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Anal. Chem. 1980, 52, 1260-1264
(5) Hadeishi, T. Appl. Phys. Lett. 1972, 21, 438-40. (6) Prugger, H.; Torge, R. United States Patent 3676004, filed Dec. 17, 1970; filed in Germany Dec. 23, 1969. (7) Parker, C.; Pearl, A. Australian Patent 474204, filed Jan. 5, 1971. (8) Hadeishi, T.; McLaughlin, R. D. Science 1971, 174, 404-7. (9) Held, A. M. Ph.D. Thesis, Montana State University, 1972. (10) Hadeishi, T.: Church, D. A.; McLauahlin, R. D.; Zak, B. D.; Nakamura, M.; Chang, R. Science 1975, 187,-348-9. (11) Stephens, R.; Ryan, D. E . Taianta 1975, 22, 659-62. (12) Koizumi, H.; Yasuda, K. Anal. Chem. 1975, 47, 1679-82. (13) Koizumi. H.: Yasuda. K. Soectrochim. Acta. Part81978. 3 1 . 237-55. i14j Koizumi; H:; Yasuda, K. Anal. Chem. 1976, 48, 1178-82. (15) Uchida, Y.; Hattori, S. Bunko Kenkyu 1977,26, 266-71; Chem. Abstr. 1978, 88, 163211a. (16) Stephens, R. Talanta 1978, 25,435-40. (17) Stephens, R.; Ryan, D. E. Talanta 1975, 22, 655-8. (18) Stephens, R. Talanta 1977, 24. 233-9. (19) Murphy, G. F.; Stephens, R. Talanta 1978, 25, 223-5. (20) Koizumi, H.; Katayama, M. Phys. Lett. 1977, 63A, 233-4.
(21) Koizumi, H.; Katayama, M . Phys. Lett. 1977, 64A. 285-6. (22) Koizumi. H. Japanese Patent 145 181-1975, filed May 10, 1974. (23) Dawson, J. 8.;Grassam, E.; Ellis, D. J.; Keir, M. J. Ana/yst(London) 1978, 101, 315-16. (24) Koizumi, H.; Yasuda, K. Spectrochim. Acta, Part 8 1978,31, 523-35. (25) Koizumi, H.; Yasuda, K., Katayama, M. Anal. Chem. 1977, 49, 1106-12. (26) Grassam, E.; Dawson, J. B.; Ellis, D. J. Ana/yst(London) 1977, 102, 804-18. (27) Brodie, K. G. Unpublished work, Varian Techtron, 1971. (28) Otruba, V.; Jambor, J.: Komarek, J.; Horak, J.; Sommer, L. Anal. Chim. Acta 1978, 101, 367-74. (29) de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta, Part 8 1978, 33. 495-512. (30) Kubasik, N. P.; Volosin, M. T. Clin. Chem. 1974,20, 300-301.
RECEIVED for review December 21, 1979. Accepted March 7 , 1980.
Determination of the Absolute Quantum Efficiency of Luminescence of Solid Materials Employing Photoacoustic Spectroscopy M. J. Adams,' J. G.
Highfield,* and G. F. Kirkbright"
Chemistry Department, Imperial College of Science and Technology, London S W7, U.K.
Photoacoustic spectroscopy is employed as a calorimetric method for the determlnation of the luminescent quantum efficiency of several solid materials: tetraphenylbutadiene (TPB), yellow liumogen, and sodium salicylate. The values obtained for the quantum efficiency of TPB are shown to be dependent upon the physical nature of the sample. The technique employed utilizes the phenomenon of photoacoustic saturation, in which the amplitude of the photoacoustic signal is independent of the sample absorption coefflcient. With the aid of a graphical extrapolation procedure, the proposed method is rapid, accurate, and precise. The method may be employed for the examination of solid samples and materials in solutlon.
