1552
Anal. Chem. 1992, 64, 1552-1555
Chemical Sensing Using Concentration Gradient Transients Produced during Diffusive Transport of Analytes Janusz Pawliszyn Department of Chemistry and Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
The formation of concentratlongradientsduringthe extraction of analytes from an aqueous sample into an organic liquid phase Is the principle of thIs new chemical sensing method. The refractive Index gradient is probed by measuring the deflection of a focused laser beam which passes close to the interface between the two phases. The maximum of the concentration gradlent trandent is proportional to the concentration of analyte In the sample. The large concentration gradients generated at the interface durlng the initlai stages of the maw transport process ensure good sensltlvlty. The detection limit of this technique Is proportionalon the distance of the probing laser beam from the interface and the diffusion coeffkknt of the analyte in the organk phase. The seiectivtty of the methodIs relatedto the properties of the organic phase as described by the distribution constant. I n addition, separatlon of species varying substantially in diffusion codfldent Is achieveddncethe thw cwrerpondlngto the maxhum of the trandent is inversely proportional to the diffusion codkient. This approach allows very rapid analytlcal determlnatlondnce quantitatlonIsperformedIn the initialstages of the sample preparation step.
The design of new chemical sensors which facilitate rapid analysis of small sample volumes is one of the primary objectives of modern analytical research. Electrochemical methods have such properties. In these techniques, the current generated during the transport of electroactive species from the bulk solution to the electrode followed by a redox reaction is measured.' Similar information can be obtained by investigating the concentration gradients above the electrode ~urface.~s From Fick's first law; F = -D aclax, the flux of the species, F, transported to the electrode surface is proportional to the magnitude of the gradient dC/ax above it, where D is the diffusion coefficient of the analyte in the electrolyte. The concentration gradient approach is more general than electrometer detection. In the gradient method all types of chemical processes, not only those involving electrons, can be studied. For example, the electrode surface can be replaced by a catalytic surface or membrane to measure mass transport properties of the membrane or the selectivity of the catalytic reaction. The concentration gradient sensor can be utilized onlywith a detection technique capable of probing the small volumes associated with diffusion layers, which in most cases are less than 100 pm. A recent implementation of Schlieren optics,2 a refractive index gradient method known for over a century? requires only a single light beam to propagate through the volume being investigated. The deflection of the beam, 8, is (1) Rieger, P. H. Electrochemistry; Prentice-Ha& Inc.: Englewood Cliffs, NJ, 1987. (2) Pawliszvn, J. Spectrochim. Acta Reu. 1990.13. 311-354. (3) Pawliszyn, J. A d . Chem. 1988,60,1751-1758. (4)Toepler, A. Ann. Phys. Chem. 1866,127, 556. 0003-2700/92/0364-1552$03.00/0
proportional to the magnitude of the concentration gradient
aciax:2
e = L-(-)(-) an aC n a C ax where L is the path length of the beam through the concentration gradient and n is the refractive index of the medium. High spatial resolution and sensitivity of this type of measurement can be achieved by using modem optical components, such as lasers, to produce narrow beams and by using silicone position sensors.2 In addition, a selective technique of the concentration gradient measurement based on photothermal processes, which can identify and selectively monitor a target species forming the gradient, has recently been developed.2~~In this article basic procedures and instrumentation are described to monitor the concentration gradients produced during the diffusive transport of analytes through the interface between two phases.
