INSTRUMENTATION

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INSTRUMENTATION Infrared absorption spectrum techniques are ideal in medical research, criminological investigation, or forensic tests

by Ralph H. THE several methods and techOFniques for the identification of substances, few are as direct and con-

Figure 1.

We had the opportunity of seeing striking examples of this technique at Kodak Park last fall, and through the courtesy of Don H. Anderson of the Industrial Laboratory of the Eastman Kodak Co., we are able to present some of the principal features of the method. A beam-condensing lens system for handling microspecimens was developed several years ago [Anderson, D. H., and Miller, O. E., J. Opt. Soc. Amer. 4 3 , 777 (1953)]. This system employed planoconvex silver chloride lenses. The high index of refraction of silver chloride makes it easy to attain a high relative

aperture system with a reasonable degree of correction for spherical aberration. The Special Products Sales Division of Eastman Kodak can supply a set of infrared microsample optics for this purpose, consisting of an unmounted pair of plano-convex silver chloride lenses of 24-mm. diameter and 22-mm. focal length, coated with a black silver sulfide smoke that cuts out

Typical silver chloride lens

vincing as an infrared absorption spectrum, and wherever the nature of the substance permits, such characterization leaves little to be desired. In medical or biochemical research, criminological investigations, or forensic tests those methods are ideal, provided one can extend the techniques to extremely small specimens. A combination of techniques has made this possible, so that 10 to 50 y of material will suffice for quantitative spectra and simple identification can be made in the fractional microgram range. VOLUME

Müller

2 8 , N O . 4, A P R I L

1956

Figure 2. Typical spectrograms 41 A

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Cary Applied

INSTRUMENTATION

Physics

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Double Monochromator Provides L o w Stray Light, High Resolving Power Spectrophotometer users can measure high absorbance values with the same speed, accuracy and relia­ bility a s other measurements by using a Cary Recording Spectrophotometer which incorporates a double mono­ chromator to virtually eliminate stray light and provide unusually high re­ solving power. With single monochromator in­ struments, stray light interferes so greatly with measurements a t high absorbance values t h a t the d a t a ob­ tained and conclusions drawn a r e often meaningless. Frequently, such data are reported in the literature and it is impossible to confirm this data with careful work using a relia­ ble instrument. S t r a y light can be corrected for, but a great deal of extra data and t e ­ dious calculations a r e required which largely negates the convenience of a recording spectrophotometer. In the Cary Recording Spectrophotometer, the problem of stray light is handled by prevention r a t h e r than correction. The double monochromator reduces

s t r a y light to a negligible amount over the entire range of t h e instru­ ment—less than 1 ppm for the most part. H i g h r e s o l v i n g p o w e r in t h e double monochromator results not only from dispersion which is twice as great, but aberrations in one-half of the monochromator m a y be concelled in t h e other half. Also, by com­ bining two different types of dispers­ ing elements, the advantages of each m a y be combined to give better per­ formance over a wide range. In the Cary Model 14, for exam­ ple, a prism and a grating a r e com­ bined to give high resolving power over t h e entire ultraviolet, visible and near infrared regions. Resolving power is about 1A in the visible and ultraviolet and 3A in the n e a r infra­ red—nearly t e n times the accuracy offered by single monochromator in­ struments. F o r complete design and per­ formance information on Cary Re­ cording Spectrophotometers, write for bulletin ACS-l 2.

The HC1 spectrum, above, is the rotational fine structure, and the pattern of a large peak followed by a smaller one is due to the presence of two chlorine isotopes. These peaks a r e separated by about 10A (3cm-Ο VIBRATING REED AMPLIFIER IMPROVES MASS SPECTROMETER PERFORMANCE

The Cary Vibrating Reed Ampli­ fier, Model 36, is being used in a n increasing number of mass spectrom­ eter installations where high molec­ ular weight analyses make rapid scanning of m a s s numbers desirable. The Model 36 combines rapid re­ sponse with high sensitivity. Response is critically damped, with an 0.1 sec. natural period (98.6 percent response in 0.1 second). Thus a range of 100 mass numbers can be accurately scanned in a s little as one minute. Sensitivity and range a r e such t h a t as little a s 10- l s amperes and up to 10- ' ' amperes can be measured to a

Cary SPECTROPHOTOMETERS VIBRATING REED AMPLIFIERS APPLIED PHYSICS CORPORATION

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reproducibility of 0.2 percent with­ out change of range. Stability of t h e Model 36 is superior too —zero drift is less than 10- '5 amperes. Another Cary instrument of in­ terest to mass spectrometer users is the Model 31 Vibrating Reed Elec­ trometer. I t is preferred for meassuring ion currents in work where extreme response speed is not re­ quired, such as isotope determina­ tions. Sensitivity of t h e Model 31 is 10-" amperes, and like the 36 it h a s high stability — less t h a n 5 χ 1 0 - " amperes zero drift. Write for bulletin ACA-12.

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radiation below 1 micron. The silver chloride itself is transparent well be­ yond 17 microns. Still more conven­ ient, for those who wish a completely mounted assembly, are commercial beam-condensing units supplied by both Baird Associates and Beckman Instru­ ments, Inc. These can be placed in position in standard twin-beam instru­ ments in a matter of a few seconds. They are patterned after the design of Anderson and Miller. A typical silver chloride lens is illustrated in Figure 1. The crystal on the pinhead is about 20 times larger than that required for infrared analysis. Sample preparation involves a sim­ plified potassium bromide pellet tech­ nique. Anderson and Woodall have described a complete procedure for the preparation of round and rectangular pellets weighing only 5 mg. and con­ taining as little as 10 y of the compound under test. These are mounted very easily in the silver chloride beam-con­ densing system of cross-sectional area less than 4 sq. mm. [Anderson, D. H., and Woodall, Ν . Β., ANAL. CHEM. 25,

1906 (1953)]. Further details of a simple die for preparing potassium bromide pellets under vacuum, are given by Anderson and Smith [ANAL. CHEM.

