Infrared sample beam

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

The light from the infrared source C is made to split into two beams one of which passes through the sample i.e., the sample beam) while the other caters as the reference beam. This sort of double-beam arrangement facilitates in measuring difference in intensities between the two beams at each wavelength,... 

The reaction of 03 with C2F4, C3F6, and C4F8-2 was studied at room temperature by Heicklen.72 He introduced 03 into the perfluoroolefin in a cell situated in the sample beam of an infrared spectrometer. The products included 02 and CFzO in all cases. With C3Fe and C4Fe-2, CF3CFO was also formed but it was always less important than CFaO. 

In the pressed disc technique a known weight of sample is intimately ground with pure, dry potassium bromide and the mixture inserted into a special die and subjected to pressure under vacuum. The concentration of sample in the disc is usually in the region of 1.0 per cent. The disc so produced may be mounted directly in the sample beam path of the spectrophotometer and the spectrum recorded. This method has the advantage that the spectrum so produced is entirely due to the sample since pure dry potassium bromide is infrared transparent in the 2-25 /xm region. To eliminate the possibility of impurities in the potassium bromide, however, a blank disc (no sample) can be made and mounted in the reference beam path of the spectrophotometer. Care should be taken to ensure that both discs are of equal thickness otherwise inverse peaks may occur if the potassium bromide is damp or impure, and this will be particularly noticeable if the reference disc is thicker than the sample disc. 

The Nd-YAG infrared laser beam can be focused down to 10 pm, producing a microplasma from the sample and the carrier gas (Ar) that ablates the sample surface. The laser can be operated in two different modes with repetitive firing high magnification that produces a pit diameter of 20-40 pm, and low... 

A Mutiple Internal Reflection Accessory for use with Perkin-Elmer infrared spectrophotometers was used to introduce the sample to the IR beam. To correct for losses in energy transmittance through the sample beam resulting from the use of the MIR Accessory, a comb-type reference beam attenuator was employed. A LFE Corpora-... 

An infrared spectrometer measures the frequencies of infrared light absorbed by a compound. In a simple infrared spectrometer (Figure 12-4), two beams of light are used. The sample beam passes through the sample cell, while the reference beam passes through a reference cell that contains only the solvent. A rotating mirror alternately allows light from each of the two beams to enter the monochromator. 

Run the infrared spectrum of an unknown carbonyl compound obtained from the laboratory instructor. Be particularly careful that all apparatus and solvents are completely free of water, which will damage the sodium chloride cell plates. The spectrum can be calibrated by positioning the spectrometer pen at a wavelength of about 6.2 p without disturbing the paper, and rerunning the spectrum in the region from 6.2 to 6.4 p while holding the polystyrene calibration film in the sample beam. This will superimpose a sharp calibration peak at 6.246 p (1601 cm ) and a less intense peak at 6.317 p (1583 cm ) on the spectrum. Determine the frequency of the carbonyl peak and list the possible types of compounds that could correspond to this frequency (Table 2). 

The measurement of diffuse reflectance effectively involves focusing the infrared source beam onto the surface of a powder sample and using an integrating sphere to collect the scattered infrared radiation.59 The technique requires careful attention to sample preparation, and often one must dilute the analyte with KBr powder to reduce the occurrence of anomalous effects.60 In practice, one obtains the spectrum of the finely ground KBr dispersant, and then ratios this to the spectrum of KBr containing the analyte. The relative reflectance spectrum is converted into Kubelka-Munk units using standard equations,61 thus obtaining a diffuse reflectance spectrum that resembles a conventional IR absorption spectrum. 

The key component in the FTIR system is theMichelson interferometer, as schematically illustrated in Figure 9.16. The infrared radiation from a source enters the Michelson interferometer. The interferometer is composed of one beam-splitter and two mirrors. The beam-splitter transmits half of the infrared (IR) beam from the source and reflects the other half. The two split beams strike a fixed mirror and a moving mirror, respectively. After reflecting from the mirrors, the two split beams combine at the beam-splitter again in order to irradiate the sample before the beams are received by a detector. 

