Anal. Chem. 1989, 6 1 , 1405-1410
E j / m V = 2.2 - 17m~,,cl f 0.1 mV
(20)
Since Ej for the physiological phoephate buffer is 2.5 mV, then the pH values obtained by the operational methods, i.e., the cell with flowing junction and the cell with glass electrodepH meter, may be corrected by evaluating ~ E . J ( ~asH ) 6Ej = 2.5 - (2.2 - 1 7 m ~ , c J (21) Equation 16 becomes PHX= P H + ~ [(Ex- Es + 6Ej)/kI (22) With this ~ E correction, J the p H values from the cell with flowing junction and those computed with eq 9 are in agreement to within f0.005 pH unit, as shown in Table VII. The overall uncertainty is about f0.015 pH unit, which is caused by the experimental error (about f0.2 mV) and by the assumptions for the evaluation of the activity coefficients (about f0.6 mV). The consistency of the three sets of experiments allows one to have confidence in the pH values of HEPES solutions.
LITERATURE CITED (1) Good, N. E.; Winget, G. D.; Winter, W.; Connolly, T. N.; Izawa, Singh, R. M. M. Biochemistry 1988, 5 , 467.
S.;
1405
Vega, C. A.; Bates, R. G. Anal. Chem. 1978, 4 8 , 1293. Taylor, M. J.; Pignat, Y. Cryobioiogy 1982, 19, 99. Roy, R. N.; Gibbons, J. J.; Baker, G. E. Cryo-Left. 1985, 6.265. Cohn, E. J.; Edsall, J. T. Proteins, Amino Acids and PeptMes; Hafner Publishing Co.: New York, 1965. Scatchard, G.; Kirkwood, J. G. Phys. 2. 1932, 33, 297. Wu, Y. C.; Koch, W. F. J. Solution Chem. 1988, 15, 461. Harned, H. S.; Owen, B. B. The Physical Chemistry of Elecfrolytic Solutions; ReinhoM Book Co.: New York, 1958. Wu, Y. C.; Koch, W. F.; Marinenko, G. J. Solution Chem. 1988, 75, 675.
Wu, Y. C.; Feng, Darning: Koch, W. F. J. Solution Chem., in press. Bates, R. G. Determination of pH; John Wiley 8 Sons, Inc.: New York, 1954. Lather, W. M. Oxidation Potentials, 2nd ed.; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1952.
RECEIVED for review November 14, 1988. Accepted March 8, 1989. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified is necessarily the best available for the purpose.
Fluorescence Studies of Energy Transfer in Sodium Taurocholate and Sodium Dodecyl Sulfate Micellar Solutions Kasem Nithipatikom' and Linda B. McGown* Department of Chemistry, P. M . Gross Chemical Laboratory, Duke University, Durham, North Carolina 27706
The use of mlcellar reagents to solublllze fluorescent molecules In solution can lead to determlnatlon errors If the solublllzatlon Is accompanied by promotlon of photophyslcal lnteractlons between the molecules. The fluorescence spectral studles of energy transfer between polycycllc aromatlc hydrocarbons described here lndlcate that sodlum taurocholate (NaTC) Is less llkely than sodium dodecyl sulfate (NaDS) to promote photophyslcal lnteractlons. Fluorescence llfetlme studles and vlbronlc band ratlos of pyrene lndlcate that the probe microenvironments are less polar In NaTC than In NaDS. Interpretatlon of these results In terms of structural dlfferences between the bile salt NaTC and the detergent NaDS suggests that (1) the NaTC Interior Is more closely packed, less permeable to external solution, and better able to Isolate probes from each other and (2) NaTC Is preferable to NaDS for the sdublllzatlon of analytes In direct fluoromelrlc determinations.
INTRODUCTION Micellar reagents have been used in fluorescence analysis to solubilize hydrophobic molecules in aqueous solution (1-3). At low concentrations of solubilized molecules, it is unlikely that more than one molecule will bind per micelle. Multiple occupancy may occur a t higher concentrations, thereby increasing the probability of intermolecular interactions such as excited-state complex formation, complexation quenching,
* Author to whom correspondence should be addressed.
