INSTRUMENTATION Reader's comments offer information on micro infrared spectrophotometry as practiced b y experts in the field instrumental can soon become alarmingly unfamiliar with the techniques of a n y one branch of analysis. This column cannot become a forum or a repository for t h e concentrated and distilled wisdom in each specialty; nevertheless we are always h a p p y t o pass on additional information or another point of view whenever an3 7 one is kind enough t o direct us to it. Three m o n t h s ago we mentioned t h e elegant techniques of Anderson of K o d a k P a r k in handling very small samples for infrared analysis, b y m e a n s of silver chloride accessory lenses.
AGREEABLE and fortunate conANsequence of writing this column is t h e frequent comment which we receive from readers. Professional pride keeps us from describing in detail the several occasions on which we have been shown to be downright wrong or grossly negligent. M o r e frequently we are presented with supplementary information or suggestions which, if passed on to the reader, can m a k e the discussion more widely useful. T h e analytical applications of the science of instrum e n t a t i o n are now so extensive t h a t anyone whose concern is primarily
by Ralph H. Müller
M
W e believe it m a y serve a useful purpose to quote the comments received recently from Alexander D . M e b a n e , Ortho Research Foundation, R a r i t a n ,
WAVE
NUMBERS IN
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INDEX SAMfLfc
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LENGTH IN
MICRONS
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WAVE 1200
1100
1000
Î.ENGTH
IN
MICRONS
NUMBERS IN C M - '
900
100 NO. DATE
1
INDEX
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5 - J O - 5 6
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W A V E NUMBERS IN C M > 2000 1500
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WAVE
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NUMBERS IN C M · '
1400 1300 ' ι Ι Ι ι I
SAMPLE ACETYL5ALICYUCACID (25 Τ ) | . 0 % in KCL, mad* into r a t h e r cLoudy wefErOSï'î.lrt-ji Rurt at 51oW = « " s p e e d
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2 8 , N O . 7, J U L Y
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1956
LENGT
WAVE
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37 A.
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INSTRUMENTATION N . J. I n doing so, it is possible t h a t we m a y clarify some aspects of micro infrared spectrophotometry as it is practiced b y experts in t h e field. T h a t there m a y be additional viewpoints we have no doubt, b u t this is beyond our ability to judge. M e b a n e says : I t does not seem to be widely known that with the unmodified Baird spectro photometer, satisfactory spectra can be obtained from samples as small as 15 7. Generally speaking, about 6 γ per sq. mm. of sample must be placed in the beam to produce an infrared absorption curve of good strength; so the minimum usable sample size is determined by the minimum usable sample-beam cross section. Now, experiment shows that the beam at the Baird microcell bracket can be masked down to ca. 1 X 3 mm. before its energy be comes too attenuated for proper operation. (On our instrument, with both beam shutters wide open, an aperture of this size shows a transmittance of 2 4 % from 2 to 12 microns, falling off to 1 3 % at 15 microns). Α 2Ο-7 sample suffices to fill a 3-sq. mm. aperture; and as may be seen from the examples enclosed, an acceptable curve can indeed be obtained from a sample of this size. For most compounds, 8 7 is enough to produce a recognizable (though weak) I R curve. No doubt this is known to many users of the Baird, but I have yet to see it stated in print. Some Perkin-Elmer users have recently reported that the beam in their machine can likewise be masked down to 1 X 3 mm. [D. L. Wood, Rev. Sci. Instr. 26, 787 (1955)], although in their case this cannot be done directly at the sample; so presumably the minimum sample size is the same for both instru ments. I believe, though, that semicloudy KC1 wafers such as I use for sample mounting would not give satis factory results on the Perkin-Elmer, which seems to be much more sensitive to tur bidity than is the Baird. I have had no opportunity to examine the Beckman IR-4 in this respect. Of course, anyone who intends to do quantitative work on the 20-γ scale, or to use samples smaller than 15 7, would be well advised to resort to Anderson's accessory lenses. Without them one is working on the borderline of the unaccept able; it becomes advantageous, for ex ample, to use the slower scan speed, and even with this the peaks at the extreme long-wave length end will be poorly re produced. With a condensed beam, much more energy can be put through a sample of this size, and the benefit of this is apparent on comparing the enclosed curves with those reproduced in your article [April column]. Moreover, with the beam condensed by lenses to about V» its original cross section (and a lens loss of 50%), it ought to be possible to use wafers as small as 0.6 sq. mm., containing 4 7 for a good curve and 1.5 7 for a recog nizable one, which is truly micro scale operation. To be sure, there will cer tainly be some practical difficulty in sample preparation oil this scale, and the heat produced by the concentrated beam will be injurious to some compounds; but for work below the IO-7 level, beam condensation is indispensable. In the range 19 to 50 7, it may be desirable but, as the enclosed curves show, it is certainly not essential.
Some of Mebane's curves are repro duced on page 37 A. T h e actual sam ples are attached to t h e charts in t h e lower right-hand portion of the legend a t t h e left. Of t h e samples, our corre spondent says, "These are m a d e sim ply b y brief grinding in an agate mor tar, followed b y filling into an aperture cut in blotting paper, and pressing (10 seconds a t 10,000 pounds per square inch) in a Carver press between two small pieces of hard steel. N o special equipment is required. M o u n t i n g in t h e b e a m is done with M y s t i k t a p e . Whether this simple technique can be m a d e reprodueibly q u a n t i t a t i v e is a question I can't answer, not having tried. Probably it could, with an internal s t a n d a r d (lead thiocyanate or some such c o m p o u n d ) . " We hope our exuberance over the achievements of modern instrumental analysis can be forgiven. So m a n y of these methods afford intimate a n d unquestioned detail about the charac teristics of substances and leave little or no d o u b t about their identity. It leaves us increasingly cold and in different to t h e pleas of some professors who yearn for t h e emphasis on analytical fundamentals. Are "fundamentals" necessarily simple and confined to pre cipitation and filtration? Perhaps these push-buttons are the real offenders; perhaps if our instruments were fitted with h a n d cranks instead of motors one could feel more intimate companion ship with t h e instrument and gain t h e impression t h a t his good j u d g m e n t and timing were indeed contributing to t h e excellence of the results. Polyethylene
in Precision
Capacitors
T h e labors of t h e synthetic organic chemist provide the instrument designer with m a n y useful products, t h e im portance of which is not always too evident. A large portion of t h e sig nificant progress in scintillation counting arises from t h e wealth of liquids having useful properties as liquid scintillators, and t h e same holds t r u e for plasties developed for similar purposes. A striking example is afforded b y superior quality polyethylene which is used as t h e dielectric in precision capacitors. Polyethylene capacitors are now avail able up to 4 or more microfarads capacitance. T h e y possess extremely low absorption and charge distribution effects, and resistivities as high as 10 12 ohms. As a consequence it is not too surprising to note t h a t one can charge such a capacitor and come back days later to find t h a t it has retained the bulk of its original charge. Such capacitors have extensive possibilities as integrator or memory storage elements.
Circle No. 38 A on Readers' Service Card, page 51 A
38 A
ANALYTICAL
CHEMISTRY
