tribromide bromination is about 1-275 that of molecular bromine. Therefore tribromide ion is a less reactive brominating agent than molecular bromine as was fouiid previously for aromatic bromination ( 1 ) . The above results for tribromide ion activity in substituted 8-quinolinols agree with a previous observation (4, that in 8-quinolinols substitution in the seven position Occurs via the attack of the tribromide ion as
well as by molecular bromine, whereas substitution in the &position takes place only via attack of a bromine molecule. LITERATURE CITED
( I ) Bell, R. P., Iiawlinson, D. J., J . Chem. Sot. 1961, 63. (2) Corsini, A., Graham, R. p., Anal. Chzm. Acta 28, 583 (1963). (3) Kozak, G. S., Fernando, Q., J . Phys. Chem. 67, 811 (1963).
(4)Kozak, G. S., Fernando, Q., Freiser, CHEM. 36, 296 (1964). H., (5) O’Dom, G., Fernando, Q., Zbid., 37, 893 (1965). (6) Petrow, Y., Sturgeon, B., J . Chem. Soc. 1954, 570. ( 7 ) Prasad, R., Coffer, H. L. D., Fernando, Q., Freiser, H., J. Org. Chem. 30, 1251 (1965).
RECEIVED for review December 9, 1965. Accepted March 16, 1966. Work supported by the National Institutes of Health.
IntermoIecuIar Energy Transfer as a Means of Chemical Analysis Sensitization of Rare Earth Emission in Dilute Solution by Aromatic Carbonyl Compounds W. J. McCARTHY and J. D. WINEFORDNER Department o f Chemistry, University o f Florida, Gainesville, Fla. The fluorescence emission of terbium, europium, samarium, and dysprosium ions is selectively sensitized by various aromatic carbonyl compounds. The system is excited by monochromatic light at the wavelength of maximum absorption for the phosphorescence transition of the carbonyl compound, and emission is measured at the wavelength corresponding to the rare earth ion emission line. The influence of donor-acceptor concentration on the emission intensity of the rare earth ion is studied, and limits of detection earths are given. An for the empirical equation is developed to facilitate prediction of sensitization by a specific aromatic carbonyl compound for a specific rare earth. The analytical possibilities of the new phenomenon are discussed with regard to sensitivity, selectivity, and speed of analysis and other analytically important considerations.
rare
A
ISVESTIGATIOS of the emission of several rare earth ions in chelates by the method of intramolecular energy transfer resulted in the discovery of a new phenomenon. This phenomenon, the intermolecular energy transfer from excited organic compounds to rare earth ions in solution (7, 9, IO I S ) , seems to possess great analytical possibilities for several reasons: The method should be nearly as selective as any atomic emission procedure because the line emission from an ion in solution is being measured, and even more selectivity should arise because various
x
848
ANALYTICAL CHEMISTRY
organic compounds transfer their energy with widely different efficiencies to different rare earth ions. Analytical considerations such as speed, accuracy, precision, and convenience have not been previously studied for the phenomenon and were included in this investigation. Commercially available instrumentation is used for these studies. THEORY
The phenomenon of intramolecular energy transfer in rare earth chelates is well known, but it has received little attention from an analytical standpoint, probably because of the stoichiometric dependence of the sensitized emission, The process of intramolecular energy transfer has been described in the past few years in detail by Weissman ($1) and Crosby and coworkers (4-6, 8). The ligand molecules are raised to the first excited singlet state by absorption of a photon which approximately corresponds in energy to the absorption energy of the uncomplexed ligand; the spectral shift is, of course, due to the presence of the ligand in the chelate rather than simply the uncomplexed ligand. After a radiationless crossover to the triplet state of the ligand, energy is transferred to the metal ion in the chelate, raising the metal ion to an excited (luminescent) level. As the excited ion undergoes a radiational deactivation to the ground state, the sensitized emission of the metal ion is observed. This type of energy transfer has been applied to a number of cases of analytical interest. Two articles (2, 18)
