Beam condenser

KBr pellets are made in dies of various types, most of which require laboratory presses capable of delivering at least ten tons of pressure. Frequently, the pellets will require a special holder to enable the operator to mount them in the spectrophotometer. In other cases, the pellet is made directly in the holder, and the holder will require a special device for mounting in the spectrophotometer. This latter is especially true of microsize pellets, which usually are mounted in a beam condenser rather than directly on the instrument. 

Most dies produce circular pellets, although one manufacturer makes a die that produces rectangular pellets. Plastic-on-paper forms can be obtained that will fit into standard circular dies, and convert the die to produce rectangular rather than circular pellets. In this case, the pellet is made in the form and the form plus the pellet are transferred to a standard holder. 

As the die or pellet holder frequently requires certain features for optimum operation with a given spectrophotometer, it is an excellent idea to check the literature put out by the various manufacturers before purchasing these accessories. It should be borne in mind that as macro- and microdies may not be interconvertible, the anticipated sample sizes available should also be considered. 

A unique entry in this area of accessories is the MiniPress of Wilks Scientific Company. This accessory makes the pellets without a press and directly in the holder in fact, the press is the holder. A special adapter is required to mount the holder in the spectrophotometer. The MiniPress has the disadvantage of vignetting the beam. 

In using these accessories, the operator should follow the manufacturer s operating instructions to be certain that he does not damage the various parts. He should also remember that KBr is corrosive, and therefore he should thoroughly clean the die after each pellet has been pressed. If the die will not be used for a period of time, it should be stored in a dessicator. Care must also be exercised in handling the polished faces of the rams, as scratches, pits or dents in these faces will usually result in poor pellets. 

Fourier Transform Infrared Spectrometry, Second Edition, by Peter R. Griffiths and James A. de Haseth Copyright 2007 John Wiley Sons, Inc. 

Beam condensers have proven to be very useful when the sample can be prepared as a KBr disk, since devices are available for pressing disks as small as 0.5 mm in diameter. For the examination of very small regions of larger samples, however, beam condensers are less benehcial. Take, for example, the identihcation of an impurity in a polymer him. To examine samples of this type with a beam condenser, the impurity must be mounted immediately behind a pinhole aperture drilled in a metal plate. The smaller the region to be examined, the more difficult it is to mount the pinhole so that it is located exactly over the impurity. Because more and more samples of this type are being examined every day, beam condensers have fallen out of favor and a device with a remote aperture is used instead. This device is the infrared microscope. 

The optics of many FT-IR microscopes are such that three identical Cassegrains are used, in which case the size of the sample image at the detector is identical to the size of the sample bein observed. Since the SNR of the baseline of an FT-IR spectrum is proportional to (see Eq. 7.8), it is beneficial to use detectors that are as small as possible for microscopy. The linear dimension of most detectors that are installed in FT-IR microscopes is 250 pm, which may be compared to a size of 1 or 2 mm for the detectors installed in the spectrometer. Had a 2-mm detector been installed in the microscope, the SNR of the spectrum would have been eight times worse. 

The spatial resolution of the microscope is determined by the wavelength of the radiation, X, and the numerical aperture (NA) of the Cassegrain. Less fundamentally, spatial resolution is also limited by the SNR that can be achieved when the size of the aperture is very small. The NA is the sine of the acceptance half-angle at the sample. At the beginning of this chapter it was stated that the diffraction-limited spatial resolution is approximately equal to the wavelength, X. More accurately, it is given by 

Since the largest NA that can be achieved on most microscopes is about 0.6, the spatial resolution can be approximated by the wavelength of the radiation. Fortunately, the SNR of spectra measured by most contemporary FT-IR microscopes is sufhciently high that acceptable spectra of samples as small as 10 pm in diameter can be measured in less than 5 minutes. 

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In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

Infrared microscopy combines an optical microscope with an FT-IR spectrometer enabling pico- to femtogram (10 12—10 15 g) quantities of substances to be characterized or very small areas of larger samples to be analysed. Beam-condensing optics focus the radiation onto an area of the sample identified using the optical microscope and either reflectance or transmittance spectra can be recorded. The highly-sensitive MCT detector (p. 283) is normally used as its size can be matched to that of the radiation beam to maximize its response. 

