11). How far linearity extends I do not know; in a rough preliminary trial with diluted trichloroethylene vapors, the emission of the I n line a t 451 mp was proportional to the 0.65th power of the concentration between 2500 and 4500 pg./l. It must be keut in mind t,hat in a spectrophotomet& capable of high gain, the shot noise may exceed the flame flicker noise if the slit is too narrow or the emission too weak. The wide slit mentioned in Table I was needed because the indium had been largely vaporized or absorbed into the copper by repeated attempts to remove the residual chloride. The emission per unit of chlorine concentration was by then only 0.01 of its value at the start, when a large area of fresh indium had been present. The data of Table I1 were also obtained with the aged indium. Ordinarily, a slit of 0.1 mm. would be wide enough for the flame flicker noise to exceed the shot noise a t a concentration of 10 pg./l., and it is narrow enough to isolate the InCl band a t 360 mp efficiently. It is obvious that a burner with clean indium would afford much better sensitivity than that reported here; probably about 0.001 pgJ1. of chlorine could be detected in air. DISCUSSION
Indium was tried as a substitute for copper and with a view, a t first, to using its resonance line at 451.1 nip because it was expected to be uniquely superior to any other element in the Beilstein test, for the following combination of reasons: indium has a low resonance potential; the resonance lines have high transition probability; indium is largely dissociated to atomic vapor in a hydrogen flame; the strongest radiation is
(or was expected t o be) concentrated in a sharp line; indium should react easily with the hydrogen halides; the halides are readily volatile; the solid oxide is reduced by hydrogen, so that the surface remains accessible; the vapor pressure of indium is quite low; the nietal is liquid a t the temperature of use, presenting a surface having constant properties; indium is easy to apply and support upon an inert substrate. The expectation was borne out; the 451.1-mp line afforded better sensitivity for chlorine than did the copper band emission in an equivalent van der Smissen burner, despite the somewhat greater volatility of indium, which results in a weak background emission of In in the absence of chlorine. The unexpected discovery of the InCl emission increased the sensitivity of the method still further by largely eliminating the background and a t the same time made it specific for chlorine. The higher dissociation energy of TnCl (4.5 e.v. as against 3.7 e.v. for CuC1) may account for its persistence in the flame. Moreover, it is likely that the InCl is abnornially excited by chemiluminescence due possibly to recombination of H atoms, because the total power output in the InCl spectrum exceeds by perhaps an order of magnitude that in the In spectrum in the lower part of the flame. Other Possible Applications. The instrument can certainly be adapted t o the analysis of gases other than air, and to liquid and solid samples by spraying them into the primary air. Solids can be sprayed reproducibly in suspension (3, 6). Crider ( I ) , whose method for sulfur resembles the present
method in its use of an air-hydrogen flame, has shown that a liquid aerosol gives the same response as a vapor per unit concentration. A good sprayer can spray 1 ml. of solution or suspension with 1 liter of air, and it is therefore reasonable to expect a detection limit of 0.01 p.p.m. of chlorine in a liquid sample or 1 pap.m. in a solid sample sprayed as a suspension a t 1% solids content. LITERATURE CITED
(1) Crider, W. L., AXAL.CHEM.3 7 , 1770 (1965). \ - - - - I
(2) Dragerwerk, German Patent 1,095,552 (Dee. 1960). (3) Gilbert, P. T., ANAL.CHEW34, 1025 (1962). (4)Herrmann, R., Alkemade, C. T. J.,
“Chemical Analysis by Flame Photometry,” pp. 29-31, Interscience, New York, 1963. (5) Mason, J. L., AXAL. CHEM.35, 874
(1963). (6) Pearse, R. W. B., Gaydon, A. C.,
“The Identification of Molecular Spectra,” 3rd ed., Wiley, New York, 1963. ( 7 ).Rosen, B., Editor, “Constantes shlec-
tionnCes. Donnhes spectroscopiques concernant les molkcules diatomiques,” Vol. 4 of “Tables de constantes et donn6es numhriques,” Hermann, Paris, 1951. -~
(8) Van der Smissen, C. E., U. S. Patent 3,025,141 (March 1962).