have reviewed the state of the method through 1962; recently a new method utilizing intramolecular energy transfer has been developed for the determination of Sm, Tb, Dy, and E u ( I ) . Recently, however, sensitized emission from nonchelated rare earth ions in solution has been observed. hlatovick and Suzuki ( I S ) observed that EU(hT03)3 and Tb(N03)3 fluoresce strongly in solutions of acetophenone, propiophenone, and benzophenone when irradiated with long wavelength ultraviolet light (3000 to 4000 A,). They found that water strongly quenched the emission of the rare earth ions. They concluded that the mechanism of excitation for the rare earth ion was through a weakly bonded ketone complex. They suggested that the process n-as probably endothermic, which would account for the observed rise in the quantum efficiency with increased temperature. El-Sayed and Bhaumik (7) observed that benzophenone efficiently transferred its energy to an europium chelate (europium trishexafluoroacetylacetonate) a t room temperatures. They suggested initial intermolecular transfer of triplet state energy from the benzophenone to the ligand and subsequent intramolecular transfer of energy to the europium ion as the mechanism of the process. A tenfold increase in europium ion emission upon addition of the benzophenone donor was reported by these authors. Because of temperature variations of the intensity of rare earth emission, a diffusion-controlled process was shown to exist rather than the weak
chelate previously suggested for the mechanism of energy transfer. Similar results have been obtained by Gallagher, Heller, and Wasserman ( 9 ) in studying the nonradiative resonance exchange process (19) from terbium to europium ions after initial transfer of energy from 4,4’ -dimethoxybensophenone. The authors found that other rare earth ions would not participate in this two-step type of mechanism. Heller and Wasserman (10) have performed an extensive investigation of direct transfer of triplet state energy from various organic carbonyl compounds to rare earth ions in solution. A qualitative examination af the efficiency of transfer from 25 organic compounds to europium and terbium in dilute solutions was presented. The possibility of a chelate formation mechanism for energy transfer was shown to be improbable, because no changes in emission spectra are observed from one donor to another. The criteria for efficient energy transfer were qualitatively discussed, and, of greatest importance, the lack of dependence of rare earth emission on the stoichiometry of the system was also considered. They also suggested that a second-order diff usion-controlled process was the probable mechanism for excitation of the rare earth ions. It has been shown for a limited number of systems (IO)that the process observed under the experimental conditions described above is the transfer of triplet state energy from the donor to the acceptor. This process may be schematically represented as shown in Figure 1 . The donor absorbs incident radiation and is excited to the lowest energy excited singlet state. The molecule then crosses over to the lowest energy triplet state. After an encounter with a rare earth ion which has an absorption level of slightly lower energy than the energy of the triplet state of the donor, energy is transferred. This transfer of energy populates one of the luminescent levels of the rare earth ion. Finally, emission of energy as the line fluorescence of the rare earth ion ( I O ) results. Hence, in an analytical study3 the system would be excited a t the absorption maximum for production of the lowest energy triplet state of the donor molecule, and the emission would be measured a t the fluorescence wavelength of the rare earth ion emission line (or, where there is more than one line, a t the most intense line). Experimental evidence supports this type of process (7, 9, 10, I S , 1 9 ) : Excitation occurs a t the wavelength maximum for the excitation of the phosphorescence of the donor. Emission occurs a t the wavelength(s) for rare earth ion luminescence. There are no shifts in the rare earth ion emission spectrum from one donor to
Excited Singlet State
r
-
Intersystem
1
Tfidat Triplet Stote
Crossing
I L umirescent State
b P
Excitation
Emission
Grwrd Singlet State
~
I Donor Electronic
J Diagram]
Ground State
[Acceptor Electronic Diagram1
Figure 1 . Schematic representation of mechanism for intermolecular energy transfer from aromatic carbonyl donor compound to rare earth acceptor Step 1 . Absorption of photon, hr,,, to produce excited singlet state of donor Step 2. Rodiotionlesscrossover from excited singlet state to triplet state of donor Step 30. Upon collision of donor with acceptor, donor loses its energy b y a radiationless deactivation to the ground singlet state Step 3b. Rare earth accepts energy lost b y donor in step 3 0 and is raised to its luminescent electronic level(s) Step 4. Rare earth loses its energy b y emitting a photon, hvem. This produces the emission spectrum of the rare earth. Other radiational and radiationless processes are involved but are not important in the transfer mechanism, although they affect the efficiency of energy transfer
