1410
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
ANALYTICAL CHEMISTRY. VOL. 61. NO. 13. JULY 1. 1989
demonstrated signal enhancement factors exceeding 20 with 5-min dialyses for a variety of cations (7). Enrichment factors were shown to increase with tubing length, lower rereiver pHs, and temperature (7J. Further, the analyte was concentrated into a normalized matrix. in which easily ionizable elements W E )could he included for ioniiation suppresqion (7). Finally, since co-ions are rejected with Donnan dialysis. interferences such as PO;+ on Caz' could be alleviated in an on-line fashion (7). In a later study, this approach was employed with inductively coupled plasma atomic emission spectrometry (ICP-AKSJ; high signal enhancement factors (>50) and alleviation of intra-alkali interferences were reported (8).A further characteristic of this approach w a s the minimal additional hardware requirementq for conducting these experimenta (Le. the tubing, a peristaltic pump, and a stirrer) compared to that required for the normal FAA or ICP-AES experiments. \Vith either of these previous on-line experiments, rereiver solution w&s amtinuously pumped through the tubing (bathed in sample solution) and into the nehulizer of the atomizer. Sine typical receiver solutions contain 3-10s dissolved solids, this continuous introduction may periodically result in nebulizer, burner, or torch blockage (8).In addition, Donnan dialysis is optimized under static conditiuns while typical nehulizers are optimized at flow rates of 1-10 mL/min. Cunsequently, compromised flow rates were required for the on-line experiments ( 7 , R ) . In addition. enrichment factors inneased with tuhing length, which necessitated larger sample volumes and a hulky dialysis cell. Finally, typical turnaround analysis times were on the order of 20 min (7.8). Described herein is a flow injection approach to the Donnan dialysis FAA experiment, which is intended to overcome these less desirahle characteristics. The coiled, tubular membrane is used as a direct replacement for the injection loop of a conventional sample injector. In this manner, the dialysis is conducted under static flow conditions, while the preconcentrated sample is injected at the optimum nebulizer flow rate and the receiver is introduced only during the injection. Characterization of relevant operating parameters and applications of the optimized experiment to drinking water analysis are described. EXPERIMENTAL SECTION The cation-exrhange membrane employed for must nf the Dunnan dialysis experiments was 0.64 mm i.d. x 0.89 mm o.d. tubing of various lengths made of Nafion 81I (Du Pont Polymer Prnducw, Wilmington, DE). For some experiments, a thin-wall Nafion membrane (0.33mm i.d. X 0.48 m m 0.d.J obtained from Perma-Pure Pioducts (Farmingdale. N J I was employed. The dimensions were for the dry tuhing. I n either case. the tubulnr membrane was affixed to a Hheodyne 7125 sample injector HS depicted in Figure I . The membranes were loosely coiled (tu maximize solution contact) around a three.prong holder; the tubing ends were inserted into short lengths of 1.6 m m 0.d. tetrafluoruethylene (TFEi tuhiny of appropriate inner diameter to f i t snugly around the chosen memhrane and were connected tu the injector hy using 1.6-mm Kel-F fingertight fittings from Lpchurch Scientific (Oak Harbor. WAi. When the compression type fittings me tightened, effective seals are made, preventing leakage at the operating pressures of the experiment, even with > 3 ml. min of liquid flow. Atumic absorption ineawrements were made hy wing a Varian AA-473 with the nutomatic gas control unit set at I3 L, min air and 1.7 Limin wetylene for Ph analvses nnd 13 I./min air and 1.4 L/min'acetylenefor Cu analyses. Analytical wavilengths were 217.0 nm fur Pb and 224.7 nm fur c'u. To begin a dialysis experiment, the injector was placed in the LOAD mode and the tubular membrane was manually flushed and filled with the receiver solution (typically 0.5 M Sr(NO&, 1.20 mM AI(N0,). and 0.1 M HNO, in deionized, distilled water (DDW))by using a syringe. The dialysis cell could then be lowered into the stirred sample solution and the dialysis conducted for
* 1411
LOAD
n
CARRIER SOLUTION FLAME +
.