The problems associated with the study and determination of photoluminescence quantum efficiency values of solid materials have always been more difficult to overcome than for materials in solution. Tregellas-Williams (1)has reviewed the methods employed for the determination of luminescence efficiencies for inorganic phosphors and, more recently, Lipsett has provided an extensive summary of the subject ( 2 ) . The majority of techniques employed today use photometric methods, Le., the detection and measurement of a certain fraction of the emitted luminescence following excitation of the sample and an examination of the literature suggests that the most serious errors in these methods occur in applying corrections necessary to account for the experimental geometry employed. Wrighton et al. ( 3 ) have utilized a conventional scanning emission spectrophotometer to determine absolute fluorescence efficiences of powdered samples. By measuring Present address, M a c a u l a y I n s t i t u t e for Soil Research, Craigiebuckler, Aberdeen AB9 2QJ, U.K. P r e s e n t address, C h e m i s t r y D e p a r t m e n t , U n i v e r s i t y of B a t h , B a t h . U.K. 0003-2700/80/0352-1260$01 .OO/O
the diffuse reflectance of the sample relative to that of a nonabsorbing standard material at the excitation wavelength and by recording the emission of the sample under identical conditions, the luminescence efficiency was determined as the ratio of the intensity of emitted radiation to the difference in intensity of the diffuse radiation from the sample and the nonabsorbing standard. Following calibration of the detector sensitivity as a function of wavelength, however, and following introductions of corrections for the nonideality of the absolute reflectance standards, the error in the method was reported to be k25%. Considering the extensive application of photometric methods, and their inherent disadvantages, there have been few developments in alternative techniques for determining luminescence quantum efficiencies of solids. Most calorimetric methods are modified versions of Bodo's ( 4 ) and employ thermocouples or thermistors to monitor the heating effect within the sample following absorption of electromagnetic radiation. A piezoelectric calorimeter has been described by Callis ( 5 ) and used to determine the triplet yield for anthracene dissolved in a rigid matrix of polymethylmethacrylate. In this paper we wish to report the use of photoacoustic spectroscopy, PAS, for the determination of absolute quantum efficiencies of solid materials. PAS employs the optoacoustic effect in which the absorption of modulated electromagnetic radiation produces a periodic temperature wave within the sample. The magnitude of this temperature fluctuation is dependent upon the optical absorption characteristics of the material under study and upon the efficiency of radiationless conversion following excitation. In PAS the temperature of the sample, contained within a sealed cell, is monitored with the aid of a microphone transducer via the periodic pressure wave produced in the gaseous atmosphere surrounding the sample (6, 7). Adams et al. have employed the PAS technique for the determination of the absolute fluorescence quantum efficiency of aqueous solutions of quinine bisulfate (8), and Malkin and Cahen have used PAS to study radiant energy 8 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
conversions in photosynthesis (9). By utilizing the phenomenon of photoacoustic signal saturation, we wish to demonstrate that photoacoustic spectroscopy may provide a relatively rapid and precise method for the study of luminescence in solid materials. Luminescence quantum efficiency data are determined here for 1,1,4,4-tetraphenyl 1,1,3-butadiene, 2,2’-dihydroxyl-l,l’-naphthaldiazine, and sodium salicylate. THEORETICAL CONSIDERATIONS T o derive from the experimental data the absolute quantum efficiency of fluorescence, Q, for a sample, it is necessary to consider the theoretical expression relating the amplitude, A, of the photoacoustic signal a t wavelength X to the characteristics of the sample, Here P a b s is the radiant power absorbed at this wavelength by the sample, y is a n efficiency factor which is a measure of the conversion efficiency of absorbed power into heat by nonradiative mechanisms, and K is a constant given by the thermal transfer characteristics of the sample and the instrumental arrangement employed. For a sample capable of fluorescence, then
where Q is the fluorescence quantum efficiency, vo is the frequency of excitation, and uF is the mean frequency of the fluoresced radiation (8). Combining Equations 1 and 2,
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Rosencwaig and Gersho (10) and McClelland and Kniseley (11) have defined the thermal depth, L , of a photoacoustic signal as that thickness of sample contributing to the production of the photoacoustic signal at the samplegas interface. The thermal depth is controlled by the thermal diffusivity, a cm2 s-l, of the sample and the instrumental modulation frequency, w rad sec-l, viz,
