Anal. Chem. 1998, 70, 3906-3911
Incomplete Trapping in Supported Liquid Membrane Extraction with a Stagnant Acceptor for Weak Bases Luke Chimuka, Negussie Megersa,† Jan Norberg, Lennart Mathiasson, and Jan A ° ke Jo 1 nsson*
Analytical Chemistry Department, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
Supported liquid membrane extraction is a powerful analytical methodology for continuous extraction and enrichment of ionizable organic pollutants from aqueous environmental samples. Sample workup with this technique can be hampered by low enrichment factors due to incomplete trapping in the stagnant acceptor solution for weak basic compounds. The different parameters, such as acceptor pH and ionic strength of both the donor and acceptor solutions, related to this problem were studied theoretically and experimentally. A simple equation for the maximum enrichment factor was developed. Results showed that the ionic strengths of the two aqueous phases in the system have an effect on the maximum enrichment factor, which can be increased by increasing the ionic strength of the donor phase. For chloro-s-triazines, a satisfactory agreement was obtained between the experimental measurements and theoretical estimations. This study defines the practical analytical applications and limitations of the system where the analytes are partially ionized in the acceptor solution. The use of liquid membranes for selective removal of ionizable organic compounds from aqueous solutions is a very promising alternative to liquid-liquid extraction and solid-phase extraction methods because of the high selectivity and relatively high flux values that can be obtained. One such approach originally described by Audunsson1 is the supported liquid membrane (SLM) extraction technique, where an organic liquid is immobilized in a thin porous membrane which forms a barrier between two aqueous phases. Compared to polymeric membranes, this offers high mass-transfer rates and the possibility for a wide variety of organic liquids and additives without compromizing the stability of the membranes. The technique has been used for sample preparation in a number of applications.2-5 Typically, the uncharged analytes are extracted from the flowing * Corresponding author: (fax) +46 46 222 45 44; (e-mail) jan•ake.jonsson@ analykem.lu.se. † Permanent address: Department of Chemistry, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia. (1) Audunsson, G. Anal. Chem. 1986, 58, 2714-2723. (2) Knutsson, M.; Nilve´, G.; Mathiasson, L.; Jo ¨nsson, J. A° . J. Chromatogr., A 1996, 754, 197-205. (3) Lindegård, B.; Bjo¨rk, H.; Jo¨nsson, J. A° .; Mathiasson, L.; Olsson, A. M. Anal. Chem. 1994, 66, 4490-4497. (4) Pa´lmarsdo´ttir, S.; Thordarson, E.; Edholm, L. E.; Jo ¨nsson, J. A° .; Mathiasson, L. Anal. Chem. 1997, 69, 1732-1737.
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aqueous donor stream into the organic phase followed by a second extraction into the acceptor phase, which is usually stagnant. The detailed general theory of the SLM extraction process with a stagnant acceptor has been thoroughly described by Jo¨nsson et al.6 Several parameters govern the extraction in such a system. One of these is the pH of the acceptor phase. The acceptor pH is seen to be critical and is ideally set so that the analyte molecules are completely ionized and therefore trapped, i.e., prevented from re-entering the membrane. If such a condition is met, the extraction efficiency is constant with time and the enrichment factor increases virtually infinitely. In particular, the conditions for complete trapping of the extracted analytes in the stagnant acceptor in ionized form require that the acceptor pH is at least 3.3 pH units below the pKa of basic analytes.6 Recently, in our laboratory, the application of the SLM technique for sample preparation of different types of weak bases has been investigated. Studies undertaken for the chloro-s-triazine and some aniline derivatives have revealed that the extraction efficiency in such a system decreases with time 7,8 so that the enrichment factor increases only to a certain limit. The reason for this is incomplete trapping in the stagnant acceptor of the very weakly basic analytes (pKa e 2). Thereby, complete ionization cannot be fully accomplished. The maximum enrichment possible is further limited due to differences in partition coefficients between the two aqueous phases as shown in this paper. The objective of this study is therefore to investigate the effects on maximum enrichment in the case where the requirement for the acceptor-phase pH cannot be fully met and to define the factors that limit the extraction in an SLM system where the analytes are partially trapped. With this background information, it was anticipated that a general solution would be found which will extend the analytical applications even under partial trapping of the analytes in the acceptor phase. THEORY The extraction efficiency,6 E, is defined as the fraction of analyte extracted from the donor phase into the acceptor phase. (5) Djane, N.; Bergdahl, I. A.; Ndungu, K.; Schutz, A.; Johansson, G.; Mathiasson, L. Analyst 1997, 122, 1073-1077. (6) Jo¨nsson, J. A° .; Lo¨vkvist, P.; Audunsson, G.; Nilve´, G. Anal. Chim. Acta 1993, 277, 9-24. (7) Chimuka, L.; Nindi, M. M.; Jo ¨nsson, J. A° . Int. J. Environ. Anal. Chem., in press. (8) Norberg, J.; Zander, A° .; Jo ¨nsson, J. A° . Chromatographia 1997, 46, 483488. S0003-2700(97)01327-9 CCC: $15.00
© 1998 American Chemical Society Published on Web 08/14/1998
Figure 1. Different compartments of the SLM extraction unit. For explanation of the labels, see text.
