Relative Efficiency of a N e w Liquid Scintillation Fluor, p-Bis-(o-Methy lstyryI) Benzene SIR: This report compares a recently announced secondary solute for liquid scintillation counting, p-bis-(o-methylstyryl) benzene (bis-MSB) with two previously available fluors. The conditions which must be specified when comparing fluors are discussed in a recent study ( I ) of relative efficiencies among some primary and secondary solutes in liquid scintillation counting. The usefulness of a secondary solute depends upon the optics of the counter being used ( 4 ) and upon the kind and degree of quenching in the sample. At low concentrations, a secondary solute functions as a wavelength-shifter by absorbing the emission of the primary fluor and reemitting a t a longer wavelength. Because this process is less than 100% efficient, the resulting loss of light is compensated only if the longer wavelength results in a sufficient increase in multiplier phototube sensitivity, in reflectivity of the counting chamber, and/ or transmission in the counting chamber and sample. A counter in which Lucite was interposed between the sample and the multiplier phototube was found ( I ) to be more wavelength-sensitive than those which had an air gap only. I n some modern commercial counters the addition of a secondary solute to an unquenched sample of primary solute caused a small decrease in counting efficiency, and in others an increase. At higher concentrations the secondary solute may function in some cases to increase the light output from the sample. Two additional excitation mechanisms become important at high secondary concentration : radiationless transfer of energy from the primary fluor (9) and direct solvent excitation. I n the latter case the secondary is acting as a primary solute. (Because it is usually considerably less soluble than a good primary, it will not, in general, give as great an efficiency when used alone. A high concentration of a secondary may be between 0.2 and 4 grams/liter.) These last two mechanisms contribute to the increase m efficiency observed when a secondary is added in sufficient quantity to samples containing certain chemical (colorless) quenching agents. Although solvent quenching by chemical quenchers predominates over direct quenching of fluors, some halogen compounds, ketones, acids, and bases quench the fluor molecules as well as the solvent. The primaries which emit a t short wavelengths are in general more quenched than the secondaries. If a secondary is present in sufficient concentration, it may compete with the
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Figure 1. Tritium counting efficiencies as a function of concentration of secondary solute in unquenched samples containing 4 grams/liter PPO, in three liquid scintillation counters
quencher for the excitation energy of the primary through radiationless transfer, recovering for light production some energy which would otherwise be lost. It may also act as a less-quenched primary. It was of interest to test the effectiveness of the new fluor in reducing quenching, as well as in the role of wavelength-shifter. EXPERIMENTAL
Bis-MSB samples and PPO (2,5diphenyloxazole) were obtained from Pilot Chemicals, Inc. POPOP (p-bis2-(5-phenyloxazolyl)-benzene) and dimethyl-POPOP were obtained from Arapahoe Chemicals, Inc. Reagent grade toluene was used as the solvent, and toluene-H3 was used to label all samples, which were in equilibrium with air. Counting was performed in Nuclear-Chicago Corp. Model 725 spectrometers with both quartz-face and glass multiplier phototubes, and in a Packard Instrument Co. Model 3002. RESULTS AND DISCUSSION
A range in counter response to wavelength-shifting is shown in Figure 1, where counting rates of unquenched samples containing secondary solute 4 grams/liter PPO are given relative to that of a sample containing the PPO only. The top two sets of curves were obtained with Nuclear-Chicago counters and the lower set with a Packard counter. The latter shows the greatest change in sensitivity with wavelength, as indicated by the steeper initial rise in efficiency with secondary concentration
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[at a concentration of 0.05 gram/liter wavelength-shifting is virtually total, as shown by spectrofluorometric measurements by the author and by others (S)]. Differences between counters with identical optics are attributable to variation among individual multiplier phototubes. Counter I shows the least change in sensitivity with wavelength and illustrates the loss in efficiency which may be suffered when only a small amount of secondary solute is added. The increase in efficiency at higher concentrations is the contribution from the secondary solute functioning as a primary. In the absence of PPO, at concentrations of 0.6 and 2 grams/ liter, respectively (nearly maximal a t 4 ' C.), POPOP and bis-MSB gave H3 efficiencies of 17y0and 28.5% in Counter I. A saturated solution of dimethylPOPOP ( dimethyl-POPOP > POPOP. Tritium efficiencies in samples with bis-MSB were 1.1to 1.2 times those in POPOP samples, while dimethylPOPOP samples were 1.03 to 1.07 times more efficient than POPOP. The relative efficiencies vary somewhat with the kind and degree of quenching, but the values observed in these experiments are indicative of the order of magnitude to be expected. When comparisons were made a t equal concentrations by weight of secondary solute, efficiencies with POPOP were 1.01-1.03 times those with bis-MSB in samples with 30 grams/liter PPO, and were 1.08-1.18 times those with dimethyl-POPOP with each of the
quenching agents. The order POPOP > bis-MSB > dimethyl-POPOP is the same as for unquenched samples. The chief advantage of the new fluor, bis-MSB, is its higher solubility. It should find its greatest usefulness a t high concentrations in samples in which the primary fluor is quenched by some other sample component or through selfquenching. LITERATURE CITED
(1) Bush, E. T., Hansen, D. L., “Radio-
isotope Sample Measurement ‘Techniques in Medicine and Biolo p. 395, International Atomic %kergy Agency, Vienna, 1965. (2) Birks, J. B., “The Theory and Practice of Scintillation Counting,” p. 31615, Macmillan, New York, 1964. (3) Hayes, F. N., et al., U. S. At. Energy Comm. Rept. LA-1639. (19531. (4) Ott,. D. G., “Liquid Scintillation Counting,” C. G. Bell, F. N. Hayes, eds., p. 101-7, Pergamon Press, Oxford, 1958. ELIZABETH T. BUSH Nuclear-Chicago Corp. 333 E. Howard Ave. Des Plaines, Ill.
