Anal. Chem. 1992, 64, 1183-1187 (5) Mizutani. T.; Yamamoto, K.; Tajima, K. J . A@. FwdChem. 1978, 28, 862. (6) Haraguchi, K.; Kuroki, H.; Masuda, Y. J . chromew. 1988, 381, 239. (7) Haraguchi, K.; Kurokl, H.; Masuda, Y. chemapphwe 1989, 19, 487. (8) Masuda, Y.; Kuroki, H.; Haraguchl, K.; Nagayama, J. fnvkon. Haelth Perspect. 1985, 59, 53. (9) Bakke, J. E.; Bergman, A. L.; Larsen, Q , L. ScMce 1982, 217, 645. (10) Lindetrinn, K.;Schubert, R. M C 8 CC,J . H@ Resolut. chrometogr. Unvmetogr. C 0 " u n . 1984, 7 , 88. (11) Nagayama. J.; Klyohara, C.; Mohrl, N.; Hkohata, T.; Haraguchl, K.; Masuda, Y. Fukmka Acta Med. 1987, 78, 199. (12) Lund, J.; Brandt. I.; Poeiiinger. L.; Bergman, A.; Kiassen-Wehler, E.; Qustafsson. J. A. Md. Uk3rt%COl. 1985, 27, 314. (13) Bergman, A.; Jansson, B.: Bramford, I. Blamed. Mess Spectrom. 1988, 7 . 20. (14) Haraguchi. K.; Kurokl, H.; Masuda, Y. J. A@. Fwd Chem. 1987, 35. 178. (15) Haraguchi, K.; Kurokl, H.; Masuda, Y. Chenwspke 1987, 18. 2299.
1183
(16) Buser, H. R. Anal. Chem. 1985, 57, 2801. (17) Huckins, J. N.; Tubergen, M. W.; Lebo, J. A.; Gale. R. W.; Schwartz, T. R. J . A m . Off. A&. them, 1990, 73, 290. (18) Zook,D. R.; Buser, H. R.; Olsson, M.; BereqviSt, P. A.; Rappe, C. Ambio, In press. (19) Haraguchl, K.; Bergman, A.; Athenasladou. M.; Jacobsen, E.; Oisson, M.; Mesuda, Y. mxln 1990 Otganohelogen Compounds 1 1990.415. (20) Stemmier. E.; Hitee, R. A. Homed. Envfron. Mess Specbom. 1988, 17, 311. (21) Sears, L. J.; Campbell, J. A.; Wmsrud, E. P. Bbmed. Mess Spec&om.1987, 14, 401. (22) Bergman, A.; Brandt, I.; Haraguchi, K.; Larsen, 0.; tkn, U. D&xh 1990 Ckgen0he-n Compounds 1 1990, 23.
RECEIVED for review November 11,1991. Accepted February 13, 1992.
Peroxidase-Incorporated Polypyrrole Membrane Electrodes Tetsu Tatsuma,* Masayuki Gondaira, and Tadashi Watanabe* Institute of Industrial Science, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
Pyrrole was elecbopdymerlzed onto an Sn02ekcbock In the presence of horreradlsh peroxidase (HRP) and KCI. The HRP-hcoqmated polvpyrrole (PPy) w a n e deckode thw obtained respond8 to H202In a few aocondr and can sense 10-*-10-4 M H202In electrolyte solution8 containing no mediator. AII -atkn charge of around 5 mC ans was found to be optimal regardhgthe 8emWty andS/N ratb of the sensor output. PPy functbned not only as an enzyme supporl but ako as a part of the electrode material. We obtained evidence that an oxidation product(8) of pyrrole, most probably a pyrrole OlIgOmw(8), In the PPy membrane can modlate tho HRP-PPy electron trader, though a dlrecl electron transfer may also be possible. The deadyetate ekctrodo klnetlcr were qualitatlvdy analyzed.