Usually, the extraction conditions are set so that RD is close to 1 and RA is a very small value. cA is zero from the beginning of the extraction and increases successively, usually to values well over cD. Thus, ∆C will decrease as the extraction is going on and eventually approach zero, which leads to the following expression for the maximum concentration enrichment factor (as cI and cW are then equal):
Ee(max) ) (cA/cI)max ) (RDKD)/(RAKA) It is a measure of the rate of mass transfer through the membrane which is constant at specified extraction time, flow rate, phase composition, and ionic strength. It is given by
E ) nA/nI ) (cAVA)/(cIVI)
(1)
(7)
If KD is assumed to be equal to KA, which is the case if the compositions of two aqueous phases are similar, then
Ee(max) ) (cA/cI)max ) RD/RA
(8)
or
E ) 1 - nW/nI
(2)
Here, nA and nI are the total amounts of analyte found in the acceptor and present in the incoming (extracted) sample, respectively; cA and cI are the corresponding concentrations. VA is the volume of the stagnant acceptor phase, and VI is the volume of the extracted sample that has passed through the donor channel. nW is the total number of moles in the donor waste accumulated from the start of the experiment. The extraction efficiency can be calculated from experimentally measured quantities using both or either of these equations. The possible presence of a significant fraction of the analyte dissolved in the membrane or being adsorbed to various surfaces can be estimated by comparing the extraction efficiency determined using the above equations. The concentration enrichment factor, Ee, is defined as follows (see also Figure 1):
Ee ) cA/cI
(3)
which together with eq 1 leads to
Ee ) EvI/vA ) (nAVI)/(nIVA)
(4)
Ee ) (1 - nW/nI)VI/VA
(5)
or
The rate of mass transfer and thus E is proportional to the concentration difference, ∆C, over the membrane, which can be written6
∆C ) RDcD - RAcAKA/KD
(6)
where RD and RA are the fractions of the analytes that are in extractable (uncharged) form in the donor and acceptor phases, respectively. cD is the mean concentration in the donor phase and approximately equal to the mean of cI and cW. KA and KD are the partition coefficients for the analytes in the acceptor and donor phases, respectively.
However, if the ionic strengths of the donor and acceptor phases are very different, this assumption may not be valid. The differences in partition coefficients may contribute significantly to the maximum enrichment factor possible as observed in the present work. The rate of mass transfer is constant only when ∆C is constant, i.e., when RDcD . RAcAKA/KD. Thus, for the purpose of reproducible analytical application of SLM extraction, it is necessary that RA is very low. It was found that if RA < 0.0005,6 the change in mass-transfer rate, and thus extraction efficiency, is negligible. For basic analytes, as in the present study, this corresponds to the condition that pH in the acceptor phase should be at least 3.3 units lower than the pKa of the least basic analyte. If this condition cannot be met, as is the case with some of the compounds studied here, incomplete trapping is experienced, which limits the linearity of extraction efficiency versus time, as well as the maximum enrichment factors attainable. EXPERIMENTAL SECTION The detailed experimental procedures in this work for chloros-triazines, alkylthio-s-triazines, and aniline derivatives are all described elsewhere.7-9 For chloro-s-triazines,7 a Waters 600 LC pump (Waters Assoc., Milford, MA) was used to pump the mobile phase for reversed-phase separation connected to an automatic sample injection system (Waters Assoc., WISP, model 710B). The analytical column was a C18 column, 5 µm × 250 mm × 4.6 mm Techsphere (Macclesfield, United Kingdom) followed by UV detection (Waters Assoc., model 480) at 235 nm. A mobile phase consisting of 50% acetonitrile and 50% 0.05 mol/L sodium acetate adjusted to pH 7 with 0.5 mol/L sulfuric acid at a flow rate of 1.0 mL/min was used. For alkylthio-s-triazines,9 a similar system was used, however, with an Iso Chrom LC pump (Spectra Physics, San Jose, CA) and a Kratos Analytical Instruments (model 757, Ramsey, NJ) UV detector. The aniline derivatives8 were determined using an automated LC system consisting of two model 600 HPLC pumps (Waters Assoc.) and an autosampler (WISP, model 710B). The LC system was controlled by a Millennium 2.15 (Waters chromatographic computer system) and was connected to the SLM system via a 10-port high-pressure valve (Valco, Houston, TX). (9) Megersa, N.; Jo¨nsson, J. A° . Analyst 1998, 123, 225-231.
