INSTRUMENTATION by Ralph H. Müller
N e w developments in chromatographic scanners, thermal conductivity apparatus, thermistors, and infrared detectors
GRADUATE school we were once told I Nthat a professor of analytical chemistry could be expected to do one of two things during his career. One was to devise a new scheme of qualitative analysis, the other to invent a new hydrogen sulfide generator. Times have changed. These days there seems to be an epidemic of chromatographic scanners, many of them described without much reference to prior art. Yes, we described one too, about 10 years ago. The subject is almost ripe enough for a definitive review. Despite this, we wish to mention another one. It is new; the author says so, and we believe him. Blake, of Sydney, Australia, who is well known for his pioneering work in radio-frequency titrimetry and for his monograph on that subject, is the author [Anal. Chim. Acta 17, 489, 492 (1957)]. The new method is based upon the preferential dissipation of static charges by zones in a paper chromatogram. In the first arrangement, an incandescent lamp with a collector electrode cemented to the outside of the lamp bulb acts as the source of static charge. Presumably, this is a consequence of the Edison effect, wherein electrons emitted by the filament strike the inner wall of the bulb and induce a charge on the outer surface. The collector is connected to a fine electrode under which the chromatographic strip can be moved. A grounded electrode is placed adjacent to or below the feeder electrode. Charge dissipation is indicated by an electrometer or simple gold-leaf electroscope. As the chromatogram is moved, the presence of a zone is indicated by a decrease in electroscope deflection. In a second arrangement, Blake uses any convenient source of high potential, a swamping resistor, and small
neon bulb. In this case, the neon bulb indicates the presence of a zone. When so located, each zone can be excised and analyzed by any one of the dozen physical or microchemical procedures which have been used in the past. The author emphasizes the simplicity of this scheme, but does not regard it as better than his earlier method of zone detection by radio-frequency methods. For some time we have felt that analysts, and our ACS Division of Analytical Chemistry, should seek wider acquaintance with the work and aims of the Society for Non-Destructive Testing. There was, not too long ago, a distinct impression among analysts that the two subjects had very little to do with each other. It is becoming apparent that, in many respects, they often have the same goal but different methods of approach. No analyst has to be reminded that many of our instrumental methods of analysis are indeed nondestructive. In fact, chromatography, including vapor-phase chromatography, affords direct examples. Likewise, a number of techniques which the nondestructive testing people use, give answers which the analyst would be prepared to furnish by entirely different techniques. Under these circumstances the question arises —which is the better approach from the point of speed, precision, and economy ? In recent months some Americans have been fortunate enough to hear the talk by Fritz Fôrster, a prominent exponent of nondestructive testing and director of the Institut Dr. Fritz Fôrster, Reutlingen, Germany. While his electrical, magnetic, and electronic techniques have specific meaning for people charged with routine testing, inspection, and control, it became evident to any analyst that these prob-
lems would have been approached by the chemist in an entirely different way and in most instances not nearly so rapidly or precisely. Thermal Conductivity Determination
In this connection we would like to describe a simple but elegant technique recently developed at the National Physical Laboratory in Teddington. It is concerned with the rapid determination of thermal conductivity. This, of course, is a nonspecific property of matter, but in its various applications it can supply information which the analyst might be tempted to gain by more laborious methods. The thermal comparator, for measuring thermal conductivity, surface roughness, and the thickness of foils or surface deposits is described by R. W. Powell [J. Sci. Instr. 34, 485 (1957)]. The thermal comparator consists of two metal balls similarly mounted in a block of balsawood, but one is mounted at a slightly lower level, so that it touches any surface on which the block rests. After the Mock has been heated to a small fixed temperature excess (70° C.) in an oven, it is laid in contact with the test surface. Differentially connected thermocouples attached to each ball measure the increased rate of cooling of the ball which makes contact. The differential e.m.f. is observed 10 seconds after contact is made and is shown to be a function of the thermal conductivity of the material on which the ball rests. Actually, when the block is removed from the oven, the twro balls will be at the same temperature and the e.m.f. will be zero. As cooling of the ball in contact with the specimen occurs, the e.m.f. will rise rapidly, and the rate will then fall off, VOL. 30, NO. 3, MARCH 1958 ·
