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containing quality factors that express the reliability of the results. For ease of interpretation, the information about correlation may be displayed in the form of a spectral representation like in Figure 12. We hope that further research will allow us to use our correlation matrices in order to identify molecular fragments and ultimately to build up entire structural formulas of unknown compounds.
ACKNOWLEDGMENT We are indebted to Martial Rey for valuable assistance with instrumentation, to FranGois Claret and Pierre Vogel for providing a sample of the bicyclo[2.2.2]octane derivative, and Ruud Scheek, Rolf Boelens, and Robert Kaptein for their kind permission to use their software package.
LITERATURE CITED (1) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear Magnetlc Resonance in One and Two Dimensions ; Clarendon Press: Oxford, 1987. (2) Meler, B. U.: Bodenhausen, G.: Ernst, R. R. J. Magn. Reson. 1984, 60, 161. (3) Neidig, K. P.; Bodenmuller, H.; Kalbitzer, H. R. Biochem. Biophys. Res. Commun. 1984, 125, 1143. (4) Pfandler, P.; Bodenhausen, G.: Meier, B. U.; Ernst, A. R. Anal. Chem 1985, 5 7 , 2510, (5) Pfandler, P.; Bodenhausen, G. J. Magn. Reson. 1986, 7 0 , 71. (6) Bolton, P. H. J. Magn. Reson. 1988, 7 0 , 344. ( 7 ) Mhdl, 2.; Meier, 8 . U.: Ernst, R. R. J. Magn. Reson. 1987, 72, 584. (8) NoviE, M.; Oschklnat, H.; Pfbndler, P.: Bodenhausen, G. J. Magn. Reson. 1987, 7 3 , 493. (9) Glaser, S.; Kalbitzer, H. R. J. Magn. Reson. 1987, 7 4 , 450. (10) Meier, B. U.; M6d1, 2.; Ernst, R. R. J. Magn. Reson. 1987, 74, 565. (11) Hoch, J. C.; Hengyi, S.; Kjaer, M.; Ludvigsen, S . : Poulsen, F. M. Carlsberg Res. Commun . 1987, 5 2 , 111. (12) Eggenberger, U.; Pfandler, P.: Bodenhausen. G. J. Magn. Reson. 1988, 7 7 , 192.
(13) Grahn, H.; Delaglio, F.; Delsuc, M. A.; Levy, G. C. J. Magn. Reson. 1988. 77. 294. (14) NovIE, M.; Bodenhausen, G. Anal. Chem. 1988, 6 0 , 582. (15) Pfandler, P.; Bodenhausen. G. J. Magn. Reson. 1988, 79, 99. (16) Meier, 8. U.: Ernst, R. R. J. Magn. Reson. 1988, 79, 540. (17) Pfandler. P.; Bodenhausen, G. Magn. Reson. Chem. 1988, 26, 888. (18) Szalma, S . ; Pelcrer, I . J. Magn. Reson. 1988, 76, 416. (19) Maudsley, A. A.; Ernst. R. R. Chem. Phys. Lett. 1977, 5 0 , 368. (20) Maudsley, A. A.: Muller, L.; Ernst, R. R. J. Maagn. Reson. 1977, 2 8 , 463. (21) Freeman, R.; Morris, G. A. Bull. Magn. Reson. 1979, 1 , 5 . (22) Muller, L. J. Am. Chem. Soc. 1979, 101, 4481. (23) Bax, A.; Griffey, R. H.: Hawkins, B.L. J. Am. Chem. SOC.1980, 105, 7 188. (24) Bolton, P. H.: Bodenhausen, G. Chem. Phys. Lett. 1982, 8 9 , 139. (25) Ssrensen, 0. W.: Ernst, R. R. J. Magn. Reson. 1983, 5 5 , 338. (26) Morris, G. A.; Freeman, R. J. Am. Chem. SOC. 1979, 101, 760. (27) Burum, D. P.; Ernst, R. R. J. Magn. Reson. 1980, 3 9 , 163. (28) Wimperis, S.; Bodenhausen, G. J. Magn. Reson. 1986. 69. 264. (29) Le Cocq, C.; Laliemand, J.-Y. J. Chem. Soc.. Chem. Commun. 1981, 150. (30) Doddrell, D. M.; Pegg, D. T.: Bendall. M. R. J. Magn. Reson. 1982, 48. 223. (31) Kogler, H.; Ssrensen, 0. W.; Bodenhausen, G.; Ernst, R. R. J. Magn. Reson. 1983, 5 5 , 157. (32) Levitt. M. H. Pmg. Nucl. Magn. Reson. Spectrosc. 1986, 70, 518. (33) Friboiin, H. €in- und zweidimensionale NMR-Spektroskopie; Verlag Chemie: Weinheim, 1988. (34) Claret, F.; Carrupt, P.-A.; Vogel, P. Helv. Chim. Acta 1987, 70, 1886. (35) Marion, D.; Wuthrich. K. Biochem. Biophys. Res. Commun. 1983, 713, 967. (36) Morris, G. A.; Smith, K. I. Moi. Phys. 1987, 6 1 , 467. (37) Bax, A.; Freeman, R.: Kempsell, S . P. J. Am. Chem. SOC. 1980, 102. 4849.