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A typical membrane unit used consisted of two circular poly(tetrafluoroethylene) (PTFE) blocks (diameter 120 mm, thickness 8 mm) with grooves arranged as an Archimedes spiral, (depth 0.25 mm, width 1.5 mm, length 2.5 m) each with a total volume of ∼1.0 mL. Aluminum blocks with 6-mm thickness were used on both sides of the PTFE blocks to stabilize the construction. Preparation of the supported liquid membrane was usually done by soaking a porous PTFE membrane with pore size 0.2 µm, total thickness of 175 µm with 115 µm polyethylene support and a porosity of 0.70 (Millipore FG, Bedford, MA) in an organic solvent to be immobilized for a period of 30 min. The membrane was then placed between the two PTFE blocks with the rough side of the membrane facing the donor side. The whole construction was clamped tightly together with eight screws. After the membrane had been placed between the PTFE blocks and clamped, any excess organic solvent on the outside of the membrane was flushed by pumping 20 and 10 mL of water through the donor and the acceptor channels, respectively. The flow system typically used for triazine compounds7,9 consisted of two peristaltic pumps (Minipuls 3; Gilson Medical Electronics, Villiers-Le-Bel, France) which were used to pump solutions at constant flow in acid-resistant tubing (Acid-Flexible; Elkay Products, Shrewsbury, MA) with internal diameters of 2.0 mm for the donor solution and 1.0 mm for the acceptor solution. The flow system was connected with 0.5-mm-i.d PTFE tubing and Altex screw fittings. A mixing coil and a tee connector used in the experimental setup were also made of PTFE. For the aniline derivatives, a similar setup was used, however, automated and connected on-line as described in ref 8. All experiments were repeated at least two times. Determination of the Enrichment Factors and Partition Coefficients. The enrichment factor at specified times during each extraction was followed and calculated according to eq 5 by measuring the concentration of the analytes in the donor solution entering the membrane unit and that leaving the unit while the acceptor was kept stagnant. The influence of the increase in ionic strength for the analytes in the donor solution onto the enrichment factor was studied with the sample solution at ionic strengths of 0.1, 0.7, 1.6, and 3.1 by additions of 0.0, 0.2, 0.5, and 1.0 M Na2SO4, respectively. The partition coefficients for the analytes between the aqueous solutions and the organic solvent used as a membrane were found using batch extraction. Aqueous donor solutions were shaken in separatory funnels with dihexyl ether (in 5:1 volume ratio) for 15 min. The mixture was left to stand and the aqueous solution was separated. The concentration in the aqueous donor phase was measured before and after extraction using the LC system described above. To study the influence of the ionic strength in the donor solution on the partition coefficient, this procedure was repeated but with the different ionic strengths mentioned above. The partition coefficient of the analytes from the donor solution to the membrane was calculated using
KD ) ([A]D(i) - [A]D(f))/[A]D(i)
(9)
[A]D(i) and [A]D(f) is the concentration of the analyte in the aqueous donor solution before and after extraction, respectively. 3908 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
Table 1. pKa’s of the Studied Weak Bases and rA values at Different pH Values RA values at pH calcd by eq 12 compound
pKa
atrazine simazine terbuthylazine ametryn desmetryn dimethametryn terbutryn aniline 3-methyl-5-nitroaniline 3-chloro-4-methylaniline 3,5-dichloroaniline
1.6810 1.6510 2.010 4.111 4.011 4.011 4.311 4.61a 2.34a 3.97a 2.48a
∼1.0
∼0.7
∼0.0
9.13 × 9.52 × 10-2 4.57 × 10-2
2.04 × 10-2 2.13 × 10-2 9.8 × 10-3
10-2
6.82 × 10-4 9.22 × 10-4 6.82 × 10-4 4.62 × 10-4 2.26 × 10-4 4.21 × 10-2 9.86 × 10-4 2.96 × 10-2
2.42 × 10-5 4.51 × 10-3 1.06 × 10-4 3.27 × 10-3
a Calculated by the program ACD/pH (Advanced Chemistry Development, Inc. Toronto, Canada).