Present address: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409. 0003-2700/89/0361-1405$01.50/0
and nonradiative energy transfer. Inaccuracy in the determination of a fluorescent compound may arise from any of these processes. In the case of nonradiative energy transfer, error may occur if the analyte is located near another fluorescent molecule that has an excitation spectrum that overlaps with the emission spectrum of the analyte. We have been investigating sodium taurocholate (NaTC), a trihydroxy bile salt, as a micellar reagent for fluorescence analysis. The aggregation properties and physical characteristics of NaTC are quite different from those of conventional detergents and may prove to be preferable for solubilization of analytes. Specifically, NaTC aggregates are smaller and have lower aggregation numbers than do common detergent micelles, and are therefore less likely to bind multiple probes per micelle. In addition, the structure of the NaTC monomer ( 4 ) suggests a higher concentration of mass in a more rigidly structured micellar interior, which could provide NaTC with a greater ability to isolate solubilized molecules from the aqueous solution and from each other. In this paper, we describe studies of energy transfer between polycyclic aromatic hydrocarbon (PAH) molecules in aqueous NaTC and sodium dodecyl sulfate (NaDS) solutions, in order to compare the extent of energy transfer in the two different micellar media.
EXPERIMENTAL SECTION The NaTC (ULTROL grade, >98%) was purchased from Calbiochem,the NaDS (puriss, >99%) was from Fluka, perylene and NaCl (both gold label grade) were from Aldrich, phenanthrene, 9,lO-dimethylanthracene (DMA), benzo[k]fluoranthene (BkF), and pyrene were from Ultra Scientific, and 9-methylanthracene (9MA) was from Molecular Probes. All of the compounds were used without further purification. Stock solutions 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
Table I. Energy-TransferFarameters( 6 ) and Aqueous Solubilities ( 7 ) of the PAH Compounds
donor phenanthrene phenanthrene phenanthrene pyrene pyrene 9MA
BkF (I
spectral Ro, C,, overlap, acceptor 8, mM cm6/mol DMA 9MA 23 SPA 24 perylene 33
37 32 12
8.59 11.20 31.34
9
46.46
BkF perylene 36 perylene
solubility,” &f
7.210.27 7.211.4 0.72/0.0012 0.72/0.003 1.4/0.0012 0.003/0.0012
Aqueous solubilities of donor/acceptor.
of the PAH compounds were prepared in absolute ethanol. All aqueous solutions were prepared in demineralized, HPLC-grade water. Stock solutions of NaTC and NaDS in water, with or without NaCl, were prepared fresh each day. Solutions of the PAH compounds in NaTC or NaDS were prepared by gently evaporating the ethanol from the appropriate volume of the PAH stock solution, diluting with the NaTC or NaDS stock solution in a volumetric flask, and sonicating for at least 1 h. Solutions were not deoxygenated. Fluorescence spectral and lifetime measurements were made with an SLM 48000s multifrequency, phase modulation specwith a 450-W trofluorometer (SLM Instruments, Inc., Urbana, E), xenon arc lamp source and photomultiplier tube detectors. The sample chamber was maintained at 25.0 A 0.1 OC. Fluorescence excitation and emission spectra were collected with 1-nm scanning intervals, in the “10-average”mode, in which each measurement is the average of 10 samplings made over a 3-s interval. For each donor/acceptor pair in NaTC or NaDS, spectra were recorded for the donor, the acceptor, and the donor/acceptor mixture. The individual spectra of the donor and the acceptor were subtracted from the spectrum of the donor/acceptor mixture, producing a fluorescence difference spectrum. Fluorescence lifetimes were determined from the phase shift and demodulation of the emission signal,relative to the modulated excitation beam, at three or more modulation frequencies (5). Each lifetime was determined in the “100-average”mode (100 samplings averaged over a 30-s interval), and each reported lifetime is the average of five determinations.
