INSTRUMENTATION Most rapid progress occurs when the man with the problem and the man with assorted techniques discuss new approaches (~\K TWO previous occasions we have ^ reported techniques for the identification of organic compounds on the basis of their characteristic infrared absorption spectra. With the improved optics now available and the use of KBr pellet techniques, these methods afford precise characterization well into the microgram region. Related to these studies, but restricted to a special class of compounds, is a compilation of the ultraviolet absorption spectra of 56 narcotics. Originally prepared by C. G. Farmilo, P. M. Oestreicher, and R. Levi of the Food and Drug Laboratory, Ottawa, Canada, these data were published in the Narcotics Bulletin of the United Nations [6, 56-69 (September-December 1954)]. Because of the rather limited circulation of this journal, they have been reproduced, by permission, by Applied Spectroscopy [10, No. 1, 15 (1956)]. Each of the 56 spectrophotometric curves presents log e values versus wave length. The common name, structural formula, Geneva nomenclature, and solvent used, are specified. In most cases the curve is given for the free base and also for the hydrochloride. According to the editors, these spectra will be coded on IBM cards and distributed through ASTM Committee E-13. Progress in Instrumentation We have always been awed and impressed by the accomplishments of the organic chemist, the biochemist, the pharmacologist, and the toxicologist, and could never rightly understand their corresponding astonishment at the capabilities of a remote-cutoff pentode or other simple electronic device. These are happy and delightful times in which we live, for, despite an almost hopeless degree of intense specialization, active and progressive research people are drawing on the resources of other fields to an astonishing degree. In a moment of aberration, the instrument designer might be foolish enough to imagine that his recording and automatic instruments are beginning to put order and precision into these complicated fields of inquiry. Actually they have been progressing in elegant and orderly fashion, and for a long time. The most rapid progress seems to occur when the man with the problem spends enough time with the man with an asV O L U M E 28, NO. 8, A U G U S T
sortment of techniques to discuss new approaches. Some analytical instrumentation is becoming downright dreary and repetitious. Much of our instrumentation is an attempt to mechanize classical procedures and as such is more of a problem in engineering. It is likely to be dreary, especially if its aim is to mechanize and speed up simple operations which were originally dreary and monotonous—but necessary. We continue to wonder whether the punch-card assimilation of data is the best solution. To be sure, it is an excellent interim solution, because the collating and sorting equipment is standardized and can be used for other purposes of a business or accounting nature. There exists an increasing probability that the newer storage and memory elements can be incorporated in many of our instruments. In the example quoted on the narcotics data, it would seem to be not too difficult to refer absorption maxima and minima to a simple memory matrix or reference tape and secure immediate identification. In this connection we should remember that the cathode-ray spectrophotometer does exist, even if it does not seem to sell too well. As long as we consider the latter solely as a means of seeing conventional spectra in a "great hurry" and nothing more than that, it is not particularly impressive. However, if its response is differentiated or otherwise electronically shaped to present distinct numbers as a function of wave length, we have the rudiments of a fast identification scheme. Classical Spectroscopy The work of the classical spectroscopist continues unabated. As G. H. Dieke of Johns Hopkins has stated in a recent paper, "Practically all work in spectroscopy is based on wave length and intensity measurements. The quality of these basic measurements determines the usefulness of spectroscopic data for applications in atomic and molecular physics, technology, and industry. In some phases of spectroscopy further progress can be expected only if the existing wave length tables are thoroughly revised and new material added. Because sometimes hundreds or thousands of individual lines are involved, the necessary measurements constitute no small task. Progress is 1956
by Ralph H. Müller
often impeded because of the unwillingness of the persons concerned to perform such uninspiring measurements." Dieke, with Dimock and Crosswhite, has developed semiautomatic methods for performing this important chore. Their approach contrasts with the elegant machines which Harrison has built for this purpose at M.I.T., in that they have endeavored to employ only readily available parts and to keep the complexity of the equipment down. As described [J. Opt. Soc. Amer. 46, 456 (1956) ], their arrangement uses a photoelectric scanner in place of the microscope of the comparator. The line contour is presented on an oscillograph together with its mirror image. The direct and mirror image will coincide, if the line is exactly in the middle of the scanned interval. The observer simply moves the screw of the comparator until the images on the scope coincide. Ordinarily, the reading on the comparator is then read by the operator and written in a notebook. This fatiguing operation is eliminated by coupling an analog to digital converter to the comparator screw. This magnetic readinghead produces an electric pulse for each thousandth of a revolution. For the given screw pitch and employment of a 2 to 1 gear ratio, an output pulse is generated for each half-micron motion of the spectrum plate. A scaler circuit counts these pulses and when the operator presses a button (foot pedal) the accumulated counts are transmitted to an electric typewriter or IBM summary punch, thus recording the comparator setting. The scaler circuit is bidirectional and will therefore subtract counts if the comparator is moved in the opposite direction. So far, the principal advantage lies in relieving the operator of the tedium of reading and recording the comparator settings. It is intentionally semiautomatic, because the authors prefer to retain operator 37 A
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INSTRUMENTATION
PLASTIC PHOSPHOR