another, which is expected if a chelate is present. The process as described above in spin-invariant in the transfer stepLe., the donor multiplicity changes by 2 [from 3 to 1 (triplet to singlet)]and the rare earth ion multiplicity also changes by two-e.g., trivalent Tb absorption a t 491 mp is 7F,-,5 D,. The efficiency of transfer increases with temperature-i.e., the number of encounters increases. The process of intermolecular energy transfer from organic molecules to rare earth ions is very similar to the transfer of energy between organic molecules in solution (14-1 7 , 20). Application of intermolecular energy transfer between molecules to analysis has been discussed by Parker, Hatchard, and Joyce (17’). EXPERIMENTAL
Apparatus. An Aminco-Bowman spectrofluorometer (American Instrument Co., Silver Spring, Md.) equipped with an iiminco solid sample accessory (Catalog No. C73-62140), an Aminco-Kiers phosphoroscope attachment (Catalog No. C27-62140), a potted RCA 1P28 multiplier phototube, and a n X-Y recorder (Model 1620-814, manufactured for American Instrument Co., by Electro Instruments, Inc., San Diego, Calif.) were used for obtaining all spectra. The source was a high pressure xenon arc powered by the solid state power supply provided with the instrument. Relative emission intensities were obtained
directly from the Aminco photomultiplier - microphotometer also supplied with the instrument. Luminescence decay times for molecules with relatively long lifetimes (greater than 0.2 second) were measured by manually terminating the excitation beam with the aid of a blank slit placed in the excitation path. For species with short luminescence decay times-Le., less than 0.2 second-the multiplier phototube was powered by batteries, and the photoanodic current of the multiplier phototube was amplified by a Textronix Type 122 low level preamplifier (Textronix, Inc., Portland, Ore.). The output of the preamplifier was displayed on a Textronix Type 545 oscilloscope, equipped with Type L plug-in preamplifier and the decay of the species during the repetitive cycles of the phosphoroscope was recorded on Polaroid 3000 speed Type 47 film using a Textronix trace-recording- camera (Type C-12). Reagents. The oxides of cerium, praseodymium, neodymium, samarium, lanthanum, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, (Rare Earth Division, American Potash and Chemical Corp., West Chicago, Ill.) were obtained in purities of 99.9% or better, except for Dy and Ho which were of 99% minimum purity. Reagent grade acetic anhydride was used as the solvent. All donor organic carbonyl compounds were reagent grade and were readily available from several sources. Stock solutions of all donor compounds were made to be 0 . 0 2 X with acetic anhydride as the solvent. VOL. 38, NO. 7, JUNE 1966
849
Wavelength ( m Q )
Excitatiop
-I-+
Wavelength Figure 2.
ll
'
Wavelength (my
1
Emission
W a d e ng t h ( m d
(mu)
Luminescence spectra of 4,4'-dimethoxybenzophenone (donor) and terbium(ll1) ions
Fluorescence excitation and emission spectra for donor at room temperature (2 X 10-M) Phosphorescence excitation and emission spectra of donor at 77' K. (2 X 10-6M) Line spectrum of Tb+J a t room temperature (2 X lO-*M) C. d. Excitotion and emission spectra for donor and Tb+a together at room temperature. (Donor a t 10%l and terbium ions at 1O"M) N o intended correlation from plot to plot of relative intensities a.
b.
The rare earth oxides were dissolved in HC1 or HNOa and evaporated to dryness on a hot plate. The rare earth chloride or nitrate was then dissolved in acetic anhydride with the aid of deionized distilled water with care to prevent spattering due to localized heat production. I n this manner, stock solutions of the rare earth ions were prepared to be 0.02M in the rare earth ion. Procedure. Relative emission intensity signals of the rare earth-donor systems were obtained by adding 1 ml. of the stock solution of the donor and 1 ml. of the stock rare earth solution to a 1-sq. cm. quartz cell after deoxygenating with a stream of nitrogen gas. Oxygen is well known as a quencher for the triplet state and Heller and Wasserman discuss the necessity for deoxygenating solutions (10). This procedure produced a 0.OlM solution of both donor and rare earth ion. The mixture in the quartz cell was placed in the solid sample cell compartment, and the relative emission intensity signal of the rare earthdonor system was determined. Phosphorescence excitation and emission spectra were obtained as previously described (12, 22) for the donor molecules in ethanol solvent at 77' K. Phosphorescence decay times were also 850
a
ANALYTICAL CHEMISTRY
measured as previously described (1.2) for the systems with lifetimes greater than 0.2 second and as mentioned above for systems with lifetimes less than 0.2 second. Fluorescence excitation and emission spectra were obtained similarly a t room temperature of the donors alone and for the donor-acceptor systems.
