Optical null

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. 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

In optical null instruments, radiation is passed through the sample and reference paths simultaneously or the instrument may be designed to pass the radiation through each path alternately (pre-sample chopping). A wedge or comb attenuator is moved in or out of the reference beam until absorption in both beams is equal. The movement of the... 

FIG. 1 Schematic diagram of a double-beam, optical null, infrared spectrometer. 

In polarimetry and ORD, the sample is placed between the first polarizing element (the polarizer), which remains fixed, and the second element (the analyzer), which can be rotated about the axis of propagation. Maximum intensity of the transmitted light is observed when the principal axis of the polarizer and analyzer are colinear and exactly parallel. The intensity is zero when they are crossed that is, when the principal axes are orthogonal to each other. The most accurate way to determine the rotation angle a is to set the polarizer and analyzer in the crossed position using an achiral substrate and to measure the extent to which the analyzer has to be turned to restore the optical null position when the achiral sample is replaced by a chiral substrate. 

Mattson also found that the 1375-cm 1 peak had the largest analytical error of those peaks measured. Part of the reason is that the peak is so strong that its absorbance approaches 1 in a 0.05-mm cell and 2 in a 0.1-mm cell. According to Hannah (36), thick cells in optical null instruments give nonlinear absorbance. They are linear only up to absorbances of 0.8 to 1.0. Ratio recording systems, such as that used by Mattson, are... 

The Use of 0.1-mm Cells. For routine use, we recommend the 0.05-mm sealed-demountable cells because some of their absorbances in 0.1-mm cells are in the nonlinear range for optical null instruments and the standard deviations rise significantly. Furthermore, a single sealed cell changes with continued use, particularly when some intractable spill samples may have traces of residual water. We have demonstrated that triplicate analyses of a No. 2 fuel oil show not even line width variations in a properly used sealed-demountable cell. 

Figure 16-11 shows schematically the arrangement of components in a typical IR spectrophotometer, l.ike many inexpensive dispersive IR instruments, it is an optical null type, in which the radiant pow cr of the reference beam is reduced, or attenuated, to match... 

Measurement of higher departure aspheres using beam conditioning optics (nulls) which may be diffractive, refractive, or reflective... 

The workhorse infrared instrument used for routine characterization of materials in the undergraduate organic laboratory is the optical-null double-beam grating spectrometer (Fig. 8.3a). For a discussion of double-beam spectrometers, see the UV-vis instrumentation discussion (p. 604). Although many... 

This instrument optically balances out the differential between the two beams. Therefore this kind of instrument is called optical null recording spectrometer. More sophisticated instruments are called ratio-recording instruments. In these instruments the intensities of both sample and reference beams are measured and ratioed. 

Answer. This idea that single-beam instruments are better for quantitative analysis arises from the fact that a single-beam instrument measures beam intensity directly, while in a double-beam optical null instrument it is the movement of a comb that is amplified as a measurement of intensity. The comb must be made linear throughout the range in which it is used, otherwise it will not give a linear relation between the intensity and the comb movement, and this will lead to errors in intensity measurements. 

Double-Beam Optical Null Systems. Virtually all instruments used for analytical chemical applications utilize a double-beam optical null photometric system. In these instruments an electrooptical. servo-system continually attenuates the energy in the reference beam so that there is no net signal difference between the reference beam and the sample beam. The recording pen indicates the position of the reference beam attenuator and therefore the relative transmittance of the sample. 

Figure 2-8 shows a schematic diagram of an optical null system. Some of the key components in this system may now be described. 

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. 

Figure 2-8. Schematic diagram of an optical null double-beam system. (Courtesy of Perkin-Elmer Corporation.)...

Having examined the working basis and requirements of a doublebeam optical null system, we shall next examine some of the properties of such systems of which the analyst should be aware. The accuracy of calibration of these systems is generally quite constant until the open area of the attenuator becomes very small. For very small openings it is virtually impossible to ensure precise calibration. The principal cause for this is the effect of slit width. If the slit is very narrow, the zero position of the attenuator is sharply defined. As the slit widens, the attenuator will have to move farther to stop the energy and produce zero signal. There is an analogous effect near 100%. 

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