Anal. Chem. 1999, 71, 1210-1214
Lowering the Detection Limit of Solvent Polymeric Ion-Selective Membrane Electrodes. 2. Influence of Composition of Sample and Internal Electrolyte Solution Tomasz Sokalski,†,§ Alan Ceresa,† Monia Fibbioli,† Titus Zwickl,† Eric Bakker,‡ and Erno 1 Pretsch*,†
Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universita¨ tstrasse 16, CH-8092 Zu¨ rich, Switzerland, and Department of Chemistry, Auburn University, Auburn, Alabama 36849
The influence of the composition of the internal electrolyte solution on the response of Pb2+- and Ca2+-selective membrane electrodes is investigated. It is shown that the lower detection limit is improved by generating, in the membrane, ionic gradients that lead to a flux of primary ions toward the inner reference electrolyte solution. If the ion flux is too strong, it may cause analyte depletion at the membrane surface and, as a consequence, apparent super-Nernstian response. Such electrodes are not adequate to measure low analyte activities but can be used to determine unbiased selectivity factors. The results are interpreted in terms of a steady-state model, introduced in the companion paper, that describes the influence of concentration gradients generated by ion-exchange and coextraction processes on both sides of the membrane. Ion-selective electrodes (ISEs) have found widespread use for the direct determination of ionic species in complex samples.1-4 In the early days, their selectivity was often the limiting factor in determining low levels of analyte ions. In unbuffered samples, the current polymer membrane ISEs show lower detection limits that are much higher, typically in the micromolar range.3 This poses serious limits to their use at trace levels as required, e.g., for determining heavy metals in environmental samples. Moreover, the detection limits hitherto found do not allow to obtain true selectivity coefficients for discriminated ions by the usual methods.5-8 As shown recently,6,8 many ionophores induce much better selectivities than anticipated so far so that the influence of †
Swiss Federal Institute of Technology. Auburn University. § On leave from Department of Chemistry, Warsaw University, ul. Pasteura 1, PL-02-093 Warsaw, Poland. (1) Umezawa, Y. Handbook of Ion-Selective Electrodes: Selectivity Coefficients; CRC Press: Boca Raton, FL, 1990. (2) Morf, W. E. The Principles of Ion-Selective Electrodes and of Membrane Transport; Elsevier: New York, 1981. (3) Bakker, E.; Bu ¨ hlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083-3132. (4) Bu ¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593-1687. (5) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1995, 67, 507-518. (6) Bakker, E. Anal. Chem. 1997, 69, 1061-1069. (7) Bakker, E. Electroanalysis 1997, 9, 7-12. (8) Sokalski, T.; Ceresa, A.; Zwickl, T.; Pretsch, E. J. Am. Chem. Soc. 1997, 119, 11347-11348. ‡
1210 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
interfering ions often is no longer the limiting factor for assessing low levels of analytes. There is experimental evidence that small amounts of electrolytes are continuously released from ISE membranes into the measuring solution.9 It can be expected that the ISE is insensitive to changes in analyte activity if the concentration of these ions at the membrane surface exceeds that of the sample bulk. This interpretation is now well established by numerous facts, such as the detection of analyte ions in a sample bulk that originally contained none,9,10 as well as the influences of analyte ion buffering in the sample11,12 and of the composition of the internal electrolyte solution.13 Recently, this leaching process has been successfully suppressed by lowering the analyte activity in the inner compartment through an ion buffer.8 As a consequence, analyte ions on the inner side of the membrane are, to a certain degree, exchanged by interfering ions so that a flux is induced that prevents leaching and improves the detection limit by up to 6 orders of magnitude.8 The method also makes true selectivity coefficients accessible.8 A steady-state model introduced in the companion paper14 shows that both ion exchange and coextraction13 contribute to the