Anal. Chem. 1994,66, 1713-1717
Bulk Optodes Based on Neutral Dithiocarbamate Ionophores with High Selectivity and Sensitivity for Silver and Mercury Cations Markus Lerchl, Elmar Reltter, Wllhelm Simon,? and Ern0 Pretsch' Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Universitatstrasse 16, CH-8092 Zurich, Switzerland Dldarul A. Chowdhury and Satsuo Kamata Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, Korimoto, Kagoshima 890, Japan
Membranes for optically sensing Ag+ and Hg2+ based on plasticized poly(viny1 chloride) (PVC) containing a metal ionselective ionophore, a hydrogen ion-selectivechromoionophore, and lipophilic anionic sites are presented. Different dithiocarbamate ionophores have been used to develop novel optical sensors for environmental analysis. Optode membranes based on the ionophore methylene bis(diisobuty1dithiocarbamate) (MBDiBDTC) reversibly respond to Ag+ and Hgz+ and show extremely high selectivities over Li+, Na+, K+, Mg2+, Cu2+, Cd2+,and Pb2+. The detection limit for Ag+ is 2.5 X M at pH 4.7. The dynamic range, response behavior, reversibility, and selectivity of the optode membranes are discussed. Worldwide, there is a growing awareness of the need to reduce environmental pollution. Especially for the quality control of drinking water, sewage, and other samples, inexpensive analytical methods are required. The demand for chemical sensors of low detection limits and high selectivities, e.g. for toxic metal ions such as silver(1) and mercury(11), therefore is on the increase. Recently, solvent polymeric bulk optode membranes with high selectivity for lead1q2and uranyl ions3have been described whose response behavior is based on a cation exchange systeme4 The present work reports on analogous membranes containing, besides lipophilic anionic sites, two cation-selective carriers. One of them, the ionophore, shows selectivity for the analyte cation, and the other, the chromoionophore, is a lipophilized H+-selective indicator which drastically changes its optical properties upon protonation. Like all optodes whose recognition mechanism involves an ion exchange, the membranes described here respond to the activity ratio of the two ion species involved. Hence, to allow the measurement of the analyte cation activities, the pH of the sample solution must be either monitored or kept constant by buffering. For the developing of new sensors, a number of chromoionophores of t Deceased, November 17, 1992. (1) Lerchi, M.; Bakker, E.; Rusterholz, B.; Simon, W.Anal. Chem. 1992, 65, 1534-1540. (2) Lerchi, M.; Simon, W.Proc. SPIE Int. SOC.Opt. Eng. 1992,1716,336-345. (3) Lerchi, M.; Reitter, E.; Simon, W.Fresenius' J . Anal. Chem. 1994, 348, 272-276. (4) Morf, W. E.;Seiler, K.;Ssrensen,P. R.;Simon, W.InZon-SelectiueElectrodes; Pungor, E., Ed.; Akademiai Kiad6: Budapest, 1989; Vol. 5 , pp 141-152.
0003-2700/94/0386-1713$04.50/0 0 1994 Amerlcan Chemical Society
different basicities are a ~ a i l a b l e . As ~ ionophores, several dithiocarbamate compounds have been evaluated. Some of them had been used successfully in ion-selective electrodes (ISEs) for copper(II)6 or lead(II).7
EXPERIMENTAL SECTION Reagents. Aqueous solutions were prepared with doubly quartz-distilled water and metal chlorides or nitrates of the highest purity available (E. Merck, Darmstadt, Germany, and Fluka AG, Buchs, Switzerland). For membrane preparation, poly(viny1 chloride) (PVC, high molecular), potassium tetrakis [3,5-bis(trifluoromethyl)phenyllborate (KTm(CF&PB), bis(2-ethylhexyl) sebacate (DOS), and tetrahydrofuran (THF, freshly distilled prior to use) were purchased from Fluka AG (Buchs, Switzerland) and tetraethylthiuram disulfide (TETDS) together with sodium diethyldithiocarbamate (for synthetic purposes) from Tokyo Kasei Kogyo (Tokyo, Japan). The syntheses of the ionophores methylene bis(diisobuty1dithiocarbamate) (MBDiBDTC) and tetramethylene bis(diisobuty1dithioarbamate) (TMBDiBDTC)7 as well as of the chromoionophores ETH 5418 and ETH 5315l have been reported earlier. Synthesis. SOctyl N,N'-Diisobutyldithiocarbamate (ODiBDTC). Diisobutylamine (0.14 mol) and 2-propanol (25 mL) were dissolved in water (300 mL) containing NaOH (0.14 mol). While stirring at room temperature, carbon disulfide (0.14 mol) was slowly added and allowed to react for 2 h. The precipitate was filtered and crystallized from 2-propanol, yielding sodium N,N'-diisobutyldithiocarbamate (18.4 g, 64%, mp 36-37 "C) which was dissolved in acetone (0.02 mol in 100 mL). To this solution, n-octyl bromide (0.02 mol) in acetone (1 5 mL) was added dropwise while stirring at 40 OC. After 5 h, the mixture was cooled to room temperature and the white precipitate (NaBr) was filtered off. Upon addition of water (500 mL) to the filtrate, an oily liquid separated at the bottom. It was extracted with diethyl ether (20 mL) and concentrated on a rotary evaporator at 35 (5) Bakker, E.; Lerchi, M.; Rosatzin, Th.; Rusterholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211-225. (6) Kamata, S.;Bhale, A.; Fukunaga, Y.; Murata, H. Anal. Chem. 1988, 60, 2464-2467. (7) Kamata,S.; Onoyama, K. Anal. Chem. 1991, 63, 1295-1298.
AnalyticalChemistry, Vol. 66,No. 10, May 15, 1994
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Table 1. Membrane ComporHlona
composition I I1 I11 IV
v
VI VI1 VI11
ionophore amt (mmol/kg) 47.6 MBDiBDTC 12.5 MBDiBDTC 57.5 MBDiBDTC 59.8 MBDEDTC 47.6 TMBDiBDTC 56.3 TETDS 54.8 0DiBDT C 80.8 ODiBDTC
chromoionophore amt (mmol/kg) ETH 5418 11.0 ETH 5418 3.1 ETH 5313 11.1 ETH 5418 12.4 ETH 5418 11.0 ETH 5418 16.4 9.2 ETH 5418 10.2 ETH 5315
amt of KTm(CF3)2PB (mmoUkg)
12.1 3.5 12.9 14.2 12.1 16.8 9.5 10.9
a The iono hore, chromoionophore, and borate salt together with PVC (80 mg) and plasticizer DOS (160 mg) were dissolved in freshly distilled THP(1.5 mL).
OC. The liquid product was purified by flash chromatography (silica gel, benzene/diethyl ether 9:l (v/v)) and bulb-to-bulb distillation togive pure, slightly green ODiBDTC (6.4g, 57%), bp 140 OC (40 Pa). Anal. Calcd for C I ~ H ~ S (317.60): NS~ C,64.29;H, 11.11;N,4.41. Found: C,64.45;H, 11.27;N, 4.35. Theconstitution wasconfirmed by 'H-NMR (300 MHz, CDCl3), IR, and FAB-MS. Methylene Bis(diethy1dithiocarbamate) (MBDEDTC). In a round-bottomed flask fitted with a reflux condenser and a mechanical stirrer, sodium diethyldithiocarbamate (0.04 mol) was dissolved in ethanol (250 mL) anddibromomethane (0.02 mol) added dropwise to the refluxing solution under vigorous stirring. The reaction was allowed to proceed for 4 h. On cooling to room temperature, white crystals of NaBr and the target compound precipitated. Deionized water (200 mL) was added to the reaction mixture and MBDEDTC was separated by repeated extraction with 100-mL portions of chloroform. After evaporating the solvent from the combined extracts, white MBDEDTC was obtained which on recrystallization from EtOH appeared as white needlelike crystals (8.6 g, 69%), mp 72-73 OC. Anal. Calcd for CllH22N~S4 (310.55): C, 53.96; H, 7.04; N, 6.99. Found: C, 53.90; H, 7.02; N, 6.98. The constitution was confirmed by 'H-NMR (300 MHz, CDC13), IR, and FAB-MS. Optode Preparation. The membrane components (see Table 1) were dissolved in T H F (1.5 mL). By use of a spin-on device, two identical membranes of about 1-3 pm thickness (depending on the rotation frequency) were cast on glass plates and mounted in a measuring cell of about 600 p L inner volume. Apparatus. UV-vis spectra were run on a UVIKON Model 8 10 double-beam spectrophotometer (Kontron AG, Zurich, Switzerland). The pH values were determined with a pH glass electrode (Orion Ross Model 8 103, Orion Research AG, Uetikon am See, Switzerland). M) were Standard Solutions. All standard solutions ( prepared by dissolving the salts in diluted magnesium acetate buffer of pH 4.7 (ionic strength I = M). The solutions were further diluted by weight and stored at room temperature in polyethylene bottles to avoid contamination. Absorption Experiments. One or two cells (depending on the membrane type) were mounted in the sample path of the spectrophotometer and connected to a peristaltic pump to obtain a flow-through system. This arrangement allowed experiments to be performed with two or four membranes in the optical path. The flow was kept constant at about 5 mL/ min. The empty cell in the reference path of the spectro1714