RECEIVED for review April 25, 1989. Accepted July 14,1989. This research was supported in part by a grant of Spectrospin AG, F h d e n , Switzerland, and by the Swiss National Science Foundation.
Fiber-optic Dipping Sensor for Organic Solvents in Wastewater F. L. Dickert* and S. K. Schreiner Institute of Physical and Theoretical Chemistry, Erlangen University, Egerlandstrasse 3, 0-8520 Erlangen, FRG
G. R. Mages and Heinz Kimmel Siemens AG, Central Research and Development, ZFE Fl AMF 3, Paul-Gossen-Strasse 100, 0-8520 Erlangen, FRG
Triphenylmethane dyes were used as sensor materials for the detection of organlc solvents and gases (ammonia) in water vla changes In optical absorbance. For this purpose these sensor layers were separated from the aqueous solutions by a gas-permeabk membrane. The measurements were performed wlth a quartz fiber arrangement using optical detection In the vlsible range. The dlstlnct vapor pressures of the organic solvents In water lead to an enhancement of the sensor selectlvity In comparlson to the analogous gas-phase studies. Down to approxknately 30 ppm organic solvents In water can be analyzed In favorable cases.
Substituted 3,3-diphenylphthalides (I)form a highly colored triphenylmethane dye (11) by interaction with an acid component, e.g., a phenol (1). This reaction can be applied for the design of an optochemical sensor system for the detection of solvent vapors in the air (1,2). The sensor materials are used in thin layers (thickness 0.2-0.6 pm) on a carrier for optical transmission measurements. The absorption of solvent vapors leads to a decrease in the optical absorbance due to
a shift of the following equilibrium toward the colorless phthalide (I). R1
(I)
in)
These ideas are transferred to the development of a sensor arrangement for the detection of solvent traces in water, since huge amounts of wastewater are produced in the chemical industry that have to be continuously analyzed according to their organic ingredients. For this purpose the sensor layers are separated from the water by a permeable membrane (3). The solvent is allowed to penetrate the membrane, and its vapor yields the same sensor effect as described for gas-phase measurements (1). A fiber optical system is used as a transducer for the light (4, 5 ) . In this way the light of the emitter is transferred to the
0003-2700/89/0361-2306$01.50/0@ 1969 American Chemical Society
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dipping sensor A'Giiiifier
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Flgure 1. Bifurcated glass fiber bundle and sensor head: 1, wave guide end; 2, sensor layer; 3, metal cover with O-ring and mounting; 4, Teflon membrane; 5, windows; 8, gas volume: 7, mirror. layer and then back to the detector element. Remote monitoring of poisons in water can be accomplished by this procedure. The phthalides are practicable sensor materials for several organic solvents, and many water pollution problems can be solved by using the proposed system. EXPERIMENTAL SECTION Sensor Materials. Two distinct phthalides, the crystal violet which yields lactone (111)and a derivative with an indolyl ring (IV) especially large sensor effects for ammonia, were selected from a large variety (6) of compounds because of their pronounced absorbance in the visible range (6). Again bisphenol A (I)(Fluka) was an acidic ingredient for the formation of the dyes (11). The linearity of the characteristiclines over the whole measuring range can be drastically improved by the addition of 4-nitrophenol (Merck), which is a stronger acid in comparison to bisphenol A. In this way no plateau formation is observed at low solvent content in the air. The stability of the layers was improved by 10 wt % poly(vinyl chloride) (Fluka).
