Potassium Bromide Capillary Cell for Infrared Microspectroscopy Edward D. Black, Pioneering Research Division, Quartermaster Research and Engineering Center, U. S. Army, Natick, Mass. absorption cells fabricate.d C from silver chloride, sodium chloride, aiid polyethylene have been reAPILLARY
ported (f-8). Such crlls can also be conveniently made by the potassium bromide pelleting terhnique. Sufficient potassium bromide to form a pellrt 1 t o 2 mm. thick is used. Half this amount is compacted with mi)derate band pressure on the bottom fa ce of the partially assemblrd die. AII Lshaped wirc, shorter than the Fiellet diameter or length, is centered on this layer of salt and the remainder 01' the potassium bromide is added and Iland prrssed. The pellet is completed ii1 the usual maimer under vacuum and high prcwne. Thr result is represent€d in Figure 1, in which the dashed lines indicate d i e r e cuts are to be made with a razor blade. The pcllet is reitdily fracturd along line 1 without brea king the wire. Manual draning apart of the tivo sections rrsults in extricating one arm of the L. Should the longrr length rrinain embedded, it may be withdi'awn by grasping the exposed vire with flatfacrd plirrs or forceps. A cut ahlong line 2 opens tlu, other end of the capillary. 130th end faces of th6 pelleit are then sliavcd smooth. Rubber fi.nger covers are usrd to prevrnt foaging __ - of the prllet. Capillarics 0.075 mm. in dianirter have been formed with S o . 40 Chrmomel :"" h "-"..-:-- .... 11 -c -:.IA .k- ... ),IIC. A ~ L L ~ ~ p PuL~ iL ULL111~~s ~ mre breaks it into straight sections, burr-free a t one end, that are suitable for the 1,-shaped wires. Two commonly available dirs, K13r die IIK.2 of the Research and Industrial Instruments Co., London S.W. 9 , England, and Prrkin-Elmer die, Part 021-0106, which form pellets 13 mm. in diametw, have been used.
The final length of the 0.075-mm. diameter capillary is usually 7 to 8 mm. Figure 2 is a photomicrograph of a section of capillary partially filled with carbon tetrachloride. The microscopic imperfections evident throughout the pressed salt do not interfere with the spectra. Figure 2 illustrates the proper focus in the microscooe of the soectrometer (Perkin-Elmer' Model i l 2 with Model 85 microscope attachment). Because of the loss of transmission at the opaque curved sides of an empty capillary, a solid portion of the pellet serves as a background against which to compare a spectrum of a liquid in the condensed phase .
Figure 2. Photomicrograph of 0.075-mm. diameter potassium bromide capillary containing carbon tetrachloride Special procedures for filling and sealing capillary cells have been describrd
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I
, (1-8).
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Figure 1. Diagram of potassium bromide pellet with embedded 0.075-mm. wire
If sufficient nonvolatile sample is available, it can be transferred to the capillary without specialized equipment-for example, 20 y of geraniol, dissolved in 10 ml. of isopentane (hoiling point 28' C.),has been readily manipulated. Evaporation in a 20ml. pear-shaped flask with occasional cooling to effect washing down of the walls with condensed solvent vapor brings the solute to a small area a t the bottom of the flask. Carbon tetrachloride (0.05 to 0.10 ml.) is then added and this solution is taken up in a micropipet. For the final concentration, the solvent is evaporated dropwise on a smooth nonwettable surface such as flat Teflon film (0.006 mm. thick) stretched over a 8/,inch hole in a metal or plastic plate and secured with tape. As evaporation proceeds, the concentrate tends to agglomerate in a small area. Alternatively, the carbon tetrachloride may be evaporated within a
glass capillary or micropipet. At the appropriate conc.entration, the sample is introduced to the capillary by touching the end to the solution on the film or by using a glass capillary for transfi:r. Thus, sufficient solute (3 to 4 y) 111 carbon tetrachloride solution evriituatrs in the capillary for spectra in good agreement, with those ubtained by infrared techniques on the macro scale. An additional method for sealing capillary cells is the rapid, firm pressing of molten polyethylene to the ends of the freshly filled capillary. Glass rods (3 to 4 mm. in diameter) which have been standing in a few millimeters' depth of the molten plastic a t about 125' C. are used. Both ends of the capillary need not be touched simultaneously. The melt solidifies instantly on the potassium bromide surface and makes an effective seal, as indicated by a clear, glassy appearance of the polyethylene. If the pressing of the melt to the pellet is not sufficiently even or firm, a frosty incomplete seal might result and the solvent rvaporates too rapidly. Such a seal may br removed and another sealing attemptrd. A good polyethylene seal still allows very gradual evaporation of carbon tetrachloride. However, the process is slow enough to give ample time to record the spectrum. On occasion, loss of solvent over a period of hours or overnight has advantageously allowed spectra to be obtained at diffrrrnt concentrations and eventually in the condensed phase in the same capillary. Other fmtures of thr method are the ease of location and focusing in the microscope, the clarity of the absorption cell, which is unaffected by visual light and has good transmission from 2 to 15 microns, and the ease of partial recovery of the sample by dissolving the pellet in water and subsequently extracting with organic solvent. ACKNOWLEDGMENT
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The author is grateful to J. D. Loconti for the suggestion of a polyethylene seal, to A. S. Wrigley for the photomicrographic work, and to D. R. A. Wharton, M. L. Wbarton, and A. L. Bluhm for their helpful interest. LITERATURE CITED
(1) Blout, E. R., Parrish, M., Jr., Bird, G. R., Abbate, M. J., J . Opt. Soe. Ani. 42, 966 (1952). (2) Jones, R. N., Nadeau 4 Spectroehim. Acta 12, 183 (1958); . " (3) Molnar, W. S., Yarbomugb, V. A., Appl. Spectroscopy 1 2 , 143 (1958). VOL. 32, NO. 6. MAY 1960
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