Infrared spectra were recorded on the resist film spun onto a silicon wafer using a JASCO IR-810 spectrometer equipped with a JASCO BC-3 beam condenser or a JASCO A-3 spectrometer. In the measurements on the latter spectrometer an uncoated silicon wafer was placed in the reference beam in order to balance the silicon absorption band. The subtraction between the spectra was carried out on a built-in micro-processor attached to the IR-810 spectrometer, and the resulting difference spectrum was used to detect structural changes in the polymer molecule upon exposure. The subtraction technique was also used to balance the silicon absorption band. 

When only very small samples are available, ultra-microcavity cells are used in conjunction with a beam condenser. A spectrum can be obtained on a few micrograms of sample in solution. When volatility permits, the solute can be recovered for examination by other spectrometric techniques. The absorption patterns of selected solvents and mulling oils are presented in Appendix A. 

The quality of the spectrum depends on the intimacy of mixing and the reduction of the suspended particles to 2 /mm or less. Microdisks, 0.5-1.5 mm in diameter, can be used with a beam condenser. The microdisk technique permits examination of samples as small as 1 fig. Bands near 3448 and 1639 cm-1, resulting from moisture, frequently appear in spectra obtained by the pressed-disk technique. 

Beam Condensers.4c Beam condensers are used to focus the IR radiation from a beam that is typically 8 mm in diameter to one that is around 2 mm at the sample plane. This allows the analysis of 50-100 resin beads without KBr dilution. A diamond compression cell is used to flatten beads and to support the sample throughout the measurement. The same diamond cell without beads is then used to record a background spectrum. 

One approach to following reaction kinetics on a solid phase is as follows. A trace amount of resin beads is taken out of a reaction vessel, rinsed briefly with solvent, and subjected to single-bead FTIR analysis or analysis by FTIR with a beam condenser. As an example, the kinetics of the reaction shown in reaction 1 was studied,4 that is, a combination of Wang resin 1 with succinimidyl 6-(iV-(7-nitrobenz-2-oxa-l,3-diazo-4-yl)amino)hex-anoate 2 to produce compound 3. The IR spectra for this transformation are... 

Figure 7.1. IR spectra of the reaction product at various times during reaction 1 obtained by (a) the single-bead FTIR and (b) the beam condenser FTIR.

Infrared Spectra. A Perkin-Elmer model 237 spectrometer was used. Solutions of 5 to 10 mg. per ml. of lipid in spectral grade chloroform were placed in a NaCl microcell of 1.0-mm. path length for study in the region from 2.5 to 6.0 microns. The film technique was used for observations between 5 and 15 microns, with a beam condenser and attenuator (Perkin-Elmer, Norwalk, Conn.). The lipid, 75 to 100 /i.grams in either chloroform or chloroform-methanol 85 to 15, was deposited on 1 sq. cm. of the NaCl plate, and the solvent was removed by evaporation under an infrared lamp for 10 minutes. 

Infrared spectra of individual fractions were determined by means of a Beckman IR-5 spectrophotometer equipped with a 5 X KBr lens-type beam condenser. Infrared spectra of selected reference compounds were obtained from samples which had been purified by chromatography. On the basis of identity of infrared spectra and retention data with those of authentic reference compounds, most of the peaks shown in Figures 1 and 2 were identified (15,16). To obtain information about minor components not detectable in the infrared spectra, mass spectra were obtained as components of an irradiated odor concentrate were eluted from a 10-foot, J/g-inch 5% Carbowax 20M column programmed from 20° to 160°C. at 1° per minute. These spectra were obtained on a modified model 14 Bendix Time-of-Flight mass spectrometer. Electron energy was set at 70 e.v., and spectra were scanned from m/e 14-200 in 6 seconds. 

A Perkin Elmer model 257 infrared spectrophotometer with a beam condenser and normal slit program was used to identify some reaction products. Infrared (IR) spectra of the compounds were obtained from neat liquids between KBr crystals. 

Infrared spectroscopy KBr discs (3-mm) were made with a micropelleting kit (Model 198854) and mounted in a micropellet holder (Model 195465), used in conjunction with a C621 beam condenser (all ancillary equipment by Beckmann RIIC Ltd, Purley, Surrey, U.K.). Spectra were obtained on a Pye Unicam SP200G spectrometer. 

The situation, however, is different for the infrared spectroscopic measurements with opposed anvil cells. The source beam in commercial Fourier transform infrared spectrometers is generally focused to about 1 cm diameter at the sample, whereas the diameter of the gasket hole in the high pressure cell is only about 0.3 mm. Therefore, a source beam condensing system is required in order to obtain infrared spectra with a good signal-to noise ratio. Commercial beam condensers (4X, 6X) could, in principle, be adapted for these purposes. In practice, however, the mirrors of the... 