McClelland and Kniseley (11 ) have demonstrated that the photoacoustic signal is directly proportional to the sample absorption coefficient, /3, for values a t p < 27r/ L but becomes progessively less sensitive to increases in p, ultimately becoming independent of @ at @ = 20.1rlL. The photoacoustic signal is then said to be “saturated”. Many solid materials and powders have high absorption coefficients owing to their highly condensed form and it may be anticipated that the photoacoustic signals derived from such samples will be independent of their absorption coefficients. Therefore, the serious problems of absorbance matching of samples and reference materials may be avoided by making use of photoacoustic saturation. Thus, if the photoacoustic signals are monitored in saturation and with identical instrumental conditions, and Equation 7 reduces to
A plot of AF/ANF vs. Xo (Le., the normalized photoacoustic spectrum) should yield a straight line of slope m and an intercept, a t A0 = 0 , of KF/KNF,where
for a fluorescent material and ANF
=
(4)
KNFPabs(?iF)
for a nonfluorescent sample, i.e., y = 1. The subscripts F and NF refer to the fluorescent and nonfluorescent conditions, respectively. Thus, combining Equations 3 and 4 and rearranging, in terms of wavelength,
”.(
AF KNFPabs(NF)
I--.-.-
Q = -
ANF
A0
K F
Pabs(F)
)
(5)
I n our earlier studies examining aqueous solutions of quinine bisulfate, the fluorescence was effectively quenched by the addition of chloride ions, without changing the absorption spectrum of the sample. Thus, the thermal and optical absorption characteristics of the fluorescent sample and nonfluorescent reference were identical and Equation 5 may then be reduced to
”(
Q = -
A0
I-i:F)
Q may then be readily determined. Unfortunately, for solid samples the accurate matching of thermal and optical characteristics of the sample with a nonluminescent reference material is not easily achieved and i t is necessary to develop a new approach. This may be attained by utilizing the phenomenon of photoacoustic signal saturation. Rearrangement of Equation 5 provides
which predicts a h e a r relationship between AF/ANF and A,, the excitation wavelength.
Q, the quantum efficiency of luminescence, may be calculated from Equation 11. EXPERIMENTAL All photoacoustic measurements were made with the aid of a single-beam photoacoustic spectrometer similar to that described in our earlier studies (8). Radiation from a 300-W xenon arc source was focused through a rotating sector onto the entrance slit of an f / 4 monochromator. The photoacoustic cell containing the sample or reference material under study was positioned at the exit slit of the monochromator. The photoacoustic signal was monitored with the aid of a sensitive capacitor microphone, mounted within the cell, and led to a lock-in amplifier unit. The frequency-reference signal was generated by the rotating-sector unit. The output from the lock-in amplifier was led to a digital scan-recorder system and the amplitude of the PAS signal recorded. Nonluminescent reference samples were prepared by “smoking” a thin glass sheet to produce a carbon-black film. As only relative signal magnitudes are necessary for this study, the amount of luminescent sample examined is not critical and a sufficient amount was used to completely cover the base of the photoacoustic sample tray (ca. 500 mg). Employing an amplifier time-constant of 10 s and a modulation frequency of 18 Hz, signal amplitude data sets were recorded at 10-nm intervals within the spectral range of interest. The procedure was repeated several times for both sample and reference materials, producing signal amplitude data of ca. 1 % precision. A spectral half-bandwidth of 20 nm was employed for all studies. The luminescent materials examined included, 1,1,4,4-tetraphenyl-1,3-butadiene, TPB (99% purity, general purpose grade and “Gold Label” scintillation grade, Aldrich Chemical Co. U.K.), 2,2-dihydroxy-l,l’-naphthaldiazine (yellow liumogen, Aldrich
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
Table I. Effects of Particle Size o n Luminescence of TPB luminescence sample material quantum efficiency TPB (99% Duritv) L
“
I
TPB (scintillation grade) Yellow Liumogen sodium salicylate a Sample as received. powder.
50
I 300
350
LOOnm
h.
Figure 1. Normalized photoacoustic spectra within the wavelength range for photoacoustic saturation of (a) Congo Red, (b) Malachite Green, (c) TPB, (d) Yellow Liumogen, (e)sodium salicylate
Chemical Co.) and sodium salicylate (Aldrich Chemical Co.). Nonluminescent reference materials included Congo Red and Malachite Green (Hopkins and Williams Ltd., U.K.). Corrected fluorescent emission spectra of the samples were obtained using a spectrofluorimeter (American Instrument Co., Silver Spring, Md.) equipped with a front-surface emission accessory. The excitation monochromator was calibrated using a M in 0.05 M H,SO,) by quinine bisulfate solution (2 X comparison with the published spectrum by Gill (12). The emission monochromator was calibrated against the excitation monochromator using doubly-distilled water. The spectral sensitivity curve for the instrument was obtained by comparison of the experimentally measured emission spectrum of quinine bisulfate with the corrected spectrum recommended by Melhuish (13).
