Fixed-wavelength spectrophotometer

Block diagram for a single-beam fixed-wavelength spectrophotometer with photo of a typical instrument. 

Visual methods have been virtually displaced for most determinations by methods depending upon the use of photoelectric cells (filter photometers or absorptiometers, and spectrophotometers), thus leading to reduction of the experimental errors of colorimetric determinations. The so-called photoelectric colorimeter is a comparatively inexpensive instrument, and should be available in every laboratory. The use of spectrophotometers has enabled determinations to be extended into the ultraviolet region of the spectrum, whilst the use of chart recorders means that the analyst is not limited to working at a single fixed wavelength. 

Kinetic measurements were performed on a Hitachi 150-20 UV/VIS spectrophotometer. Dehydrobrominations were studied in DMF solution using cyclohexyl amine (CHA) as the base. Applied CHA concentrations were 2, 2.5, 3, 3.5, 4 and 5 10 3 mole.dm-3, initial concentration of 1 was 5 10 5 mole.dm-3 in every case (pseudo-first-order conditions). Ionic strength was adjusted to lO l mole.dm 3 with potassium nitrate. Kinetic curves / D(t) / were recorded at fix wavelength, X = 290 ran and the temperature was maintained at 30, 35.5, 40°C. Stock solutions were made daily for la and freshly for every measurement of Ih. The reaction was started by injection of solution of 1 to the thermostated solution of CHA. 

Photometers are more sensitive than spectrophotometers, are cheaper and more robust and are well suited to routine work where monitoring at 254 nm or some other fixed wavelength is acceptable. Spectrophotometers, however, allow tuning to the most favourable wavelength either to maximize sensitivity for a particular solute or to detune the response to other solutes. By allowing monitoring down to 190 nm, weakly absorbing or saturated compounds can be detected. 

Figure 11.12—Schematic ant optica path of a single beam spectrophotometer equipped with electronic regulation (Hitachi U-1000). Measurements in solution are often carried out at a fixed wavelength after a calibration curve has been plotted. The use of higher performance double beam UV/Visible spectrometers is not necessary for these measurements in which the spectrum is not recorded. On the other hand, quantitative measurements from mixtures represent a different type of analysis.

IR spectroscopy is one of the oldest spectroscopic measurements used to identify and quantify materials in on-line or near-line industrial and environmental applications. Traditionally, for analyses in the mid-IR, the technologies used for the measurement have been limited to fixed wavelength NDIR filter-based methods and scanning methods based on either grating or dispersive spectrophotometers or interferometer-based FTIR instruments. The last two methods have tended to be used more for instruments that are resident in the laboratory, whereas filter instruments have been used mainly for process, field-based and specialist applications, such as combustion gas monitoring. 

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. 

Two types of photometers are used in HPLC— the fixed-wavelength filter photometer and the variable-wavelength spectrophotometer. These use a low-pressure mercury arc lamp as a light source, a transmission or interference filter to isolate a narrow band of wavelengths of light with which to excite the sample, and a photodiode to detect the light transmitted through the sample. 

It is probably best to use a variable-wavelength UV-Vis spectrophotometer and not a fixed wavelength instrument. These detectors are very stable and have excellent sensitivity. We have obtained an excellent baseline and chromatograms when the detector was set on 0.003 AU full scale. The variable-wavelength feature is useful because detectors of this type can be used to take advantage of rather small differences in absorbance spectra of various ions. 

An example of the former is the ultraviolet spectrophotometer which may be of fixed wavelength (usually 254 or 280 nm) or variable wavelength design. The detector functions by monitoring the change in absorbance as the solute passes through the detector flow cell, i.e. it utilises the specific property of the solute to absorb ultraviolet radiation. 

Circular variable filters can be used in scanning monochromators they are easily calibrated, have constant resolution, are rugged, and require little maintenance. The filters also have application in radiometers, where they have replaced sets of filters of fixed wavelengths. Another application is as order sorter for spectrophotometers. 

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