P ~ UT. L GILBERT^ Beckman Ins‘muments, Inc. Fullerton, Calif. RECEIVEDfor review -4ugust 29, 1966. Accepted September 30, 1966. The author wishes to express his appreciation of the support of the E. S.Naval Applied Science Laboratory in sponsoring a part of this work. 1 Present address, Beckman Instruments, Inc., Spinco Division, Palo Alto, Calif.
of External Heavy Atom Effect to Increase nsitivity of Measurement in Phosphorimetry SIR: Phosphorimetry has been shown to be a sensitive, selective method of measurement of many drugs (3, id, I S ) , pesticides (9), and aromatic hydrocarbons (6). However, the sensitivity of measurement of some aromatic hydrocarbons is too lo^ to be analytically useful. In such cases, it is possible to increase sensitivity by breaking down the spin-quantization restriction to transitions-Le., by increasing the probability of the transition occurring between states of different multiplicity. This may be accomplished by perturbation of the molecular system through solvent molecule interaction, by introduction of a heavy atom into the molecule in concern or into the solvent, or by introduction of a paramagnetic species into the molecular system or into the 1922
0
ANALYTICAL CHEMISTRY
solvent. Use of the external heavy atom effect to increase the transition probability of niultiplicity-forbidden transitions and to increase the sensitivity of measurement is reported here. The external heavy atom effect has been studied only from a physical sense. However, hfcGlynn, Daigre, and Smith ( 5 ) have suggested that it could have analytical uee. Graham-Bryce and Corkhill ( 2 ) have investigated the effect of ethyl iodide on a series of dinitronaphthalenes, coumarin, fluorescein, and 9,iY-dimethyl aniline. In ‘ all instances, they observed an increase in the phosphorescence intensity for these compounds in a 21:l ( v . / ~ . ) mixture of ethanol-ethyl iodide as compared to ethanol alone. hIcGlynn and coworkers (4, 6, ?, 8) have studied
in detail the physical significance of the heavy atom effect. The effect of ethyl iodide on naphthalene, phenanthrene, and ten other polynuclear aromatic hydrocarbons is described here. The objective of this work is t o improve the sensitivity of phosphorimetric measurement of polynuclear aromatic hydrocarbons and to indicate the possible use of the heavy atom effect in phosphorimetric studies. Many of the molecules influenced most significantly by the heavy atom effect are of biological importance. For esample, several of the compounds studied, e.g., 3,4-benzpyrene, 1,2,5,6dibenzanthracene, and lJ2-benzathracene, are of special interest because they are carcinogens.