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ANALYTICAL CHEMISTRY
roughly in accord with a Newton's law behavior. The rate of rise for the first 10 or 20 seconds is so high that it is indistinguishable from true linearity, for which reason, the author uses the e.m.f. rise in the first 10 seconds. The re sponse cannot be calculated to sufficient precision. It is necessary to calibrate the assembly with at least two speci mens of known thermal conductivity. When the e.m.f. attained in 10 sec onds is plotted against thermal con ductivity, a rapidly rising curve re sults which suggested the true relation ship. A plot of Ε (10 seconds) against the square root of the thermal con ductivity is linear and affords a simple means of calibration. The precision of a single determination is ± 6 % . Lead deviates considerably because, it is thought, of its softness. Surface con ditions affect the results and it has been shown that it is possible to meas ure surface finish in this way. Simi larly, it is possible to measure the thickness of thin foils, especially if they differ appreciably in thermal conduc tivity from the block or anvil on which they rest. Surface coatings can also be measured in this way. The author points out that the actual measurement requires no more than 10 or 20 seconds but as many minutes may be required to reheat the measur ing block in the oven. Modifications have been made to accelerate the re heating of the cooled ball by electrical heating. The system is ideally suited for nondestructive sorting or identifi cation of solid materials. There are several means of rendering some of these operations automatic without destroying the essential sim plicity of the method. We have been studying these and will describe them elsewhere. Thermistor Applications
As we have emphasized repeatedly, thermistors have many uses other than the measurement of temperature. A very stimulating article by A. M. Hardie [J. Sci. Instr. 34, 58 (1957)] shows how series-connected thermistors of the indirectly heated type can be used as a phototube load resistor in order to compensate for the slow drift of illumination from a lamp source and changes in photocell characteris tics. The inherent limitation of com mercially available themistors of hav ing room temperature resistances some what low for this purpose is easily overcome by connecting as many as six in series. By means of feed-back systems, the thermistors can be heated and thus change their resistance. One thus has an automatic gain control for the photoelectric system. Hardie's in
strument was intended for vibration studies and his system succeeded in stabilizing to the extent of 0.03%. For his purpose, the light beam was sym metrically modulated by the vibration under study. The method would seem to have wide application in other pho tometric problems. Better Infrared Detectors
The rapid advances in solid-state physics are paying off in the develop ment of better infrared detectors. Several papers have dealt with the be havior of indium antimonide as a photoconductive detector and of InSb as a p-n junction device behaving as a pho tovoltaic cell. Limit of response is somewhere in the vicinity of 7 microns. Thus, P. W. Goodwin [J. Sci. Instr. 34, 367 (1957)] has studied a single crystal of InSb between room temperature and 90° K. At the higher temperature a typical resistivity is 120 ohms, which rises to 20,000 ohms at 90° K. The relative response increases 1300-fold at the lower temperature with a de crease in wave length of the peak sensi tivity from 6.7 to 5.6 microns. Al though the cell resistance increases on cooling, its value makes the cells still suitable for matching into transistor amplifier circuits. The significant prop erty is the small time constant, which is of 4 X 10~7 second as measured with a pulsed spark source. Related work on InSb is described in another paper by Avery, D. G., Good win, D. W., Rennie, A. E., [J. Sci. Instr. 34, 394 (1957)]. Among idle thoughts for the month, we return to praise of the American telephone system—but in the form of a question. How can it serve the sci entist in the form of audio-telemeter ing? It's being done, of course, in transmitting information from remote pumping stations, etc., but what other useful things could it do for us? As a stunt, we once sent trichromatic data as audio "beeps" to upper Manhattan, where they were relayed over another 'phone line to our laboratory in lower Manhattan and were reconverted to trichromatic beams to reproduce the original color. Will subscribers some day have access to costly computers which can be interrogated in a similar fashion? These are simple things as far as communication scientists and engineers are concerned. It is a ques tion of what sort of information re positories would be useful and desir able, and it is an important question because the widespread and individual use of elaborate computers is, and will continue to be, out of reach for many scientists.