RESULTS AND DISCUSSION
The use of intermolecular energy transfer as an analytical tool depends upon the knowledge of several factors which influence a determination. The optimum excitation wavelength for any donor-acceptor system is the wavelength at which the donor is excited to the state which allows it to transfer its energy to the acceptor. The optimum wavelength of emission is the wavelength a t which the acceptor emits the greatest amount of its energy devoid of interfering lines. The excitation spectrum of the mixed system is a superposition of the phosphorescence excitation spectrum and the rare earth excitation spectrum (10).
In Figure 2, the energy transfer process is more clearly demonstrated. Four spectra are shown: fluorescence excitation and emission spectra of the donor a t room temperature; phosphorescence excitation and emission spectra of the donor a t 77" K.; the line spectrum of the rare earth ion a t room temperature; and the excitation and emission spectra of the donor-acceptor system a t room temperature. The great increase in the emission intensity of the rare earth ion in the presence of a donor as compared to the emission in the absence of the donor is a direct result of the transfer of energy from the triplet state of the aromatic carbonyl to the rare earth ion. Intermolecular energy transfer is not the only means of excitation, as can be seen from Figure 2, d. The excitation spectrum of the rare earth emission is a combination of the phosphorescence excitation spectrum of the donor and a line absorption spectrum of the rare earth, and so some direct excitation of the rare earth should also occur. Direct demonstration of intermolecular energy transfer can be made by referring to Figure 3, where three
Table I.
\
DONOR
~
LI
ALONE
v,
z
a
(lO'*N
W
It
IQ
i W [L
Tb'3(5xlO%41 /-=me 400
450
ALONE K n U i v i t y a s "0" md "b"above
5cO
550
Enhancement Factors for Various Rare Earth Ion-Donor Systems and Phosphorescence Lifetimes of the Donors
600
-* 6M
WAVE L E NGT H(mp) Figure 3. Illustration of sensitized rare eorth ion emission due to aromatic carbonyl compound Donor is 4,4'-dimethoxybenzophenone and RE acceptor is Tbi3 a. Fluorescence spectrum of donor (10"M) alone at room temperature b. Emission spectrum of donor 110-2M) and acceptor (5 X 1 0 - 8 M ) at room temperature c. lower curve. Emission spectrum of acceptor alone a t 5 X 10-%4 obtained at same instrumental sensitivity as used for a and b Upper curve. Emission spectrum of acceptor alone at same concentration but at 50 times greater sensitivity, both spectra taken at room temperature
spectra are shown: emission of a solution containing no acceptor, of a solution containing donor a t 0.01M and acceptor at a concentration 5 times the concentration a t the limit of detection, and of a solution containing no donor but containing an acceptor at 5 times the concentration as at the limit of detection-i.e., limit of drtection resulting when donor is present. These spectra also indicate the relative ease with which one may make quantitative measurements of rare earth emission in the presence of a fluorescent donor. Quantitative measurements of the degree of enhancement of the rare earth ion emission by various donors are presented in Table I, in terms of the enhancement factor obtained by dividing the relative intensity of the rare earth ion emission in the presence of the donor by the relative intensity of the background a t that wavelength. This is an analytically important value because the limit of detection is defined as a n enhancement factor of unity-i.e., the limit of detection is that concentration which produces a luminescence signal exactly equal to the background signal. Actually this is a very conservative
Donor Benzophenone 4,4'-l)imethoxybenzophenone 2-Acetonaphthone 2-Bromoacetophenone Vanillin T'eratraldehy de 2-Bromo-4'-phenylacetophenone 2,4'-Dibromacetophenone 9-Fluorenone 7,8-Benzoflavone o-Nitrobenzaldehyde 4'-Bromoacetophenone Benzaldehyde Anisaldehyde o-Tolualdehyde m-Tolualdehyde 1-Acetonaphthone Acetophenone p-Chloroacetophenone
Phosphorescence lifetime, sec.=Ib 0.008
Eu +a 3.3
0.035 0.90 0.038 0.025 0.22
7.3 2.0 8.0 2.7 2.0
0.028 0.008 . . .c 2.9 2.2 0.004 0.020 0.14 0.022
0.034 0.6
0.036 0.022
Enhancement factorcsd Tb + 3 Sm+3 Dy+3 150 12 2 0 37 14 2.0
23 0 0 23 33
17
10
21
0
23
0 0 0 0
0 0 0 0
1100
27 4.7
0
0
5
0
2.3 3.3 25
0
76 425 50 370
2.0
28
20 10
480 0
9.3 5.0
195 44
0
2.0 2.0 0
2.0
2.5 3.5 19
0 6.0 10 0 0
8 11 0
3 4
8
2.5