26, 1674 (1954)]. Extensive experi­ ence with the pellet techniques has led these authors to abandon the mineral oil mull almost completely, an experi­ ence no doubt shared by most workers in recent years. Some typical spectrograms obtained with this microprocedure are shown in Figure 2. Although we have quoted concentration ranges above, the desig­ nations applying to Figure 2 have been quoted in somewhat more striking fashion by Anderson, when he states, "If one twentieth of a grain of sugar could be split off, this would be enough to give a recognizable infrared curve for sucrose. Less than one thousandth of an aspirin tablet contains enough acetylsalicylic acid to give a recognizable curve, and less than one one-hundredth of a cup of coffee contains enough caf­ feine to give a good infrared record." These superb achievements are, of course, not the sole means of approach to microsamples. Elegant reflecting microscope systems have been developed for the same purpose. However, the latter are inclined to be more expensive and difficult to couple with existing in­ struments, particularly double-beam in­ struments. I t would seem that these techniques could be extremely useful to the micro­ biologist. He has already perfected micromanipulators for dissection and excision on a microscopic scale. Now it is no longer necessary for him ANALYTICAL

CHEMISTRY

INSTRUMENTATION

to depend upon morphological criteria, color tests, and other inferential procedures for final analysis.

MODEL VRSAC750 0-750 VOLT RANGE

Forensic Analysis For many years we had the privilege of being a colleague of the eminent toxicologist, Alexander 0. Gettler. In meeting the exacting demands of the Office of the Chief Medical Examiner of the City of New York, he was faced almost daily with the problem of determining "the cause of death beyond all reasonable doubt." If this literal statement of the law seems vague, or none too specific to the analyst, a typical day in the criminal court would allay his worries. As a matter of fact, the amount of scientific evidence brought into court would seem superfluous to the average scientist. One easily gained the impression that it would be easier to fool or mislead a scientist than the average lay juror. It was always strikingly evident how profound an impression was made by photographic evidence— in ballistics, rifling marks, or by spectrograms. A learned witness could talk all day about activity coefficients, pH, solubility products, and other respectable concepts and elicit nothing but yawns. Even routine cases were supported by elaborate pathological, bacteriological, chemical, and physical evidence and most of these were classics in their completeness. It is comforting and reassuring to note how the average juror, under such an elegant medicolegal system, responds immediately to sound, vivid, and convincing scientific evidence. Under such circumstances, it matters little that he is unimpressed or even bored by high authority.

low

cost

VOLTAGE REFERENCE SOURCE FOR RESEARCH LABORATORIES · SCHOOLS · PRODUCTION TESTING AMPLIFIER GAIN CHECKS · OSCILLOSCOPE CALIBRATION SERVO TESTING The Sorensen VRSAC750 V o l t a g e Reference Source is a l o w cost, h i g h l y accurate r e g u l a t o r p r i m a r i l y d e s i g n e d f o r c a l i b r a t i n g AC v o l t m e t e r s in the 0-750 v o l t r a n g e . It is i d e a l f o r use w i t h n e a r l y every p o w e r meter c o m m e r c i a l l y a v a i l a b l e . Its compact d e s i g n , s i m p l e o p e r a t i o n a n d accurate p e r f o r m a n c e m a k e it e x t r e m e l y useful to the l a b o r a t o r y or the p r o d u c t i o n shop. The VRSAC750 is specifically d e s i g n e d f o r bench-top opera t i o n . . . a l l controls are w i t h i n easy reach of the o p e r a t o r , a n d the reference meter is clear, easy to r e a d , t i n d p l a c e d a t eye level to insure m a x i m u m accuracy of a d j u s t m e n t . Input voltage range Input frequency nput current Output voltage Output voltage accuracy Harmonic distortion S I Z E 205/ e " h i g h , 1 9 % '

105-125 VAC, 1 0 60 ± 0 . 5 cps 7 amperes maximum 1-799 volts in 1-volt steps ± 0 . 2 5 % at any voltage in 2Ct°-30°C ambient 1 % maximum introduced by the unit w i d e , 12" deep W E I G H T 115 pounds net

Gettler was faced with the prodigious job of keeping up with countless new drugs and preparations, always certain in the knowledge that one day he would be confronted with the problem of isolating them, intact or modified, from large amounts of tissue or body fluids. In the past, an almost excessive number of confirmatory tests were necessary to build up a convincing exhibit. The micro infrared techniques would, in most cases, be far simpler and much more convincing.

VR5AC1Q — A versatile i n s t r u m e n t f o r l o w e r v o l t a g e a p p l i c a t i o n s f e a t u r i n g h i g h accuracy at e x t r e m e l y l o w cost. Output voltage 10 mvto lOvIEMS in three ranges range Calibration ± 0 . 1 % at f i l l scale at 60 cps accuracy Input voltage 115v ± 1 0 % , single phase Input frequency 50-60 cps; tc 400 cps with slightly less accuracy Waveform Distortion is negligible Regulation with load 0.25% max., with load resistance higher than 0.5 megohm Regulation with line ± 0 . 2 5 % ma:c.

Write today for complete spécifications, performance data, and quotations.

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Our digression on forensic analysis should not divert attention from the widespread applications of these techniques, but in another sense, the whole question of medicolegal analysis is of tremendous importance. Aside from a dozen large cities and the Federal Bureau of Investigation, which utilize the best resources of science, the abomination of the coroner's system flourishes to a degree which is little short of a national disgrace. ANALYTICAL

CHEMISTRY