To ∼2 mL of the organic solution left in the funnel, 4 mL of 1 mol/L H2SO4 was added and then the resultant mixture was shaken as above. To 3.0 mL of the aqueous phase was added 1.2 mL of 7 M NaOH to raise the pH to 6.0, and its analyte concentration was determined by LC. This procedure was used to calculate the partition coefficients of the analytes from the acceptor solution to the organic solution according to
KA ) kD/RA
(10)
kD is the distribution constant of analytes between the organic liquid and the acceptor solution. kD and RA were calculated from the following equations:
kD ) [A]m/([A]a + [AH+]a)
(11)
RA ) Ka/([H+] + Ka)
(12)
[A]m is the equilibrium concentration of the analyte in the organic liquid. [AH+]a and [A]a are the equilibrium concentrations of the analyte in the ionized and nonionized forms in the aqueous acceptor solution, respectively. Ka is the dissociation constant of the analyte, and [H+] is the concentration of hydrogen ions in the acceptor solution. RESULTS AND DISCUSSION Optimization of the SLM Extraction. The extraction efficiency and selectivity of the SLM extraction is controlled by several parameters, such as the acceptor and donor pH and membrane composition, which have been studied elsewhere.6 The optimized factors given in refs 7-9 were used for the chloro-striazines, aniline derivatives, and alkylthio-s-triazines, respectively. For the basic compounds with pKa e 2 (see Table 1), the acceptor pH was found to be the limiting factor in attainment of high extraction efficiency because the condition for complete trapping (10) Cutt, R. C. H.; Schabacker, D. J. Mechanisms of pesticide movement into ground water; Lewis Publishers: Boca Raton, FL, 1991; pp 18-21. (11) Esser, H. O.; Dupius, G.; Vogel, C.; Marco, G. J. In Herbicides: Chemistry, Degradation and Mode of Action, 2nd ed.; Kearney, P. C., Kaufman, D. D., Eds.; M. Dekker: New York, 1976; Vol. II, pp 129-208.
Figure 2. (A) Variation of enrichment factor, Ee, with extracted sample volume of simazine (0.40 ppm each) at two concentrations of sulfuric acid as an acceptor solution: ([) 0.2 M H2SO4 (pH ∼0.7) (9) 1.0 M H2SO4 (pH ∼0.0) (B) (C) in Supporting Information.
in the acceptor solution was not fully met, which requires that the acceptor pH is at least 3.3 pH units below the pKa of the basic analyte.6 The extraction efficiency decreased with time as the partially ionized analytes increased in the stagnant acceptor solution (see details below). Influence of the Degree of Trapping on Enrichment Factor. The variation of enrichment factor with sample volume extracted at different concentrations of sulfuric acid in the acceptor solution was studied for chloro-s-triazines (Figure 2) and aniline derivatives (Figure 3). The results obtained are in good agreement with the developed theory that the amount extracted increases with increase in the degree of trapping for basic analytes.6 It is interesting to note (Figures 2 and 3) that, in the early extraction periods, there is insignificant difference in enrichment factors at two trapping solutions. However, as the extracted sample volume increases, this difference becomes more pronounced. This effect is more evident with simazine (Figure 2A), 3,5-dichloroaniline, and 2-methyl-5-nitroaniline (Figure 3), which are the least basic compounds (see Table 1 for pKa values). The difference in enrichment factors can be attributed to the increasing amount of untrapped analytes in the acceptor decreasing the concentration difference ∆C (cf. eq 6) and thus the analyte flux as the extraction continues. This effect is therefore expected to become more pronounced with the lowest trapping capacity solution, least basic compound, and at higher extracted sample volume. The degree of trapping could still be increased by using more acidic acceptor solution (more than 1 M sulfuric acid). The disadvantage with this approach is decreased stability for both the membrane and the analytes. Triazines especially are known to undergo acidic hydrolysis.12 Furthermore, increasing viscosity and ionic strength of more concentrated acceptor solutions may also hamper permeation of the analyte from the membrane to the bulk of the acceptor solution. Maximum Enrichment Factor. The maximum enrichment factor that can be obtained for an analyte is influenced by the pH (12) Pacakova, V.; Stulik, K.; Jiskra, J. J. Chromatogr., A 1996, 754, 17-31.