induced fluxes. The composition of the internal solution influences both the achievable lower detection limit and the shape of the response function. The model also predicts that too strong gradients toward the internal solution are expected to induce apparent super-Nernstian response slopes due to depletion of the sample at the membrane surface. In this contribution, the influence of the composition of the internal electrolyte solution and of the sample on the response of Pb2+- and Ca2+-selective membrane electrodes is investigated. The results are in line with the model calculations and show experimentally that different internal (9) Bu ¨ hlmann, P.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.; Umezawa, Y. Electroanalysis 1995, 7, 811-816. (10) Maj-Zurawska, M.; Erne, D.; Ammann, D.; Simon, W. Helv. Chim. Acta 1982, 65, 55-62. (11) Schefer, U.; Ammann, D.; Pretsch, E.; Oesch, U.; Simon, W. Anal. Chem. 1986, 58, 2282-2285. (12) Sokalski, T.; Maj-Zurawska, M.; Hulanicki, A. Mikrochim. Acta 1991, 285291. (13) Mathison, S.; Bakker, E. Anal. Chem. 1998, 70, 303-309. (14) Sokalski, T.; Zwickl, T.; Bakker, E.; Pretsch, E. Anal. Chem. 1999, 71, 12041209 (preceding paper in this issue). 10.1021/ac9809332 CCC: $18.00
© 1999 American Chemical Society Published on Web 02/12/1999
electrolyte solutions are suitable for determining true selectivity coefficients and measuring submicromolar activities. EXPERIMENTAL SECTION Reagents. Poly(vinyl chloride) (PVC), 2-nitrophenyl octyl ether (o-NPOE), bis(2-ethylhexyl) sebacate (DOS), sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), the ionophores N,N-dicyclohexyl-N′,N′-dioctadecyl-3-oxapentanediamide (ETH 5234) and 4-tert-butylcalix[4]arene-tetrakis(thioacetic acid dimethylamide) (lead ionophore IV), and tetrahydrofuran (THF) were from Fluka AG (CH-8071 Buchs, Switzerland). Aqueous solutions were prepared with freshly deionized water (18.0 MΩ cm specific resistance) obtained with a NANOpure reagent-grade water system (Barnstead, CH-4009 Basel, Switzerland). The acids and salts Ca(NO3)2, Mg(NO3)2, Ba(NO3)2, Sr(NO3)2, NaCl, LiCl, and KCl were of Suprapur grade from Merck (Darmstadt, Germany), and Mg(OAc)2, AcOH, disodium ethylenediamine tetraacetic acid (Na2EDTA), H2SO4, and the other salts were of puriss p.a. or Microselect quality from Fluka. Membranes. Membranes contained 1.0 wt % (9.88 mmol kg-1) lead ionophore IV, 0.8 wt % (9.36 mmol kg-1) NaTFPB, 64.5 wt % o-NPOE, and 33.7 wt % PVC or 0.17 wt % (1.68 mmol kg-1) lead ionophore IV, 0.04 wt % (0.47 mmol kg-1) NaTFPB, 66.51 wt % DOS, and 33.28 wt % PVC for Pb2+ ISEs and 1.3 wt % (13.6 mmol kg-1) ETH 5234, 0.6 wt % (5.9 mmol kg-1) NaTFPB, 65.3 wt % o-NPOE, and 32.8 wt % PVC for Ca2+ ISEs. Membranes of ∼200 (50) µm thickness were obtained by casting a solution of ∼240 (48) mg of the membrane components, dissolved in ∼2.5 mL of THF, into a glass ring (28 mm i.d.) fixed on a glass plate. Electrodes. Conventional Ca2+ ISEs were prepared with Philips IS 561 electrode bodies (Mo¨ller Glasbla¨serei, Zu¨rich, Switzerland). For all other ISEs, a disk of 6-mm diameter was punched from the above ion-selective membranes and glued to plasticized PVC tubing with a THF/PVC slurry. The respective internal filling solutions were in contact with the reference halfcell (Ag/AgCl in 3 M KCl) through a 1 M KCl bridge electrolyte. These and the conditioning solutions for the conventional Ca2+ and Pb2+ ISEs (A) were 10-2 M CaCl2 and 10-3 M PbCl2, respectively. For the low detection limit ISEs, the internal filling solutions were 10-2 M Na2EDTA, 10-3 M PbCl2, and 10-2 M (B) or 2 M NaCl (C), pH 4.5, calculated aPb2+ ) 1.8 × 10-12 (B) and 1.1 × 10-13 M (C) for the Pb2+ ISEs, and CaCl2 and Na2EDTA in different concentrations for the corresponding Ca2+ ISEs: 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, adjusted to pH 5.39 with NaOH, calculated aCa2+ ) 3 × 10-8 M, aNa+ ) 7.3 × 10-2 M (B); 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, adjusted to pH 6.91 with NaOH, calculated aCa2+ ) 1.3 × 10-10 M, aNa+ ) 10-1 M (C); 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, adjusted to pH 8.6 with NaOH, calculated aCa2+ ) 3 × 10-12 M, aNa+ ) 1.2 × 10-1 M (D). The low detection limit Pb2+ ISEs were conditioned first during 2 days in a solution 10-3 M each in PbCl2 and HCl and then 1 day in pH-buffered 10-5 M PbCl2 (10-4 M each in NaOAc and HOAc, pH 4.7), and the corresponding Ca2+ ISEs in 10-2 M CaCl2. For each membrane composition, three electrodes were measured. EMF Measurements. Except for the Pb2+ calibration curves, measuring solutions were prepared by successive automatic dilution of stock solutions with two Metrohm NET-Titrino 721 (10mL burets) or a Metrohm Liquino 711 and two Metrohm Dosino 700 instruments equipped with 50-mL burets. These experiments
