Photometers double-beam

The photometers are installed in the upper part of the analytical console. Each analytical channel is fitted with a double-beam filter photometer. The wavelength range is 340—900 nm. Flow cells are available in lengths of 10, 30 and SO mm. Detectors such as a flame photometer, conductivity cell or pFI electrode can also be connected externally when required. 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

Each correction may be applied separately. It must be noted that the application of a protein factor is essentially independent of the application of a paper factor, which accounts for deviation from Beer s law. This last correction is a systematic error due to reading on paper and can be fitted either into the scale of the photometer or into the wedge of a double-beam automatic reader. The protein factor may then be applied afterward to particular fractions. In the case of planimetric integration a corrected base line can be drawn, but this is rarely done. 

All. Ash, K. C., and Piepmeier, E. H., Double-beam photon-counting photometer with dead-time compensation. Anal. Chem. 43, 26-34 (1971). 

Modern instruments isolate a narrow wavelength range of the spectrum for measurements. Those that use filters for this purpose are referred to as filter photometers those that use prisms or gratings are called spectrophotometers. Spectrophotometers are classified as being either single or double-beam. 

Many modem photometers and spectrophotometers are based on a double-beam design. Figure 25-20 shows two double-beam designs (b and c) compared with a... 

Figure 25-20 Instrument designs for UV/visible photometers or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector. In (b), a double-beam-in-space instrument is shown. Here, radiation from the filter or monochromator is split into two beams that simultaneously pass through the reference and sample cells before striking two matched photodetectors. In the double-beam-in-time instrument (c), the beam is alternately sent through reference and sample cells before striking a single photodetector. Only a matter of milliseconds separates the beams as they pass through the two cells.

Rhodamine B was added to the tank inflow, and the resulting effluent concentration was recorded as a function of time in a double-beam (Zeiss) spectral photometer. These measurements were evaluated statistically in order to describe the hydraulic characteristics of the tanks. Effluent concentrations of the tracer, dosed as a delta impulse, observed for different (hydraulic) surface loading (q = 0.28, 0.58, and 1.0 m/h) were evaluated in terms of a characteristic (dimensionless) number, the dispersion number (4), and shown in Figure 3. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

A schematic diagram of the optical layout of a typical LC spectrophotometer is shown in Figure 6.18. The instrument is described as a double-beam photometer, although the reference beam in fact passes directly onto a reference photocell. 

Figure I3-I6b is a schematic representation of a double-beam photometer used to measure the absorbance of a sample in a flowing stream. Here, the light beam is split by a iwo-branched (bifurcated) fiber optic, which transmits about 50% of the radiation striking it in the upper arm and about. 50% imho lower arm. One beam passes through the sample, and the other passes through the reference cell. Filters are placed after the cells before the photodiode transducers. Note that this is the double-bcam-in-spacc design, which requires photodiodes with nearly identical response. The electrical outputs from the two photodiodes arc converted...

FIGURE 13-16 Single-beam photometer (a) and double-beam photometer for flow analysis (b). In the single-beam system, the reference cell is first placed in the light path and later replaced by the sample cell, tn the double-beam system (b). a liber optic splits the beam into two branches. One passes through the sample cell and the other through the reference cell. Two matched photodiodes are used in this double-beam-in-space anangement. 

The infrared spectra of penicillin analogues have been discussed. The infrared spectrum of ampicillin trihydrate was measured on a Perkin-Elmer Model 21 double-beam spectro-photometer S. Ttie infrared absorption of penicillin derivatives has been recorded and discussed. Figure 1 and Figure 2 are the spectra of the Squibb Primary Reference Substances of ampicillin trihydrate and anhydrous ampicillin recorded as mineral oil mull and potassium bromide pellets with a Perkin-Elmer Model 21 spectrophotometer. Interpretation of the spectrum of ampicillin trihydrate has been re-ported °and is given in Table 1. 

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. 

The following schematic diagram shows a different 90 scattered light photometer by the same company, working on the double-beam principle. 

The dispersive infrared analyzer, which is similar to the double-beam spectrometer, is capable of analyzing components of liquid and gaseous process streams. In contrast, the nondispersive infrared analyzer, which is a filter photometer, is more suited to the selective analysis of components of gaseous process streams. In general, the design and operation of nondispersive infrared analyzers are simpler than those of dispersive infrared analyzers. Also, the use of filtering techniques with the nondispersive infrared analyzers increases the selectivity of the analysis, but this can also reduce sensitivity. These infrared analyzers are commonly used for the analysis of... 

Ultraviolet photometers are also available for continuously monitoring the concentration of one or more constituents of gas or liquid streams in industrial plants, rhe instruments are double beam in space (sec Figure 1.3-16b) and often employ one of the emission lines of mercury, which has been isolated by a filter system. Typical applications include the determination of low concentrations of phenol in wastewater monitoring the concentration of chlorine, mercury, or aromatics in gases and the determination of the ratio of hydrogen sulfide to sulfur dioxide in the atmosphere. 

Many mixlern photometers and spectrophotometers are based on a double-beam design. Figure 13-13b illustrates a double-beam-in-space instrument in which two beams are formed in space by a V-shape mirror called a beamspHuer. One beam passes through the reference solution to a pholodetector, and the second simultaneously traverses the sample to a second, matched detector. The two outputs are amplified, and their ratio (or the logarithm of their ratio) is determined electronically or by a computer and displayed by the readout device. With manual instruments, the measurement is a two-step operation involving first the zero adjustment with a shutter in place between selector and... 

Figure 10. Schematic diagrams for (a) simple fixed wavelength UV photometer-flow-through cell module and (b) variable wavelength double beam UV photometer-flow-through cell module.

These detectors may be divided into the three types shown in Section 1 of Table 2, namely (i) simple fixed wavelength, (ii) dispersion type (prism or grating) variable wavelength double beam UV rectrc hotometers and (iii) diode array UV photometers. Figure 10 consists of schematic diagrams of the first two types of detectors. 

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