EXPERIMENTAL
An Aniinco-Bowman spectrophotofluorometer with an Aminco - Keirs phosphoroscope attachment, a 150-watt xenon arc lamp, and a potted RCA 1P28 multiplier phototube (American Instrument Co., Inc., Silver Spring, X d . ) and an X-Y recorder were used throughout this work. Relative phosphorescence signals (P is phosphorescence intensity signal of sample in presence of solvent with heavy atom, and PO is phosphorescence intensity signal of sample in presence of solvent containing no heavy atom) were obtained directly from the photomultiplier photometer unit. Reagents. The hydrocarbons were obtained from the following commercial sources: naphthalene and anthracene (K and K Laboratories, Inc., Plainview, T's. Y., 11803); acenaphthene and phenanthrene (Distillation Products Industries, Rochester, X. Y., 14603) ; 1,2-benzfluorene, 2,3-benzfluorene, retene, and triphenylene (City Chemical Co., S e w York, h'. Y., 10011); and 3,4-benzpyrene, 1,2,5,6-dibenzanthracene, and 1,Zbenzanthracene (Kutritional Biochemicals Cosp., Cleveland, Ohio). The ethyl iodide (Fisher Scientific Co., Pittsburgh, Pa.. 15219) was purified by passing through activated alumina in a dark room and stored over copper away froin room light. Stock solutions of the hydrocarbons in ethanol were prepared in the concentration range of lo-% to 10-4M depending on their solubilities in ethanol. The stock solutions were stored a t 20" C. The alcohol solvent was distilled technical grade ethanol (13). Procedure. Phosphorescence excitation and emission spectra [uncorrected for instrumental response (11)] were obtained for the ethanolic solutions of the polynuclear hydrocarbons a t 77' K. Spectra were measured only on concentrations of the hydrocarbons falling on the linear portion of the analytical curve. The slit arrangement specified by the manufacturer for routine analysis was used ( 1 ) . Analytical curves (phosphorescence intensity signal, P or Po us. sample concentration. C) were determined for each hydrocarbon as previously d e scribed (19). Limits of detection, in micrograms per milliliter, were obtained using the slit arrangement for maximum sensitivity. as specified by tlne manufacturer ( I ) . The limit of detection was defined as that concentration giving a signal twice the mean fluctuation in the background (14). All spectra, analytical curves, and limits of detection $yere recorded for each polynuclear aromatic hydrocarbon in ethanol and in ethanol-ethyl iodide of various proportions. Ethanol-ethyl iodide mixtures in the proportions L.,sedformed a clear glass a t 77" K. Apparatus.
RESULTS AND DISCUSSION
The phosphorescence excitation and emission peaks (uncorrected for instrumental response), the analytically useful region (near linearity) of the
Table 1.
Phosphorescence Characteristics of Polynuclear Aromatic Hydrocarbons in Ethanol at 77" K.
Analytically useful .. .
Hydrocarbons Naphthalene Anthracene Phenanthrene Triphenylene Ret,ene Napht,hacene 1,2,5,6-Dibenzanthracene 1,2-Benzanthracene 3 4-Benzpyrene Acenaphthene 1,2-Benzfluorene 2,3-Benzfluorene
Excitation peak," mp 290, 230 300 300, 260 290, 270 265, 305 300, 320, 400
Emission peak,a mp 505, 475, 545 462, 435, 500, 580 499, 468, 535 461; 435' 510, 470, 540 515, 560
340, 310, 355 310. 280 325; 275, 310 320, 240 315, 275 325, 280
550, 595
.51 n
562, 540 515, 485, 555 500, 540 500, 540
range of Limit of analytical detection, curve, pg./ml. fig./mLb 6.4-0.13 180,-1.8 18.-0.018 230.-0.23 120.-0.12 120.-0.12
0.06 0.05 0.002
28.-0.28
0.03 n- . n.1 --
2.1
-n
0.0002
0.001 0.001
92
50.-2.5 77. -3.0 44.-1.1 22.-0 22
3.0 0.2 0.2 0.2
First value is most intense peak, second value is next most intense peak, etc. Limits of detection and analytical curves determined at most intense excitation and emission peaks. a
~
Table II.
~
~~~~~
Effect of a Heavy Atom Solvent on Phosphorescence Intensity of Polynuclear Aromatic Hydrocarbons at 77" K.
Hydrocarbons Naphthalene Anthrscene Phenanthrene Triphenylene Retene Naphthacene 1,2,5,6-Dibenxanthracene
1,2-Benzanthracene 3,4-Benzpyrene Acenaphthene l12-Benzfluorene 2,3-Benzfluorene
Concentration, /%/ml. 13.
18.
1.8
2.3 6.0
6.0 2.8 23.
25. 15. 22, 22.
