Infrared Identification of Materials in the Fractional Milligram Range By Means of a Beam Condensing Syztem and Pressed Potassium Bromide Pellets Contairiing the Sample DON 11. ANDERSON AND N. 13. WOODALL Industrial Laboratory, Eustrnan Kodak Co., Rochester,
N. Y.
Infrared records on samples weighing less than 1 mg. frequently are needed. Previously available macrotechniques are oompletely unsuited to examining these small samples. A pair of silver chloride lenses was arranged to form a heam condensing system which has a cross-sectional area of less than 4 sq. mm. Provision was made to place the unit quickly and precisely in tho sample heam of a Baird infrared double-beam spectrophotometer. Simple microdies were designed and fabricated so that potassium bromide tablets weighing only 5 mg. can be pressed. These may contain as little as 1Oy of the compound under test. Only a few seeonds are required to convert from macro to micro scale operation. The manipulation techniques employed are simplor and easier than any previously available.
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BEAM-CONDENSING SYSTEM
N WORKING with photographic goods, foreign particles
sometimes need to be identified and it is usually not possible to collect. enough particles to make macro infrared records. The use of a reflecting microscope system w a ~ first considered in order to make infrared measurements on tiny samples. Reflecting microscope systems arc, however, expensive: they must be aligned verb- precisely and, most important, h a w not as yat been used x-it.h double-beam instruments. It was thareforo dceided t u see if m m e infrared-transmitting mstorial could be used to eonstruct lenses lor B heam-condensing system that would substsntially reduce the area of the sample beam.
The beam-condensing system ( 1 ) consists of a pair of planoconvex, silver chloride lenses coated v i t h an antihalation and antireflection coating, and a reference plate sinularly coated. The lenses hnvc diameters of 28 mm. and radii of curvature of 22
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Figure 1. Silver Chloride Lei~ s e sMounted on Base Plate with Sample Holders .
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Kremers ( 2 ) reviews the properties and usefulness of Silver chloride as a lens material for infrared work. With the techniques outlined, satisfactory lenses were formed and a beam-condensing unit was constructed. While i t does not provide the degree of sample reduction provided by a reflecting microscope system, a great many problems do not require testing samples smaller than 107. The reflecting microscope system can, however, be used for samples st lesat as small 8s l y . The lens system is not expensive and does not require any instrumental adjustments in going from macro to micro scale work with the Baird double-beam spectrophotometer.
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Figu,re2. Dies and Plungers Used to Prepare Round and Rectangular Potassium Bromide Pellets
Material. Die shell.
AIS1 Type 44QC afainloss steel or methacrylate
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Plunger. AIS1 Type MOC stainleessfcel. can be cut fmm a steel dowel pin
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T h e round plunge
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V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3 mm. They are assembled on a holder, shown in Figure 1, \yhich has locating pins so t h a t it can be placed precisely in position in the instrument in a rapid, reproducible manner. The beamcondensing unit is placed between the source and the focusing mirror in the sample beam. The sample under test is then placed between these lenses a t the point where the beam has the minimum cross-sectional area by means of plastic blocks that fit over locating pins. It requires only seconds to convert the Baird double-beam spectrophotometer from macro to micro scale work and all the advantages of double-beam operation are retained.
in the base plate carrying the silver chloride lenjes. This pcrniits placing the samples accurately in the beam. This system is used with the 6-mm. potassium bromide disks and the narroiv potassium bromide strips as well as film or oil samples that require micro testing. The block is shown in position between the lpnses in Figure 1 , PREPARATION OF SAMPLE
Sample preparation techniques have developed t o the point where good records can be obtained Tl-ith only 107 of a chemical. I t is known that this can be pushed with some sacrifice in resolution to a t least 5y, but for the authors' work this has not yet bcen necessary. The development of potassium bromide pressings ( 3 , 4 )has been of great. help in furthering this micro work. The greatest advantage of the technique is in the increase i n w n p l e bulk obtained by mixing the inert potassium bromide with the sample. Ordinarily 100 to 200 parts by weight of potassium bromide are mixed with the sample under test and in this may it is possible to handle and manipulate the material conveniently. The mising is carried out in mullite mortars with hand grinding. The p o t a s sium bromide is ground fresh each morning by hand or with a mechanical mortar grinder to a fineness of about 100 mesh. After the sample t o be tested is added, further grinding reduces the mixture to a fineness of about 200 mesh. It has not been necessary to dry the potassium bromide in this laboratory, where the air is maintained a t about 40% relative humidity and 74" F. 9 25-y sample is mixed with about 5 mg. of potassium bromide and pressed in a micro die to form a strip about 0.75 by 6 mni. Only about 3.5 mm. of this strip are actually on the beam; the rest of the strip is used in holding it in position.
The sample area covered is usually 1.5 by 3.5 mm. Occasionally preparations half this width are used. T h e n enough sample is available, a potassium bromide disk about 6 mm. in diameter is used. Strips even narrower than these can be used TYhen necessary because the slit on the Baird varies from about 0.06 mm (0.002 inch) a t 2 microns to 2.5 mni. (0.1 inch) a t 16 microns As the dimensional reduction with the beam-condensing system is about l/3, the reduced slit image varies from about 0.02 to 0.8 mm. (0.0007 t o 0.03 inch). I t is therefore possible to fill completely the slit image betmen the lenses over a considerable wave-length range by strips even smaller than the half-n idth strips reported. -4s sample size is reduced, more attention must be paid to holding and working with the samples as well as to positioning them in the instrument. Sample positioning, using a mechanical stage as sold for use with a microscope, is described by Anderson and Miller ( 1 ) . Since the initial disclosure was made, i t has been possible to dispense with the mechanical stage for holding most samples. The sample holder in current use is a plastic block with a small central aperture and two guide holes that fit over pins
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