in their systems. Fabrication of reagentless enzyme electrodes has been claimed by use of conducting polymers;le18 the polymers may electrochemically communicate with the enzyme active center,'6 or they can entrap an electron mediator.16 Lately, Wollenberger et al.'O fabricated a horseradish peroxidase (HRP)-incorporated polypyrrole (PPy) membrane on a pyrographite or platinum electrode (HRP/PPy electrode) for reagentless HzOzsensing. The electrode, however, is apparently insensitive to HzOzat concentrations below 0.1 mM. They claim the occurrence of a direct electron transfer between HRP and PPy (or base electrode) on the basis of the observation of sensor responses in test solutions containing no mediator. However, a possibility of HRP-electrode electron mediation by monomeric and/or oligomeric pyrrole, which may be present in the PPy membrane, has not been examined. In the preaent work we deposited an HRP/PPy membrane on a tin oxide electrode. As a result of optimization of fabrication parameters, we obtained electrodes exhibiting linear INTRODUCTION cathodic current responses in an HzOzconcentration range Sensing of H20z is important in several fields including of 10-alod M in electrolyte solutions containing no mediator. biochemistry and environmental chemistry. Often a signifiIn addition, we present pieces of evidence that an oxidation cantly low level of HzOzshould be detected since, e.g., subproduct(s) of pyrrole, contained within the membrane, memicromolar Hz02can damage mammalian cells.' Further, a diates the HRP-PPy (or SnOz)electron transfer. A result of sensitive HzOzsensor is useful to develop sensors for various steady-state response analysis is also given. substances by coupling it with H202-generatingoxidases.24 EXPERIMENTAL SECTION A glucose sensor with glucose oxidase is a typical example. Materials. Tin oxide (go00 A thick, F-doped) coated glass Peroxidases provide electrochemical H202 sensors with a plates as the base electrode were obtained from Nippon Sheet detection limit as low as lo4 M by use of a dissolved electron mediator for the peroxidase-electrode charge t r a n ~ f e r . ~ ~ ~Glass (Japan). Horseradish peroxidase (grade II)from Boehringer (FRG) was used without further purification. Each pyrrole sample Mediator-free peroxidase electrodes can be fabricated, but they (Tokyo Kasei, Japan) had been once distilled and stored under are insensitive to submicromolar HzOzat present.'-1° Pera nitrogen atmosphere, unlese otherwise noted. (3-Aminooxidase model electrodes as reagentless HzOzsensors, which propy1)triethoxysilane (APTES) and glutaraldehyde were purwe developed recently," also exhibit a sensitivity much lower c h a d from Shin-Etsu Chemical (Japan) and Sigma, respectively. than that of mediated peroxidase electrodes. Electrode Preparation. An Sn02-coatedglass plate (1.0 cm2) was pretreated with sulfuric acid that had been heated by dilution Conducting polymers have been proposed as electrode (1:l) for a few minutes. HRP/PPy electrodes were prepared by materials or enzyme supports for eledrochemical sensors.1s16 electropolymerizationin an electrolyte solution (5 mL) containing Electropolymerization of an appropriate monomer facilitates 0.06 M KC1,0.05 M distilled pyrrole, and 0.6 g L-' HRP at room one-step fabrication of enzyme electrodes with well-controlled temperature. Reference and counter electrodes were Ag/ membrane thickness, location, and area. A close contact of AgCl/KCl (0.06 M) and platinum black, respectively. Electroenzyme molecules with the conducting polymer chains gives polymerization was done in a gdvanostatic mode (current density, another potential advantage, namely the promotion of en0.1 mA cm-2),which yielded films with more uniform thickness zyme-electrode charge transfer, though some workers17J8 than in a potentiostatic mode. The electrode potential was +800 stated that the polymers did not work as conducting materials to +900 mV during polymerization. PPy electrodes without HRP 0003-2700/92/0364-1183$03.00/0
0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
51
d
4 N h
:
0 -8
v
-1
-6
-5
-4
-3
-2
Log ([H2O2I9M)
Flgure 1. Cathodic current increase vs H202concentration proflles O), PPy (A),and bare SnOl (V)electrodes at pH 7.4 for HRP/PPy (0, (0)or 6.4 (others). Electrode potential is +150 mV vs Ag/AgCI.