Analytlcal Chemistry, Vol. 66,No. IO, May 15, 1994
photometer consisted of two glass plates without membranes. All spectra were recorded at 21 f 1 OC in the transmittance mode from 800 to 400 nm or 700 to 350 nm, the membranes containing ETH 54 18 or ETH 53 15 as chromoionophore. Calculations. All calculated curves were fitted to the experimental data points by varying K t t h in eq 2. The maximum and minimum absorbance values required for determining the degree of protonation of the chromoionophore wereobtained by contacting the membranes with 0.01 M HCl and 0.01 M NaOH, respectively. Since all measurements were made with solutions of high buffer ion concentration as compared with that of the analyte ion, changes in activity coefficients could be neglected. In cases where different chromoionophores were involved, their basicity difference was considered for calculating the selectivity coefficients (pKa(ETH 5418) - pKa(ETH 5315) = 3.85).
RESULTS AND DISCUSSION F'rinciple of Operation. The working principle of the present sensing systems is identical to that of the lead-selective optode described recently.' Therefore, only the final equations adapted to Ag+ as the analyte ion are given here. The PVC/ DOS optode membranes studied contain a Ag+-selective ionophore L and, competing with it, a H+-selective chromoionophore C (ETH 5418 or ETH 5315) which on contact with an aqueous silver salt solution give rise to the following equilibrium: K?Ch
Ag+(aq)+ 2L(org) + CH+(org)
The experimental data are in agreement with the assumption of the dithiocarbamate ionophores yielding 2: 1 complexes with the metal ions under study. By adding a salt with a highly liophilic anion (R-, tetrakis [3,5-bis(trifluoromethyl)phenyl]borate) to the membrane, the presence of a sufficiently high amount of cations in the organic phase is ensured. The absorbance of the chromoionophore is determined at the wavelength of maximum absorption of its protonated form. With K&th as the overall equilibrium constant and 1 - a = [CH+]/ [C,,,] as the degree of protonation of the chromoionophore, the response function of the optode to Ag+ is given by
I
-0.01
M HCI log CM = -8.5
IONOPHORE MBDiBDTC CHROMOIONOPHORE ETH 5418
ODiBDTC
MBDiBDTC W
y
0.2 -
2 B TETDS
i!0.1 -
MBDEDTC
-4.5 0.1M NaOH
o.ol A
U
A
I
400
TMBDiBDTC
"'
500
600
700
800
WAVELENGTH [nm]
Q 1 2 % N & 70
ETH 5418
\
0
A
owETH 5315
Figure 1. Constitutions of ionophores and chromoionophores.
where [Ltot], [C,,,], and [RJ are the total concentrations of ionophore, chromoionophore, and anionic site, respectively. Activities refer to species in the aqueous phase, whereas brackets indicate concentrations of compounds in the organic membrane phase. Calibration curves were calculated with eq 2 varying Kkth for optimum fitting to the experimental data. As all measurements were carried out at constant ionic strength given by the buffer solution, activity coefficients in the aqueous solutions were assumed to be constant so that total concentrations of the respective measuring ions were used for calculations. For determination of the selectivity coefficients k Z h l by the separate solution method (SSM), t &, of the interfering ion M has to bedetermined as well. The selectivity coefficients are then calculated as follows8
with z as the charge of the interfering ion M. The values of log ~ A ~ , Mare ~ P represented ~ graphically by the horizontal distances between the two respective calibration functions if plotted in the logarithmic form. In order to compare the selectivity coefficients of membranes containing chromoionophores of different basicity, the difference in their pK values was taken into account as described elsewhere.' Ionophore Evaluation. Four bis(dithi0carbamate) compounds (Figure 1) were tested for their use as silver-selective ionophores. For a first evaluation, optode membranes of composition I and IV-VI were equilibrated with 0.01 M HCl to fully protonate the chromoionophore. After gently wiping (8) Bakker, E.; Simon,W.Anal. Chem. 1992, 64, 1805-1812.