All lifetimes measured in an ethanol solution at 77" K. Lifetimes greater than 0.2 sec. measured on recorder; lifetimes less than 0.2 sec. measured with oscilloscope. c Enhancement factor is defined as ratio of RE intensity to background intensity a t wavelength of maximum R E emission. d Concentration of all donors and acceptors is 10-2M in acetic anhydride solvent. Front surface illumination used to obtain maximum intensity. All enhancement factors measiired at room temperature (293" K.). e No phosphorescence observed. a b
definition for the limit of detection. I n most isolated experimental cases, a considerably lower limit of detection can be obtained. Also included in Table I are the measured triplet state lifetimes at 77" K. The lifetimes a t room temperature are much shorter. Even so, there is an inverse correlation between the enhancement factor and the triplet state lifetime. This observation seems o contradict the energy transfer scheme which has been invoked for an explanation of the phenomenon, However, because the energy transfer process is both collision and lifetimelimited, the contradiction fades-Le., the time for a collision must in the cases studied here be much less than the lifetime of the triplet state, and so the more triplet singlet transitions, the greater the enhancement of the rare earth emission. Acetic anhydride was the so'vent used for all measurements except the phosphorescence spectra. Although the energy transfer process under examination here has been reported (10) to occur in other less objectionable solvents (ethyl alcohol, methanol, pyridine, acetic acid, and dimethylformamide) , the greatest emission intensities were observed in acetic anhydride. The presence of water in the solvent system decreased the efficiency of sensitization because water in the coordination sphere of the rare earth ion probably prevented an intimate encounter with the donor.
Wide variation is observed in the efficiency of energy transfer for various systems. In an attempt to explain this in terms of physical parameters characteristic of each system, a multiple regression analysis of experimental data was performed by means of the stepwise multiple regression with variable transformation computer program (ERMPR 3) obtained from SHARE distribution number 1194 modified for the University of Florida IBM 709 digital computer. The program produces an empirical equation relating input parameters to the enhancement factor. The programmer determines which of the factors produce the best fit by examining such statistical information as simple correlation coefficients between the various parameters and the standard error of the input data and the resulting equation. The reliability of the equation produced by this procedure is measured by examining the multiple correlation coefficient and the F level. From a consideration of the standard errors and multiple correlation coefficients obtained from over 20 trials, the best empirical equation for predicting the enhancement factor appears to be: h E j = -18.7
1.5
(,)"*
- 0.085
(2
+
+ 2.8 X IO3 (&) +
VOL. 38, NO. 7, JUNE 1966
851
I
CTd+l.O.Ol M
16
165 lo4 lo-3 I d2 io-' 4,4-d imethoxy benzophenone Conc n. (mole , h e r )
10-
Figure 4. Variation of terbium(ll1) ion luminescence with changes in donor (4,4'-dimethoxybenzophenone) concentration Terbium concentration 0.01 M at all points. rurfoce illumination
1.9
x
los
(k)'+
- 1.1 x (ZkDY 1'5 (&D)'
1.3 X 10'
lo+)1'2+ 1 MAAIfD
- 1.0 x
10-5
x
where E , is the enhancement factor, T i s phosphorescence decay time in seconds, M A is the atomic weight of the acceptor, and M a is the molecular weight of the donor. The equation can be used to predict approximate values of the enhancement factor for a specific donor-acceptor system. Only the decay time and the molecular weights prove significant in determining E,. Various functional relationships between phosphorescence and fluorescence excitation and emission wavelengths were constructed, but all were of little significance in determining El. By determining the decay time and the two weights, an approximate enhancement factor can be predicted. The standard error of In E! is +l.lOi.e., the standard error of El is 1 3 . 1 . The above equation was calculated to have a multiple correlation coefficient equal to 0.65 and an F level of 3.52; both of these statistical measures place the equation well within a 95% confidence interval, assuming a normal distribution. Only europium, terbium, samarium, and dysprosium were found t o display any sensitized emission. The effect of donor concentration on the acceptor
852
ANALYTICAL CHEMISTRY
Dota taken a t room temperature with front
luminescence was studied. With the concentration of the terbium held constant, the concentration of the donor, 4,4'-dimethoxybenzophenone, was varied. The results of this study are shown in Figure 4. The relative
lo5
lo-?