were performed under full computer control in a single 50-mL polyethylene beaker. For the Pb2+ calibration curves, manual gravimetric dilutions were made using for each concentration in separate polyethylene beakers which were pretreated with 10-2 M HNO3 for 2 weeks. The concentrations of the sample solutions were confirmed by electrothermal AAS measurements in the range of 10-6-10-8 M. Potentials were measured with a custom-made 16-channel electrode monitor at room temperature (20-21 °C) in stirred solutions during 30 min after sample changes. Except in the region of apparent super-Nernstian response, the potential drifts were smaller than 1 mV/h in all cases. Reference electrodes (either double-junction free-flowing calomel or Metrohm Ag/AgCl type 6.0729.100) with 1 M LiOAc or 3 M KCl as bridge electrolytes served for the Pb2+ or Ca2+ measurements, respectively. All EMF values were corrected for the liquid junction potential using the Henderson equation. Selectivity coefficients were determined by the separate solution method15 in metal chloride or nitrate solutions. For each interfering ion, a calibration curve was taken (see Results and Discussion). Activity coefficients were calculated according to Meier16 or the Debye-Hu¨ckel approximation. The Pb2+ concentrations were calculated by taking into account the complexation with AcO-, Cl-, and OH-.17,18 RESULTS AND DISCUSSION The effects of varying inner electrolyte and membrane compositions on the detection limit of membrane electrodes are systematically investigated with Pb2+ and Ca2+ ISEs based on a calix[4]arene derivative19 and an oxaglutaramide (ETH 5234),20 respectively. The responses of five Pb2+ ISEs to PbCl2 in a 10-4 M acetate buffer (pH 4.7) are shown in Figure 1. The lower detection limit of the ISE with conventional inner filling solution A is, according to the IUPAC definition, 10-7.2 M Pb2+. The inner filling solution B has a sufficiently low Pb2+ and high Na+ activity to induce a gradient in the membrane with decreasing Pb2+ concentration toward the inner compartment of the ISE. The two ISEs having this inner solution were made with membranes of equal compositions but different thicknesses. The conventional 200-µm-thick ISE membrane B exhibits an improved lower detection limit but an apparent super-Nernstian slope of ∼45 mV/ decade between aPb2+ ) 10-7 and 10-8 M. Both the improvement of the detection limit and the occurrence of the apparent superNernstian slope have been predicted by the steady-state model in the companion paper.14 While the increase of the gradient reduces the outflow of Pb2+, at very low sample activities it induces a depletion of the analyte at the membrane surface because of a strong inward flux at intermediate levels. A decrease of the membrane thickness to 50 µm (B′) has a strong influence on the shape of the response curve. Such an effect is not expected and has never been observed for conventional membranes but has been predicted by the steady-state model for ISEs with strong (15) Buck, R. P.; Lindner, E. Pure Appl. Chem. 1994, 66, 2527-2536. (16) Meier, P. C. Anal. Chim. Acta 1982, 136, 363-368. (17) Sille´n, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes; Special Publication 17; The Chemical Society: London, 1964. (18) Sille´n, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes, Supplement No. 1; Special Publication 25; The Chemical Society: London, 1971. (19) Malinowska, E.; Brzo´zka, Z.; Kasiura, K.; Egberink, R. J. M.; Reinhoudt, D. N. Anal. Chim. Acta 1994, 298, 253-258. (20) Gehrig, P.; Rusterholz, B.; Simon, W. Chimia 1989, 43, 377-379.
Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
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Table 1. Potentiometric Selectivity Coefficients, log -3 Kpot PbJ, Obtained by the Separate Solution Method in 10 M Chloride or Nitrate Solutions Compared with Literature Valuesa 19 ion J
log Kpot PbI
H+
-5.60 -10.20 -5.57 -6.55 -8.80 -13.80 -12.30 -12.10
Li+ Na+ K+ NH4+ Mg2+ Ca2+ Sr2+
SDb 0.05 0.64 0.07 0.10 0.42 0.57 0.62 0.48
ref 19 -3.8 -3.8 -4.2 -4.1 -5.2 -4.8
ion J
log Kpot PbJ
SDb
ref 19
Ba2+
-11.50 9.53c -4.50 -5.78 -11.20 -13.40 -13.10
0.42 0.23 0.13 0.17 0.35 0.42 0.40
-4.9 1.5 -3.3 -3.8 -4.9 -5.2 -5.2
Ag+ Cu2+ Cd2+ Zn2+ Co2+ Ni2+
a Values estimated from the graph shown in Figure 3a of ref 19. Standard deviations calculated from measurements with three electrodes based on the same master membrane and internal solution C (see Experimental Section). c Calculated from the response at 10-5 M Ag+, see text.
b
Figure 1. Response of four Pb2+ ISEs containing the lead ionophore 4-tert-butylcalix[4]arene-tetrakis(thioacetic acid dimethylamide). (AC) Identical o-NPOE-PVC membranes but with different internal electrolytes: A, 10-3 M PbCl2 (conventional); B (200 µm thickness) and B′ (50 µm thickness), 10-2 M Na2EDTA, 10-3 M PbCl2, and 10-2 M NaCl; C, 10-2 M Na2EDTA, 10-3 M PbCl2, and 2 M NaCl. (D) Same inner electrolyte as C but DOS-PVC (2:1) membrane with lower concentration of ionophore and ionic sites.
gradients in the membrane.14 The thicker membrane exhibits a more robust behavior with a reduced flux-induced bias. The inner solution C contains an even lower Pb2+ and a higher Na+ activity in the inner solution than B so that it generates a stronger concentration gradient in the membrane. The shape of the response curve is, therefore, even more extreme than that for the thin ISE B′. Because of drifting signals (see below), it cannot be used for reliable measurements below 10-6 M activities. The same inner solution as for C but a different membrane composition were used in ISE D: o-NPOE was replaced by DOS and the concentrations of ionophore and sites were reduced by about 1 order of magnitude. Since ion fluxes in the membrane are directly dependent on these concentrations, they are reduced in ISE D relative to that in C so that the super-Nernstian slope is eliminated and reliable measurements are possible until the detection limit of 10-10.1 M. Selectivity coefficients have been determined by the separate solution method3 in 10-3 M solutions using ISEs with the inner filling solution C (larger Pb2+ flux toward the inner solution) and are given in Table 1 together with literature values.19 The corresponding electrode slopes for most of the measured ions are close to Nernstian, indicating that the membrane reversibly 1212 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
Figure 2. Response of the Pb2+ ISE C (see Figure 1) to Pb2+ and various interfering ions.
responds to them (Figure 2). This is a necessary condition to yield true selectivity coefficients. Only for the highly discriminated ions Mg2+, Ca2+, Sr2+, Co2+, and Ni2+ (log Kpot PbJ < -12), slopes in the activity range of 10-2-10-3 M are somewhat reduced (∼20 mV/ decade), likely because of interferences by H+ and not by Pb2+ leaching from the membrane, as can be concluded from Figure 2. The selectivity coefficients obtained with ISEs for which the bias through leaching of the primary ion is avoided are better by up to 8 orders of magnitude than the literature values. On the pot other hand, log KPbAg ) 9.53 is by 8 orders of magnitude worse
Figure 4. Time-dependent response of Ca2+ ISEs B, C, and D (cf. Figure 3) with internal buffers. The corresponding sample concentrations are shown on the upper x axis.
