INSTRUMENTATION by Ralph H. Müller
Balance between classical a n d instrumental methods desirable . . . . . . Automatic counting equipment described E 12th Annual Louisiana State T HUniversity Symposium on Analytical Chemistry a t Baton Rouge, January 26 to 29, was well attended. Philip West and his colleagues at LSU continue to maintain the fine tradition of these meetings combined with hospitality which is already legendary. We have had the honor of participating in the first, the fifth, and this, the twelfth symposium. For some years this event has h a d an international flavor. This year, Great Britain was represented by Ronald Belcher of Birmingham and by Cecil Wilson of Belfast, by Herbert Weisz of Vienna and Wilhelm Fresenius of Wiesbaden. The presence of Fresenius was a historical event. He is the great-grandson of Remigius Fresenius— the contemporary of Liebig, Wôhler, and Berzelius. The four generations of this family encompass almost the entire history of analytical chemistry. With characteristic thoughtfulness West arranged t o have Fresenius inducted into the Phi Lambda Upsilon honorary society and to deliver the first Fresenius Lecture. As all Phi Lambda's will recall, t h e society's patron "saints" are Fresenius, Liebig, and van't Hoff. If we are not mistaken, the Fresenius family represents a unique record in t h e annals of science. The triumvirate of Becquerels in France was another example. The grandson was the discoverer of radioactivity. The details of this meeting were discussed earlier (ANAL. C H E M . , March, page 39A). One of the most useful by-products of such meetings is the opportunity to talk shop with analysts. We were pleased to learn from our good friend Jay Lohr, now a t D u Pont, that he has succeeded in introducing a broad approach in the solution of analytical problems. As he explained it, "some of Larry Hallett's (Editor, ANALYTICAL CHEMISTRY) broad philosophy had brushed off on him," a n d in his own competent hands, it is paying off handsomely. We, among others, were priviliged to witness for some six or seven years what Hallett accomplished as director of analysis and analytical research a t General Aniline a n d Film Corp. laboratories. I t was a perfect balance be-
Typical automatic windowless flow counting instrumentation for low level beta counting tween the classical methods and the instrumental approach. At the time, one could have wished that the same rational and sensible attitude could have prevailed in our academic institutions. With all the information to be gleaned from elementary analysis, from gravimetric and volumetric methods, the results were supplemented by the techniques of chromatography, electron microscopy, x-ray, infrared, spectrophotometry, electron diffraction, polarography, a n d other electroanalytical techniques. With such perfect coordination of techniques it was impossible to go "overboard" in veneration of precipitates or to fall over the other rail in behalf of Buck Rogers electronics. The same philosophy is now being expounded from t h e editorial chair and the Editor's Column and it is one in which all progressive analysts will concur. Automatic Counting Equipment From Paul McNulty and Alex Ε . Η . Piranian, both of Tracerlab, 1601 Trapelo Road, Waltham 54, Mass., we learn of the several economies which can be achieved by the use of automatic count ing equipment. I n any work requiring the counting of radioisotopes, the skill of an analyst or radiochemist is con fined to schemes of isolation, sample preparation, counting geometry and such effects as backscattering, self-ab sorption, etc. Once these are settled,
the actual process of introducing the samples, starting and stopping the counting, and recording the elapsed time or number of counts is a procedure which requires no skill a t all. For weak samples, it may consume much of the total time. When this time becomes excessive, one is faced with the need for duplicate equipment or additional tech nicians. This is precisely the time a t which automatic sampling and auto matic counting should be considered. To quote McNulty and Piranian: Two important benefits thus accrue. The first is that laboratories which must meet definite work completion sched ules are able to telescope their work by utilizing time which formerly (for all practical purposes) did not exist. A second benefit is t h a t it permits fuller utilization of the radiation detection equipment already available and often eliminates the need for duplicate scalers, detectors, and other equipment. In addition to the economic benefits gained by the users of automatic sample counting equipment, there are other ad vantages which are worthy of consid eration. Most important among these is convenience; that is, the freedom from constant attendance a t a sample changer. The machine-printed results which are furnished by the data printer provide permanent records and greatly reduce t h e probability of error in r e cording and transcribing results. In view of the advances and wide spread acceptance which have been gained by equipment of this type over VOL.