were fabricated in a similar manner. Electrodes covalently modified with HRP6were prepared as follows. An SnOTcoated glass plate was treated successively with a 10% aqueous solution of APTES for 30 min, then with a 2.5% aqueous solution of glutaraldehyde for 30 min, and finally with a pH 6.4 phosphate-buffered solution of HRP for 30 min, at room temperature. Measurement of the Sensor Performance. Electrochemical measurements were performed in a 1/15 M phosphate-buffered solution at 30 O C . Reference and counter electrodes were Ag/ AgCl/KCl (saturated) and platinum black, respectively. The solution was kept under stirring, unless otherwise noted. Calibration curves were obtained by stepwise injections of H20z standard solutions with Eppendorf pipets. RESULTS AND DISCUSSION Response Properties of the HRP/PPy Electrode. The HRP/PPy electrode generated a cathodic current by addition of H202to a mediator-free electrolyte solution at +150 mV vs Ag/AgCl. This sensor worked at potentiah up to +300 mV vs Ag/AgCl, beyond which the output current was canceled by the direct oxidation current of H202on PPy and/or SnOz. The response was complete in a few seconds. In a quiescent electrolyte, the output current took 3-6 min to reach a steady state. Depletion in H202inside and in the vicinity of the HRP/PPy membrane appears to determine the response time; a thicker diffusion layer in the quiescent solution retards to H202depletion in the diffusion layer. The steadystate cathodic currents at +150 mV vs Ag/AgCl (pH 6.4 or 7.4) are plotted against the H202concentration in Figure 1. The electropolymerization charge was 5 mC cm-2 for all the electrodes presented in this Figure. The background current (as cathodic current) was about 350 nA cm-2 at pH 6.4 or about 250 nA cmT2at pH 7.4 and has been subtracted from the observed currents. As seen, the HRP/PPy electrode can sense H20zat a level of 10-7-104 M. The detection limit of the HRP/PPy electrode prepared with an electropolymM HzOzbecause erization charge of 0.5-1 mC cm-2 was of a lower noise level. These detection limits are much lower than that (ca. M) of an HRPIPPy electrode fabricated by Wollenberger et al.'O even though the working potential of the former is more positive than that of the latter (-10 mV vs SCE). Though the difference between the two studies lies in the base electrode material, polymerization potential, and other polymerization conditions, which might affect the property of the electropolymerized film, we are at present unable to identify the principal cause(s) for such a large discrepancy. The cathodic current observed on the neat PPy electrode at pH 6.4 is 104-foldlower than that on the HRPIPPy electrode, and is comparable with that on a bare SnOz electrode.
v
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
was observed in mediator-free electrolyte solutions. However, they operated the sensor at a potential of -10 mV vs SCE, namely with an overvoltage of some 700 mV for the reduction of compounds I and 11. It is puzzling whether the HRP-PPy direct electron transfer needs such a large overvoltage. They did not discuss the pyrrole monomer or its oxidation products, such as pyrrole oligomers, which may be present in the PPy film, a~ possible electron mediators between HRP and the electrode. The latter point was examined in this work. For this purpose, SnOzelectrodes modified covalently with HRPG were employed in pH 7.4 phosphate-buffered solutions of pyrrole (0.05 M on the monomer basis) at +150 or +300 mV vs Ag/AgCl. Pyrrole was used after distillation and subsequent storage for about 40 days under nitrogen or under air. By use of pyrrole stored under nitrogen, the response of the HRPmodified electrode to 7 X lo-' M HzOzwas 1nA cm-z at +150 mV vs Ag/AgC1. Under identical conditions, the response was 1 order-of-magnitudehigher by use of pyrrole stored under air. Bare SnOzelectrodes exhibited no response to 7 X lo-' M HzOzin the presence of 0.05 M pyrrole, which had been stored under nitrogen or under air. These results evidence that an oxidation product(s) of pyrrole, formed more rapidly during storage under air as seen from the darkening of the liquid, mediates the HRP-electrode electron transfer. Since the amount of the oxidation product(s) is much less than monomeric pyrrole even in pyrrole stored under air, the mediation ability of the latter is negligible as compared to the former. Use of pyrrole stored under air gave a response to HzO2even at +300 mV vs Ag/AgCl, though the current was not reproducible, most probably because of an interference from the direct Hz02oxidation on Sn02 Thus the HRP-PPy electron transfer may be