&Pi Zinc Sulfide Alpha C o ~ n i i ~ N. Irving Sax and David Rosi, Division of 4n alpha counting with efR ficiencies in excess of 90% is obtainable by the use of Mylar and OUTINE
plastic-dispersed zinc sulfide film [N. A. Hallden and J. H. Harley, ANAL. CHEM.32, 1861 (1960)] and a scintillation spectrometer (Tri-Garb, Packard Instrument Co., La Grange, A successful technique consists of coating two pieces of the plastic film on the dull surface with the sample, placing them face to face, and inserting into the well of the spectrometer. Based upon a National Bureau of Standards standard for radium-226 and a standardized solution of plutonium-239, high 4 r efficiencies are obtained. The instrument used in this laboratory accommodates plastic films with a total area of 40 sq. em, It has been determined experimentally that when
Laboratories and Research, New York State Department of Health, Albany,
the sample to be counted is spread out on this area, up to 50 mg. of carrier weight can be employed with very little decrease in efficiency. The spreading technique consists of transferring the barium radium sulfates precipitate with the aid of 95% ethyl alcohol and a transfer pipet from a centrifuge cup to the zinc sulfide-impregnated film and drying under an infrared heat lamp. Care is taken to obtain an even deposit on the film. A series of six or seven transfers is sufficient to make the total transfer and deposit it evenly on the film. By setting the discriminators of the spectrometer to exclude extraneous pulses, background levels can be maintained a t approximately 0.1 count per minute, enabling the measurement of 10-14 gram of radium per sample. Increases
N. Y.
in the sensitivity of the system may be obtained by allowing for a 3@day accumulation of radon daughter products. Further development of this counting technique is proposed. This laboratory is experimenting with a powder technique for counting radium as the barium radium sulfates precipitate. I n this procedure, which seems to produce high 4.tr efficiencies, silver-activated zinc sulfide is mixed with the mixture of sulfates as a slurry in 95% ethyl alcohol, transferred to the inside of a glass or plastic counting vial, and allowed to dry. The thin even coating thus produced on the inside bottom of the counting vial works very well. Furthermore, the use of B counting vial automatically keeps the sample from either being contaminated or contaminating the outside.
Free Diffusion Sa Paul
D. Gam and Jo
!I Telephone Laboratories, Inc., Murray Hill, N. J.
the authors pointed out certain advantages in restricting deconiposition products in thermogravimetric studies [ANAL. CHEW 32, 1563 (1960)l. Subsequently, the advantages of the opposite extreme became apparent. This paper describes the results obtained when diffusion of gas to and from the sample is as nearly unrestricted as is experimentally convenient. That the traditional crucibles are useless in thermogravimetry is essentially true. I n a few cases crucibles are acceptable. I n vacuum thermogravimetry any open vessel is satisfactory. I n controlled atmosphere work, where the atmosphere is solely ECENTLY,
TEMPERATURE IN DEGREES CENTIGRADE
errnol decomposition of lead carbonate
the gas involved in the reaction, the geometry of the container is immaterial. I n displacement problems such as the “blowing” of polyethylene the crucible will probably be a convenience. In the work reported here the sample is heated in the form of a thin layer on a flat surface. While this arrangement is not yet ideal, the wall of the traditional crucible is no longer a barrier. Under these conditions the atmosphere has ready access to the sample as well as the product gases to the atmosphere. Again the decomposition will be more rapid than in a crucible but a t a lower temperature, because the partial pressure of the decomposition product will be more nearly that of the ambient atmosphere. The only region in which the partial pressure will build up appreciably is within the bulk of the sample, but, with the thin layer of powder, diffusion to the atmosphere is easy and the decomposition will be comparatively unhampered. The sample holder is simple: a disk with a slight lip, supported by a rod from the center. The results are equally simple. Ordinary reversible decompositions occur, as we could predict, a t B significantly lower temperature than when the products are confined purposely or inadvertently.
