Sampling Technique for Infrared Spectroscopy of Solids James E. Stewart, Beckman Instruments, Inc., Fullerton, Calif.
alkali-halide pellet technique T has become a standard procedure in infrared spectroscopy of solid samples. HE
Unfortunately, the technique has a few defects: It is difficult, and occasionally impossible, to obtain clear pellets containing volatile solids. Certain substances react with alkali halides or undergo other changes under pressure. Some materials have refractive indices vastly different from those available with alkali halides, resulting in excessive scatter of radiation; in any case, the variation of refractive index in the vicinity of vibrational frequencies is often large enough to distort absorption bands. Eyen in the absence of the effects of scatter. the intensity of the absorption bands of particles suspended in a solid medium is dependent on particle size. The most carefully treated alkali-halide powder inevitably contains a niinute amount of water, which produces weak absorption bands a t 2.9 and 6.1 microns. These bands are troublesome when it is desired to study weak Q-H or K-H ab$orptions of the sample. The sampling technique described here was developed in an attempt to aroid some of these difficulties. The fact that the relatively hard alkali halides can be pressed into clear
pellets suggests that this can be done with other materials, particularly the softer organic solids. The obvious difficulty is that such pellets, to be suitable for infrared spectroscopy, must be thin and, therefore, are mechanically fragile. However, the pressure necessary to produce clear pellets of many materials is not excessively high and these can be squeezed between alkali-halide windows and observed directly, if precautions are taken to aroid cracking the windows.
A 10 X 15 X 5 mm. sodium chloride microcell window, of the type described by Davison [ J . Opt. SOC.Am. 45, 227 (1955)], was constructed with a clear area of 1 X 5 mm. surrounded by a wide moat cut in the face of the salt with a sharp tool. The small size of the clear area requires that the cell be used with a beam condensing device [White, J. U., Veiner, Seymour, Alpert, N. L., Ward, hf., A N S L . CHEU.30, 1694 (1958)], unless diaphragming of the spectrometer can be tolerated. I n preparing a sample, a quantity of powdered material is placed on the clear area of the window and a cover window is placed on top of it. Tlic assembly is mounted in a microcell frame and squeezed by turning the mounting screws until the sample becomes clear. A spacer placed between the windows, outside the moat, regu-
w.
lates thickness and avoids distortion of the windows. Surplus powder is forced from the clear area of the window into the moat, aiding further in the prevention of window breakage. This technique has proved useful in obtaining satisfactory spectra of a number of difficult solid samples. For example, samples of camphor and menthol n-ere fractionated from a coiiimercial product by gas chromatography. Only about 0.1 mg. of each was available and the volatility of the materials prevented the production of good alkali-halide pellets. Menthol melts a t 35.5’ C. and camphor, although it melts at 176” C.. sublimes readily a t room temperature. The pressed solids provided good spectra, making identification of the fractions certain. Other materials prepared in this way include 2,6-xylenol, o-cresol, biphenyl, n-tetracosane, zinc stearate, phthalic anhydride, and 2-methyl-2-npropyl-l,3-propanediol. This sampling technique avoids all the diaculties listed above, but is successful only with relatively soft materials. The reproducibility in thickness of samples prepared in this way is about the same as for viscous liquid samples in standard demountable cells.
Transistorized Dead-Stop End Point Detector Robert W. Freedman, 5250 Keeport Drive, Pittsburgh 36, Pa. HE “dead stop” titration method of T F o u l k and Bawden (1) is still the method of choice for Karl Fischer and other titrations involving elemental bromine and iodine. The apparatus used ranges from their simple batteryoperated system to more complex equipment with vacuum tube amplification and “magic eye” end point detection (4). A two-transistor amplifier was used by Phillips (3) for dead-stop and amperometric titrations. This author has used the FoulkBawden apparatus for the titration of bromine and iodine with sulfur dioxide solution ( 2 ) . The demand for greater sensitivity, particularly in precision Karl Fischer titration, subsequently led to the design of a compact transistorized version of this simple circuit employing one transistor. The unit is powered by a single battery. Electrode polarization voltage can be varied over a n-ide rnnge. Variation of
current due to temperature change can be compensated for rapidly. The schematic of the transist,orized meter is presented in Figure 1. The Figure 1.
titration assembly is similar to that used previously (2). Operation of Meter. The instrument is turned on (switch S,, Figure
Schematic of transistorized meter
Battery, 3-volt, Burgess 422 MI. Microammeter, 100-pa. d.c. (Triplett 321T) Meter case, Bud CM1966 Meter jocks (2). (E. F. Johnson) 10520 (red) 10521 (block) Meter pins (2). E. F. Johnson 105-41 5 250 pot. (IRC Q1 1201) R1. Rz. 1,000,000-ohm 1 /2-watt 613. 22,000-ohm. 1/2-watt cr-; , R ~ . I 000 pot. (Mailory ~ 4 ) SKI. Transistor socket (3-hole) SI. Rotary switch, 2-position 4-contact (Mallorv 31 4251 S2. Black push button, “normally on” (1 141 Spemco) SS. Red push button, “normally off“ (1 141A Spemco) TI. Transistor (Raythean CK722) 6. Base C. Collector E. Emitter TO ELECTRODES 61.
B IRAI S
t
5.
.l:s
1
14 TEMP
1
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1) and allowed t o warm up foi about 10 minutes. The disconnect button, S2,is depressed, isolating the electrodeb and the temperature control knoll, RB, used t o zero the needle. Drift is minimized by this readjustment after each titration. The electrodes are then shorted by depressing button Sa, and the bias. R1, is adjusted to full scale deflection. The mixture is titrated as described (1). For Karl Fischer or other titrations in which an exccss of halogen is present a t the end point, an unchanging maximum generally above 90 pa. is taken as the point of inflection.
Greater sensitivitj ir c11)tainc.dby the use of a transistor. -4current :miplification of 10 to 20 time!, i- thus available. Onlv :t