The Rate of Oxidation of Platinum Electrodes SIR: Feldberg, Enke, and Bricker (2) have reported that the oxidation
charge accumulated a t constant potential in the anodic oxidation of platinum in perchloric acid is proportional to the logarithm of oxidation time over a wide range of time and applied potential. Quite similar results were obtained by Smith (6) in sulfuric acid solution. The first authors have given an interpretation of this behavior by writing a conventional electrochemical rate equation and making the assumption that the forward rate constant decreases with increased oxidation of the surface in an exponential manner. The origin of this Temkin-like behavior is uncertain, but the introduction of an exponential term of this kind appears to be justified by experimental data. Such a treatment implies that the slope of the oxidation charge us. log time curve should be independent of potential at high oxidation potentials. At lower oxidation potentials, the curves are less linear, and the slopes depend upon potential. The work reported here attempts to extend the treatment to lower potentials. It is well known that the nature of an anodically oxidized platinum electrode is not completely specified by a specification of the oxidation charge present upon the electrode, but rather, depends strongly upon the time and potential history leading to the formation of the oxide charge. The hysteresis in the 1242
ANALYTICAL CHEMISTRY
anodic and cathodic curves for platinum oxidation and reduction may be cited (3). The magnitude of charge capable of existing on smooth platinum electrodes corresponds in many cases to nine or ten electrons per surface platinum atom, if a roughness factor not much greater than one is taken. This large charge cannot be accommodated by any reasonable oxidation state of surface atoms alone. The fact that oxygen atoms readily reach positions below the surface has been demonstrated by low energy electron diffraction (7) and by field ion microscopy (6). Recently, Warner and Schuldiner have used the term “dermasorption” for such subsurface oxides (9). It is proposed here that the exponential decay of the rate constant may be taken as the limiting form in a two-step process, the first of which is regarded as an electrochemical reaction a t the surface. This is followed by a potential-independent transfer of oxide to the underlying platinum structure. Alternatively, film growth by metal atom transfer may be involved. The two steps are quite different from those discussed by Feldberg, Enke, and Bricker ( 2 ) . EXPERIMENTAL
Apparatus. The circuitry used for constant potential electrolysis and integration of charges accumulated was similar to that previously descri.bed (1).
Electrodes. Two types of test electrodes were used. A single crystal platinum electrode approximately 2 mm. in diameter was mounted in the end of a piece of glass tubing with black sealing wax. This electrode had an exposed geometric area of 0.091 cm.2 The second electrode was polycrystalline platinum wire sealed into soft glass tubing. After annealing, this electrode was polished using kerosene-levigated alumina supported on a felt disk mounted on a dental engine. The exposed geometric area was 0.110 cm.2 Electrochemical measurements made with the single crystal were found to be the same as those made with the wire electrode within experimental uncertainty, after allowing for differences in area. Most of the data reported here were obtained with the wire electrode. Cell and Electrolvte. All electrolyses were made h 0.5M H$Oc prepared by diluting reagent grade acid with water distilled from alkaline permanganate in a borosilicate glass still. The electrolyte was freed of oxygen by purging with nitrogen obtained from evaporation of liquid nitrogen and passed over heated copper. The working electrode of platinum gauze in 0.5M H2S0, was in a compartment separated from the test electrolyte by a glass frit. Potentials were measured against a saturated calomel electrode connected by a capillary filled with 0.5M H2S04. Potentials are herein referred to saturated calomel. Stirring the electrolyte had no effect upon the charges observed;