Figure 3. Variation of enrichment factor with extracted sample volume for aniline derivatives (0.10 ppm) at different concentrations of the acceptor solution: (A) 0.1 M sulfuric acid (pH ∼1.0) (B); 1.0 M sulfuric acid (pH ∼0.0) Extracted compounds: aniline (1), 3-chloro4-methylaniline (2), 3,5-dichloroaniline (3), and 3-methyl-5-nitroaniline (4).
of the acceptor solution and the pKa of the analyte (see eqs 7 and 12). The rate at which the maximum enrichment is reached on the other hand is dependent upon such factors as the flow rate of extraction, the partition coefficient of the analyte between the donor solution and the organic liquid, and the composition of the acceptor solution. To determine the maximum enrichment factor, various SLM systems consisting of different types of weakly basic analytes at more or less constant ionic strength of the donor solution were studied. These consisted of chloro-s-triazines, representing an incomplete trapping situation, alkylthio-s-triazines, representing a complete trapping situation, and aniline derivatives, representing both of these situations. Figures 3 and 4 shows the results of these experiments. In an incomplete trapping situation (Figure 4), the maximum enrichment factor was attained easily especially with simazine and atrazine, which are more weakly basic and therefore not trapped much with 1 M sulfuric acid. These results agree well with Table 1, where at this acceptor pH (∼0.0), simazine has the highest value of the fraction of uncharged analyte (RA value) while terbuthylazine has the least. As the extraction of analytes continues, the concentration of uncharged analytes in the acceptor builds up, which in turn decreases the flux across the membrane. At the plateau, the concentration of uncharged analytes in the acceptor solution and membrane equals that in the donor solution reaching an equilibrium and the flux ceases. For alkylthio-s-triazines (Figure 4B), the situation changes since these compounds are more basic than the chloro-s-triazines. At Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Table 2. Comparison of Maximum Enrichment Factors Experimentally Obtained from Supported Liquid Membrane Extraction (Ee(SLM)) and Calculated from the Measured Partition Coefficients Ee(Cal) (cf. Eq 7) at Various Ionic Strengths for the Chloro-s-triazines
KP(A)a KP(D)a
Ee(SLM)
Ee(Cal)
ionic strength
simazine
atrazine
terbuthylazine
0.1 0.7 1.6 3.1 0.1 0.7 1.6 3.1 0.1 0.7 1.6 3.1
13 (n ) 2) 14 (n ) 4) 21 (n ) 4) 28 (n ) 4) 47 (n ) 3) 40 76 110 150 53 79 106 180
44 (n ) 2) 45 (n ) 4) 80 (n ) 4) 140 (n ) 4) 260 (n ) 4) 50 92 150 >147 50 90 160 290
300 (n ) 2) 350 (n ) 2) 610 (n ) 2) 1120 (n ) 2) 1990 (n ) 2) 115 210 >370 >350 118 210 380 680
a K P(D) and KP(A) are the partition coefficients of the analyte into the organic liquid from the aqueous donor and acceptor solutions, respectively, measured as described in the text.
Figure 4. Variation of enrichment factor with extracted sample volume for chloro-s-triazines (0.40 ppm each) (A) and alkylthio-striazines (0.25 ppm each) (B): ([) simazine, (9) atrazine, (b) terbuthylazine, (4) dimethametryn, (O) terbutryn, (9) ametryn, and (]) desmetryn. The acceptor solution contained 1.0 M (pH ∼0.0) and 0.1 M sulfuric acid (pH ∼1.0) for chloro-s-triazines and alkythio-striazines, respectively.