RESULTS AND DISCUSSION The donor/acceptor pairs were selected on the basis of nonradiative energy-transfer probability, which can be expressed in terms of several parameters, including Ro, which is the mean distance between donor and acceptor a t which the probability for nonradiative energy transfer is equal to the probability of emission, Co, which is the concentration of acceptor at which there is an average of one acceptor molecule within a sphere of radius Ro about the donor, and the overlap between the emission spectrum of the donor and the excitation spectrum of the acceptor. The relative orientation of the transition dipoles of the donor and acceptor molecules is also an important factor. In micellar systems, the probability of energy transfer will depend upon the location and concentration of the donor and acceptor in the micelles and on their orientation and freedom of motion a t the binding sites. Aqueous solubility of the donor and acceptor may be an important factor in determining their relative location and distribution within the micellar solutions and could therefore also affect the energy-transfer probability. Energy-transfer parameters (6) and aqueous solubilities (7) are listed in Table I for the donor/acceptor pairs used in our studies. Energy Transfer in NaTC and NaDS and Dependence on Acceptor Concentration. Spectra are shown in Figure 1 for five different donor/acceptor pairs, each with a given donor concentration and three or more different acceptor concentrations, in NaTC and NaDS. The concentration of the micellar reagents, expressed in terms of total monomer added, is 30 mM for both NaDS and NaTC, which is well
above the critical micelle concentration (cmc) for both reagents (8.1mM for NaDS (8)and ea. 12 mM for the “quasi-cmc” of NaTC (9,101). The spectra for a given donor/acceptor pair are all normalized to the spectrum of the donor in NaDS to eliminate effects due to variations in quantum yield and molar absorptivity in the different media and are shown on the same intensity scale to permit direct comparisons between spectra in NaTC and those in NaDS. As discussed above, each spectrum is the difference between the mixture spectrum and the spectra of the individual components (donor and acceptor in NaTC or NaDS, a t the same concentrations as in the mixture). The difference spectrum, therefore, indicates only those spectral features that result from mixing the donor and acceptor together and does not include any fluorescence produced by direct excitation of the acceptor a t the wavelengths used to excite the donor. The subtraction also eliminates any contributions due to reagent impurities. The difference spectra contain two different regions, corresponding to the emission spectrum of the donor and, at longer wavelengths, the emission spectrum of the acceptor. In the event of nonradiative energy transfer, the intensity of the emission spectrum of the donor decreases with increasing acceptor concentration, and this portion of the difference spectrum becomes increasingly negative. Other processes that may contribute to a negative difference include (1)a filtering effect, in which the acceptor exhibits significant absorption a t the wavelengths used to excite the donor, resulting in a decrease in the exciting light available to the donor, (2) radiative transfer, i.e., reabsorption by the acceptor of light emitted by the donor, and (3) competition for micellar binding sites, resulting in displacement of donor molecules by acceptor molecules. The region of the difference spectrum corresponding to the emission spectrum of the acceptor is zero in the absence of acceptor and, in the event of energy transfer, increases as acceptor concentration increases. It is interesting that all of the difference spectra have negative regions in the presence of acceptor, but not all of them have significant positive regions. For example, the 9MA/perylene system does not exhibit significant acceptor fluorescence, despite its having very fworable characteristics for energy transfer (long Ro,low Co, and high spectral overlap) and a large negative difference at the emission wavelengths of 9MA. The absence of nonradiative energy transfer in this system could be due to (1) a large separation between the donor and acceptor molecules, especially in light of the 1000-fold difference between their aqueous solubilities, (2) an unfavorable relative orientation of their transition dipoles in the micellar aggregates, or (3) displacement of the donor from the micelle by the much less soluble acceptor. In terms of analytical applications of micellar solubilization, the most significant observation from Figure 1 is that the magnitudes of both the negative and positive portions of the difference spectra are greater in NaDS than in NaTC, indicating that solubilization in NaDS is much more likely than in NaTC to promote energy transfer and other photophysical interactions between fluorescent molecules. Effects of Micelle Concentration and NaC1. Parts a-d of Figure 2 show the difference spectra for the phenanthrene/9MA system at two postmicellaf concentrations of both NaTC and NaDS. Significant energy transfer is observed in the phenanthrene9MA system, despite relatively unfavorable Ro, Co, and overlap parameters; perhaps the very similar solubilities of the donor and acceptor promote energy transfer in the micelles by locating the two molecules in close proximity. Again, as was the case for the systems shown in Figure 1, energy transfer is significantly greater in NaDS than in NaTC; this is true at both concentrations of NaTC and NaDS.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
Emission Wavelength
1407
(nm)
Figure 1. Difference spectra of donor/acceptor systems. Acceptor concentrations in the key correspond to spectra, listed in order of increasing negativelpositive spectral magnitudes. Key: (a) phenanthrene (2.5 pM)/9PA (1, 5, 10 pM), in 30 rnM NaDS; (b) donor/acceptor as in a, in 30 rnM NaTC; (c) pyrene (10 pM)/perylene (2, 10, 20 pM), in 30 rnM NaDS; (d) donor/acceptor as in c, in 30 rnM NaTC; (e) BkF (2.5 pM)/perylene (1, 5, 10 pM), in 30 mM NaDS; (f) donor/acceptor as in e, in 30 rnM NaTC; (9) 9MA (2.5 pM)/perylene (0.5, 1.0, 2.5, 5.0, 7.5 pM), in 30 rnM NaDS; (h) donor/acceptor as in g, in 30 rnM NaTC; (i) pyrene (5.0 pM)/BkF (1, 5, 10 pM), in 30 rnM NaDS; (i) donor/acceptor as in i, in 30 rnM NaTC.
