Optical systems double-beam optics

Double-beam AA spectrophotometers are still marketed by instrument manufacturers. A double-beam system compensates for changes in lamp intensity and may require less frequent re-zeroing than a single-beam instrument. These considerations had more merit some years ago when hollow cathode lamps suffered from some instability. It should be noted, however, that the optical... 

The need for improved background correction performance has generated considerable interest in applying the Zeeman effect, where the atomic spectral line is split into several polarised components by the application of a magnetic field. With a Zeeman effect instrument background correction is performed at, or very close to, the analyte wavelength without the need for auxiliary light sources. An additional benefit is that double-beam operation is achieved with a very simple optical system. 

The above two modifications have been duly incorporated in a double-beam spectrophotometer. Figure 21.6, depicts the schematic diagram of the optical system involved in a Lambda-2 microcomputer-controlled UV-VIS Spectrophotometer (Perkin-Elmer). 

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

FIGURE 3.4. Optical system of double-beam IR spectrophotometer. 

The crystal absorption spectra were obtained with a Zeiss instrument modified for double beam operation. It was equipped with a calcite polarizer and quartz optics which allowed accurate spectral measurements up to 45,000 cm.-1. The optical system was designed to record spectra of very small crystals (about 2 mm.). The details of the design and operation of the crystal spectrophotometer will be reported elsewhere. ... 

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. 

A true double beam spectrometer requires separate optical paths for sample and reference, and involves simultaneous comparison of the two. It is possible to construct a pseudo-double beam spectrometer, employing only one detector and optical path, taking advantage of the capabilities of a computer system attached to the detector. 

Forward optic spectrophotometers are either singlebeam or double-beam spectrophotometers. The singlebeam instruments can be either very simple or expensive depending on the sophistication desired or needed. Simple single-beam instruments have poor stability and excessive drift. These advantages are eliminated in systems equipped with a high-resolution monochromator with adjustable slits, controlled by microprocessors for rapid data acquisition and evaluation of data. 

Using a mode-locked Nd + YAG laser system to generate picosecond sample excitation pulses and picosecond probing continuum pulses in their double beam spectrometer, Spalink et. al. (30) were able to measure difference absorption spectra of irradiated samples of 11-cis-rhodopsin and 9-cis-rhodopsin at selected times after excitation by means of a PAR OMA-2 optical multichannel detection system. The difference absorption spectral data were obtained over the entire spectral range from 410 nm to 650 nm at one time with an OMCD as opposed to the... 

Early UV-Vis and IR spectrophotometers, back in the 1950s, were big clunkers that usually had double-beam monochrometers to compensate for opti-cal drift and electronic noise They were slow and only moderately sensitive. Improvements in optical and electronic technology have reduced the necessity for double-beam optical systems that reduce, the energy of the transmitted beam. Modem single-beam instruments are smaller faster, more sensitive, and more economical than the older versions. But double-beam instruments still provide the optimal stability and the choice depends on your need. All modem dispersive IR instmments are single beam. 

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. 

There are two main advantages of double-beam operation over single-beam operation. Very rapid monitoring of sample and reference helps to eliminate errors due to drift in source intensity, electronic instability, and any changes in the optical system. Also, double-beam operation lends itself to automation—the spectra can be recorded by a strip-chart recorder. 

The double-beam system is used extensively for spectroscopic absorption studies. The individual components of the system have the same function as in the single-beam system, with one very important difference. The radiation from the source is split into two beams of approximately equal intensity using a beam splitter, shown in Fig. 2.28. One beam is termed the reference beam, the second beam, which passes through the sample, is called the sample beam. The two beams are then recombined and pass through the monochromator and slit systems to the detector. This is illustrated schematically in Fig. 2.28. In this schematic, there is a cell in the reference beam that would be identical to the cell used to hold the sample. The reference cell may be empty or it may contain the solvent used to dilute the sample, for example. This particular arrangement showing the monochromator after the sample is typical of a dispersive IR double-beam spectrophotometer. There are many commercial variations in the optical layout of double-beam systems. 

Figure 2.28 Schematic diagram of a double-beam optical system.

The two main sources of background absorption (i.e., absorption from material other than the sample) are the solvent used for liquid solutions and the air in the optical light path. In a conventional double-beam dispersive system, comparing the sample beam to the reference beam and recording the difference spectrum in real time automatically eliminate absorption from air and solvent. If the sample is a liquid solution, a matching liquid cell with pure solvent is placed in the reference beam. The absorption from the solvent and from the air is measured simultaneously and subtracted from the sample beam signal. 

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