EXPERIMENTAL SECTION Figure 1shows the design of two diffusion cells used in the experiments discussed in this article. The initial results were obtained with a cellulose nitrate membrane (0.2 pm pore size, Whatman Limited, Maidstone, England). A small round piece of the membrane about 2 mm in diameter was glued carefully onto one end of a piece of 2-mm-0.d. and about 1-mm-i.d.glass tubing using high-temperature epoxy (Epo-tek 353ND, Epoxy Technologies, Billerica, MA). This capillary waa then inserted into about a 5-cm length of 3-mm4.d. glass tubing (see Figure 1A). The aqueous sample was introduced to%hesystem by a piece of 0.5-mm-0.d. fused-silica capillary (PolymicroTechnologies, Tuscon, AZ). The pure solvent (water)waa deliveredusing microtubing made of polyfluorocarbon resin (Cole-Palmer,Wella sley, MA). The second design shown in Figure 1B consisted of asingle silicone hollow fiber membrane of 0.3-mm4.d. and about 0.6-mm-0.d. (Dow Corning CanadaInc., Mississauga, ON) placed in a 2-cm-longsquaretubing of 1-mmi.d. (VitroDynamics,Rockaway, NJ). The stripping fluid, hexane or water, flowed through the center of the fiber, while the aqueous sample flowed around the exterior of the fiber and entered through a fused-silica capillary. The diffusion modules were attached to a vertical translation stage (Klinger Scientific, Montreal, Quebec) to allow precise adjustment of the position of the laser beam versusthe membrane surface. The systems were mounted on a vibration-isolated optical table. A light beam, focused by a 5-cm focal length lens, from a helium-neon laser (Model 1303p, Uniphase, San Jose, CA) propagating 0.1 mm above the membrane waa used to probe the concentrationgradients. The detector used consisted of two photodiodes as described in ref 5. The detail description of the deflection sensor is proved in ref 2. A syringe operated manually or a syringepump with a 10-port valve (Valco Instruments Co. Inc., Houston, TX) facilitated delivery of the sample and the solvent. The hexane and benzene used were optima grade (Fisher Scientific,Napean, ON), and the sucrose was reagent grade (BDH, Toronto, ON). Poly(acry1ic (5) Pawliszyn, J. Rev. Sci. Instrum. 1987,58, 245-248.
@ 1992 Amerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1002
aqueous phase
1559
organic phase
\
u
-10
-8
-6
-4 -2
0
2
4
6
8
10
DISTANCE [a] Flgwo 2. Spatial concentration profiles of an anaiyte formed at the interface between two lmmlscible phases assuming K = 10 and Z = 1 at different times after contact (a) 0, (b) 17, (c) 107, (d) 1007, (e) 10007, (f) 100007, (g) -. The distance from the membrane surface is expressed in u = (2&)’/* units. describing Fick’s second law of diffusion+ ac,/at = D,
a2cwiaX2
(2)
a c j a t = D,
a2cjaX2
(3)
where D, and Doare the diffusion coefficients of the compound being extracted in water and organic phase respectively. C, and C, are the concentration distributions of the analyte in water and organic phase respectively. If it is assumed that before contact of the two phases the analyte is present only in the aqueous phase: C&,t = 0) = Ci,C,(x,t = 0) = 0 (Figure 2a), where Ci is the initial concentration of analyte in the aqueous phase and the distribution constant is defiied as K = C,(x = O,t)/C,(x = 0,t). This problem can be solved analytically using the Laplace transform method when it is assumed that the diffusion coefficient is independent of the analyte concentration:
Robe Beam
Flguro 1. Experimental arrangements wlth planar porous membrane (A, top) and wlth slilcone hollow fiber membrane (B,bottom).
(m
acid) = 15OOOO) was obtained from Polysciences, Inc., Warrington, PA. The samples were prepared by dissolving appropriate amountsof analytein deionized water. The samples and water were allowed to equilibrate to room temperature prior to their use in the experiments. The standard deviations of the methods were estimated on the basis of seven measurements.
RESULTS AND DISCUSSION In analytical practice, prior to the determination of organic compounds in aqueoussamples,a separation step is necessary to isolate target analytes from the matrix. This step usually consists of an extraction process using organic liquids. In many cases this process determines the overall time required for analysis since it is slow and difficult to automate compared to the actual determination step. In this paper we would like to consider another approach, where the determination is performed simultaneously with the separation step. This is facilitated by using an optical technique which is able to probe the extraction rates in such a way that the signal is proportional to the concentration of analyte in the sample. The concentration distribution of an analyte between two immiscible phases, for example organic (liquid fluid or polymeric membrane) and aqueous phases, in contact with one another can be calculated by solving differential equations
ZIK - erf(x/2D,1/2t1/2) C,(x,t) = ci 1+ ZIK
x
