| 1.To become acquainted with the operation of the IR spectrometer and
with calibration of the wavelength scale.
2. To prepare and to measure various samples: polymer films, nujol (mineral oil) mulls, KBr pellets. |
| Two Perkin Elmer different IR spectrophotometers are to be used: Scanning
model
1420 and Fourier Transform model
1620.
The Model 1420 instruction manual and a polystyrene film. The Model 1600 Tutorial |
| The Model 1420 is quite simple to operate, and will quickly produce quality spectra. There is, however, one set of spectra which should be obtained at least once a week, and more often if the instrument receives heavy use. These spectra should be dated and kept, since they provide a continuous record of instrument performance. Variations in these spectra can be used as guides for modifying analytical procedures, or to detect incipient troubles. Comparison of the most recent spectra with those obtained when the instrument was new will allow quick detection of deterioration in instrument performance that would probably go unnoticed for a considerable time without such comparison. |
| Part I - Operation of the
Scanning IR Instrument; Performance Checks
Turn the instrument on and place a sheet of paper on the recorder. Carefully align the 4000 cm-1 line on the chart paper with the index mark on the recorder scale. The gain should be set according to the instructions in the manual. Set the paper size to 0.5 (we simply use this particular size!). Without a sample in either the sample or reference beam, adjust the 100%o control until the pen reads 95% on the transmission scale. Start the scan by pressing the SCAN switch and record the Io, or baseline. It should be flat within the specification noted in the manual. At the completion of the scan, return the recorder to 4000 cm-1, and adjust the 100% control until the pen reads 100%. Obtaining a Spectrum of the Polystyrene Calibration Sample With the l00% control adjusted for a 100% transmission reading, place the polystyrene calibration sample in the sample holder of the instrument and scan the spectrum of polystyrene on the same piece of chart paper used for the Io scan from 4000 to 650 cm-1. Remove the chart paper from the recorder as described in the instruction manual, and fill in the appropriate information on the upper part of the chart. The spectrum of polystyrene contains a convenient set of absorption bands which may be used to verify the calibration of the frequency scale of the instrument. These peaks are numbered in Figure, and their positions should be compared with the frequencies tabulated below. The experimentally determined frequencies should agree with the tabulated values to within 20 cm-1 from 4000 to 2000 cm-1 and within 10 cm-1 from 2000 to 650 cm-1. |
| Peak Number | Wavenumber, cm-1 | Peak Number | Wavenumber, cm-1 |
| 1 | 3027 | 7 | 1181 |
| 2 | 2851 | 8 | 1154 |
| 3 | 1944 | 9 | 1028 |
| 4 | 1802 | 10 | 907 |
| 5 | 1601 | 11 | 699 |
| 6 | 1495 |
| Part 2. Operation of the Fourier
Transform IR Instrument; Performance Checks
Following Tutorial, run the baseline and standard polystyrene film using FTIR instrument. Compare the results with the Table and with your data from the sacnning instrument. |
| Part 3. Prepare a mull in mineral oil and KBr pellets of your samples. Record the spectra. Repeat the sample preparation until you have a high quality spectrum. For sample preparations, follow Tutorial. |
| Appendix A. Care and Handling of NaCI Crystal Windows
OBJECTIVE To become acquainted with procedures for cleaning and polishing optical grade NaCl windows to be used in infrared analysis. MATERIALS For Cleaning: Solvent for the sample material which will not affect crystal. For Polishing: Finger cots or rubber gloves; Wet-or-Dry sandpaper, grit for routine surface imperfections, 400 grit for badly scratched surfaces; soft cloth such as Fisher Polishing Cloth 12-284; 6-inch square of plate glass or other flat plate; Linde type 0.05 B alumina polishing compound; 30% water in alcohol solution. The care and handling of the crystal windows used in demountable cells and demountable sealed cells is critical to the quality of any infrared analysis. Window fogging, caused mainly by etching of the window surfaces by water vapor, may result in a sloping baseline or in excessive reduction the energy transmitted by the cell. Sample residues occluded on window faces may absorb energy in a way which will seriously hinder the interpretation of other spectra or reduce the accuracy of quantitative analyses. These difficulties can be overcome by keeping the windows clean and dry, and by polishing them when necessary using the procedures described. Furthermore, windows that are cracked or broken may still be useable. Large pieces may be found suitable for running samples harmful to the crystal material, where it is not desirable to ruin a good window. Smaller pieces can be cleaved as described and used in microsampling cells. |
Properties of Crystal Materials
| Crystal | Useful Range(cm-1) | Water Solubility(g/100mL H2O) | Other Properties |
| NaCl | 10000-650 | 35. 7 | Cleaves and polishes easily |
| KBr | 10000-400 | 53. 8 | Cleaves and polishes easily |
| CaF2 | 10000-1110 | 0. 0017 | Does not cleave, difficult to polish |
| BaF2 | 10000-760 | 0. 17 | Obtained as sawed blanks, moderately easy to polish |
| Irtran-2 | 10000-715 | Insoluble | Glasslike, withstands severe thermal shock. Difficult to polish |
PROCEDURE
| Cleaning the Windows
Sodium chloride crystal windows can be cleaned easily by washing with a solvent (not water) for the film or sample on the window. However, if the sample is insoluble, or if the crystal surface has become fogged or scratched, it may be necessary to polish the crystal as described below. If the window is merely fogged, omit grinding on the sandpaper and proceed to the polishing operation. Polishing Cell Windows Place a sheet of 600 grit Wet-or-Dry sandpaper on a flat surface. Grind the window surface by moving it back and forth, in a straight line, over the sandpaper until any surface imperfections, such as scratches or insoluble sample residues, disappear. If the windows are badly scratched, use a coarser sandpaper, such as 400 grit, for the first grinding. Follow with a second grinding using 600 grit paper and a motion perpendicular to that of the first grinding. Continue with the 600 grit paper until the scratches from the coarse paper are no longer visible. After grinding, put on rubber gloves or finger cots and polish the window surface as follows: Stretch the piece of cloth tight over the flat plate and hold them together with one hand. Sprinkle a little polishing compound over a 2-in. diameter circle on the cloth surface and moisten the compound with several drops of the water-alcohol solution. Polish the window surface by rubbing it over the compound with a circular motion. After two or three minutes of polishing, wipe off the window quickly using a dry, clean portion of the cloth. Inspect the window surface to see whether additional polishing is required. The surface should be clear and free from scratches. A small amount of "orange peel" may be evident; this is acceptable if barely noticeable. If no further polishing is needed, grind and polish the other side of the window. After both sides of the window have been polished, obtain a spectrum of the window to detect any residual polishing compound which might remain on the window |