RESULTS AND DISCUSSION T o examine the validity of Equation 10, it is necessary to satisfy the conditions expressed in its derivation. The most important of these is that photoacoustic signal saturation be achieved. As many luminescent organic materials have high absorption coefficients within their excitation wavelength range, this class of compounds should prove suitable for study. Such a material is tetraphenylbutadiene, T P B . Burton and Powell (14) have described TPB as a useful scintillator for photometric detectors for operation in the vacuum ultraviolet. TPB has a n extremely high absorption coefficient and its absorption spectrum extends through the near-ultraviolet, a spectral region ideal for photoacoustic studies due to the intense emission from the xenon source providing high sensitivity at a high signal-to-noise ratio. The luminescence efficiency of TPB is reported to be high and constant over a wide range of excitation wavelengths (14). The photoacoustic measurements were made as described above and a plot of AF/ANFvs. Xo, for the wavelength region 320 to 380 nm employing a carbon-black as the nonluminescent reference absorber, is shown in Figure 1. The plot is linear in accordance with Equation 10. As a comparison, Figure 1 also shows the normalized spectra of two nonluminescent organic dyes, Congo Red and Malachite Green, as obtained under identical experimental conditions. Both dyestuffs are intensely colored and exhibit photoacoustic saturation within this spectral region. As expected, these materials exhibit a flat spectrum, Le., linear with zero slope, characteristic of saturation. T o calculate the absolute luminescence efficiency of T P B from this data, it is necessary to know the mean emission
0.94 +. 0.01“ 0.86+. O . O l b 0.86 c 0.01” 0 . 8 6 k 0.01 0.33 f 0.01 0.55 t 0.04
Sample crushed to produce fine
wavelength, XF, of the material. This may be obtained from the corrected emission spectrum. This spectrum was recorded using the calibrated fluorescence spectrometer and was found to be consistent with that reported by Burton and Powell (14). I t exhibited a maximum emission at 432 nm and a mean emission wavelength of 450 nm (calculated manually). The reproducibility of the photoacoustic measurements was checked by conducting the studies on several successive days and by using least-squares analysis on the data. With the aid of Equation 10, the luminescence quantum efficiency of T P B was calculated as 0.94 0.01. Comparison of this value with the relative quantum efficiency reported by Burton and Powell is good, assuming sodium salicylate to have a quantum efficiency of ca. 0.6. Following these initial studies a second source of T P B (scintillation grade) was examined. Using a n indentical procedure, the absolute quantum efficiency was determined. In this case the value obtained for Q was 0.86 f 0.01, i.e., lower than that for the general purpose (9970 purity) grade reported above. The physical nature of the two samples was different, however. The scintillation grade material was a fine powder while the general purpose grade consisted of visible crystalline needles. T o examine the possible effects of particle size on the luminescence of T P B , both samples were subjected to manual grinding for several minutes; the results are shown in Table I. It is evident from these data that reducing the particle size by grinding the general purpose grade sample produces a reduction in the luminescence quantum efficiency of TPB. Further grinding produces no changes in the values determined for Q and a limiting value of 0.86 f 0.01 was attained for each sample. Kristianpoller and Dutton (15) have reported the use of yellow liumogen (2,2’-dihydroxyl-l,l’-naphthaldiazine) as a scintillator for ultraviolet detectors, and Kristianpoller (16) has employed this material in a quantum counter for quantum efficiency measurements. Although the absorption spectrum of yellow liumogen exhibits some band structure, the absorption coefficient is greater than lo4 cm-’ a t wavelengths less than 480 nm, its absorption band edge. I t is to be expected, therefore, that yellow liumogen should exhibit photoacoustic saturation under the conditions employed for this study. Samples of crystalline yellow liumogen and samples ground to a fine powder were examined and photoacoustic measurements were made in the manner reported above. The mean emission wavelength of yellow liumogen was estimated from published spectra as 557 nm (15). Replicate measurements were made on successive days and a typical graph of AF/ANFvs. Xo, employing a carbon-black reference absorber, is shown in Figure 1. From these data, the absolute quantum efficiency of fluorescence of yellow liumogen was determined as being 0.33. Particle size and grinding of the sample appeared to have no effect on the values determined for Q. The final study of the potential of this photoacoustic method for the determination of the quantum efficiency of solid materials was concerned with an examination of sodium salicylate. T o date, this material has been the most popular