Measurements. The central part (Figure 1)of the equipment is a statistically mixed bifurcated glass fiber bundle (Oriel, Model 77533). The sensor material was applied in a tetrahydrofuran solution (Merck) to the common end of the wave guide (Figure 1, (1)).Sensor layers (2) of 0.2-0.4 pm thickness are formed after solvent evaporation. The sensor layer was protected from the liquid phase by a metal cover (3). The penetration of solvent vapor is possible by means of Teflon windows (5) (foils of 100 pm thickness, (4)) at the sides of the sensor head. The gas volume (6) in front of the sensor layer was minimized to realize short sensor rise times. A mirror (7) was adjusted at the bottom of the cap. The light passes the sensor layer twice due to a reflection (Figure l), and the sensitivity is enhanced in this way. A light-emitting diode (LED, A,, = 590 nm, Siemens) was taken as the light source. The rectangular signals (600 Hz) of a wave generator (Wavetec 164) were wed as a power supply. The normal operation current for the LEDs was 5 mA. Improved stability
-
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Figure 3. Dipping sensor output slgnal at 20 O C as a function of the addition of tetrahydrofuran (in volume percentages) to water at the marked time points. The sensor layer contained lactone (111) with equimolar 4-nitrophenoi and a 7.5 excess of bisphenol A. was achieved by applying an increased direct current of 20 mA for 2 days. A silicon photodiode was used as a detector followed by a self-built preamplifier. Modulation of the LED current (600 Hz) enables a lock-in amplifier (Ithaco 399) to be used and thus the stray light to be eliminated (Figure 2). The temperature stability of the sensor head was controlled with an accuracy of f O . l "C by immersing it in a bath to realize constant vapor pressures. RESULTS AND DISCUSSION Sensor Effect a n d Reversibility. A typical sensor response, the output voltage of the lock-in amplifier, as a function of the addition of tetrahydrofuran at the marked time points can be seen in Figure 3. Crystal violet lactone (111) with an excess of phenols was chosen as the sensor material. Decreasing absorbance leads to an increase in the light intensity at the detector diode, and the lock-in amplifier output voltage is enhanced. The water vapor also penetrates the Teflon membrane and therefore causes a diminution of the sensor layer absorbance (I) in comparison to dry air. The addition of 0.2% by volume of the organic solvent yields steps that slightly increase in height in parallel with the amount of tetrahydrofuran in water. This sensor signal can easily be converted to absorbances, and the steps attain an equal height in these units. The sensor is removed from the solution after the measuring procedure and is then again immersed in pure water. In this way the starting condition is reestablished and the absorbance becomes identical with that before the measurements. Spikes are observed for sensor responses as a function of time if organic solvents are studied that are only slightly soluble in water, such as ethyl acetate. Sensor Characteristics. Excellent linear characteristic lines are obtained if absorbances calculated from data as presented in Figure 3 are plotted versus the solvent content in water. For example, the addition of 1 vol % of tetrahydrofuran to water halves the initial absorbance and therefore appreciable effects are observed even for small amounts of organic solvents. Time Behavior. The exact rise times of the sensor responses can be determined from measurements as demonstrated in Figure 3 if the time scale becomes expanded. The
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Figure 5. Dipping sensor output signal for pure water as a function of temperature.