judgment in setting on a spectral line. Entirely automatic setting on lines is entirely feasible and works well with isolated symmetrical lines but is less reliable when only partly resolved and asymmetrical lines are involved. Such general information as rough estimate of intensity, whether the line is broad, diffuse, shaded to red, re versed, etc., can be set in on the printed record by pressing a series of buttons which permit as many as 16 units of in formation to be printed. Inasmuch as the entire installation has reduced the time required for precise measurement by a factor of 10, it is not surprising to note that it was necessary to provide the operator with an audible or visual signal when he was running out of typei writer paper! Precise measurement of line intensity is equally important for many applica! tions of spectroscopic data. As a line is scanned for wave-length position, a density measurement is also obtained, but its mere oscillographic presentation is not precise enough. The authors I therefore use a portion of the scanning COMBINES HIGH EFFICIENCY beam to excite a photomultiplier tube and LOW COST for a precise measure of intensity. Designed for scintillation counting ap ' This part of the problem can be handled plications requiring large masses of by any number of well established phosphor, Sintilon brand plastic phos ' methods of microphotometry. In a phor provides an efficient, economical alternative to stilbene, anthracene, and previous publication the authors have shown how a photoelectric microphosodium iodide crystals. Pulse height and efficiency for detection tometer can be arranged to record the of radiation compare favorably with modified Seidel function. This func other crystals and response is linear to lower energies. Self-absorbtion of tion is directly proportional, over the Sintilon to its own fluorescent radiation whole density range, to the logarithm is less than 10% in 4" sections. of the intensity. They point out the Sintilon plastic phosphor is thoroughly alternative of recording data on a linear polymerized; extremely stable and will scale and leaving reduction to an auto not cloud. It may be machined to any size or shape required without difficulty. matic computing machine. For grating wave-length measureC O M P A R I S O N CURVE S i n t i l o n Plastic Phosphor vs. S t i l b e n e C r y s t a l : ments, the relation between wave 4000 Ι ; ;i η —] ί . • = ;-. length and comparator reading is linear to a first approximation. A Ai 2000- ~ ^ - Û ^ > ^ j — - - - - - - - - - - :;v v·^: quadratic interpolation is better and is readily done even with a simple calcu 1000- !;ύ.^.~·.^-~; J ^ O * ^ ·.••-•: • —— ——ϊ-ΐ-τ lating machine, but very cumbersome -.. ..,. ,,,. .....^., -., - : N > S K ^ · . : ν'" '' '" ι"""? when done with a desk calculator. ϊ 600— ; ;., • .;:• ;:-•" •: -.• - ^ s ^ S e .-.-r^ Thus if χ is the comparator setting, the interpolated value for wave length is •u 4 0 0 - ^ r - ~ H τ ^ — ^ - Ι.·..·:;.·.:·"· " :: ^ s - ~ ^ ^ / ; Γ ; - : . given by λ0 = a + bx + ex2, where a, b, 200 >'•- Ι " ••- •"" I "< ': I —li—: -V·,' Vi.Si.l and c are constants determined from .S 1.0 1.S 2.0 2.5 3.0 three standard lines. The calculating Discriminator Volts A —Stilbene, β Grams B —Sintilon, 8 Grams machine—for instance, IBM Type Radiation source, gamma rays from radium 604—is set up for quadratic interpola SPECIFICATIONS tion with the constants a, b, and c. Sintilon Plastic P h o s p h o r Principal Emission Spectrum A.U 4600 When the cards containing direct read Relative Light Yield to Betas β ings of comparator setting are fed into Decay Constant χ 10—8 second 8 Density 1.05 the machine, it will print (at a rate of Melting Point °C 110 100 cards per minute) the interpolated • N a t i o n a l Radiac, Inc. brand name wave lengths λ0 and all additional in For L i t e r a t u r e a n d D e t a i l e d Specifications, w r i t e : DepfA-8 formation relating to intensities, line characteristics, etc. National Radiac INC. The authors point out that their ' rather flexible set of standard com1 475 Washington St., Newark 2 , N. J. j ponents permits the choice of those I variables which are most important for Circle No. 38 A on Readers' Service Card, page 53 A
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a particular application. The spectroscopist is more likely to be interested in the precise evaluation of wave lengths, including the best possible interpolated values. The analyst would be inclined to show a preference for precise esti mates of relative intensities. In any case, a complete system of this sort would enable one to get every possible bit of useful information and the sys tem has the inherent possibility of dispensing entirely with the intermedi ate stage of photographing the spectra. This is particularly attractive in the infrared region at high resolution. The direct-reading spectrographs, of which there are several eminently prac tical examples, have been serving the analyst for many years. Their function is relatively simple compared with the Dieke system. Automatic
Computation
We find ourselves straddling several fences in an attempt to evaluate con temporary practices in automatic com putation. The foregoing example, in handling the prodigious amount of im portant spectroscopic data, requires no argument or justification, because the computational elaborations are amply justified by the importance of the data and the great burden which the older methods of measurement impose. Most of our concern arises from other examples, in which prodigious sums have to be invested to calculate résulte that have so little fundamental importance. It is not uncommon to hear computer experts speak of machme solutions of data for which no a priori reasons are apparent for the proper mode of solution. The excuse in such cases seems to be that the machine can find an answer or correlation by a. process of successive approximations, and, at machine speeds, these can run into an enormous "number of such approximations. The arguments on this subject are countless and it must be said that if ample funds are available more than half of the arguments are pointless. The analyst has many fine contributions to his credit in this field, and in general he has been extremely conscious of what is reasonable. Conversely, it would be interesting to see what some autotitrators, and analyzers of all sorts, would look like if they were supplemented with a half-million-dollar array of data assimilators, computers, and predictors. There may come a day when analytical data have to be funneled into a central control agency at microsecond speed for prompt interpretation, but this is a problem of communication and may well leave the analyst cold and unimpressed. ANALYTICAL
CHEMISTRY
