Sector mirror

The splitting and recombination of the beam is accomplished by means of two rotating sector mirrors which are geared to the same electric motor so that they work in unison (Fig. 17.12). The microprocessor which is used to operate such an instrument will automatically correct for the dark current of the photocell, i.e. the small current which passes even when the cell is not exposed to radiation. 

The second type of double-beam instrument is one where the light source is divided into two beams by a rotating sector mirror that alternately reflects and transmits the light. This results in a chopped beam of light that alternately passes through the reagent blank and the sample as shown in Fig. 5.18. 

Figure 25-20c illustrates a double-beam-in-time spectrophotometer. Here the beams are separated in time by a rotating sector mirror that directs the entire beam through the reference cell and then through the sample cell. The pulses of radiation... 

Sector mirror A disk with portions that are partially mirrored and partially nonreflecting when rotated, directs radiation from the monochromator of a double-beam spectrophotometer alternately through the sample and the reference cells. 

I igure 13-21 shows construction details of a typical, relatively inexpensive, manual, double-beam ultraviolet-visible spectrophotometer In this instrument, the radiation is dispersed by a concave grating, W hich also focuses the beam on a rotating sector mirror. The instrument design is similar to that shown in Tig ure 13-13c. 

Flipping a polariser between 0° and 90° can be accomplished at a rate of about one turn per second. Faster multiplexing rates can be obtained from the system shown in Fig. 5.17. Ip and Is are separated by a beamsplitter and two polarisers or a polarising beamsplitter and multiplexed into one detector by a rotating sector mirror. The routing signal is derived from the rotation of the mirror. An optical system of the design shown in Fig. 5.17 is described in [28]. 

The drawback is that the optical system is complicated. A sector mirror is not commonly available, and it must be well aligned on the driving shaft to avoid wobbling of the reflected beam. Although the same detector is used for both light paths, it is not simple to obtain exactly the same IRF in both channels. In particular, the illuminated areas of the detector for the 0° beam and the 90° beam must coincide exactly. 

By means of a rotating chopper with sector mirror, the radiation from the analyte line-like source and the radiation from a continuum source are passed alternately through the atomizer (Figure 75). Both radiation beams fall on the same detector after passing through the monochromator. The ratio of both radiation intensities is measured. 

After leaving the monochromator the radiation is directed to the sample compartment by a rotating sector mirror, where it is alternately focused on the substance to be examined (which is contained in a cell with quartz windows) and a reference cell (which holds the pure solvent used to dissolve the sample). The system now has two beams, hence the name double-beam spectrophotometer. After passing through the sample where the absorption of radiation may occur, the beams are recombined. 

The beam splitting usually occurs after the monochromator. Rotating sector mirrors are commonly used for splitting or chopping the beam (Figure 8.18). The chopped beams reach sample and reference and subsequently to the detector at intervals which depend upon the rotational frequency of the chopper. The device then records the ratio of the reference and sample signals. 

The sector mirror C alternately reflects reference-beam energy and transmits sample-beam energy through the remainder of the system. The rate at which a spectrophotometer can be scanned is essentially limited by the speed of this sector, which in turn is determined by the speed of response of the detector. If the latter is a thermocouple or a metal bolometer, the most commonly used detectors in analytical instrumentation, the chopping frequency is generally in the 10-13 Hz range. 

The synchronous rectifier S is mechanically or electrically coupled to the sector mirror. It converts the amplified low-frequency output of the detector to direct current. The rectifier is phased with the optical chopper mirror so that the polarity of the rectified output indicates the condition of unbalance of the optical null system. That is, one polarity indicates more energy in the reference beam than in the sample beam. 

Some lamps can be used for several elements. A low pressure mercury lamp can be used for the determination of mercury. However, because of some instability of die lamp output, a double beam spectrophotometer gives more reliable results than single beam instruments. In the former, a rotating sector mirror chopper splits die beam from the lamp into a reference beam and a sample beam, which passes through die burner or atomiser. A mirror combines the two beams which pass through the monochromator. Then the ratio of the intensity of the two pulses is electronically measured, thus elminating any fluctuation of the lamp output. Instruments display absorbance and/or u.v. transmission. 

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