results in Figure 4 indicate that the sensor response to an increase of 0.2% ethanol is established in less than 10 s if the rise time is defined via a signal coming up to 90% of the end point. This finding is very favorable for chemical sensors and the rise times are faster than those observed for gas-phase measurements with low solvent amounts and low humidity. The sensor material behind the membrane incorporates water according to the saturation vapor pressure. The rate-determining process for the sensor effect is the diffusion of solvent in the layers and not the formation of the lactone from colored triphenylmethane dye (7). The sensor films are soaked, and therefore a high mobility of particles is realized, leading to very favorable time behavior. Temperature Dependencies. The sensor response is directly related to the vapor pressure of both water and the organic component. For this reason the temperatures of the solutions under investigation have to be exactly controlled, and defined conditions are established in this way. The effect of temperature on the sensor output for pure water is shown in Figure 5. A characteristic maximum is observed, which can be explained by two opposing effects. The vapor pressure increases in parallel with temperature. Thus a diminishing dye concentration in the layer is accompanied by a rise of voltage (Figure 5). Another effect, however, has to be included, namely the temperature dependence of absorption of solvent vapor in the sensor layers. This phenomenon obviously shows a larger temperature dependence than the vaporization process does, and therefore the signal decreases above 22 "C. These data for pure water are the ordinate intercepts of the sensor characteristics which describe solvent sensitivities at variable temperatures (Figure 6). The slopes of these straight lines increase with temperature, this behavior being due to the vapor pressure of the organic solvent in water. The absorption of ethanol in the sensor layers is favored by an increasing
temperature since the vaporization of the solvent is more strongly influenced than the desorption process. This phenomenon is quite plausible since ethanol and other organic solvents are more favorably adapted in their polarity to the sensor layers than water. Detection Limits. The sensitivity for minute amounts of organic solvents in water can be inferred from the slope of the characteristic lines and the noise amplitude of the measuring signal. The noise level in typical measurements absorbance was 0.5 mV, which can be converted to 4 X units (AU). The detection limits can be defined as twice this level. According to Figure 3 a limiting value of 15 ppm tetrahydrofuran can be detected in water by the proposed method. The sensitivity of the dipping sensor arrangement to nonaqueous solvents is not only affected by the noise level, but temperature control also has to be discussed. The pressure of water and its absorption in the sensor layer are temperature dependent as shown in Figure 5 . An accuracy of hO.1 "C in controlling the temperature of the water solution and the sensor element can easily be achieved. These variations lead to changes of f1.2 X lo4 and f4.1 X low4AU for the range of the positive and the negative slope of the curve in Figure 5 . Additionally the temperature dependence of the organic solvent vapor pressure has to be included since different slopes for the characteristic lines in Figure 6 are observed. These AU at changes in the slopes lead to a variation of 1.7 X a 0.5% content of ethanol in water if the temperature is altered by f O . l "C. This effect is negligible in comparison to the noise level. The ordinate intercepts, however, of the characteristics (Figure 6) have to be included also in discussing temperature dependencies. These effects are larger than the noise level, as can be deduced from Figure 5. This temperature dependence of the water sensor response simulates up to 30 ppm tetrahydrofuran in water. If the temperature of the sample or of the sensor layer is less exactly controlled or defined, the sensitivity and the inaccuracy of the dipping sensor will of course increase. Selectivity. A large number of different solvents were used to test the selectivity of this sensor equipment. The relative changes of absorbances that are caused by the addition of 0.5% organic solvent or ammonia to water are presented in Figure 7. A better visualization of the data is achieved by dividing all values by the minimum response of all chosen solvents observed for methanol. The sensitivities differ by a factor of up to 30. Three different effects have to be considered for the explanation of
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
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Figure 7. Dipping sensor sensitivities; relative changes of absorbance AA / A referred to the minimum response (AA /A o), (methanol) for the addition of 0.5 vol YO (ammonia; weight percentages) ingredients to water at 20 OC.