*
ANALYTICAL CHEMISTRY, VOL 52 NO 8, JULY 1980
fluorescence agent for UV radiation detectors and has been studied extensively, with a marked inconsistency in values for its luminescence quantum efficiency. In solution, sodium salicylate has an absorption coefficient of ca. 5 X lo3cm-’ a t 310 nm, the wavelength of maximum absorption, and the wavelength region in which the condition of photoacoustic saturation may be expected to apply is limited. Furthermore, the absorption spectrum is centered about a region (ca. 300 nm) where the intensity of radiation from the xenon source employed is considerably less than at the longer wavelengths used for the other materials examined. The sodium salicylate was examined as a crystalline solid (with no pretreatment) within the wavelength range 275 to 315 nm, at 5-nm intervals. Below this wavelength region, the signal-to-noise ratio of the photoacoustic measurements was too low for accurate results, and a t longer wavelengths photoacoustic saturation could not be assumed as the normalized signals decreased rapidly. The plot of A F / A m vs. Xo for the wavelength region of interest is shown in Figure 1. T h e mean emission wavelength was estimated from the fluorescence emission spectrum to occur a t 437 nm and Q was calculated from the photoacoustic data and from Equation 10. Three successive determinations of the absolute luminescence quantum efficiency of sodium salicylate provided results of 0.50, 0.59, and 0.55. While these values are in general agreement with those reported in the literature, the precision attained is obviously inferior to that observed for T P B and yellow liumogen. Possible Sources of Error. The results of this study of t h e luminescence quantum efficiencies of solid materials demonstrate the high precision attainable using photoacoustic spectroscopy. The accuracy of the technique, however, is more difficult to assess. Two possible sources of instrumental error are readily identified: (a) absorption of radiation, both from the source and fluorescence emission by the photoacoustic cell walls, and (b) reflection of incident radiation by the sample. In principle, the contribution to the photoacoustic signal from a sample due to background absorption by the cell walls may be estimated by examining a nonabsorbing reference compound. Unfortunately, in practice, this may be difficult t o achieve as the more common reflectance standards, e.g., magnesium oxide, barium sulfate etc., all exhibit weak absorption in the near ultraviolet region. At wavelengths greater than ca. 400 nm, however, the measured signal from a magnesium oxide sample is typically about 2% of that from the materials examined here. T h e design of the photoacoustic cell is such that the sample, contained within the sample tray, is close t o the entrance window of the unit and the majority of emitted and reflected radiation from within the sample tray, would be directed out from the cell. Furthermore, the highly polished nature of the photoacoustic cell walls also tends to reduce t h e absorption here. T o a first approximation, therefore, we may assume the contribution to the photoacoustic signal from background absorption to be negligible. T h e reflectivity of t h e luminescent samples within the wavelength region of interest presents a second possible source of error as the reflectance will be expected to vary as a function of the absorption coefficient across the absorption band. The reflectance spectra of the luminescent samples were recorded using the Aminco-Bowman spectrofluorimeter and a frontsurface illumination accessory. All samples were observed to be of low and constant reflectance within the working wavelength range. As only changes in reflectivity across the absorption band may contribute a n error to the photoacoustic measurements, a negligible systematic error from this source may be assumed from these studies. Other sources of error in the determination of Q values may arise from the choice of AF, the mean fluorescence wavelength, and self-absorption of the fluorescence emission by the sample.