(P/Po,a Ethanol/et,hyl iodide (v./v.) 19/1 9/1 5/l 0.3 0.2 0.1 0.6 0.7 0.9 0.6 0.4 0.2 0.2 0.1 0.05 1.1 1.1 0.8 2.4 3.1 4.6 1.2 1.2 1.3 1.2 1.3 1.4 1.5 2.2 3.5 0.9 0.9 0.7 6.4 9.8 13. 8.7 15. 25.
a Ratio of phosphorescence intensity in ethanol-ethyl iodide ( P ) mixture t o phosphorescence intensity in ethanol ( P o ) ,
analytical curve, and the limits of detection, in micrograms per milliliter, for twelve polynuclear aromatic hydrocarbons in ethanol at 77" K. are listed in Table I. From the wavelengths of the peaks, it should be possible to excite and to measure selectively almost any compound in the presence of one or more of the other compounds. For example, McGlynn, Neely, and Neely (6) demonstrated selective determination of napthalene and phenanthrene in various proportions. The influence of various concentrations of ethyl iodide in the solvent on the phosphorescence intensity signal for twelve polynuclear aromatic compounds is given in Table 11. The data in Table I1 were obtained by adjusting the excitation and emission wavelengths t o obtain the maximum intensity signal for each solution. Of particular interest is the large increase in phosphorescence signal observed for the benzfluorenes and the large decrease in phosphorescence signal observed for triphenylene, naphthalene, and phenan-
threne. These data contradict to some extent the generalizations made by Graham-Bryce and Corkhill (8). The analytical curves for those compounds showing enhanced phosphorescence determined in 9/1 ethanol/ethyl iodide were linear over ranges greater than or equal t o those obtained in ethanol, and the limits of detection were lower by an amount predictable on the basis of the P/Po ratio. The quenching effect of ethyl iodide on certain of the hydrocarbons is also significant. This effect is not completely undesirable because it is often possible t o use quenching of one or more compounds to improve the selectivity of measuring a certain compound. 811 excitation spectra shifted toward longer wave lengths as the concentration of ethyl iodide increased in the solvent mixture. Excitation of most of the hydrocarbons in the presence of the heavy atom solvent occurred between 320 and 330 mp. Ethyl iodide absorbs quite strongly at wavelengths below 325 mp which may explain this grouping VOL. 38, NO. 13, DECEMBER 1966
e
1923
of the excitation maxima. A similar effect mas noted by Sawicki, Stanley, and Elbert (IO) in their quenchofluorometric studies. The use of ethyl iodide in the ethanolic solvent is simple and improves the sensitivity of measurement and often the range of linearity of the analytical curves. Even though ethyl iodide is light sensitive, no particular problems with photodeconiposition were encountered during these studies. The choice of ethyl iodide as a heavy atom solvent diluent was based largely on the formation of clear, rigid glasses for various proportions of ethanol and ethyl iodide a t 77” I(.(see Table 11). It is possible that other alkyl halides may also be useful as external heavy atom solvent diluents and should be studied more extensively.
S I R : The theory and practice of the electrochemical technique named chronocoulometry have been described in detail (1-6). The technique offers significant advantages in studies of reactant, adsorption a t electrodes and the measurement of electrode reaction rate constants, but it shares the common failings of all potentiostatic techniques : The unavoidable uncompensated resistance in the cell prevents the electrode potential from reaching the value desired instantaneously (IO). In solutions of high conductivity with electrode reactions having high exchange currents, it is usually possible t o obtain results that are not significantly degraded by this source of potential errors. In less conducting solutions, however, the effect of the uncompensated resistance can be very serious and extremely difficult to eliminate. Described in this note is a new technique which combines the advantages of the chronocoulometric approach with the immunity