Lead carbonate (Figure I) behaves rather differently, No inflection for the basic lead carbonate appears. Instead, the material decomposes first to the dioxycarbonate. Before decomposition to the ovide is complete, the oxygen from the atmosphere, not being kept out by the carbon dioxide, oxidizes PbO to Pb304. FinalIy the sample reyerts to PbO. Again, because the oxygen is not restricted, this decomposition occurs a t a lower temperature than in the deep vessel. The curves from the previous paper are included for comparison. The cobalt oxalate hydrate, too, behaves essentially as predicted (Figure 2) ~
Figure 2. Decomposition of cobalt oxalate hydrate
The water comes off a t a low temperature and oxalate decomposition occurs a t a much lower temperature. The final decomposition is alniost instantaneous. The evolution of carbon monoxide and dioxide does not appreciably inhibit the approach of oxygen to the sample, so the ox4dation of the cobalt occurs very rapidly, providing the energy for the decomposition of the oxalate. The significance of this and the previous work is that me have shown that the most common method of supporting a sample is not a t all satisfactory. The wall of the traditional crucible restricts the product gases sufficiently t o affect the partial prewire over the
sample. Either of two techniques wi11 provide a substantial improvement in weight-loss curves for reversible and irreversible decompositions and without recourse to controlled-atmosphere or vacuum thermobalances. Significant improvement is obtained even on pyrolysis of organic materials. The shallow vessel will provide partial pressures that approach the ambient pressures, but these partial pressures will remain nearly constant, so that the weight loss will occur at essentially constant pressure. The closed vessel also provides a constant pressure, but in this case the partial pressure of the product gas is 1 atm. In addition, the restriction of the product gas leads to the interesting and useful condition
A Semiautomatic Plotting Attachment
in which the atmosphere during a decomposition is always that which is pertinent to the reaction. The importance of sample holder design in thermogravimetry is closely related to the problems of reproducibility (between laboratories) of differential thermal analysis curves and correlation cf thermogravimetric and differential thermal analysis data. Both problems have plagued workers in thermal analysis. We expect to develop differential thermal analysis techniques permitting closer correlation of the data with thermogravimetric data and enabling the virtual duplication of differential thermal analysis data in apparatus dissimilar except for the sample holder.
for the Beckman DU Spectrophotometer
George N. Bowers, Jr.l, and Samuel Raymond, The William Pepper Laboratory of Clinical Pathology, University of Pennsylvania Philadelphia 4, Pa.
s MANY APPLICATIONS of spectrophotometry, it is necessary to plot absorbance readings as a function of time. S n example is the determination of transaminase activity by measurement of the rate of change of reduced diphosphopyridine nucleotide absorbancy in a suitable enzyme-substrate mixture. To obtain valid results, the enzyme concentration must be estimated from the zero-order slope of a plot of absorbancy against time. The attachment (Figure 1) eliminates recording and manual plotting of such Present address, Hartford Hospital, Hartford, Conn.
Figure 1.
readings. A steel tape on the circumference of the cam, A , which is attached t o the transmission-density knob of the Beckman DU spectrophotometer, positions the typewriter carriage according to the absorbance. The synchronous motor, B, moves chart paper a t a uniform rate past the printing area. When the spectrophotometer is balanced, a manually operated momentary-contact switch, C, activates a solenoid, D, on the period key of the typewriter, thus imprinting a dot. The only modification of the spectrophotometer is a 3/32-inch centering hole drilled into the transmission-density knob. The plastic cam is centered on the knob by
a pin and held in place with two clamps. The instrument is not impaired for other uses and, if need be, the steel tape and the cam are easily removed. Any typewriter may be easily adapted for holding and moving the chart paper. To make this adaptation, release the mechanisms controlling movement of the carriage and rotation of the paper roller. Attach a constant speed drive motor (1/5 r.p.m., from Haydon Motors, Torrington, Conn.) to the shaft of the paper roller to drive the paper a t a speed of approximately 1 inch per minute. The transverse motion of the carriage
Cam-typewriter arrangement for semiautomatic plotting attachment VOL. 32, NO. 13, DECEMBER 1960
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