For both NaDS and NaTC, the magnitude of the positive portion of the difference spectrum is smaller a t the higher concentration of the micellar media, indicating a lower degree of energy transfer a t the higher concentration. As micellar concentration increases, the number and/or size of the micelles increases, thereby reducing the number of molecules solubilized within a single micelle and increasing the average dis-
tance between donor and acceptor. The magnitudes of the negative portions of the difference spectra, which correspond to phenanthrene emission, are slightly less at the higher concentration. The spectral features in the negative regions are slightly different at the two concentrations, indicating a difference in donor microenvironment. Figure 2e shows the phenanthrene/gMA system in absolute
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
I
1
1
I
I
1
Emission Wavelength (330
-
I
520 nm)
Figure 2. Difference spectra of phenanthrene donor (2.5 pM)/SMA acceptor (1, 5, 10 pM, in order of increasing spectral magnitude). Key: (a) 15 mM NaDS; (b) 30 mM NaDS; (c) 15 mM NaTC; (d) 30 mM NaTC; (e) ethanol: ( f ) 15 mM NaTC and 0.10 M NaCI; (9) 30 mM NaTC and 0.10 M NaCI; (h) 15 mM NaTC and 1.0 M NaCI; (i) 30 mM NaTC and 1.0 M NaCI.
ethanol, in the absence of micellar media. The magnitude of the negative difference is much neater in ethanol thnn in nnv of the micellar solutions, indicating that the acceptor is absorbing at the donor excitation wavelengths (the filter effect, discussed above). No energy transfer is evident in ethanol, which suggests that energy transfer in the micellar media is a nonradiative process. Parts f-i of Figure 2 show the effect of NaCl on the phenanthrene/9MA system in NaTC. The stabilizing effect of
NaCl on NaTC aggregation has been well-described (11) and is nn imnnrtant mneidnratinn fnr AnQioninu anolT+ionl
nv-
periments in which NaTC is used for solubilization. At 15 mM NaTC, which is just above the quasi-cmc region, NaCl significantly increases the number and/or size of the NaTC aggregates, thereby decreasing the probability of nonradiative energy transfer and reducing the magnitudes of both the negative and positive portions of the difference spectrum. This is different from the effect of increasing NaTC concentration
ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
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Table 11. Fluorescence Lifetimes of Donor in the Presence of Acceptor in 30 m M Micellar Solutions donor,
acceptor,
MM
MM
BkF 2.5
perylene -
10.6
11.2
11.1
5.0
10.7 10.7
10.0
10.7
1.0
pyrene 10.0
fluorescence lifetime, ns NaDS NaTC
perylene 2.0
10.0 20.0
PFene
BkF
5.0
1.0 5.0
10.0
11.5 11.1
165 161 163 164
290 290 290 291
162 163 161 163
292 292 292 292
Table 111. Vibronic Band Intensity Ratio of Pyrene in the Presence of Acceptor in 30 mM Micellar Solutions' donor, fiM
acceptor, MM
pyrene 10.0
perylene 2.0
10.0 20.0
pyrene 5.0
BkF 1.0 5.0
10.0
fluorescence lifetime, ns NaDS NaTC 1.194
0.946
1.206 (1.0) 1.219 (2.1) 1.221 (2.3)
0.952 (0.6) 0.953 (0.7) 0.946 (0.0)
1.136 1.138 (0.2) 1.150 (1.2) 1.165 (2.6)
0.940 0.941 (0.1) 0.949 (1.0) 0.954 (1.5)
aIntensity ratio of band I (373 nm) to band I11 (384 nm). The percent change in the ratio, relative to the ratio in the absence of acceptor, is in parentheses.