mediated by the oxidation productb) of pyrrole both at +150 mV and at +300 mV vs Ag/AgCl. Of course a quantitative comparison is difficult as to the sensitivity between the HRP-modified electrode and the HRP/PPy electrode because the amount of HRP, the HRP-electrode distance, and the concentration of the oxidation product(s) are different between them. Oligomeric pyrrole(s), which may be present in the HRP/PPy membrane, are candidates for the mediators because they are stable and can be further oxidized. Oxidation of oligomeric pyrrole(s) by compound I or 11,followed by the discharge of a cation radical of it on PPy or SnOz,is envisaged as a pathway of the mediation. Cation radicals of pyrrole oligomers are more stable than that of the monomer.l93 Diaz et al.19 evaluated the oxidation potentials of pyrrole, a-bipyrrole, and a-terpyrrole in acetonitrile to be 1200,550,and 260 mV vs NHE, respectively. The redox potentials of compound 1/11and compound II/ferric HRP were estimated to be around 900 mV vs SSCE.2l Though their redox potentials in aqueous media or in a PPy membrane are not known, the redox potential of monomeric pyrrole appears to be too high to donate an electron to compound I or II. Further, the redox potential of the pyrrole trimer may be too low to envisage an electron transfer from the electrode to a cation radial of it at +300 mV vs Ag/AgCl. These considerations incite us to suppose that dimeric pyrrole is acting as a mediator in the present system, though the occurrence of direct electron transfer between HRP and PPy cannot be excluded at the present stage. Kinetic Analysis. As seen in Figure 2, the sensitivity in the linear region is almost independent of the electropolymerization charge; the H2O2transport rather than the enzymatic reaction controls the output current here. On the other hand, the current in the saturated region depends on the charge up to 5 mC cm-2; the sensor output is controlled by the rate of
1185
I
'
A
A
0' -8
-7
-6 Log
-5
-4
-3
I -2
([HzOJ,M)
Dependencies of cathodic current increase on H,Op concentration for HRP/PPy electrodes with overlying (A),underlying (V), or no addltional(0)PPy layer. Charge for flkn formatbn is 5 mC om-' for each HRPJPPy or PPy layer. Figure 3.
the enzymatic reaction. The calibration curves for the f i b s formed with charges of 5 and 10 mC cm-2 exhibit a saturation at the same level; the enzymatic reaction no longer controls the output current even at the saturated region. The Saturated current is most probably limited there by proton transport, because both reactions 2 and 3 require a proton. This is in line with the observation that a lower pH yields a higher current in the saturated region (Figure 1).Since the enzymatic reaction no longer controls the output current at 5 mC cm-z at both pH 6.4 and 7.4, the pH dependence of the saturated current does not result from that of the enzymatic activity. Further, we fabricated HRP/PPy electrodes with an overlying PPy layer. An HRP/PPy layer and a PPy layer (5 mC cm-z for both layers) were successively deposited on the SnOzsurface. The electrode thus obtained exhibited lower sensitivity to Hz02and a lower saturated current than that without the overlying PPy layer (Figure 3). Retardation of HzOzand proton transfer by the overlying PPy layer may be responsible for the reduced sensitivity and saturated current. This indicates that the sensor responses are controlled by mass transfer on the electrode prepared with an electropolymerization charge of 5 mC cm-2. In what follows we qualitatively analyze the steady-state electrode kinetics. The output current is proportional to the enzymatic reaction flux J:
i = 2FecJ
(4)
where ec is the enzyme-electrodecharge-transfer efficiency. The potential flux Jpt,Le., the flux of the enzymatic reaction where mass transport out of or within the HRP/PPy film does not control the sensor response, is given by
CEd (5) l/~,[HZOZl + (kz + W/kZMH+l where kl, k2,and k3 are the rate constants for reactions 1-3, respectively. CEis the HRP concentration in the membrane, Jpot
=
and d is the membrane thickness which can be controlled by the film deposition charge. If d is sufficiently small, J is equal to Jpt, since mass transfer does not limit the enzymatic reaction flux. Thus we estimated [H+]kzk3/kl(kz+ kS),the Michaelis constant for the incorporated HRP, to be about 4 X M from the calibration curves for thin HRP/PPy membrane electrodes. Curves a-e in Figure 4 are schematic calibration curves for HRP/PPy electrodes where the enzymatic reaction controls the output currents. The HRP/PPy membrane for curve b is 2.15-foldthicker than that for curve a, and so forth.