3 1 , NO. 4, APRIL 1959
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5:10 P.M.: Beginning
of a difficult, time-consv.ming
analysis
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the last ten years, all users of radiochemical techniques should periodically review their operation to determine whether or not they too can enjoy its benefits. There are a number of interesting ways in which automatic sample counting equipment can be used, apart from the immediately obvious one of expediting routine sample counting. Laboratories studying the half lives of short-lived isotopes obtained from atomic piles, particle accelerator, or neutron activation, have found the automatic sample changer a convenient way to make half life measurements. A single sample is inserted in an automatic changer and the unit is set to cycle continuously. Under these conditions, the sample is counted every 250 seconds. Several hours or days of counting generally provide sufficient data for an accurate determination of the half life. The scanning motor on a pulse height analyzer is set to complete its scan in, say, 10 minutes. A scaler-ratemeter with preset time feature is. also set for 10 minutes and a strip-chart recorder is attached to the ratemeter. The recorder thus automatically provides a permanent pulse height spectrum record for all samples inserted. A less elaborate system incorporates a dual channel analyzer into the unit and analyzes samples of two separate energy peaks and reports its results separately. With regard'to spectrometry, it is important to remember that most spectrometry requirements can be incorporated into an automatic sample counting system with but minor modifications to the standard instrument setup. An impressive number of instruments and devices have been developed to perform automatic sample counting. Typical of these are the standard automatic sample changer and the automatic windowless flow sample counter. The operation of both units is simple and reliable. On the former up to 25 standard 1-inch samples are placed into slots on the circumference of_ a large circular turntable. An arm with a 1-inch ring is lowered over the sample and slides it into the shielded detector unit. When fully inserted, a relay starts a scaler. When the scaler reaches either a preset time or a preset count, the scaler stops counting and relays this information to a data recording printer. Simultaneously, the arm withdraws the sample, the turntable is advanced one position, and the next sample is inserted for counting. The entire operation (after loading) is performed without attention from working personnel. The unit can be set to cycle once, twice, three times, or continuously. The detector shield used with this type of sample changer is particularly versatile and will accommodate standard Geiger, proportional, scintillation, and flow-type Geiger detectors.
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ANALYTICAL CHEMISTRY
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An automatic windowless sample counter is usually used when a large number of relatively weak samples must be counted with maximum efficiency. These counters may operate in either the Geiger or proportional region. Up to fifty samples of any size up to 2 inches in diameter can be accommodated by this unit. The samples are placed on special planchet holders and loaded into a clear glass sample tower which is clamped onto the main unit. The sample changing mechanism takes a sample from the bottom of this tower and inserts it into the gas filled counting chamber. When the sample has been counted the information is fed to the data recording printer, the sample is removed from the detector and raised into a glass collecting tower. This operation is repeated until all samples have been counted. Geiger or proportional gas is fed first to the detector and from the detector is split into two streams and vented after passing through the sample towers, thus keeping all samples in a sealed atmosphere of counting gas at all times. The importance of a detector's ability to accommodate extra large samples is one that cannot be overemphasized. If sufficient active material is available to prepare 2-inch samples a marked reduction can be made either in counting time or the lower limit of specific activity detectable. For example, 1inch samples of BaC0 3 (infinitely thick) with an absolute specific activity of approximately 1.0 d.p.m./mg. can be counted to a 5% probable error in approximately 100 minutes on the SC-50B. If 2-inch samples of the same material are prepared, only 22 minutes are required for similar results. In addition to the basic equipment described above, these automatic sample counting systems include a scaler and data recording printer. The scaler used contains special connectors and relay circuits, which are necessary to activate the changer itself and the data printer. It should also contain both preset count and preset time controls. Because it is usually desirable to count all samples to the same probable error, the preset count is an essential feature. Since on occasions a very weak sample is encountered, preset time is a desirable feature to cut short any unprofitably long counting run. The data recording printer provides a permanent record of the sample number and the time required for a given number of counts. In our own nuclear researches, for lack of manpower, it has been found necessary to embark on an extensive program of automation and the design of mechanisms using relays and pneumatic devices to perform routine manipulations. These enable one to collect data overnight and leave regular hours for setting up new experiments and collating data.