to uncompensated resistance possessed by the coulostatic technique. The experimental arrangement i s essentially that employed for ordinary coulostatic measurements (7, 8). A charge, Q, is instantaneously injected into an electrode in a solution containing one half of a redox couple. However, in contrast with the usual practice in coulostatics, the value of Q is chosen to be large enough so that after it is injected the electrode potential is well out on the diffusion plateau of the corresponding polarogram. The resulting open circuit potential-time transient is recorded oscilloscopically and then converted into the corresponding open circuit chargetime variation with the aid of the ap-
1
@
ANALYTICAL CHEMISTRY
Uses of Phosphorescence” from “Fluorescence and Phosphorescence Analysis: Principles and Applications,” D. M. Hercules, ed., Interscience, New York,
LITERATURE CITED
(1) American Instrument Co., Pnc., Instructions No. 838, Silver Spring, Md. (2) Graham-Bryce, I. J., Corkhill, J. M., Nature 186, 965 (1960). (3) Nollifield, W. C., Winefordner, J. D., Talanta 12, 860 (1965). (4) McGlynn, S.P., Azumi, T., J . Chem. Phys. 40,507 (1964). (5) McGlynn, S. P., Daigre, J., Smith, F. J., Ibid., 39,675 (1963). (6) RlcGlynn, 8. P., Neely, B.T., Neely, C., Anal. Chim. Acta 28, 472 (1963). ( 7 ) ILlcGlynn, S. P., Reynolds, M. J., Daigre, G. W., Christodoyleas, N. D., J . Phys. Chem. 6 6 , 2499 (1962). (8) McGlynn, 8. P., Sunseri, R., Christodoyleas, N., J. Chem. Phys. 37, 1818
1966.
(12) Winefordner, J. D., Lata, H. W., ANAL.CHEM.35, 1517 (1963). (13) Winefordner, J. D., Tin, M., Anal. Chim. Acta 31, 239 (1964). (14) Winefordner, J. D., Vickers,’ T. J., ANAL.CHEY.36, 1939 ‘(1964). L. v. S. H O O D J. D. WINEFORDNER Department of Chemistry University of Florida Gainesville, Fla. 32601 Received for review August 11, 1966. Accepted October 10, 1966. Research was carried out as a part of a study on the phosphorimetric anal sis of drugs in blood and urine supported cy a W. S. Public Health Service Grant (GM-11373-03).
(1962).
(9) Moye, H. A., Winefordner, J. D., J . Agr.. Fpod Chem. 13, 516 (1965). (10) Sawicki, E., Stanley, T. W., Elbert, W. C Talanta 11, 1433 (1964). (11) Wihefordner, J. D., “Analytical
0
I
1
I
I
I
I
I
I
10
20
30
40
50
60
70
80
t“Z,
psec.”2
Figure 1. Charge-(time)’I2 plot for charge-step chronocoulometry in 0.2mM cadmium solutions A charge step of 0.98pcoulombs was applied in less than 1 Mcrec. to a which was held a t hanging mercury drop electrode (area = 0.032 -0.3 volt vs. SCE until the instant before the charge injection. The resulting open circuit potential-time transient was recorded oscilloscopically and analyzed to give the corresponding charge-time transient Supporting electrolyte was A. 1 M N a N O a B. 0.9M NONO.? 0.1 M NoSCN
+
propriate charge-potential data. (With electrolytes for which the chargepotential data have not been determined, for example, by double layer capacity measurements, they can readily be obtained by injecting known charge increments in supporting electrolyte solutions free of reactant and measuring the resulting step changes in potential.) The charge-time data is then analyzed as in standard potential-step chronocoulometry by plotting charge vs. the square root of time (1, 5 ) . The slope of this plot is -2n FACD112/& and the zero-time &-intercept should be the amount of charge initially injected. In case the reactant is initially ad-
sorbed, the &-intercept will be smaller than the amount of charge injected by the amount of adsorbed reactant. Figure 1 shows Q - t 1 l z plots for cadmium ion in sodium nitrate in the absence and presence of thiocyanate. I n the pure nitrate, the &-intercept, 0.97 pcoulomb, agrees well with the amount of injected charge, 0.98 pcoulonib, indicating the absence of adsorbed cadmium. When thiocyanate is added, the intercept is much smaller than the amount of injected charge and the difference between the two, 0.27 pcoulomb, agrees, within the present experimental reproducibility, with the amount of cadmium found to be ad-
