Monochromators UV/VIS

Only in the case of some alkali metals which have some few widely spaced resonance lines across the visible spectrum, may this be achieved by simple filter monochromators. For the determination of most elements, however, high quality UV/VIS monochromators are required that are capable of achieving a spectral bandwith of the order of 0.1 nm. Such monochromators are nowadays implemented in most commercial AA instruments. The optical components used (particularly, monochromators and detectors) are very similar to those used for emission spectroscopy and will be discussed in more detail later. 

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. 

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 optical path for flame AA is arranged in this order light source, flame (sample container), monochromator, and detector. Compared to UV-VIS molecular spectrometry, the sample container and monochromator are switched. The reason for this is that the flame is, of necessity, positioned in an open area of the instrument surrounded by room light. Hence, the light from the room can leak to the detector and therefore must be eliminated. In addition, flame emissions must be eliminated. Placing the monochromator between the flame and the detector accomplishes both. However, flame emissions that are the... 

A UV/VIS spectrophotometer consists of three components the source, the dispersive system (combined in a monochromator) and a detector. These components, which can be used independently to design a system appropriate for a desired application, are typically integrated into the same instrument to make spectrophotometers for chemical analysis. The sample can be placed in the optical path before or after the dispersive system (see Figs 11.2 and 11.3) and recorded spectra can be treated using a number of different computer algorithms. 

Fig. 7.3 Experimental setup for the nanosecond laser Flash Photolysis with a white light continuum. A Brilland-Quantel Nd YAG laser delivers the fundamental pulses (355 and 532 nm). A pulsed XBO lamp is used as white light source. The laser signal is split in order to trigger the digital storage oscilloscope (DSO) utilizing a second photodiode (PD). Two separate detection units in different geometries—photomultiplier (PMT) in front face and a PD in side face—detect the signal in the UV/vis and NIR region, respectively. The monochromator is operated by a standard PC...

In UV-vis-NIR spectrometers, the monochromator and detector are switched simultaneously. Step-like artifacts can be generated at this switch, and it is then questionable which part of the spectrum represents the correct absolute intensity. By nature, NIR detectors are susceptible to thermal radiation, and the step at the change-over to or from the NIR range and also the noise in the NIR range increase with temperature (Melsheimer et al., 2003). Sometimes authors present the UV-vis and NIR sections of the spectrum separately, disguising step-like artifacts at the transition. 

Cary 14 diagram (ca. 1953) The arrows on the optical diagram trace the path of the UV and vis radiation through the instrument. Radiation from the D2 or W lamp is directed to the monochromator entrance slit D by appropriate lenses and mirrors. From mirror E it travels to prism F where it is refracted, then to mirror G which reflects it to variable-width intermediate slit H. Mirror I reflects the radiation to grating J and from there the monochromatic beam is directed to mirror K and exits the monochromator through slit L. Semicircular mirror O, driven by motor Q, chops the beam at 30 Hz and alternately sends half the beam to the reference and half to the sample. Elements V, V1, W, and W1 pass the separated beams to the phototube. The light pulses of the two beams are out of phase with each other so that the phototube receives light from only one beam at a time. The photomultiplier for UV-vis work is shown at X and the NIR detector for 700-2600 nm is shown at Y. 

Bulk photolyses were carried out using a 1000-W, high pressure, Xe lamp (Model 6117, Oriel Corp., Stamford, Conn. 06902) and a UV-VIS grating monochromator. For purposes of comparison and evaluation of power, 8.5 mW of power at 425 nm is produced at the sample surface within the cuvette cell. The procedure for determining this is found elsewhere 3 ). Samples were loaded in a type 52-H 2mm light path quartz cell purchased from Precision Cells, Inc., Hicksville, NY. The amount of zeolite used was 0.3 grams. Solutions of 1.5 M isopropanol dissolved in acetonitrile were used for the bulk photolyses. 

The components of the fluorimeter include (Fig. 7.5) (1) power supply—powers the lamp (2) light source—UV -vis lamp (3) excitation monochromator—selects a particular wavelength of light from the lamp to excite the sample (4) sample—solution in a cuvette in a holder (5) emission monochromator—scans through a set of wavelengths where the sample emits light (6) detector—a photomultiplier tube PMT detects the number of... 

The crystal structure of as prepared samples was identified by using a powder X-ray diffractometer equipped with CuKa radiation (30kV, 20mA) and a monochromator. An infrared spectrometer was used for the chemical structure analysis. Chemical composition of samples was determined by EDX analysis. To determine the content of organic species in the composites, thermal gravimetric (TG) analysis was carried out at a heating rate of 10 °C/min in air. The BET surface area was determined by measuring N2 adsorption isotherms at 77 K. The microstructure of samples was observed by FE-SEM. Diffuse reflectance spectra were recorded with a UV-vis spectrometer. 

Figure 10.7 Schematic diagram of spectrometers and analysers in the infrared, (a) Single beam analyser containing a fixed monochromator or a filter used when a measurement at a single wavelength will suffice (b) dispersive spectrometer, double beam system. In contrast to spectrophotometers in the UV/Vis, the sample, located prior to the monochromator is permanently exposed to the full radiation of the source, knowing that the energy of the photons in this region is insufficient to break the chemical bonds and to degrade the sample (c) Fourier transform single beam model.

When the Cr02Cl2 adsorbed as chromate, such as on silica that had been calcined at 400 °C, normal polymerization activity was observed at 100 °C and a concentration of ethylene of 1.0 mol L-1 in isobutane. Indeed, the activity was nearly identical to that of Cr03/silica activated at 400 °C. The kinetics profile of the polymerization reaction was also the same, as shown in Figure 7. The polymer FILMI, MW, and MW breadth were also almost the same, as was the UV-vis reflectance spectrum. In contrast, the chlorochromate catalysts were not active for ethylene polymerization under these conditions. Thus, the monochromate species... 

As with UV-Vis detection, there are a number of variants of fluorescence detectors that largely mirror those employed in UV-Vis. The simplest fluorescence detectors employ simple fixed wavelength lamps and optical filters to set both the excitation and emission wavelengths. Designs of intermediate complexity (and cost) use filters to set the excitation wave length and a monochromator to detect the emitted light, whereas the most complex designs have monochromators for both the excitation and emission. Because fluorescence... 

So, using this approach, for a UV-Vis spectrophotometer, the source is a deuterium/ tungsten lamp, the sample is the cuvette or flowcell, the discriminator is the monochromator, the detector is a photomultiplier tube and the output device is a computer with an analogue-to-digital converter (Figure 1.2). 

---