this finding. The first step for this sensor phenomenon is the absorption of solvent by the layers. These sensor materials exhibit amphiphilic properties since an ion reaction is used (reaction 1-11), but on the other hand the carbenium ions (11) show a very effective charge delocalization. For this reason solvent molecules with relative nonpolar solvents such as ethyl acetate are also incorporated by the sensor layers. The extent of optical absorbance changes is determined by the interaction of the absorbed solvent with the network of hydrogen bonds that is formed from the phenolic acid ( I ) . Sensor measurements with solvent vapors show that large sensor effects are observed for strong donors (8) or solvents that exhibit structure-breaking effects. For this reason tetrahydrofuran shows a pronounced sensitivity due to its oxygen donor atom. This effect is even surpassed by solvents with stronger donor properties (8)such as dimethylformamide, dimethyl sulfoxide, or hexamethylphosphoric acid triamide. In this case a direct interaction between the electron-deficient compound, namely the carbenium ion (91,and the donor molecule may occur and colorless 1:l adducts are formed. Additionally, however, the donor molecules form hydrogen bonds with the phenolic ingredient. Therefore the phenol is removed from the equilibrium and the reaction (1-11) is shifted toward the covalent colorless species. Ammonia shows strong donor properties, but due to its small molecular weight no effective absorption occurs. The same effect can be responsible for the larger
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sensitivity of tetrahydrofuran in comparison to that of diethyl ether. Ethyl acetate has less pronounced donor properties but may disturb the hydrogen network in a very effective manner. Methanol has an acid proton and can form hydrogen bridges with the phenols. Therefore the acidity of the layers is only slightly changed and methanol has a smaller effect than all the other applied solvents (I). The different vapor pressures of the organic solvents in water have to be included in a discussion to explain the distinct sensitivities of the proposed dipping sensor arrangement. The extent of solvent absorption by the layers and the position of the carbenium ion formation equilibrium (1-11) are not able to explain the results in Figure 7 since ethanol and ethyl acetate vapors, for example, show a similar sensitivity as could be shown for gas-phase measurements (I). The different sensitivities presented in Figure 7 can easily be understood by a deviation of the vapor pressure from ideal behavior. This phenomenon is predominantly observed for organic solvents that are very dissimilar to water or even only slightly miscible with water. These facts apply to tetrahydrofuran and ethyl acetate, and the partial pressures of these solvents are elevated by a factor of up to 50 at infinite dilution in comparison to an ideal behavior (IO). In going from acetone to ethanol and methanol, the activitity coefficients decrease from roughly 6 to 3 to 2 (IO). The partial pressures of the organic solvents in water depend very strongly on their polarity, and it can therefore be concluded that this is the main effect for the different sensor sensitivities as shown in Figure 7. Registry No. 111,1552-42-7;IV, 121861-10-7;tetrahydrofuran, 109-99-9; ethyl acetate, 141-78-6; ammonia, 7664-41-7; acetone, 67-64-1; ethanol, 64-17-5; methanol, 67-56-1; water, 7732-18-5; 4-nitrophenol, 100-02-7; bisphenol A, 80-05-7.
LITERATURE CITED (1) Dickert, F. L.; Lehmann, E. H.; Schreiner, S. K.; Kimmel, H.; Mages, G. R. Anal. Chem. 1988, 60, 1377-1380. (2) Posch, H. E.; Wolfbeis, 0. S.;Pusterhofer, J. Talanta 1988, 35, 89-94. (3) Bruckenstein, S.;Symanski, J. S. Anal. Chem. 1986. 58, 1766-1770. (4) Sepaniak, M. J.; Tromberg, B. J.; Vo-Dinh, T. Prog. Anal. Spectros. m a , 11,481-509. (5) Saari, L. A. TRAC, Trends Anal. Chem. (Pers. Ed.) 1987, 6, 85-90. (6) Dickert, F. L.; Vonend. M.; Kimmel, H.; Mages, G. R. Fresenlus' 2. Anal. Chem. 1989, 333. 615-618. (7) Brandner, G.; Dlckert, F. L.; Lehmann, E. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 740-745. (8) Gutmann, V. The
Donor Acceptor Approach to Molecular Interac-
tions; Plenum Press: New York, 1078;Chapter 2. (9) Biumenstock, H.; Dlckert, F. L.; Hammerschmidt, A. 2.Phys . Chem, (Munich) 1964, 139, 123-132. (10)Gmehling, J.; Onken, U. Vapor-LiquM €qui/ibr/um Data Collection; Friedrch Bischoff: Frankfurt, FRO, 1977.
RE" for review May 8,1989. Accepted July 6,1989. This work was supported by Fonds der Chemischen Industrie.