15, 1992
5.
,,.................... d c
.
b a
0
ne*u 4. schematlc ceybretlon W e s for HRP/PPy electrodes. unes 1 and 2 indicate the l i m b by H,Op and proton transfer, respecttvely. Curves 8-8 are the schematlc calibration curves for electrodes with various thlckness of HRP/PPy membrane, where enzymatic reaction is assumed to control the response.
Next we consider the mass-transfer phenomena. With an increase in the membrane thickness, the sensor response rises until it is controlled by mass transfer. We confirmed experimentally that HzOztransport limits the sensitivity to about 0.45 A mr2M-' and that proton transport limitsthe saturation A level to about 4.5 X at pH 6.4. When the current is completely limited by HzOzdiffusion, the H202concentration in the HRP/PPy film and at the fi/solution interface should be nearly equal to zero. Therefore, the limited current ili, can be formulated as follows:
ili, = 2FeCDHP[H202]/1
(6)
where DHpis the diffusion coefficient of HzOz and 1 is the diffusion layer thickness. On an assumption that ec equals unity, the thickness 1 is estimated to be about 10" cm, which is a reasonable value. Equation 6 cannot be applied to the proton transfer because proton is supplied to the electrode not only by diffusion from the solution bulk but also by H20 dissociation. These limits by Hz02and proton transfer are given in Figure 4 as lines 1and 2, respectively. The output current cannot exceed these limits. Thus, calibration curves are drawn as solid curves in Figure 4. In the present analysis, we did not consider, for simplicity, the intermediate regime where the enzymatic reaction competes with the mass transfer. Nevertheless, the qualitative features of the experimental calibration curves are well reproduced by these schematic drawings. PPy as an Electrode Material. In an enzyme-incorporated conducting polymer membrane electrode, the polymer acts as an enzyme support and is expected to function also as a part of the electrode material. Belanger et al.'* and Bartlett and Whitaker," however, stated that conducting polymers did not work as electrode materials in their own glucose oxidase/PPy18 and glucose oxidase/poly(N-methylpyrrole)" systems mediated by Oz/HzOz. Here we examined whether PPy functions as an electrode material in the present system. We fabricated HRP/PPy electrodes with an underlying PPy layer, by successively depositing the two layers on SnOzat an electropolymerization each. The thickness of the underlying charge of 5 mC PPy layer was estimated to be about 13 nm.22 The H202 calibration curve obtained with this electrode was practically the same as that of the HRP/PPy electrode (5 mC cm-2) without an underlying PPy layer (Figure 3). This evidences that the PPy layer functions as a conductive material in the present system. In the case where PPy acts as a conductor, the sensor output should not be affected by the presence of
the underlying PPy layer, whether the HRP-PPy electron transfer is direct or mediated. If, on the other hand, Sn02 alone functions as an electrode material,the sensitivity should be reduced because the underlying PPy layer would retard the transport of the mediator(& and thereby the oxidized mediator(s) would be lost at least partially by decomposition during diffusion to the SnOz surface. However, there is still a possibility that the electroconductivity is different between the underlying PPy film and the HRP/PPy f i i . To examine this, we compared the sensitivity of HRP/PPy electrodes formed with charges of 10 or 20 mC cm-2. At >5 mC the system is completely controlled by diffusion and hence the enzyme works only in the vicinity of the film/solution interface. Therefore, in the case where the HRP/PPy is insulating, a lower sensitivity should be observed at 20 mC cm-2 because the diffusion lengths are longer both for the oxidized mediator from the enzyme to the SnOz surface and for the reduced mediator from the Sn02 surface to the enzyme. Similar observations have been reported by Bartlett and Whitaler" and Kajiya et a l m Nevertheless, no significant difference in the sensitivity was observed between the two electrodes. This again indicates that HRP/PPy functions as an electrode material. Belanger et d18showed that a high concentration M) of H202deteriorates the conductivity of PPy by attacking the latter. In our system operated under mass-transport-controlled conditions, however, the HzOzconcentrations is 60% of the initial sensitivity to HzOzfor at least 2 weeks when stored in the buffer. The sensitivity decayed more rapidly in air, where a loss of the enzyme or pyrrole oligomers is not expected. The faster deterioration of the sensor in air may be due to deactivation of the enzyme, the PPy film, and/or pyrrole oligomers. To retain the sensitivity for a longer term, H202solutions at concentrations above 10" M should not be tested because then HFW is readily deactivated. Further, at such high H202 concentrations, proton is depleted inside the HRP/PPy membrane as mentioned abve, leading to HRP deactivation and alteration of the PPy film property.