Figures 4a euid 4b show the infrared transmission spectra of a high-purity float-zoned silicon wafer and of a Czochralski wafer, respectively (J 3). The spectral features due to the presence of oxygen and carbon in the Czochralski wafer are clearly shown in Figure 4c, rt ich is the difference spectrum of the Czochralski wafer relative to that of the float-zoned wafer. This difference spectrum was obtained using a double beam dispersive spectrometer, with the float-zoned wafer in the reference beam and the Czochralski wafer in the sample beam. The broad band at 1107 cm and the smaller band at 515 cm" are due to interstitial oxygen, and the band at 605 cm is due to substitutional carbon.

I suggest the use of infrared spectroscopy for the laboratory tests. Samples of the him can be mounted in the path of the infrared light beam in an infrared spectrometer and the resulting infrared transmission spectra recorded. If your staff is not familiar with infrared spectroscopy or the interpretation of infrared transmission spectra, you might allow them some time to read some basic reference material on this technique. I can provide that for you. The transmission spectrum recorded by the spectrometer is like a fingerprint of the material in the path of the light. It is a pattern that is observed each time that material is tested. 

Fashion a number of mounting brackets from cardboard. The brackets should be of such size that they will fit into the slot in the infrared spectrometer s sample compartment. Cut a rectangular hole that is about 1 inch by 0.5 inch, in the center of each bracket. This is the window through which the infrared light beam will pass and where the polymer film will be located. 

The necessary instrumentation for making dichroic measurements depends on the technique used. The most common method is to use a single polarizer and an ordinary infrared or ultraviolet spectrophotometer. The sample (thin film) is then placed in the sample beam preferably before the polarizer and generally with the stretch axis at 45° to the entrance slit of the monochromator. This inclination of the stretch axis is to minimize the effect of machine polarization. Such effects have been discussed by Makus and Shareliff (42) and Jones (28). The electric vector of the polarizer is then aligned either parallel or... 

Consult your instructor on the proper operation of your instrument. Handle the infrared cell carefully, avoiding contact with water and the fingers. Fill the cell with pure m-xylene and obtain a spectrum on this from 2 to 15 p,m, being sure to record the last peak just before 15 pm (692 cm" )- Each time you run a sample, be sure to check 0% T by placing a card in the sample beam and adjust the pen to 0% T. Empty the cell, rinse and fill with p-xylene, and run a spectrum on this. Repeat for o-xylene. Run spectra on each of the standard mixtures. From the spectra of the pure substances, choose a peak of each isomer to measure. Using the baseline method (see Figure 16.11), measure PqIP for the peak for each compound. Prepare a calibration curve of the ratio of og PJP) J og PolP ri,o and of log(Po/P)para/ log(Po/E)ortho versus concentration for the meta and para isomers, respectively. See Chapter 20 and your CD for spreadsheet preparation using an internal standard. 

The chemist often obtains the spectrum of a compound by dissolving it in a solvent (Section 2.6). The solution is then placed in the sample beam while pure solvent is placed in the reference beam in an identical cell. The instrument automatically subtracts the spectrum of the solvent from that of the sample. The instrument also cancels out the effects of the infrared-active atmospheric gases, carbon dioxide and water vapor, from the spectrum of the sample (they are present in both beams). This convenience feature is the reason most dispersive infrared spectrometers are double-beam (sample -I- reference) instruments that measure intensity ratios since the solvent absorbs in both beams, it is in both terms of the ratio h / 4 and cancels out. If a pure liquid is analyzed (no solvent), the compound is placed in the sample beam and nothing is inserted into the reference beam. When the spectrum of the liquid is obtained, the effects of the atmospheric gases are automatically canceled since they are present in both beams. 

Fig. 3.12. Optical path of an infrared microspectrometer with a reflecting microscope in the sample beam and a multireflection gas cell in the reference beam. (Blout and Abbate, 1955.)...

Fig. 38. Schematic diagram of the path of the infrared light beam through the sample, using the MIR technique...

Figure 2.13 Interference pattern recorded with an empty cell in the sample beam. From Stuart, B., Modem Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. University of Greenwich, and reproduced by permission of the University of Greenwich.

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