Flgure 2. Absorption spectra of four membranes of composition I1 (Table 1) with ionophore MBDIBDTC and chromoionophore ETH 5418 after equilibration with pH-buffered Ag+ solutions: absorption maxima of ETH 5418 at 665 nm (protonated) and 521 nm (deprotonated): isosbestic polnt at 577 nm.
them dry, a few drops of metal salt solution were given onto the surface and the ensuing color changes were observed. A change from blue to red because of deprotonation of the chromoionophore at the same time indicated complexation of the metal ion by the ionophore. The following observations were made: MBDiBDTC (Membrane I). Solutions of Ag+, HgZ+,and M each) caused complete deprotonation, but the Pb2+ ( reaction with Pb2+ was very slow. In all three cases, the chromoionophore was protonated again with 0.01 M HCl, thus indicating reversible complexation. Solutions of Cuz+ (0.001 M) as well as Cuz+ or Ca2+ (0.1 M each) gave no visible change in membrane color. MBDEDTC(Membrane IV). The same response behavior was observed as with MBDiBDTC, but with Hg2+ deprotonation was very weak. TMBDiBDTC (Membrane V). Reversible deprotonation was achieved with Ag+ or HgZ+,but with Cuz+, hardly any color change was observed, and with PbZ+,CdZ+, or CaZ+ none at all. TETDS (Membrane VI). With Agz+ or Cuz+ solutions, the deprotonation of the chromoionophore was irreversible, whereas with Hg2+and CaZ+,it was only weak, and Pb2+or Cd2+ caused no color change at all. This preliminary study showed the bis(dithi0carbamates) MBDiBDTC, MBDEDTC, and TMBDiBDTC to be the most promising ionophores for Ag+. Considering on the one side that the last mentioned ligand gave unsatisfactory results in ISE membranes' and, on the other, that MBDEDTC is only weakly lipophilic, MBDiBDTC was chosen for an exhaustive investigation in optodes. For comparison purposes, the monodithiocarbamate ODiBDTC was synthesized later and also included in the study. Dynamic Range. Figure 2 shows the absorption spectra of optode membrane 11, based on ionophore MBDiBDTC and chromoionophore ETH 5418 in contact with Ag+ solutions. The neat isosbestic point indicates that during a working period of 2 days, no loss of chromoionophore occurred. The corresponding calibration curve with log Ktth = 3.4 (eq 2) is given in Figure 3. The sensor showed perfect theoretical behavior, and only for very dilute solutions, weak interference AnaEytlcal Chemlstty, Vol. 66,No. 10, May 15, 1994
1715
IONOPHORE MBDiBDTC CHROMOIONOPHORE ETH 5418
z
e
MAGNESIUM ACETATE BUFFER, pH 4.7
q
5
0.6-
5
E
0
f
s
5 U.
02-
CHROMOIONOPHORE ETH 5418
4
0.4-
O1
w
Iu
a
0.2n
I
IONOPHORE MBDiBDTC
..r" I
MAGNESIUM ACETATE BUFFER pH
470
i 1 i I
I
,
0.01
-1 0
-
Flgure 3. Degree of protonation, 1 a,of four optode membranes (composition I I) with ionophore MBDiBDTCand chromdonophoreETH 5418 as a function of log cAg. Sample, Mg(OAc):, buffer; pH 4.70, I = M; solid curves, calculated with log K& = 3.4 In eq 2. For very dilute solutions,slight interference by the buffer ion Mg2+occurred (dashed line) leading to a detection limit, log cAg(DL), of -8.6 (or 2.5 X 10-B M Ag+).