10-6
intensity of the terbium emission remained nearly constant over about a 500-fold range of donor concentrations; further decrease in donor concentration resulted in a rapid decrease in terbium emission with concentration. This observation should be of great importance in analytical studies, because small changes in donor concentration from sample to sample would not greatly affect the rare earth emission. Also, the mechanism of a diffusion-controlled encounter is supported by these data; if chelateformation were the mechanism, a wide variation of intensity of emission would be expected for donor concentrations less than acceptor concentrations (see Figure 4). Analytical curves were prepared for the rare earth ions which were found to be useful acceptors by varying the rare earth ion concentration while holding the donor concentration constant (Figure 5). All points are the averages of duplicate or multiplicate readings. The relative standard deviation for all concentrations tenfold greater than the limit of detection was less than 10%. In all cases the analytical curves were determined with the donor concentration a t 0.01M; the donor for the rare earth ions is in each case the one which most efficiently sensitized the rare earth emission (Table I). Initially very poor sensitivities were obtained for the systems while the conventional fluorescence cell arrangement was used-Le.,
6 5
IO-^
lo3
lo2
Rare Earth Ion Concn. (mole /liter) Figure 5. Analytical curves for terbiurn(lll), dysprosium(lll), europiurn(lll), and samarium(III) ions Donors. 4,4'-Dimethoxybenzophenone;. anisaldehyde; o-tolualdehyde; and veratroldehyde, respectively, held ot 0.01M for all measurements. All analytical curves taken at room temperature with front surface illumination
b Figure 6. anhydride
Luminescence spectra of ions in acetic
@
Tb3+
a.
Terbium Dysprosium c. Samarium d. Europium N o indended correspondence of relative intensity from one spectrum to another. Concentration of all RE ions 1 O-M, spectra taken a t room temperature
b.