ACKNOWLEDGMENT This work was supported in part by the Chemical Materials Research and Development Foundation. REFERENCES (1) Tatsuma, T.; Itoh. K.; Fujlshlma, A.; Kubota, Y. Unpubliehed results, Tokyo. 1987. (2) Hahn, Y.; Olson. C. L. Anal. Chem. 1979, 57, 444-449. (3) Kulys, J. J.; Pesllakiene, M. V.: Samallus, A. S. 8&e&c"?. Bloe, m g . 1981, 8. 81-88. (4) Tatsuma. T.; Watanabe. T. Anal. chkn.Acta 1991, 242, 85-09. (5) Frew, J. E.; Harmer, M. A.; HIM, H. A. 0.;Ubw, S. I. J . Electmenal. Chem. InterfacialEIectrochem. 1988, 201. 1-10. (6) Tatsuma, T.; Okawa, Y.; Watanabe, T. Anal. Chem. 1989, 61. 2352-2355. (7) Kulys, J. J.; Samallus. A. S. E l e k b o k M ~ a1984, 20, 637-641. (8) Armstrong, F. A.; Lannon, A. M. J . Am. Chem. Soc. 1987. 109, 7211-7212. (9) K&s, SchmM, R. D. J . Elecboenal. Chem. Interfaclel E k b p chem. wee. 24. 305-311. (10) Wollenberger, U.;Bogdanovskaya, V.; Bobrin, S.; ScheHer, F.; Tarasevich, M. AMI. Lett. ieeo. 23, 1795-1508.
i;
Anal. Chem. 1992, 64, 1187-1199
(l5j (18)
(17) (18)
Tatsuma, T.; Watanabe, T. Anal. Chem. 1991, 63, 1580-1585. Foulds, N. C.; Lowe, C. R. J. Chem. Soc., Faraday Trans. 1 1988, 82, 1259-1204. Umana, M.; Walk, J. Anal. Chem. 1986, 58, 2979-2983. Foulds, N. C.; Lowe. C. R. Anal. Chem. 1988, 60, 2473-2478. Iwakura, C.; Kajlya, Y.; Yoneyama. H. J . Chem. Soc., Chem. Commun. 1988, 1019-1020. Yabuki, S.; Shinohara, H.; Alzawa, M. J . Chem. Soc., Chem. Commun. 1989, 945-940. BarHett, P. N.; Whbker, R. G. J. Electroanal. Chem. InterfacialEksctrochem. 1987, 224, 37-48. Belanger. D.; Nadreau. J.; Fwtler, 0.J . Elecmnal. Chem. Intetfaclal Eksctroci". 1989, 274, 143-155.
1187
Dlaz, A. F.; Croeley, J.; Bargon, J.; Gardini, G. P.; Torrance, J. B. J . Electmanal. Chem. Interfaclel Electrodh9m. 1981, 121, 355-301. Wakman, R. J.; Bargon, J. T e b r r m 1984, 40, 3863-3970. . , Heyashl. Y.; Yamazakl. I.J . Bkl. Chem. 1979, 254, 9101-9100. (22) Dlaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W. Y. J. Ektroanal. Chem. I n t e r i a c i a l E l e c f . 1981, 129, 115-132. (23) Kajlya, Y.; Tsuda, R.; Ymyama, H. J . Ebcboenel. Chem. Interfacial EksChochem. 1991, 301, 155-104.
RECEIVED for review December 2,1991. Accepted February 18, 1992.