-
100
k
CHROMOIONOPHORE ETH 5418
YI
z
3 8
MAGNESIUM ACETATE BUFFER
60
40 W
0
100
200
300
400
500
TIME [min]
Flgure 4. Response time curves for two optodes with Ionophore MBDiBDTC and chromoionophore ETH 5418 in pH-buffered lo-' M two Ag+ solution (flow-through system; flow rate 5 mL/mln): (0) membranes of composition I and 3 pm thickness each, sample pH 4.75; (0)four membranes of composition I1 and 1 pm thickness each, sample pH 4.70. The total absorbance of the fully protonated optode membranes at 665 nm at the beginning of the experiment was taken as 100.
from Mg2+ was observed. With the definition introduced r e ~ e n t l ythe , ~ detection limit of this Ag+ sensor is 2.5 X M Ag+ at pH 4.7, with a background of 3.3 X I@ M Mg2+. This is sufficiently low for all monitoring purposes in environmental analysis.'O Response Behavior. For highly diluted solutions, bulk optodes show very long response times due to the respective saturation of the membranes with analyte i ~ n s . l .The ~ time limiting step is not the diffusion within the organic phase but the convective mass extraction from the bulk of the aqueous solution that goes on until the required absolute amount of measuring ions is reached. To achieve faster equilibration, membrane I1 was prepared with lower concentrations of membrane components and four membranes instead of only two were placed in the optical path of the spectrophotometer. Figure 4 shows the response of the resulting optode with four membranes of composition I1 and 1 pm thickness each in comparison with that of an optode consisting of two 3-pm membranes I. As expected, the response time was shortened (9) Bakker, E.; Willer, M.; Pretsch, E.Anal. Chim. Acta 1993, 282, 265-271. (10) Swiss Federalcouncil VerordnungiiberAbwassereinieirungen,No. 814.225.21, Berne, December 8, 1975 (revised October 1, 1991).
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AnalyticalChemistty, Vol. 66, No. 70, May 15, 1994
from about 500 to 130 min. For this reason, all experiments with solutions of Ag+concentrations below I F M were carried out using this improved, but more laborious technique. Figure 5 shows the response of this optode with 4 X 1 pm membranes of composition I1 to different Ag+concentrations, recorded at the wavelength of maximum absorbance (665 nm). As expected, the response times increase with decreasing Ag+ concentrations, a behavior also observed with other optodes.1J Reversibility. The above optode with membranes containing the ionophore MBDiBDTC was exposed first to 0.01 M HCl and then to Ag+ salt solutions of increasing and decreasing concentration. The theoretically expected equilibrium value of 1 - a (see Figure 3) was always obtained. When thus exposing the membranes twice to 3 X le8M Ag+, the equilibrium values of the resulting degree of protonation were found to be 0.742 and 0.735. In 0.01 M HCl, with the fully protonated chromoionophore, maximum absorbances measured at the beginning and the end of an 18-h period for recording the Ag+ calibration function (see Figures 2 and 3) were0.333 and 0.336, respectively. Thus, it can be concluded that the sensor performance is completely reversible and shows good repeatability. This is in clear contrast to the behavior of the corresponding ISE membranes which were poisoned by silver(1) and m e r ~ u r y ( I I ) . ~ Selectivity. The selectivity of optode membranes with the bis(dithi0carbamate) compound MBDiBDTC or the dithiocarbamate ODiBDTC over several interfering ions as well as that of membranes with MBDEDTC and TMBDiBDTC over Hg2+ were investigated by the separate solution method (SSM). The selectivity coefficients given in Table 2 are calculated for 1 - a = 0.64, which corresponds to 9.3 X M Ag+, the quality criteria for wastewater in Switzerland.lo The calibration functions (Figure 6) for optodes with MBDiBDTC prove that this ionophore strongly prefers Ag+ and Hg2+over K+, Na+, Li+, Cu2+,Pb2+,and Cd2+. Its selectivity over Mg2+ is extremely high as well (cf. Dynamic Range). Response to Ag+ and Hg2+is practically the same but depends on the sample pH and analyte ion concentration. In ISE membranes MBDiBDTC showed similar selectivitiesfor Cu2+, Pb2+,and Cd2+,whereas Ag+ and Hg2+were found to poison the me~nbrane.~It must be supposed that the complexation of Ag+ and Hg2+ is too strong for reversible
fable 2. SSM Sslectlvlty Coefficients, log k&, IonophoresIn Optode Membranes'
M*+
MBDiBDTC
Hg2+
0.7 -9.3 -9.6 -9.9 -11.2 -13.4 -14.6
K+ cu2+ Na+ Li+ Pb2+ Cd2+
ODiBDTC 0.9
for Different
TMBiBDTC
MBDEDTC
1.0
-1.6
-9.9 -10.7
a Normalized for pH 4.7, membrane composition 11, and 1- a = 0.64 (corresponding to 9.3 x 1 P M Ag+, the quality criteria for
wastewater in Switzerlandlo).