Emission Wavelength (330 550 nm)
-
Flgure 3. Difference spectra of phenanthrene donor/DMA acceptor (3 pM each, unless otherwise noted). Key: (a) 15 mM NaDS; (b) 15 mM NaTC; (c) 25 mM NaTC; (d) 50 mM NaTC; (e) 15 mM NaTC and 0.50 mM NaCI; (f) same as e but w& 5 p M phenanthrene, 5 p M DMA.
in the absence of NaCl (Figure 2c,d), which decreased the magnitude of the positive portion only. It would appear that the aggregates formed in the 15 mM NaTC solution with NaCl are different from those formed in the 30 mM NaTC medium. Effects of NaCl are less noticeable in the 30 mM NaTC solution, which is well above the cmc and less susceptible to the influence of NaC1. Figure 3 shows the difference spectra for phenanthrene/ DMA in 15 mM NaDS, 15 mM NaTC, 25 mM NaTC, and 50 mM NaTC. Spectra are also shown for two concentrations of probe in 15 mM NaTC solutions containing NaCl. A high degree of energy transfer occurs in NaDS, while none is evident in the same concentration of NaTC. Either increasing NaTC to 25 mM or adding NaCl to the 15 mM solution produces
a small amount of energy transfer, which is increased further in the latter case by increasing the probe concentration. No energy transfer is observed in 50 mM NaTC. Apparently, within a certain NaTC concentration range, or in the presence of NaC1, the NaTC aggregates are able to bind the donor and acceptor within the distance and orientation limits required for energy transfer. Fluorescence Lifetimes and Vibronic Band Ratios. The fluorescence lifetimes of the donor are shown in Table I1 for three different donor/acceptor systems. For all three systems, the donor lifetimes are the same in the absence and presence of acceptor, over the entire range of acceptor concentrations studied. This is not surprising, since energy transfer does not occur in any of the systems except pyrene/perylene, which exhibits a small degree of energy transfer in NaDS. More importantly, the donor lifetimes are significantly longer in NaTC than in NaDS, indicating a difference in the microenvironment of the donor between the two micellar media. A difference in microenvironment between NaTC and NaDS is also indicated by measurements of the vibronic band intensity ratio (band I to band 111)of pyrene, which increases as the polarity of the pyrene microenvironment increases (12). The ratios, shown in Table 111, indicate that the pyrene microenvironment is more polar in NaDS than in NaTC. In both micellar media, the band ratio increases as acceptor (perylene
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Anal. Chem. 1989, 6 1 , 1410-1414
or BkF) is added to the system. The percent increase is greater in NaDS than in NaTC, which is further evidence of the greater degree of interaction between donor and acceptor in NaDS than in NaTC.
CONCLUSIONS Of the seven donor/acceptor systems studied in this work, four exhibited energy transfer; in all of the latter cases, energy transfer was promoted to a much greater degree in NaDS than in NaTC. Comparison of the structures of NaDS and NaTC monomers suggests an explanation for the difference between the two media. Typical detergents, such as NaDS, have a hydrophilic head group and a long, hydrophobic tail. Micelles formed by such detergents in aqueous solution are often spherical, with the head groups at the surface, in contact with the external solution, and the tails in the interior. The polarity of the interior depends on several factors, including the permeability of the micelle to the aqueous solution. One can imagine an interior of a large micelle that is only partially filled by the hydrophobic tails, in which the remaining space could be occupied by aqueous solution and solubilized molecules. In contrast, the NaTC monomer has a relatively small, hydrophilic “head” group and a bulky hydrophobic region. It is reasonable to expect the micellar aggregates of NaTC to have very different structures than those of detergents, with the NaTC structures having a greater internal concentration of micellar bulk, which could decrease the internal capacity for solution from the outside. This supposition is consistent with the experimental results described in this paper, namely, that energy transfer is less likely to occur in NaTC than in NaDS and that the microenvironment of probes solubilized in NaTC is less polar than the corresponding microenvironment in NaDS. Our results suggest that NaTC is preferable to conventional detergents such as NaDS for solubilization of analytes in direct
fluorometric determinations, in order to.minimize error due to energy transfer and other photophysical interactions. It is also important that NaCl has little effect on the energytransfer systems in 30 mM NaTC, indicating that the salt content of a sample will not be an important factor in the determinations. Clearly, it is important to use a sufficiently high concentration of NaTC, in order to completely solubilize the probes and minimize the number of probes per micelle. The presence of NaTC monomer in these solutions should not affect the solubilization or energy-transfer processes. The 30 mM NaTC solutions used in these studies appear to be adequate for total probe concentrations as high as 30 qM.