1263
The determination of AF may be accomplished with a high degree of accuracy provided an accurate emission spectrum of the luminescent material is available. For a symmetrical the fluorescence emissinon band, XF may be equated with, ,A emission peak maxima. In cases of asymmetric band emission, as with the materials reported here, AF may be (calculated,e.g., graphically, from
J-Z,.X.d, hF = ~Ix-d’x
J-
(12)
where I x is the fluorescence emission intensity a t wavelength, A. I t is a necessary criterion of tht: photoacoustic method discussed above for determining Q 1,hat the sample strongly absorbs the incident radiation to achieve photoacoustic saturation. Thus, absorption of fluorescent radiation by the sample may give rise to a serious error if the sample exhibits severe overlap of its absorption-emission spectral profiles. Burton and Powell (14) have examined the spectra of T P B and there is little fluorescence emission at wavelengths below 400 nm, the region of intense absorption. Furthermore, the high luminescence quantum efficiency of T P B decreases the magnitude of any error caused by reabsorption. The absorption coefficient of yellow liumogen exceeds I O 5 cm a t wavelengths less than 460 nm, and its absorption spectrum exhibits a steep absorption band edge> a t this wavelength, with negligible absorption at wavelengths greater than 500 nm (15). The fluorescence emission of yellow liumogen extends from ca. 500 nm to about 650 nm; hence, as with TPH, there is little overlap of the absorption-emission profiles and negligible reabsorption may be assumed. T h e fluorescence emission spectrum of sodium salicylate extends from ca. 3’70 to 570 nm ( 1 7 ) and while spectral overlap at the lower wavelengths occurs between the absorption and emission spectra, this is outside the region of photoacoustic saturation and its effect on the magnitude of the PAS signal may not be severe. It is difficult to provide absolute magnitude values to the errors referred to above. For the samples examined here, however, a preliminary study of these possible sources of error indicates that the quantum efficiency values quoted are accurate to within a few percent.
CONCLUSION The luminescence quantum efficiency of T P B has been demonstrated to be dependent upon its physical form, in particular its particle size. As the crystatline material is ground to produce a fine powder, the quantum efficiency is reduced from 0.94, for the crystalline sample, to a value of 0.86 for the powder. The reasons for this dependence on the particle size, or the treatment employed to grind the sample, are not clear; however, it may be that the action of crushing T P B crystals creates a greater number of lattice defects at the surface of the crystals which may serve as efficient exciton “traps”. The values of luminescence quantum efficiency determined by this photoacoustic method for yellow liumogen anti sodium salicylate, 0.33 and 0.55, respectively, agree well with those reported in the literature. T h e precision attained for these values is less than that for TPB. T h e reasons for this may be appreciated by reference to Equations 10 and 11. The slope of the AF/ANF vs. Xo is directly proportional to the luminescence quantum efficiency, Q; hence, the precision of the method increases with increasing luminescence efficiency. Furthermore, in the case of sodium salicylate, the wavelength region within which the photoacoustic saturation may be assumed is of a much narrower range than the 01 her materials. We believe the data and results presented here demonstrate the value of photoacoustic spectrometry for the examination of luminescence phenomena in solid materials and for the
Anal. Chem. 1980, 52. 1264-1267
1264
determination of absolute luminescence quantum efficiencies. T h e technique is relatively rapid and simple to undertake. The major criterion for the use of the method proposed here is t h a t the sample should exhibit photoacousic saturation within the excitation wavelength region of interest.
LITERATURE CITED (1) J. Tregellas-Williams, J. Electrochem. SOC., 105, 173 (1958). (2) F. R. Lipsett, Prog. Dielectr., 7, 217 (1967). (3) M. S.Wrighton, D. S.Ginley, and D. L. Morse, J. Phys. Chem., 7 8 , 2229
(1974). (4) 2. Bodo, Acta Phys. Acad. Sci. Hung., 3 , 23 (1963). (5) J. B. Callis. J. Res. Natl. Bur. Stand., Sect. A , 80, 413 (1976). (6) M. J. Adams, 8. C. Beadle, and G. F. Kirkbright, Analyst(London), 102, 567 (1977). (7) A. Rosencwalg, in "Optoacoustic Spectroscopy and Detection", Y. H. Pao, Ed., Academic Press, New York, 1977.
