Table II.
Signal-to-Noise Ratio
Signal-to-noise Plate developer Ilford D 19 Ilford Chem. Kodak D 19 Kodak Chem.
10-12 coul. 10-11 coul.
exposure
exposure
0.627 0.056 0.222 0.053
0.752 0.364 0.495 1.324
the two plates at identical exposures (the background fog being considered as noise). The results are given in Table 11. The signal-to-noise ratios at 0.001 X coulomb leave little doubt regarding the superior sensitivity of D 19 developed plates. On the other hand, a t higher densities where one would prefer to obtain quantitative data, the signal-to-noise ratio of the plate developed by the surface developer is as good as that of the plate developed in D 19.
A model of the development process which explains the differences noted between the commonly used developers and the surface developer is as follows: Each of the commonly used developers contains an antioxidant, such as sodium sulfite, for stabilization of the solution. These materials exhibit a certain degree of solvent action for the silver halide grains in the emulsion. Any image which resides in the interior of a grain thus may become exposed to the developing solution as a result of this solvent action. Because of the low energy of the ions used to expose the plate, it is likely that the desired image will reside almost exclusively at the surface of the silver halide grain. ilny internal image thus is the result of other image formation processes and contributes only to the general fog of the plate. The results presented here show the superiority of the surface developer for quantitative work while leaving some
doubt as to its usefulness in qualitative work, except in special cases where secondary emission blackening is a problem. It should be emphasized that these results are from a limited number of plates and must, therefore, be considered preliminary. However, they seem interesting enough to encourage others to consider the photographic process in terms of low energy ions. ACKNOWLEDGMENT
I express my appreciation to J. W. Mitchell for helpful suggestions and discussions. LITERATURE CITED
( 1 ) James, T. H., Vanselow, W., Photographic Science and Technique, Series 11, 2-135 (1955). (. 2,) James. T. H.. Vanselow. W.. Photo. Sci. Eng.,2, 1-104 (1958). (3) Kennicott, P. R., General Electric RL Rept. 37660, Schenectady, N. Y., 1964. I
,
High Frequency Electrodeless Discharge System for Ashing Organic Matter Chester
E. Gleit,
Department of Chemistry, North Carolina State, of the University of North Carolina at Raleigh, Raleigh, N. C:
exists in the use of the high Ia substitute frequency electrodeless-discharge as for conventional ashing NTEREST
methods and as a means of producing free radicals and thermally unstable compounds (4). In this technique gas a t low pressure is inductively coupled to a radiofrequency field. To oxidize organic matter, oxygen is passed through an intense oscillating field and then reacted with a solid specimen a t low temperature ( 2 ) . This technique greatly reduces the volatility losses inherent in dry ashing, (2) permits low temperature decomposition of difficultly oxidizable substances such as pyrolytic graphite and other carbonaceous material (3, 6),and provides a method for ashing biological tissue without distortion of the associated mineral microstructure ( 7 ) . Applications of this technique have been restricted by lack of suitable laboratory equipment. The principal difficulty has been in the design of an efficient and flexible means of transferring power from the electronic generator to the gas. In early work a direct connection between the radiofrequency power amplifier and a resonant LC-tank was employed (1, 6). Since the plate impedance is several thousand ohms and the LC-tank and associated gaseous discharge approximates a several hundred ohm termination, only a small portion of the generated power was delivered to the 31 4
ANALYTICAL CHEMISTRY
Figure 1 .
Basic coupling circuit.
Coil Lt surrounds the gas discharge tube
load. This impedance mismatch also led to overheating of the power amplifier from reflected power and excessive radiation. Other methods such as link coupling and the use of polygonal inductors ( I ) have been employed. Such circuits are difficult to adjust, and standing-wave ratios (ratio of load impedance to line impedance) of less than two have not been achieved. A simple autotransformer-resonant LC-tank has been found to be an efficient means of transferring power to the gaseous discharge over a wide range of frequencies and gas conditions. This circuit can be used with most commercial radio transmitters and laboratory radiofrequency oscillators. The standard transmitter terminating network, which is shown on the left side of Figure 1, consists of a capacitor CI to remove d.c. high voltage from the out-
put and a r-network, composed of variable capacitors Czand CJand tapped coil L1. This network suppresses undesired harmonics and lowers the output impedance to 52 ohms. A coaxial cable of suitable impedance, such as type RG8/U or RG58/U, is generally employed to connect the transmitter to the antenna. For use in an electrodeless discharge system, the antenna is replaced by a resonant circuit composed of solenoid, Lz, and variable capacitor, C,. The inductor is tapped at an appropriate point to provide proper impedance match. CONSTRUCTION AND ALIGNMENT
For operation a t 13.56 Mc. per second an output network consisting of a Hammarlund H F B D 25 picofarad variable capacitor and a silver plated coil
consisting of 14 turns of 'I4-inch copper tubing surrounding a 4-cm. borosilicate glass tube is employed. The approximate position of the tap is two turns from the grounded end of the coil. -4 standing-wave-ratio meter facilitates initial alignment and provides a direct means of monitoring power. T o align the transmitter, the S. W. R. meter and a dummy load, consisting of a 50-ohm noninductive resistor of appropriate power rating, are attached to t'he transmitter by quarter-wave coaxial cables. Coil L1 and plate output capacitor CBare adjusted to the transmitter frequency and the loading capacitor C3 is adjusted for maximum out'put. After transmitter adjustment, the dummy load is replaced by the output tank, Ls, C4,and the gas pressure in the borosilicate tube is reduced to less than t,wo torr to establish t'he discharge. The exact position of the tap is det'ermined experimentally by varying its position and readjusting C4 until t'he highest ratio of forward to total power is found. The tap is then permanently secured. As the intensity of the discharge is directly related to received power, the brightness of the discharge can be employed in the absence of a S. W.R. meter. h minor adjustment of the loading capacitor C3 may be required to correct for stray capacitance. After completing these adjustments a small variation of Cd will correct for any subsequent changes in gas composition or pressure. If lower output power is
required, it is generally advisable to reduce transmitter output rather than to detune the output tank. RESULTS A N D DISCUSSION
Several devices employing this circuit have been in use for over a year. Gases such as oxygen, hydrogen, carbon dioxide, and methane have been used interchangeably without necessitating readjustment of the electronic system. T o ash organic matter, oxygen is passed through the discharge at a rate of approximately 25 cc. per minute S. T. P. a t a pressure of 1 torr. The specimen to be decomposed is placed in a borosilicate boat 20 cm. beyond the solenoid. Ashing rate is a function of the sample's surface area and mineral content. Three hours is usually sufficient to ash a 1-gram tissue specimen. .is the quantity of organic matter does not measurably alter the electrical characteristics of the system, realignment is not required during the decomposition process. After ashing, the generator is turned off, the gas system returned to one atmosphere, and the solid residue recovered from the combustion boat. A series of cold traps located between the boat and the vacuum exhaust pump may be employed to recover condensable gases.