an acceptor pH of ∼1.0, the analytes fulfilled the condition of having an RA value close to 0.0005 (Table 1), so the maximum enrichment factor is supposed to be reached after a much longer extraction time. Furthermore, with all other parameters equal, the compound with the smallest RA value (the more basic) should have the highest enrichment factor at a given extracted sample volume since it is trapped more. This is the trend in Figure 4B (as also seen in Figure 4A) with desmetryn and terbutryn showing the lowest and highest enrichment factors, respectively. However, ametryn is slightly more basic than dimethametryn (Table 1), but its enrichment factor is less than that of the latter. This apparent reverse in the trend could be attributed to differences in partition coefficients of the two compounds into the membrane liquid. Dimethametryn is less polar than desmetryn9 and is therefore expected to dissolve more easily in undecane, which was used as membrane liquid. Results in Figure 3 for some aniline derivatives demonstrate how the maximum enrichment factor depends in the degree of 3910 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
trapping of the acceptor solution (Table 1). By using 1 M sulfuric acid (Figure 3B) instead of 0.1 M (Figure 3A), corresponding extracted sample volumes could be increased to over 6000 mL from ∼2000 mL without reaching a plateau on the enrichment factor for the two more basic anilines. This observation seems attractive in the context of long-time field sampling of these compounds. However, for the two very weakly basic anilines, the plateau is reached at approximately the same sample volumes; so for this aspect, the gain by lowering the pH is not very obvious. As expected, the maximum enrichment factor is higher for the lower pH. However, if the maximum enrichment factors Ee(max) are calculated from eq 8, with RD ) 1 and RA as in Table 1, the results approximately agree with the observed values. Effect of Ionic Strength on the Partition Coefficient. One possibility for further increasing the enrichment factor for these weakly basic anilines would be increasing the ionic strength of the donor solution, thereby increasing the KD (cf. eq 7). The effect of ionic strength on the partition coefficients was studied as described in the Experimental Section. The results in Table 2 show that, within the ionic strength range investigated, the partition coefficients increased with ionic strength. This finding is in good agreement with the practice of traditional liquid-liquid extraction, where the salting-out effect is often used to increase the partition coefficient of the more polar analytes to the organic liquid. It can be noted that (Table 2), at constant ionic strength, terbuthylazine, which is the most hydrophobic, gave the highest partition coefficient. Simazine, which is the least hydrophobic, had the lowest value of partition coefficient. Elsewhere,13 it has been observed that increasing the ionic strength of the donor solution by addition of a salt also has a positive influence on the stability of the membrane liquid. It suppresses emulsion formation, which have been noted as one of the major causes of membrane instability. Effect of Ionic Strength on the Maximum Enrichment Factor. The effect of ionic strength on the partition coefficient (13) Neplenbroek, A. M.; Bargeman, D.; Smolders, C. A. J. Membr. Sci. 1992, 67, 133-148.
uncharged analytes build up in the acceptor, the difference becomes more evident; i.e., the concentration of uncharged analytes in the acceptor solution becomes predominant. The effect of increasing ionic strength on the enrichment factor is therefore not beneficial for short extraction times, but for longer extractions, it has important analytical applications since it allows processing of large volumes, especially where the analyte of interest may be present at low levels. Table 2 also compares the experimental maximum enrichment factors at stated ionic strengths with calculated values according to eq 7 using the KA and KD values determined. These are in good agreement. However, beyond the ionic strength of 3, the actual maximums showed indications of being less than expected ones suggesting that, in a real SLM setup, there is an upper limit where one can salt out the analytes from the donor solution to the membrane even under incomplete trapping of the analytes. Figure 5. (A) Variation of enrichment factor with extracted sample volume for simazine at different ionic strengths of the donor solution: ([) 0.23, (9) 0.69, (b) 1.59, (O) 3.1. The acceptor solution contained 1.0 M sulfuric acid. (B) and (C) in Supporting Information.
in liquid-liquid extraction experiments showed that it is possible to increase dissolution of the analytes from the aqueous donor solution to the membrane. Therefore, the ionic strength of the donor solution in SLM was varied to investigate its effect on the maximum enrichment factors. The findings (Table 2) show that the maximum enrichment factors for each compound increased with ionic strength. The increase in enrichment factor as expected was highest at any given ionic strength with terbuthylazine, which is the most basic and hydrophobic. This agrees well with eq 6, since the enrichment factor is directly proportional to the partition coefficient of the analyte from the donor solution to the membrane liquid. It can be observed once more that, in the initial extraction periods (Figure 5), there are insignificant differences in enrichment factors for all the analytes at various ionic strengths, but as
ACKNOWLEDGMENT This work was made possible by the financial support from the Swedish Institute, the Swedish Agency for Research in Developing Countries (SAREC), the Swedish Natural Science Research Council (NFR), and the European Communities (Contract ENVY-CT95-0016). SUPPORTING INFORMATION AVAILABLE Variation of enrichment factor, Ee, with extracted sample volume at different concentrations of sulfuric acid as acceptor solution (Figure 2B and C) and at different ionic strengths of the donor solution (Figure 5B and C) for atrazine and terbuthylazine (2 pages). Ordering information is given on any current masthead page.
Received for review December 10, 1997. Accepted July 6, 1998. AC971327U
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