(6) M. J. Adams, J. G. Highfield, and G. F. Kirkbright, Anal. Chem., 49, 1850 (1977). (9) S. Malkin and D. Cahen, Photochem. Photobiol., 29, 803 (1979). 10) A. Rosencwaig and A. Gersho, J . Appl. Phys., 47, 64 (1976). 11) J. F. McCleiland and R. N. Kniseley, Appl. Phys. Left.,28, 467 (1976). 12) J. E. Gill, Photochem. Photobiol., 9, 313 (1969). 13) W. H. Melhuish, J. Phys. Chem., 64 762 (1960). 14) W. M. Burton and B. A. Powell, Appl. Opt., 12, 87 (1973). 15) N. Kristianpoller and D. Dutton, Appl. Opt., 3 , 287 (1964). 16) N. Kristianpoller, J . Opt. SOC. A m . , 54, 1285 (1964). 17) R. Allison, J. Burns, and A. J. Tuzzolino. J. Opf. SOC. A m . , 54, 747 (1964).
RECEIVED for review November 26, 1979. Accepted February 29, 1980. We are grateful to the Science Research Council and B P Research Ltd. for the provision of a studentship to one of us (J.G.H.) under the CASE award scheme.
Determination of Trivalent Chromium in Seawater by Chemiluminescence C. Allen Chang and Howard H. Patterson* Department of Chemistty, University of Maine, Orono, Maine 04469
Lawrence M. Mayer Department of Oceanography, University of Maine, Ira C. Darling Center, Walpole, Maine 04573
Daniel E. Bause GCA Corporation, Bedford, Massachusetts 0 1730
Luminoi chemiluminescence (CL) analysis of aquated Cr( 111) in seawater is discussed. The major interference comes from magnesium ions. Elimination of the interference is achieved by seawater dilution and utilizing bromide ion CL signal enhancement. The detection limit is 3.3 X lo-' M (0.2 ppb) for seawater with a salinity of 35% with 0.5 M bromide enhancement. High purity reagents should be used for experiments carried out near the detection limit.
Recently the determination of metal speciation has been of increasing concern because metal ions in different forms have different reactivities and properties (1,2). We have been interested in the chromium speciation in Maine aquatic and marine systems into which chromium(II1)-rich effluents from tanneries are discharged. The chromium concentration in such environments can be as low as the parts-per-billion range (3, 4).
U p to the present, most methods used for dissolved chromium (111 and VI) analysis have involved preconcentration using coprecipitation (5-8), solvent extraction (8, 9), or ionexchange chromatographic techniques (10). Detection is usually performed by colorimetric and atomic absorption techniques. These lengthy procedures entail the possibility of inadvertent gain or loss of chromium by the sample and may result in a change in speciation. Thus, methods involving less sample manipulation before analysis to minimize sample contamination and speciation change should be developed. 0003-2700/80/0352-1264$01,00/0
We have chosen the chemiluminescence (CL) technique to examine chromium speciation. T h e CL technique not only is sensitive and selective in detecting trace aquated chromium(II1) at very low concentrations but also requires very little time in sample preparation (11,12). Previous studies have used the CL technique to determine aquated chromium(II1) only in freshwater systems (12). With luminol-H 0 basic mixture as the reagent, trace aquated chromium(I1:) o:n can catalyze luminol oxidation by H202with emission of blue light. The light intensity is proportional to the amount of chromium(II1) present in the sample. It has been found t h a t in M, the presence of excess bromide ions, i.e. greater than an increase of CL signal was observed (13, 14). This signal enhancement lowers the detection limit for aquated chromium(II1) to the parts-per-trillion range. Our early attempts t o apply CL techniques to analyze for chromium(II1) in seawater were hampered by a salt interference. This paper reports our results in studying the nature of this interference and the use of the halide enhancement effect to determine aquated Cr(II1) in seawater.
EXPERIMENTAL Apparatus. Chemiluminescence was measured using a flow system described earlier (13,14). Two 20-mL plastic syringes were
used; one syringe contains the metal analyte, the other syringe contains a mixed solution of luminol and hydrogen peroxide at the desired pH. The syringes were driven by a Sage Model 351 syringe pump. The metal ion solution was mixed with the luminol-H202 solution before entering a quartz flow cell. A Perkin-Elmer MPF-44A Fluorescence Spectrometer was used to t 2 1980 American
Chemical Society