Dynamics of Organic Compound Extraction from Water Using Liquid-Coated Fused Silica Fibers Derek Louch,Safa Motlagh, and Janusz Pawliszyn* Guelph- Waterloo Center for Graduate Work in Chemistry and the Waterloo Center for Groundwater Research] University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
&Hd-phase mkroextractlon (SPME) b an Inexpensive, raw, and solvenl-free extractbn method for the ioolatlon of organlc compounds from an aqueous sample. The technique uses a few centhwten, of poty(dknethyldkxane)coated hmd dllca optlcal flber whlch Is mounted for convenlence and ruggedness Into a mkroeyrlnge. The organic contaminants are extracted into the coatlng and transferred for thermal desorptkn and analysls into the InJector of a gas chromatograph. Mathematlcal descrlptlone of the absorptlon and derorptlon processes were developed and compared wtth experlmental results. One model assumes a perfectly agltated solutlon whkh results In extractlontlmes dependent only on dMudon of analyle In the coating. The second model considers extraction from a static solutkn. I n thls case extraction tlmes are determkml by dmudon of anatyte In water. These models facllltate a better understandlng of the extractlon process. Resutts lndlcate that when standard stlnlng equipment Is used as a means of agltatlon, the dynamlcs of the extractkn process Is controkd by dmuskn of anatyte uvough the thh sta& aqueous layer kcated around the flber. Extractlontlmes for benzene, toluene, and p-xylene wlng a coating thickness of 55 Mm are under 10 mln and can be shortened substantially when more efficient agltatlon methods are ucied. The technlque allows rub-ppb determlnatkn of organhx In water wlth flame knlzatbn detectkn. The llnear range of the method b over several orders of magnttude. The sensltivlty of the technlque Is dependent on the coatlng volume and the coatIng/water dlstrlbutlon constant. The relative precldon of the method b a few percent and b dependent on the thlckneos of the coatlng. Uslng thls technique, poly(dlmethy1slloxane)/water dlrtrlbutlon constants were determined for benzene (125), toluene (294), and p-xylene (831) and are rlmilar to correspondlng values for octand/water diarlbution constants.
INTRODUCTION The contamination of water supplies by organic pollutants has become a major concern to the public which demands better control and monitoring of this important resource. To 0003-2700/92/0384-1187$03.00/0
achieve this goal, frequent analysis of water samples is required. In the first step of the analytical process the organic contaminants are separated from the aqueous matrix. The separation techniques are divided into two major groups:' concentration methods, in which water is removed and the dissolved substances are left behind, and isolation methods in which the dissolved substances are removed from water. Isolation techniques are the more commonly used methods in the analysis of organics in water. Volatiles are usually analyzed by the use of purge and trap which is a US.Environmental Protection Agency (EPA) approved technique,2 stripping and headspace analysis. These methods either require expensive instrumentation or are not sufficiently sensitive. Nonvolatiles are analyzed primarily by the use of liquid-liquid extraction (LLE), which is an EPA approved technique? and solid-phase extraction (SPE). These methods are generally time consuming, are difficult to automate, and use expensive high purity toxic organic solvents. Recently a novel technique, solid-phase microextraction (SPME), was proposed which solves many of the problems associated with the more traditional In this procedure, a small diameter optical fiber coated with a polymeric phase is placed in an aqueous sample. The analpartition into the stationary phase and are then thermally desorbed, on-column, in the injector of a gas chromatograph. Since the coatings used are almost always viscous liquids, the extraction is, in effect, a nonexhaustive liquid-liquid extraction with the convenience that the "organic phase" is attached to the fiber. This fiber is contained in a syringe which protects it and simplifies introduction of the fiber into a gas chromatographic (GC)injector. The total analysis time is only a few minute^.^ Optical fibers are used because they are inexpensive and are made of chemically inert fused silica,the same material used to make capillary GC columns. They have a small diameter which allows convenient introduction into a chromatographic injector. These fibers coated with polymers such as poly(dimethylsi1oxane) of various thicknesses are available commercially and can be used directly in extractions. The uncoated fibel.3 can also be used or a variety of common chromatographic stationary phases can be attached to ita surfacea4This technique can be applied successfully to the analysis of volatile chlorinated organics such as chlorinated organic solvents and substituted benzenes as well as nonvo0 1992 Amerlcan Chemical Society
