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Durable Microfabricated High-Speed Humidity Sensors Petr Kuban,†,‡ Jordan M. Berg,§ and Purnendu K. Dasgupta*,†,§
Department of Chemistry and Biochemistry, and NanoTech Center, Texas Tech University, Lubbock, Texas 79409
We describe a durable microfabricated humidity sensor made of interdigitated rhodium electrodes on a silicon substrate covered with a sensing film of Nafion perfluorosulfonate ionomer. Rhodium electrodes are much less prone to oxidative degradation compared to previously described gold electrode-based sensors. Even with dc excitation, Rh electrode sensors exhibit excellent longterm response stability. It has been found that lowamplitude ((1 V) square wave excitation can prolong the usability of gold electrode-based sensors to at least several months; however, this mode of interrogation cannot provide subsecond response times. Rhodium deposition on the microsensors is much more difficult than that of gold. We were able to attain crack-free Rh deposits by adaptation of pulsed electroplating techniques. At excitation voltages of >2 V dc, the Rh sensors respond to moisture with 10 T 90% rise and fall times of 30-50 ms. These are the fastest microfabricated water vapor sensors reported to date. We demonstrate applications as a breath monitor. Such sensors should also be of utility in atmospheric eddy measurements. Short-term repeatability is better than 0.6% RSD (n ) 7). Measurement of humidity is important in a large number of application areas and spans across disciplines. A variety of transduction techniques and principles are used; Rittersma has recently reviewed such techniques focusing on present-day miniaturized sensors.1 The most common humidity transducers consist of at least two electrodes separated by a substrate with a high affinity for water, such as porous silica or alumina or a suitable polymeric material. Humidity sensors based on polymeric thin films have been specifically reviewed by Sakai et al.2 Typically, a change in capacitance of the substrate upon reversible water absorption is monitored. The sensor response time is generally limited by the rate of sorption/desorption of water from the substrate and ranges from several seconds to several minutes. Less often, a transducer of similar construction is used in the resistive mode; the impedance change of the sensor is monitored. Traversa3 has reviewed ceramic sensors of this type and distinguishes between ionic and electronic conduction modes. Nafion * Corresponding author. E-mail:
[email protected]. † Department of Chemistry and Biochemistry. ‡ Permanent address: Department of Chemistry and Biochemistry, Mendel University, Zemedelska 1, CZ-61300, Brno, Czech Republic. § NanoTech Center. (1) Rittersma, Z. M. Sens. Actuators. A 2002, 96, 196. (2) Sakai Y.; Sadaoka, Y.; Matsuguchi, M. Sens. Actuators. A 1996, 35, 85. (3) Traversa, E. Sens. Actuators. B 1995, 23, 135. 10.1021/ac0355451 CCC: $27.50 Published on Web 04/03/2004
© 2004 American Chemical Society
is a perfluorosulfonate ionomer that exhibits ionic conduction. The first use of Nafion as a solid electrolyte for a moisture sensor is now more than 25 years old,4 but attempts to use thin films cast from solution first began in this laboratory in 19905 and were followed by Nafion-P2O5 composite films for detection of very low humidities,6 cross-linked Nafion-PVA copolymers for deployment in organic solvents,7 and Nafion-polyestersulfonate composite films (that are deliberately insensitive and function only over the appropriate range) for soil moisture measurement.8 Others have continued the study of this unique, electrolytically stable, hygroscopic, ionically conductive polymer for thin-film miniaturized sensors for humidity measurement, both by itself 9 or as a sol-gel composite.10 Nevertheless, fast response has not necessarily been a goal in these studies. The importance of sensing film thickness in controlling response time is widely understood: the characteristic diffusion time varies as the square of the film thickness. However, it is less often appreciated that, given the same film thickness, the mode of measurement can affect the response time. In particular, at low face velocities because of boundary layer stagnation, the desorption time of moisture from the polymer can be slow and become the ratedetermining factor. As such, under these