with respect to an electroactive species. Such techniques have important analytical application. The distinction between potential step and potential leap should be stressed. In the former, the constancy of E is paramount, whereas in the latter, the potentiostatic condition is unnecessary, provided the magnitude of E is always sufficiently large. The tabular entry “linear voltage sweep” needs qualification. It is in the vicinity of the null potential of the electrochemical system only that a linear change of potential with time will induce a linear change in surface concentration and thereby satisfy the criterion for order four. It is not my intention to suggest that the possible orders of technique are limited to six, nor to suggest that each transient
electrochemical method necessarily corresponds to a discrete order. The utility of the order concept has been demonstrated in work on the electrolysis of nonuniform solutions (1).
KEITHB. OLDHAM Science Center North American Rockwell Corporation Thousand Oaks, Calif. 91360 RECEIVED for review May 13, 1968. Accepted July 5, 1968. (1) K. B. Oldham, ANAL.CHEM., 40, 1799 (1968).
Sampler for Collecting Gas from Blisters in Thin Metal Sections Philippe Chastagner SaGannah Riuer Laboratory, E. I. du Pont de Nemours and Co., Aiken. S. C. 29801
GAS BLISTERS are sometimes formed in metals, particularly during exposure in a nuclear reactor, as the result of inclusions in the raw materials, defects in manufacture, and various mechanical failures. Analysis of the gas within a blister assists in determining its specific cause. The sampler designed for this application is an evacuable, contained punch that is easi!y handled in a radiological containment facility, is readily shielded, and is inexpensive enough to be discarded when severely contaminated with radioactive material. EXPERIMENTAL Sampler Construction. The device shown in Figure 1 was designed to collect the gas from blisters in sections cut from thin metal specimens. The 0-ring-sealed, steel container serves as both the sample vessel and the mount for a punch and die set. The punch and die are an inexpensive commercial set modified to ensure venting of gas from punched blisters. The shank of the punch is polished to provide a good surface for the O-ring seal and is fitted with a retaining ring to limit its inward travel. “ W o n ” (E. I. du Pontde Nemours & Co.) O-rings are used; those around the shank of the punch are lubricated with “Apiezon” N stopcock grease from Associated Electrical Industries, Ltd. The valve is a Hoke TY 440, a type that has proved satisfactory in vacuum service. The sampler was shown to be free of leaks with a mass spectrometer leak detector. The sampler is designed to accept a thin metal section, the projection of which is a square about 3 inches x 3 inches with a blister in its center. This sample size allows about 1 inch of undisturbed metal to remain around the average blister and is usually sufficient to keep gas from escaping before the blister is punched. Procedure. A sample is placed convex side down on the sampler base with the blister centered over the die. With the punch partially withdrawn into the top of the sampler, the top is positioned carefully to avoid disturbing the sample. The peripheral screws are inserted and tightened, and the punch is pushed in by hand until the center point is in firm contact with the sample. The punch holds the sample in place and is in turn held by the lubricated O-ring seal on its shank. The sampler is evacuated and leak-tested on the
. 6“ A
Figure 1. Blister gas sampler
inlet system of a conventional mass spectrometer; then the valve is closed. The sampler is removed from the mass spectrometer, and the punch is pushed through the sample with an arbor press or a large vise. Then the sampler is reattached to the mass spectrometer, and the gas from the blister is vented to the inlet system where its pressure is measured with a micromanometer. The gas is then admitted to the mass analyzer for compositional analysis. The quantity of gas is calculated from the micromanometer readings and the relative volumes of the sampler and the inlet system, 250 and 3000 cc, respectively, in our case. The gas composition is calculated from the mass spectral data. RESULTS
This blister gas sampler was used successfully with samples that had been exposed to nuclear irradiation. With these samples the punch was loaded in a glove box and transferred to the mass spectrometer in a clean plastic bag. All operations with irradiated samples were planned to minimize personnel exposure; the direct handling of the bagged sampler by the analyst usually took less than 3 minutes per sample, RECEIVED for review April 20, 1968. Accepted May 31, 1968. The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission. VOL. 40, NO. 12, OCTOBER 1968
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