observing emission at right angles to illumination. Great increases in sensitivities were obtained when front surface illumination was used because the 0.01M solutions of the donors are optically thick in most cases. Analytical curves were measured for terbium and 4,4'-dimethoxybenzophenone; dysprosiuni and anisaldehyde; europium and o-tolualdehyde; and samarium and veratraldehyde. Limits of detection were found to be 10-*51, 8 X 10-7 M , 5 x 10-6, and 2 X 10-5?11 for Tbf3, Dy+3, Eu+3, and Sm':', respectively; the useful ranges of the analytical curves were over a 10,000-, 1000-, 1000-, and 100-fold concentration range, respectively (see Figure 5 ) for T b f 3 , D Y + ~ , E u + ~and , Sm+3. Extraneous ions present in the sample were found to quench the energy transfer process in relation to their concentration, as expected. This effect is probably due to a collisional competition between the extraneous ion and the rare earth ion to be sensitized. Consequently, if a system for analysis has other ions present, a standard addition technique should be employed. Such studies were extensively investigated by Alberti and Massucci ( 1 ) . Simultaneous determination of the four rare earth ions which exhibit sensitized emission should be possible. An examination of the emission spectra of the ions (Figure 6) S~OIVS that with suitable adjustment of the emission monochromator, the various lines may be isolated. For quantitative work, standard addition would be necessary t o determine the amount of each ion present. Further selectivity is available to the analyst by judicious choice of the donor (Table 1)-for example, a donor which efficiently sensitizes the emission of terbium but completely quenches the emission of other rare earth ions would be an obvious choice if terbium were to be determined in the presence of other rare earth ions. CONCLUSIONS
Intermolecular energy transfer appears to hold great promise for the determination of trace amounts of terbiIm, dysprosium, europium, and samarium in samples containing rare earth ions. Large useful concentration ranges are available for quantitative work. Great selectivity is a natural consequence of both the transfer process and the line nature of the rare earth ion
0
Wavelength emission. The low limits of detection for the rare earth ions should permit microtechniques to be used for small samples. The lack of a great dependence of emission intensity on donor concentration reduces possible gross errors in the method. The procedure for analysis is extremely fast and simple, except that acetic anhydride is a n objectionable solvent. It is conceivable that emission intensities can be increased even more (perhaps as much as 20-fold) by using deuterated solvents (3, 11). Further studies of donor molecules should yield useful enhancement factors. The empirical equation for enhancement factor given above is by no means intended as a final result but rather as an initial indication of preliminary results. ACKNOWLEDGMENT
The authors thank the University of Florida Computing Center and Jim Marcum for help in obtaining the computer program. LITERATURE CITED
(1) Alberti, G., Massucci, M. A., ANAL. CHEM.38, 214 (1966). (2) Bartholomew, R. J., Rev. Pure Appt. Chem. 8 , 265 (1958). (3) Borkowski, R. P., Forest, H., Grafstein, D., J . Chem. Phys. 42, 2974 (1965). (4) Crosby, G. A., Kasha, M., Spectrochim. Acta 10, 377 (1958). (5) Crosby, G. A,, Whan, R. E., Alire, R. M., J . Chem. Phys. 34,743 (1961).
sm3+
( mu 1
(6) Crosby, G. A., Whan, R. E., Freeman, J. J., J. Phys. Chem. 66, 2493 (1962). (7) El-Sayed, M. A., Bhaumik, 31.L., J . Chem. Phys. 39, 2391 (1963). (8) Freeman, J. J., Crosby, G. A., J . Phys. Chem. 67, 2717 (1963). (9) Gallagher, P. K., Heller, A., Wasserman, E., J. Chem. Phys. 41, 3921 (1964). (10) Heller, A., Wasserman, E., Zbid., 42, 949 (1965). ( 1 1 ) Kropp, J. L., Windsor, 31.W., Zbid., 42, 1599 (1965). (12) RlcCarthv. W. J.. Winefordner. J. ' D.,J. Asso"c: Ogic. :4gr. Chemists 915 (1965). (13) Matovick, E., Suzuki, C. K., Chem. Phys. 39, 1442 (1963). (14) Parker, C. A., Proc. Roy. SOC. 276, 125 (1965). (15) Parker, C. A., Hatchard, C. Proc. Chem. SOC.1962. 386. (16) Parker, C. A., Hatchard, C. G., Proc. Roy. SOC.A 269, 574 (1962). (17) Parker, C. A., Hatchard, C. G., Joyce, T. A., Analyst 87, 1 (1965). (18) Parker, C. A,, Rees, W. T., Zbid., 87, 83 (1962). (19) Peterson, G. E., Bridenbaugh, P. RI., J . Opt. SOC.Am. 53, 1129 (1963). (20) Porter, G., Proc. Chem. SOC.1959, 291. (21) Weissman, S. I., J . Chem. Phys. 10, 214 (1942). (22) Winefordner, J. D., hloye, H. A., Anal. Chim. Acta 32, 278 (1965).
RECEIVEDfor review February 1, 1966. Accepted March 30, 1966. Research carried out as part of a study on the phosphorimetric analysis of drugs in blood and urine, supported by a grant from the U. S. Public Health Service (GM-11373-03). W. J. McCarthy held a Gulf Oil fellowship during the academic year 1965-66 at the University of Florida. VOL 38, NO. 7, JUNE 1966
853
