improvements in instrumentation have been continually achieved which have circumvented a great number of problems encountered with mercury lamp sources. The renaissance of Raman spectroscopy has also been due to improvements in the design of monochromators and photoelectric recording systems. 

---