Double-beam spectrophotometers types

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to the single beam instruments for cloudy solutions. The bandwidth of high performance instruments can be as small as 0.01 nm. For routine measurements such as monitoring a compound on a production line, an immersion probe is employed. Placed in the sample compartment of the apparatus this accessory contains two fibre-optics, one to conduct the light to the sample and another to recover it after absorption in the media studied. Two types exist by transmission for clear solutions and by attenuated total reflection (ATR) for very absorbent solutions (Figure 9.17). 

By use of modern double-beam spectrophotometers equipped with some type of a reflectance attachment. rx is automatically plotted against the wavelength. Many investigators replot the data as percent reflectance (%R), or plot by use of a remission-function table (4) f(rx) or k/s as a function of wavelength or wavenumber. The most common method is probably the former preceding. 

One source of noise of ihls type is the slow drift in the radiant output of the source. This type of noise can be called source flicker noise (Section 5B 2). The effects of nuctuations in the intensity of a source can be minimized by the use of a constani-voUagc power supply or a feedback system in which the source intensity is maintained at a constant level. Modern double-beam spectrophotometers (Sections I3D-2and I3D-3) can also help cancel the effect of flicker noise. With many instruments, source flicker noise does not limit performance. 

Alternatively, the sample and reference may be compared many times a second, as in double-beam instruments. The light from the source, after passing through the monochromator, is split into two separate beams—one for the sample and the other for the reference. Figure 7.14 shows two types of double-beam spectrophotometers. The measurement of sample and reference absorption may be separated in space, as in Figure 7.14A this, however, requires two detectors which must be perfectly matched. Alternatively, the sample and reference measurement may be separated in time as in Figure 7.14B this technique makes use of a rapidly rotating mirror or... 

Figure 7.14. Schematic diagram of two types of double-beam spectrophotometers. A The double-beam-in-space configuration. B The double-beam-in-time configuration.

Since this is a book concerned primarily with applications, no further details are given concerning instrumentation. The reader is referred to Alpert et al. (1970), in which are discussed an optical diagram of a double-beam spectrophotometer operating variables (resolution, photometric accuracy) components of infrared spectrophotometers (sources, types of photometers, dispersing elements, detectors, amplifiers, and recorders) special operating features, such as optimization of scan time and available instruments and their specifications. The books by Martin (1966), Conn and Avery (1960), and Potts (1963), and the chapter by Herscher (1966) are also recommended for details on some of these topics. 

A proper identification of the active enzyme-substrate complex requires accurate correlations of the kinetics of the complex with the over-all activity. With this method, many obscure aspects of the enzyme action are more readily understood for example, the loss of activity due to the formation of inactive forms of the enzyme can be directly measured as a decrease in the concentration of the active enzyme-substrate compound. For simultaneous measurements, a single-beam spectrophotometer for recording the enzyme-substrate compound kinetics at 405 mu and a platinum microelectrode are satisfactory. A double-beam spectrophotometer may be used with the second beam set at 230 m i for recording the hydrogen peroxide kinetics, at 268 m/i for recording ascorbic acid kinetics, or at 610 mu for recording malachite green kinetics with peroxidase. If a doublebeam spectrophotometer is not available, the enzyme-substrate kinetics may be recorded in one experiment and the over-all activity in a duplicate experiment. Some examples of these types of studies are included. 

A dispersive instrument is patterned after a double-beam spectrophotometer. Radiation at two fixed wavelengths passes through a cell containing the process stream to provide a continuous measurement of the absorption ratio. At one wavelength, the material absorbs selectively at the other wavelength, the material does not absorb or else exhibits a constant but small absorption. The ratio of transmittance readings is converted directly into concentration of absorber and recorded. This type of instrument can handle... 

This specific type of the double-beam optical-null recording spectrophotometer is termed so because it critically balances out by the help of optical means the differential between the two beams. 

The measurement was the same as that described in the previous paper (13), except that the spectrophotometer used was a model MPS-2000 double beam type (Shimadzu Co., Tokyo, Japan) with a quartz cell (10.0 mm in light pass length). 

Spectrometer type and manufacturer Super Scan 3 (Varian) UV-VIS-spectrophotometer, double beam ... 

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 Analysis. IR absorption spectra were determined on neat samples of shale oils to furnish data for estimates of various olefinic types of compounds. Samples were run on a high-resolution, double-beam grating spectrophotometer at 0.1-mm path length between KBr plates. Quantitative measurements were made using the cut and weigh method with baselines drawn from point to point of minimum absorption. 

Determining the Infrared Spectrum. To obtain the spectrum, slide the holder appropriate for the type of die that you are using into the slot on the infrared spectrophotometer. Set the die containing the pellet in the holder so that the sample is centered in the optical path. Obtain the infrared spectrum. If you are using a double-beam instrument, you may be able to compensate (at least partially) for a marginal pellet by placing a wire screen or attenuator in the reference beam, thereby balancing the lowered transmittance of the pellet. An FT-IR instrument will automatically deal with the low intensity if you select the "autoscale" option. 

As previously indicated, most of the spectrophotometers used for analytical purposes are double-beam instruments, and most of these are of the optical null type. A few are in the ratio-recording category. Almost all utilize thermal detectors, either thermocouples or metal bolometers, although some incorporate a Golay pneumatic detector over part or all of their spectral range. Sources employed by commercial instruments run the gamut of all the sources described previously. 

Most spectra seen in the literature are of the ratioed or double-beam type. A double-beam grating instrument is called a spectrophotometer. In this type of instrument the beam from the source is divided into two beams a sample beam and a reference beam. The sample is placed in the sample beam, and the two beams are alternately passed into the monochromator through the entrance slit, usually at 13 Hz. 

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