Figure 3. Responses of four Ca2+ ISEs (ionophore ETH 5234, identical membrane composition) containing different internal electrolytes: A, 10-2 M CaCl2 (conventional); B, 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, pH 5.39; C, 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, pH 6.91; D, 10-3 M CaCl2, 5 × 10-2 M Na2EDTA, pH 8.6. Lower detection limits according to extended definition (solid bar).
than the published value of +1.5.19 The response function for Ag+ shown in Figure 2 explains this difference: in 10-2 M AgNO3 used to determine the selectivity coefficient in ref 19, the response is anionic. Owing to its strong complexation by the ionophore, Ag+ at this concentration is obviously coextracted together with the nitrate anion.3 Our value in Table 1 has been calculated from the EMF at 10-5 M Ag+, where the response is cationic (see Figure 2). This demonstrates that response functions for all interfering ions must be measured in order to judge whether the resulting selectivity coefficients are true or biased. In a second set of experiments, the influence of the inner filling solution on the ISE behavior was investigated with a Ca2+-selective membrane based on the highly lipophilic ligand N,N-dicyclohexylN′,N′-dioctadecyl-3-oxapentanediamide (ETH 5234). The potentiometric responses of four such Ca2+ ISEs with different inner electrolytes are shown in Figure 3. For easy comparison, the curves were shifted to give the same EMF value at 10-6 M Ca2+. The general trends are similar to the ones observed with Pb2+ ISEs. Decreasing Ca2+/Na+ activity ratios in the inner solution lead to lower final potentials and, at some point, to apparent superNernstian responses. The latter are more pronounced here than in the previous examples. It is clear that the low final potentials alone do not allow to assess low Ca2+ activities if the electrode is insensitive to activity changes in this region. Detection limits
according to the existing IUPAC definitions do not have a direct practical meaning with this new type of responses (e.g. with ISE D, the lower detection limit would be 10-14.5 M Ca2+ if the actual definitions were used). Therefore, values of the true lower detection limit as defined in the companion paper14 are shown in Figure 3. For the conventional electrode A (inner filling 10-2 M CaCl2), it is 10-6.8 M Ca2+, which is lowered to 10-8.5 M Ca2+ (ISE B) by buffering the activity of Ca2+ in the internal solution to a low level (3 × 10-8 M) and keeping that of the interfering ion high (here 7.3 × 10-2 M Na+). A further decrease in the Ca2+ activity of the internal solution to 1.3 × 10-10 Μ (ISE C) and 3.0 × 10-12 Μ (ISE D) leads to apparent super-Nernstian responses and worse lower detection limits of 10-8.2 and 10-6.3 M Ca2+, respectively. From the calibration curves alone, it may seem that these ISEs could be used to determine low Ca2+ activities with even higher sensitivity than the Nernstian slope of ISE B. The potential-time functions (Figure 4) show, however, that unstable potentials and strong drifts occur as soon as the response becomes super-Nernstian. Analogous results were obtained also with the Pb2+-selective electrode described above (not shown). This example demonstrates that concentration gradients of the analyte ion in the membrane must, at low analyte concentrations, be as small as possible with conventional membrane types if the maximal measuring range is to be obtained. This is in accordance with the steady-state model. Table 2 summarizes the selectivity coefficients and slopes for the Ca2+ ISEs B, C, and D having ion-buffered internal solutions. The selectivity coefficients were calculated from the potentials, measured by SSM, at four concentrations between 10-1 and 10-4 M and taking into account the slopes for the respective ions J determined in this interval. As a comparison, values obtained earlier20 with ion-buffered sample solutions are also given. It is apparent that both the slopes and selectivities gradually improve with decreasing Ca2+/Na+ activity ratio in the inner solution. The response is near-Nernstian for all ions, with ISE D having the smallest Ca2+/Na+ activity ratio in the inner solution, i.e., with the electrode that had the worst true detection limit because of strong super-Nernstian response. Except for K+ and Mg2+, this Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
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2+ ISEs with Different Ion-Buffered Internal Table 2. Potentiometric Selectivity Coefficients, log Kpot CaJ, for Ca Solutions a -1 and 10-4 M) for Ca2+ ISEsb log Kpot CaJ ( SD (n ) 4) and slope (mV/decade between 10
c
ion J
A
B
C
c log Kpot CaJ
H+ Li+ Na+ K+ Cs+ NH4+ Mg2+ Ba2+ Sr2+
-3.0 ( 0.7 (37.1) -3.1 ( 0.6 (37.0) -4.4 ( 0.5 (40.6) -3.4 ( 0.7 (34.7) -3.9 ( 0.5 (39.8) -3.9 ( 1.0 (29.3) -6.5 ( 0.6 (29.6) -3.0 ( 0.1 (21.9) -0.9 ( 0.1 (21.9)a
-3.3 ( 0.4 (43.0) -3.6 ( 0.5 (40.5) -5.2 ( 0.2 (48.4) -4.1 ( 0.4 (44.0) -4.8 ( 0.5 (52.6) -4.8 ( 0.8 (33.5) -7.5 ( 1.0 (34.8) -3.2 ( 0.1 (22.8) -1.2 ( 0.1 (22.3)a
-3.8 ( 0.2 (48.2) -4.9 ( 0.6 (58.9) -6.4 ( 0.3 (52.0) -5.6 ( 0.8 (62.0) -6.7 ( 1.1 (63.5) -6.5 ( 0.4 (42.7) -8.6 ( 0.3 (25.0) -3.1 ( 0.1 (22.8) -1.1 ( 0.1 (22.3)a
-3.1 -5.8 -5.9 -7.7 -6.5 -6.7 -4.3 -3.1 -1.2
a Mean of values obtained at four concentrations (10-1-10-4 M; SSM) using the experimental electrode slopes. b See Experimental Section. Values estimated from the graph shown in ref 20 for ion-buffered sample solutions (internal solution 10-2 M CaCl2).
