REVIEW OF FUNDAMENTAL DEVELOPMENTS
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University of California, 10s Alarnos Scientific laborafory, Ios Alamos,
brief review covers some recent advances in instrumentation which have direct or indirect bearing on the problems of the analytical chemist. T17ith few exceptions, it deals with techniques which have not been covered in the writer’s monthly column on instruCHCJIISTRY. mentation in ANALYTICAL A choice of topics becomes increasingly difficult in this field because no one can say with certainty that a new circuit, technique, or device developed by the astronomer, the physiologist, or the aerodynamacist has or has not possible interest for the analyst. As before, no attempt has been made to survey the advances in instrumentation in each special branch of analytical chemistry because the reviewers in each branch have already done this for their own specialty. Contemporary instrumentation is characterized by elegance and complexity and continues to be the despair of seielitists who face limited budgets. One would hope that more things like spot tests and paper chromatography could be discovered mhich would yield important information all out of proportion to their cost. Technology requires fast results on a wholesale scale and costly instruments are warranted. Modern pure research requires more and more sophisticated answers and few of these are obtainable by simple means. Nuclear techniques are becoming increasingly important to the analyst. These are not confined to the familiar tracer techniques, which are well established and capable of infinite extension on the basis of known principles. Some of the advances are instrumental, but significant principles have been developed, leading to new and important applications. Extensive analytical applications can be expected from the wider uses of scintillation spectrometry in m.hich gamma and x-ray emitters can be examined with multichannel pulse height analyzers. These yield count rate as a function of energy and provide a characteristic spectrum. The latest on these techniques is summarized in a book ( S I ) dealing with an informal conference held in 1956, with contributions from European and North American experts. At present multiHIS
channel pulse height analyzers are complicated and expensive, but new developments promise to reduce these factors in the very near future (64). A simple recording gamma-ray spectrometer has been described by Venable (56) who uses a sodium iodide scintillator, linear amplifier with a motor-driven base and window control. The spectrum is presented on a cathode ray tube. This is not adequate for highest precision, but is a most useful compromise. Frequently, after a complete spectrum is obtained, it is desired to set on a fixed channel and examine a gamma or x-ray peak a t the fixed energy level. The exact location is tedious to find and a clever technique for this purpose has been developed by Oak Ridge investigators (27). This is an aural counting rate nieter and a multivibrator, the rate of which is controlled by the count rate. This provides a very sensitive “howler” to find the desired channel. The audio output varies between 30 and 30,000 cycles and can easily locate peaks to within 1%. A fast “kick-sorterJ’ channel responding to pulses as short as 10-8 second and a dead time of 10-7 second is described (1).
Counters and scalers all have an upper limit to their rate of counting. I n the latter, the dead time is easily measured with a calibrated pulser. For counters, the problem is a little more involved. Once the over-all dead time is known, observed count rates are very conveniently corrected for dead time or coincidence losses by a graphical method described by Chan (10). The observed number of events detected per second by a Geiger counter depends on the dead time. The number of events, n, which would be detected per second if the dead time mere zero is related to the actual number, m, and the dead time, t, by the equation: n = m/(l
- mt)
When dealing with a large number of observations it is sometimes convenient and less laborious to use a simple graphical method of finding n from the known values of m and t instead of finding n by direct substitution. Chan has given a simple graphical method for making this correction.
N. M.
The identification of beta emitters in terms of their maximum energy is achieved with high precision by a beta spectrlimeter but absorption methods are cheaper and simpler for an approxim:tte answer. Duncan and Thomas ( I S ) compare the three common modes of calculating E,,,. The method of Blueler-Ziinti is the most generally useful. Feat her’s rule is accurate when high activitiea are available. The Harley-Hallden method is good for quick estimates or where only low activitics are available. Thickness gaging by means of beta or alpha particles finds numerous scientific uses. Industrial gaging with betas is a highly developed art, but new instruments are appearing. A new British machine (28)uses a separate beta source and ionization chamber to balance the output from the measuring head. Also i;o facilitate rapid change to a new “weight” standard, the balancing chamber is fitted with a grid electrode. This electrode can be controlled by a remotely located potentiometer. The use of krypton-85 as a source of bet:L particles has several advantages in industrial gaging. Being of lower eneryy, they are more readily absorbed than the betas from a strontium-90-yttrium-90 source or from thallium-204. The remote danger of breaking the encapsulated source is considered to present a smaller hazard because krypton45 is a gas, its only daughter element is extremely shortlived, and krypton has no known function in human physiology. Both in use and fabrication of the source holder, adequate venlilation is all that is necessary and conlamination, as in the case of liquids or scdids, is not possible (63). Thickness measurements with alpha particles cover an entirely different region, b e c a s e the range of alphas in air is from a few centimeters up to 7 or 8 cm. The method is therefore useful for extremely thin films and as such it is of interest to the microscopist, in infrared simple measurement, etc. Thus Davisor. (12) explains that in the absorption spectroscopy of polymers, it is often necessary to measure the thickness of films of 5- to 25-micron thickness. This must be known for accuracy of tlie analysis-e.g., 1 to 2% VOL. 30, I