The system is highly stable. Standing-wave ratios of 1.1 to 1.2 have been routinely achieved. A typical instrument uses an unmodified ,Johnson Viking-I1 amateur radio-transmitter and provides a forward radio-frequency power of 115 watts, with less than one watt reflected. Several transmitters based on a 4CX250 vacuum tube operating as a class-C oscillator have been constructed. These units provide a n output of 250 watts with only 0.25% reflected power. ACKNOWLEDGMENT
Mr. Harvey J. Beaudry assisted in the design and testing of this circuit. LITERATURE CITED
(1) Babat, G. I., J . Inst. Elect. Eng. 94,
(111).2711947). Gleit, C. E.; Holland, W. D., . ~ N A L . CHEM.34, 1454 (1962). (3) Gleit, C. E., Holland, W. D., Wrigley, R. C., Suture 200, 69 (1963).
(2i
( 4 ) Jolly, W., "Technique of Inorganic
Chemistry," H. B. Jonassen, A. Weissberger, ed., Tol. I, Interscience, Xew York, 1963. ( 5 ) Marsh, H., O'Hair, E., Reed, R . , Wynne-Jones, W. F. K., Nature 198, 1195 (1963). (6) Strong, C. L., Sci. Am. 209, #1, 146 (1963). ( 7 j Thomas, R. S., Fifth Intern. Conf. on Electron Microscopy, Philadelphia, Sept. 1962.
Radioautography and Scanning of Thin layer Chromatograms of Radioactive Water-Soluble Substances R. A. Schwone and R. S. Nokon, Chemistry Department, DePoul University, Chicago, 111.
of thin layer chroof radioactive watersoluble substances for purposes of radioautography and scanning is a problem which has not been solved satisfactorily. Water-base sprays of plastic ( 2 , 3) cause distortion of the spots. Collodion ( I ) or parlodion which use ethanol as a solvent cause diffusion and sometimes migration of the spot. Sprays using hydrocarbons or halogenated hydrocarbons as solvents ( 7 ) dry to form a film which is thin, brittle, and generally difficult to handle. The following method has been found satisfactory for thin layer chromatograms of radioactive sugars and polyhydric alcohols, using silica gel and kieselguhr adsorbents. A stock solution containing 11.2 grams of polystyrene per 100 ml. of benzene with dibutyl phthalate added as a plasticizer is poured over the developed plates which lie horizontally on a bench top. The plates are allowed to gel and then placed in an oven a t 70" C. for half an hour to HL
RLMOVAL
Tmatograms
dry. They are then immersed in water a t 55" C . for 5 minutes and removed. The polystyrene film is stripped off immediately by sliding a razor blade between it and the glass plate. iifter it is pressed flat, the film is ready for scanning or radioautography. Polystyrene crystalline pellets (monomer polymerized with no lubricant, Brand Plastics, Willow Springs, Ill., No. GlC1) are dissolved in benzene to form the stock solution. The concentration noted above allows for rapid drying but also gives sufficient viscosity so that the liquid does not run off the edge of the plate. Styrofoam dissolves more rapidly than the pellets, but forms a foamy solution. The optimum amount of plasticizer depends on the adsorbent. For Silica Gel G , 0.023 ml. of dibutyl phthalate is added to each milliliter of stock solution. For Kieselguhr G, 0.014 ml. is added. If the percentage of plasticizer is increased, the pliability of polystyrene film is improved, but the percentage of beta-
radiation absorbed is also increased. For quantitative work, the percentage of plasticizer should not be varied from one sample to another. A 200-mm. X 50-mm. adsorbent layer (0.008 inch thick) is adequately covered by 10 ml. of stock solution if the adsorbent is Kieselguhr G or by 15 ml. of stock solution if the adsorbent is Silica Gel G. A two-inch razor blade (Atlantic Industrial Corp., Newark) permits removal of a 50-mm. wide strip in one easy motion. I n appearance, the polystyrene film resembles paraffin and the side which was attached to the glass is smooth. Radioassaying the C14 activity from the smooth side apparently obviates differences in activity due to variations in polystyrene film thickness. This allows quantitative determinations of the amounts of solute on thin layer chromatograms. t-nder the conditions noted above, the polystyrene film absorbs about 50% of the beta-activity emanating from the surface of the VOL. 37, NO. 2, FEBRUARY 1965
315