NAFION
TUBING
SAMPLE SOLUTION STIRBAR
Flgure 1. Diagram of the flow injection Donnan dialysis cell.
the appropriate time prior to injection. Carrier solutions were metered with a pulse-dampened Beckman llOB pump. During a dialysis, pure earrier solution was provided to the FAA nebulizer; 5.4 mL/min was typically employed since this was close to the optimum flow rate for direct aspiration. In the INJECT mode the carrier solution was directed through the tubular memhrane, flushing the dialysate inta the FAA and preparing the membrnne for the next dialysis. In general, the carrier solution was pumped through the membrane until the signal returned to the base line. During this flushing, the outside of the membrane was rinsed with DDW. All solutions were prepared from analytical reagent grade (or better) salts or acids dissolved in DDW or dilute HN03. All glassware was scrupulously cleaned to include a final soak in dilute HNO, followed by a DDW rinse. The criteria employed for the selection of cations comprising the receiver solution have been detailed earlier (7). Limits of detection reported herein are based on 3 X (r signal levels. RESULTS AND DISCUSSION The evaluation of the experiments described herein is typically reported in terms of enrichment factors (EFs), calculated as the ratio of the signal for the dialysis to the signal for the direct aspiration of the same sample. More precisely, an enrichment factor is defined as the ratio of the analyte concentration in the dialysate to that in the original sample (5). Since all of the EFs in this study were based on measurements of Cu2+,for which signals were not affected by the receiver matrix, the more precise definition i s maintained. For certain measurements where dispersion was shown to influence signals, the use of the E F term is less a m a t e . For consistency however, the EF term was employed throughout the study. One of the early observations for the flow injection Donnan dialysis (FIDD) FAA experiment was that signals typically exhibited a bimodal peak shape with high time resolution, as shown in Figure 2. These different signals result from essentially two different analyte forms after dialysis: one corresponding to analyte in the receiver solution and a second corresponding to d y t e that is &xed to the membrane. The solution-phase analyte is quickly flushed into the FAA upon injection, while ion-exchange reactions are required to remove the bound analyte fraction. If a low ionic strength receiver solution is employed &e. DDW), no Donnan dialysis can occur and the first peak disappears. However, the second peak,
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 13, JULY 1, 1989
Table I. Receiver Composition Effects on Enrichment Factors for Cu2+"
-
-
15Seconds
TIME
Flgure 2. Raw data depicting bimodal signals for 5-min Donnan dialysis of 50 ng/mL Cu2+ using 3.25 m of thin-wall Nafion tubing: carrier, 5.4 mL/min 1.0 M "0,;receiver, 0.5 M S T ( N O ~ )1.3 ~ , mM AI(NO,),, 0.1 M "0,; 350-mL sample volume.
receiver solution composition
enrichment factor
0.20 M Sr(N03)2,0.5 mM Al(NO3),, 0.1 M HNO, 0.36 M Sr(N03)2,0.9 mM A1(N03),, 0.1 M HNO, 0.52 M Sr(NO& 1.3 7M A1(NOJ3, 0.1 M "0, 0.68 M Sr(N03)2,1.7 mM A1(N03),, 0.1 M HNO, 0.5 M 1.3 mM A1(N03),, 0.1 M HN03 0.5 M Sr(N03)2,1.3 mM A1(N03),, 0.2 M HNO, 0.5 M &(NO&, 1.3 mM Al(N03)3,0.3 M HNO, 0.5 M Sr(N03)2,1.3 mM Al(NO,),, 0.4 M HNO,
18.7 28.0 31.8 31.7 28.1 28.2 29.2 25.5
Five-minute dialyses of 250 ng/mL Cu2+from 400-mL samples using a 5-m coil of thick-wall tubing; carrier flow was 5.4 mL/min 0.1 M "0,.
50
Table 11. Carrier Stream and Receiver Effects on Enrichment Factors and Analysis Times" P
I
carrier compn
I
2
0
~
"
'
"
'
"
'
"
'
"
' ' ' 10 15 T I M E , MINUTES
"
20
"
.
1
25
receiver compn
0.2 M Sr2+,0.5 mM 0.2 M Sr2+,0.5 mM ~ 1 3 + 0.1 , M A13+, 0.1 M HNO, HNO, 0.1 M HNO:, 0.2 M Sr2+,0.5 mM A13+,0.1 M HNO, 1.0 M HNO, 0.2 M Sr2+,0.5 mM A13+,0.1 M HNO, 1.0 M "03 0.52 M Sr2+,1.3 mM A P , 0.1 M HNO,
enrichment factor
anal. time, min
38.7
>> 8
25.9
>>8
22.5
8
46.7
