Double-beam IR spectrophotometer

The double beam is particularly useful in qualitative analysis and most IR spectrophotometers operate on the double beam principle. A simplified double beam IR spectrophotometer has been given in the following figure with its essential units and functions. 

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

The schematic optical diagram of a double-beam infrared spectrophotometer has been shown in Figure 22.3 as per Beckman Model IR-9. 

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. 

Part—IV has been entirely devoted to various Optical Methods that find their legitimate recognition in the arsenal of pharmaceutical analytical techniques and have been spread over nine chapters. Refractometry (Chapter 18) deals with refractive index, refractivity, critical micelle concentration (CMC) of various important substances. Polarimetry (Chapter 19) describes optical rotation and specific optical rotation of important pharmaceutical substances. Nephelometry and turbidimetry (Chapter 20) have been treated with sufficient detail with typical examples of chloroetracyclin, sulphate and phosphate ions. Ultraviolet and absorption spectrophotometry (Chapter 21) have been discussed with adequate depth and with regard to various vital theoretical considerations, single-beam and double-beam spectrophotometers besides typical examples amoxycillin trihydrate, folic acid, glyceryl trinitrate tablets and stilbosterol. Infrared spectrophotometry (IR) (Chapter 22) essentially deals with a brief introduction of group-frequency... 

As shown in Figure B2.1, double-beam spectrophotometers automatically record the true absorbance by measuring log(IR/Is), thanks to a double compartment containing two cuvettes, one filled with the solution and one filled with the solvent. Because the two cuvettes are never perfectly identical, the baseline of the instrument is first recorded (with both cuvettes filled with the solvent) and stored. Then, the solvent of the sample cuvette is replaced by the solution, and the true absorption spectrum is recorded. 

Figure 11.14—Optical path between the monochromator exit and the detector for two double beam instruments (rotating mirror model and semi-transparent mirror model). Instruments with rotating mirrors are similar to those used in IR spectrophotometers. However, the light beam from the source goes through the monochromator before it hits the sample. This minimises photolytic reactions that could occur if the sample is exposed to the total radiation from the source. The optics of instruments with two detectors are simpler and only one mirror, semi-transparent and fixed, is necessary to replace the delicate mechanisms of synchronised, rotating mirrors.

Figure 9.1. (a) Double-beam recording UV/visible spectrophotometer, (b) Double-beam recording IR spectrophotometer (courtesy of Beckman Instruments,... 

Figure 9.16 Optical path from the exit of the monochromator to the detector for two double beam instruments, (a model with two rotating mirrors and a model with a semi-transparent mirror). The arrangement of the apparatus with rotating mirrors is similar to that of IR spectrophotometers apart from the fact that the light beam issuing from the source passes first through the monochromator before it hits the sample. In this way the photolytic reactions which could occur owing to an overexposure to the total radiation issued from the source are minimized. A more compact and simple optical assembly with a single beam associated with two detectors. A semi-transparent and fixed mirror replaces the delicate mechanism of synchronized, rotating mirrors.

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. 

Spectroscopic Techniques. UV-visible spectra were obtained with a double -beam UV-visible spectrophotometer (Beckmann). The asphaltenes were examined as solutions in chloroform (0.005-0.1 g per mL) with chloroform in the compensating beam. IR spectra were recorded either as potassium bromide pellets (0.1 g of asphaltene per gram of potassium bromide) or as solutions in chloroform (0.1 g/mL). 

FIGURE 16-11 Schematic diagram of a double-beam, dispersive IR spectrophotometer. The heavy black lines indicate mechanical linkages, and the light lines indicate eieclrical connections. The radiation path is designated by dashed lines. 

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. 

The use of infrared spectroscopy for the characterization of polymer blends is extensive (Olabisi et al. 1979 Coleman and Painter 1984 Utracki 1989 He et al. 2004 and references therein Coleman et al. 1991, 2006). The applicability, fundamental aspects, as well as principles of experimentation using infrared dispersive double-beam spectrophotometer (IR) or computerized Fourier transform interferometers (FTIR) were well described (e.g., Klopffer 1984). 

Catalytic ammoxidation of m-xylene was studied with aid of IR spectroscopy. The samples were pressed into self-supported plates of S mg cm thickness and placed in a high temperature vacuum IR cell connected to the vacuum line. Prior to catalytic experiments the samples were activated at 473 K under dynamic vacuum ofl.33xl0 3 Pa. Then the mixture of m- qrlene (133-670 Pa) with NHj (670-930 Pa) and air (3330-4000 Pa) was introduced into the cell and heated for 1 h at 573 K. Appearance in the IR spectrum of the nitrile group band at 2240 cm was taken as indicative of catalytic transformation. The IR spectra of adsorbed species were recorded at room temperature on a UR-20 double-beam spectrophotometer (Zeiss, Jena). 

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. 

By changing the configuration of the two cells in the san le compartment of the IR spectrophotometer this enables the determination of either recirculating gas composition or plotting of the baseline spectrum for the catalyst wafer. With the two cells in the double-beam mode, the catalyst baseline and surface spectra are recorded. If the reference cell was placed in the sample beam and an air gap in the reference beam, quantitative absorption spectroscopy was possible. The IR cells thus provide information leading to both reaction rates and mechanistic insights concerning adsorbed species at reaction conditions. 

A Unicam attachment was used with a Beckman IR-20A infrared spectrophotometer in the specular reflectance studies. The spectrophotometer was operated in both the single and double beam modes. 

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