electrode shows selectivities close to those found in ion-buffered samples.20 There, in the case of Mg2+, the ion buffer seems to pot have biased the measurement, leading to a poor value (KCaMg ) -4.3 instead of -8.6), whereas the difference in the case of K+ might be explained by the presence of impurities.20 The data in Table 2 show that the best slopes and selectivities for interfering ions are obtained with the Ca2+ ISE D having the smallest Ca2+/ Na+ activity ratio in the inner solution. However, owing to the strong super-Nernstian response (Figure 3), its true lower detection limit is not improved with respect to that of the conventional electrode. It can be concluded that, while too strong a concentration gradient may be an impediment in reaching the possible detection limit, it is useful for obtaining true selectivity coefficients. With the replacement of a large part of the primary ion at the inner membrane side, i.e., with a strong concentration gradient, the potentials in highly dilute sample solutions are lower, the slopes for interfering ions are closer to theoretical, and selectivity values are improved. Therefore, it can be concluded that, with ISE membranes having smaller gradients, the primary ions that leach from the membrane still determine the potential if their sample activity is low. As can be deduced from the steady-state model in the companion paper,14 the initial gradient with decreasing primary ion concentration toward the internal solution changes sign when samples are so dilute that primary ions are partly exchanged by interfering ions (here by H+). In contrast to traditional potentiometric interference that occurs when 50% of the analyte ion is replaced in the membrane (interfering ions of same valencies forming complexes of the same stoichiometry as the primary ion), the exchange of a small fraction on the order of 1% is already sufficient to achieve this effect.14 This finding indicates a more severe influence of interfering ions on the determinations in the submicromolar range. High selectivities, in particular with regard to H+, are important, especially for trace measurements of heavy metals where solutions often have to be acidic.
1214 Analytical Chemistry, Vol. 71, No. 6, March 15, 1999
CONCLUSIONS The influence of ion gradients within the sensing membrane on the response characteristics of ionophore-based polymeric membranes has been studied. It is shown that the lower detection limit of ISEs can generally be improved by partially replacing the primary ions at the inner membrane surface. The inner filling solution has to contain a rather high concentration of an interfering ion and a low (or virtually zero) concentration of the primary ion. Extremely low detection limits below 10-10 and 10-8 M are thus achieved for Pb2+ and Ca2+, respectively. If the induced primary ion flux toward the inner filling solution is too large, analyte depletion in the aqueous Nernstian boundary layer induces superNernstian slopes and deteriorates the true detection limits. However, such ISEs show the best responses for discriminated interfering ions and are, therefore, recommended for measuring true selectivity coefficients. ACKNOWLEDGMENT The authors thank the Swiss National Science Foundation, Hitachi Ltd. (Tokyo), Orion Research Inc. (Beverly, MA), ICSCWorld Laboratory (Lausanne), and the Petroleum Research Fund (administered by the American Chemical Society) for financial support, ETH Zurich for an internal research grant, Dr. D. Wegmann for careful reading of the manuscript, and H. Cusin and R. Frischknecht for their help with the AAS and ICPMS measurements, respectively.
Received for review August 19, 1998. Accepted December 28, 1998. AC9809332