log Cl.4
Figure 8. Selectivity of the ionophore MBDIBDTC. Calibration curves foroptodeswithmembranes I, I1,or III,alinormalizedforcompositlon 11, pH 4.7, and chromoionophoreETH 5418. The horizontal distance between the calibration curve for Ag+ and that for an interfering ion M represents the respective selectivity coefficient, log (logarlthmlc form of eq 3). Samples were metal salt solutions in ldg(OAc)p buffer, pH 4.70. Membrane composltion Iwas used with HB(NO~)~, I 1 with AgN03, and I11 with KCI, CuCi2, NaCI, LICI, Pb(N03)2, and CdC12.H20.
ISE response. This is confirmed by the large overall equilibrium constants Kzch determined from optode measurements. (Note that the calibration functions are very similar for both ions, although the respective K2ch have very different numerical values: 103.4L mol-' for Ag+, and le2 [dimensionless] for Hg2+,which is due to their different units). It seems that in this case, strong complexation of the analyte ion by the ligand is no handicap because it occurs in competition with the protonation of the chromoionophore. By choosing the latter of apopropriate b a s i ~ i t y a, ~reversible response of the sensor can be attained over a very wide range of complex formation constants for the ionophore used. This possibility of overcoming the problem of strong complexation does not exist in the case of ISEs. For comparative studies, the dithiocarbamate ODiBDTC was also tested as ionophore, giving a similar selectivity pattern as MBDiBDTC (see Table 2). As regards the selectivity of the ionophores for Ag+ against HgZ+,it is almost the same
for the three isobutyl-substituted compounds, the latter ion being preferred under the conditions chosen (pH 4.7, membranecomposition 11,l -a = 0.64). With theethyl-substituted bis(dithiocarbamate) compound MBDEDTC, on the other hand, Hgz+is strongly discriminated. This indicates that the selectivity of such compounds is mainly determined by the dithiocarbamate unit with the lipophilic side chains and not by the number of methylene groups between the two moieties. Even ODiBDTC, with only one dithiocarbamate unit, shows high selectivity for Ag+ and Hgz+ over Cd2+ and Pb2+. No preorganization of the C-shaped cavity seems to be necessary for ideal complexation.
CONCLUSIONS Optical sensing membranes based on the ionophore MBDiBDTC have response functions that are in perfect agreement with the theoretical description and show good selectivity for Ag+ and Hgz+. At pH 4.7, the detection limit for Ag+ is 2.5 X M. The use of bis(dithi0carbamate) ionophores thus allows the design of optical sensors for Ag+ and Hg2+,suitable for environmental monitoring. Nevertheless, the sensor is not usable in cases where both these ions are present in comparable amounts, except if the sum of their activities is of interest. Of special importance is the monitoring of drinking water that contains Ag+ as sterilizing agent. This technique is often used in countries with hot climate when the water is stored in large containers over prolonged periods. In order to guarantee a sufficient bacteriostatic effect, the concentration of free Ag+ should never fall below 10" M and, therefore, should be continuously monitored. With a view to the design of further ionophores, it can be concluded from the present selectivity study that changes in the lipophilic side chains have a much greater influence on the selectivity behavior of the corresponding optodes than the size of the C-shaped cavity between the two dithiocarbamate units. ACKNOWLEDGMENT We are greatly indebted to the late Professor W. Simon for contributing with his enthusiasm and encourgement to this work during the early stages. We also acknowledge financial support from the Swiss National Science Foundation and thank Dr. D. Wegmann for careful reading of the manuscript. Received for review November 12, 1993. Accepted February 19, 1994.' *Abstract published in Aduance ACS Abstracts, April 1, 1994.
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