LITERATURE CITED Love, L. J. C.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1133A. Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 769, 1. Ramis Ramos, G.; Garcia Alvarez-Coque, M. C.; Berlhcd, A,; Winefordner. J. D. Anal. Chim. Acta 1988, 208, 1. Small, D. M. I n Molecular Association in 6iologica1 and Related Systems; Gould, R. F., Ed.; Advances in Chemistry Series 84; American Chemical Society: Washington, DC, 1968; pp 31-52. Spencer, R . D.; Weber, G. Ann. N. Y . Acad. Sci. 1969, 158, 361. Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds; Academic Press: New York, 1973. Pearlman, R. S.;Yalkowsky, S.H.; Banerjee, S.J. Phys. Chem. Ref. Data 1984, 73, 555. Hinze. W. L. I n OrderedMdia in Chemical Separations;Hinze, W. L., Armstrong, D. W., Eds.; American Chemical Society: Washington, DC, 1987; p 4. Kallay, N.; Colic, M.; Simeon, V.; Kratochvil, J. P. Croatica Chem. Acta 1987, 6 0 , 555. Meyerhoffer, S. M.; McGown, L. B., unpublished results. Kratohvil, J. P.; Hsu, W. P.; Jacobs, M. A,; Amlnabhavi, T. M.; Mukunoki, Y. ColloidPolym. Sci. 1983, 267, 781. Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 87.2176.
RECEIVED for review January 13,1989. Accepted March 15, 1989. This work was supported by the United States Department of Energy (Grant No. DE-FG05-99ER13931).
Flow Injection Donnan Dialysis Preconcentration of Cations for Flame Atomic Absorption Spectrophotometry John A. Koropchak* and Lori Allen Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901-4409
The sample loop of a conventlonal high-performance liquid chromatography Injector Is replaced by a coil of tubular cation-exchange tublng, enabling Donnan dlalysis to be performed under statlc condltlons while allowing enrlched samples to be InJected Into a flame atomlc absorptlon (FAA) spectrometer at optlmum nebullzer flow rates. The recelver solutlon, contalnlng a hlgh percentage of dissolved sollds, Is only introduced Into the flame for short thnes, whlch permlts hlgher recelver concentratlons to be used. Short tublng lengths provlde hlgher enrichment, wHhln the Ilmits of sample dlsperslon, provldlng a compact dlalysls cell. Decreasingthe cation-exchange-membrane thfckness reduces the dlalysls thne requked for optlmum enrlchment. For the thln-wal tublng employed hereln, Smln dlalyses provlde 100-fold enrkhment and limit-of-detectlon (LOO) Improvement factors. The approach Is demonstrated for the trace determlnatlon of lead In drinking water.
* Author
to whom correspondence should be sent.
When an ion-exchange membrane separates a high ionic strength solution from a low ionic strength solution, ions of appropriate charge for the membrane are transported from the more concentrated solution to the more dilute solution. Since the membrane is impermeable to co-ions, ions from the dilute solution must diffuse to the more concentrated solution via a process termed Donnan dialysis in order to maintain electroneutrality ( I ) . If the volume of the high ionic strength solution (receiver) is small compared to that of the low ionic strength solution (sample), enrichment of the dilute ions results (2). This process has been shown to provide essentially matrix independent cation enrichment for samples of low to moderate ionic strength (2, 3) for both flat ( 4 ) and tubular ( 5 ) cation-exchange membranes. Tubular membranes are particularly advantageous since they have high surface area to internal volume ratios and are readily interfaced to various detectors in on-line fashion. Modest success was initially reported for coupling tubular Donnan dialysis on-line with flame atomic absorption (FAA) (6);more recently, detailed charcterization of this approach
C 1989 American Chemical Society 0003-2700/89/0361-1410$01.50/0