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophilization. The mixture is pressed with special dies under a pressure of 10,000-15,000 psi into a transparent disk. The quality of the spectrum depends on the intimacy of mixing and the reduction of the suspended particles to 2 gm or less. Microdisks, 0.5-1.5 mm in diameter, can be used with a beam condenser. The microdisk technique permits examination of samples as small as 1 fxg. Bands near 3448 and 1639 cm-1, resulting from moisture, frequently appear in spectra obtained by the pressed-disk technique. 

Specialist suppliers offer variable temperature cells which operate in the range —185 to 250°C. Micro cells with volume of 6/d are also available. The use of microscope cells of low aperture size (3.5 x 0.5 mm) and a volume of less than 0.4//1 requires a beam condenser. 

Apparatus. Infrared spectrophotometer Perkin-Elmer model 421 recording spectrophotometer or equivalent, equipped with 6X beam condenser and holder for 1.5-mm. diameter KBr, discs. 

Infrared Spectrophotometry. A research grade infrared spectrophotometer equipped with a 6-power reflectance type beam condenser and a micro-die for 1.5 mm. KBr pellets was used. 

In 1983, Shearer and co-workers (10) used FT-IR in conjunction with a beam-condensing accessory to analyze the paint chips removed from Virgin and Child, an Italian painting in the Clark Institute collection. Their spectral data supported the art conservators opinion that the painting was either an imitation of an earlier work or a heavily reworked fragment. The recent coupling of FT-IR spectrometers with microscope accessories has further allowed the analyst to obtain spectra from picogram quantities of samples and, moreover, to obtain the spectra quickly and easily (12, 13). 

An infra-red spectrum can be obtained from as little as 1 LLg or less of a compound, but greater sensitivity is obtained by placing the maximum number of molecules of the sample in the infra-red beam. Thus, where the amount of sample is limited, the area of the infra-red beam must be reduced by using a beam condenser and expanding the trans-mittance scale. 

Beam condensers, by using a pair of ellipsoid mirrors, produce very small images of the Jacquinot stop or the entrance slit at the sample position. The size of these images may be even further reduced by making use of a Weierstrass sphere. Weierstrass (1856) showed that a spherical lens has two aplanatic points . If a sphere of a glass with a refractive index n is introduced into an optical system which has a focus at a distance of r n from its center, then the beam is focused inside the sphere at a distance of r/n from the center (Fig. 3.5-9). In this case the angle O in Eq. 3.4-5 may approach 90°. Thus, a sample with a very small area can fully fit the optical conductance of the spectrometer (Fig. 3.4-2d). Microscopes usually have an optical conductance which is considerably lower than that of spectrometers. In this case, the sample and the objective are the elements limiting the optical conductance (Schrader, 1990 Sec. 3.5.3.3). 

Disc rotation can be automatically triggered from the GPC upon injection or at some preset delay so sample collection is virtually unattended. The speed of rotation of the disc is adjusted to match the time of the evolution of the sample from the chromatograph. At a speed of 10 deg/min, 36 min of sample collection can take place on each disc. The Ge discs are easily cleaned and can be reused. After collection, the disc is removed and placed in a 3X beam condenser within the PTIK optics cabinet. The beam condenser is designed to provide an optical match between the FTIR beam size, which is mm, and the size of the deposited sample, 3 mm. Once seated on the platform in the beam condenser, the disc is rotated beneath the IR beam and spectra of individual samples collected. If the FTIR system has the capability of continually collecting spectra, then the spectra of the polymer deposit can be displayed continuously, thus generating an IR chromatogram. If this software is not available, the spectra may be individually collected and displayed by scanning the disc from point to point. 

A 1-cm Beckman microcell (aperture volume of 50 /xl) of fused silica windows with a range of 220 to 2500 fx was utilized. A variable-beam condenser served to attenuate the reference beam. An actual wavelength cutoflF at 230 /x was observed for the microcell. In order to lower the cutoflF point, a matched 1-cm standard sihca cell filled with solvent was centered behind the reference beam attenuator. This allowed a lower cutoflF at 205 /x. Ethanol, 95%, was chosen because of a low cutoflF point, 205 /X, and a high dissoluble capacity for the compounds of interest. 

Specialised sampling techniques such as attenuated total reflectance (ATR) and diffuse reflectance (DR) have been found to be exhemely effective and hence have gained considerable popularity. Microsampling, for measuring very small samples, has become a common technique over the last decade as beam condensers and infrared microscopes (plus accessories) have been improved. 

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