COMMENTARY by Ralph H. Müller
DR.
SHIRLET'S DISCUSSION of the
Ra-
dioactive Detection of Nuclear Magnetic Resonance will arouse the interest of people in many fields and, hopefully, that of analytical chemists. Even with lack of comprehension of what is really going on in these nuclear phenomena, their symmetric and asymmetric distribution in angle of emitted beta or gamma radiation, the analyst will sit up and take notice of the fantastic sensitivity which these techniques permit. If only 106 nuclei are required to get a good statistical answer, this is of the order of 10~ 1 7 mole and superb trace analysis by any criterion. They will not be too discouraged by the author's statement that "its potential (if any) in chemistry is as yet unexplored." And also "its feasibility in gaseous molecules, or liquids has not yet been demonstrated." It is enough to know that N M R / R D has quickly proved its value in physics. It will command increasing interest in solid state physics where purposely introduced trace impurities can provide such a wide variety of electrical properties and functions. One thinks instinctively of alternative methods, which over the years have developed into highly useful and widely used techniques. Neutron activation analysis, which is applicable to a large number of elements, provides exact identification in terms of the decay rate of the radionuclides which are formed, and quantitative results as well. In the case of nuclei which have very large cross section values, the sensitivity is enormous. The electron probe technique of Castaing, Gunier, and others affords detailed analysis of minute specimens and definite identification of their nature. Many years ago, we heard a fascinating lecture by an eminent physiologist who used an unusual approach for delineating the distribution of calcium in striated muscle. A microtome section of muscle was deposited on a quartz slide and then carefully incinerated in a furnace. The slide containing the ash residue was then heated to something less than 800 °C and thermionic electrons emitted by the ash were then accelerated and focused on a fluorescent screen. The image was a precise replica of the gross morphological structure of the muscle structure as revealed by ordinary microscopic examination. This was no mean feat in those early days of electron optics. The method was obviously of limited use, but, pre-
sumably would work for any residue exhibiting thermionic emission at moderate temperatures. It had been known for a long time that calcium oxide as well as other alkaline earth oxides, are excellent thermionic emitters. Today, the electron probe by a scanning technique would yield the same information but it is doubtful whether the special delineation would be as precise or, for comparable precision, could be obtained as quickly. Another example is striking. Newton Harvey at Princeton, who revealed the nature of the chemiluminescence of cyprodina, showed that it arose from the oxidation of luciferon, catalyzed by the enzyme luciferase. In one experiment, he chose to furnish oxygen by its liberation at a platinum electrode immersed in the lucifer-in-luciferase system. By observation through a low power microscope, the least perceptible flash of light (dark accommodated eyes) appeared at an electrolysis current corresponding to the order of one oxygen atom. If one begins to wonder about the feasibility of detection at the single atom level, it is merely necessary to recall Rutherford's first demonstration of the transmutation of matter by the bombardment of nitrogen with alpha particles in which high speed protons were one of the products. These were detected by their very long range by visually observed scintillation on a fluorescent screen. To be sure, many such protons were observed but each flash represented a single proton. The occasional forked track in a cloud chamber again represents a single nuclear event and detailed analysis of the track reveals the components resulting from the event. If the nuclear physicist is bewildered and embarassed by the vast array of nuclear particles, some of an extremely fugitive nature, we analysts are confronted with the same difficulties when we inquire into the nature of an allegedly pure sample. Even emission spectroscopy, when applied to the examination of a very pure sample of metal will reveal the unmistakable presence of an appreciable fraction of all the known elements. Trace analysis when it approaches the single atom level is something like throwing a party for a dozen congenial friends. If one has earned even a modest reputation for hospitality, it is astonishing how many people show up.
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Circle No. 32 on Readers' Service Card VOL. 4 1 , NO. 8 , JULY 1 9 6 9
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