conditions, rather than noninvasive impedance measurement, if the water arriving to the film is actively electrolytically “burned” by applying a high enough electric field, the desorption time limitation is largely removed. This was realized by Keidel11 nearly half a century ago using films of H3PO4 that are “burned” to P2O5; such sensors are still in wide commercial use. Recently, we demonstrated that sensors microfabricated on silicon with a submicrometer-thick Nafion sensing film and interdigitated gold electrodes can easily produce subsecond response times along with other attractive attributes such as negligible hysteresis.12 Operation in the electrolytic domain requires, however, some minimum applied voltage. The response of the sensors above exhibited a gradual decrease in response over a prolonged (4) Lawson. D. D. U.S. Patent 4,083,765, April 11, 1978. Ishifuku Metal Industry Co. Ltd., Japanese Patent JP 60 36 947. February 26, 1985. Yamanaka. A.; Kodera. T.; Fujikawa, K.; Kita, H. Denki Kagaku 1988, 56, 200. (5) Huang, H.; Dasgupta, P. K. Anal. Chem. 1990, 62, 1935. (6) Huang, H.; Dasgupta, P. K.; Ronchinsky, S. Anal. Chem. 1991, 63, 1570. (7) Huang, H.; Dasgupta, P. K. Anal. Chem. 1992, 64, 2406. (8) Huang, H.; Dasgupta, P. K. Electroanalysis 1995, 7, 626. (9) Wang, H.; Feng, C. D.; Sun, S. L.; Segre, C. U.; Stetter. J. R. Sens. Actuators. B 1997, 40, 211. (10) Feng, C. D.; Sun, S. L.; Wang, H.; Segre, C. U.; Stetter, J. R. Sens. Actuators. B 1997, 40, 217. (11) Keidel, F. A. Anal. Chem. 1959, 31, 2043. (12) Su, X.-L.; Xingguo, X.; Dallas, T.; Gangopadhyay, S.; Temkin, H.; Wang, X.; Walulu, R.; Li, J.; Dasgupta, P. K. Talanta 2002, 56, 309.
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Figure 1. Process flow for fabrication of Au and Rh electroplated sensors.
operation period. Microscopic examination showed obvious oxidation of the Au anode. In this article, we show that the use of Rh as the electrode material completely overcomes these limitations. With 50-µm electrode spacing and a ∼200-nm-thick sensing film, 10 T 90% response times of 30-50 ms are routinely attained; we demonstrate its application as a breathing monitor. We also discovered that if a low-amplitude square wave excitation function and appropriate signal processing is used, Au sensors can be used over many months, albeit response time deteriorates to several seconds. EXPERIMENTAL SECTION Microfabrication. In previous work, gold electrodes were fabricated by electron beam deposition; there are limitations on how thick (and durable) an electrode can be conveniently fabricated in this manner. An alternative fabrication method that involved electroplating was used in this work (Figure 1). Humidity sensors were fabricated on 50-mm-diameter, p-type (100) silicon wafers. Wet thermal oxidation at 1100 °C for 70 min in a temperature-controlled furnace (Thermco MB-71) was used to produce a ∼500-nm-thick SiO2 layer. A 30-nm-thick adhesion layer of Ti was next deposited using a home-built high-vacuum electron beam evaporator. For sensors with gold electrodes, this was followed by electron beam evaporation of a 30-nm Ni seed layer without breaking vacuum. For sensors with Rh electrodes, an Au seed layer was deposited instead of Ni. A metal overlayer on Ti is required because Ti oxidizes upon exposure to air, resulting in a surface to which metals do not electroplate well. Nickel is used as a seed layer for electrodeposited Au electrodes to allow subsequent selective chemical etching. The Au seed layer used for the electrodeposited Rh electrodes was critical in reducing cracking in later patterning steps. A positive photoresist (Shipley 1813, Shipley, Marlborough, MA) was deposited by spin coating at 4000 rpm for 30 s and baked in an oven on a metal plate for 3 2562
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min at 115 °C. The resist-coated wafer is patterned in a mask aligner (Canon FPA-141F) using a 15-s exposure to broadband UV light from a mercury vapor lamp. A four-in. glass/iron oxide photomask defines the interdigitated electrode patterns. The mask contains an assortment of electrode patterns that have different interdigit spacing and the number of digits. The common features are 50-µm digit width and a total sensor area of 2 × 2 mm. The interdigit gap is varied between 50 and 200 µm, with the number of digits thus varying between 20 and 8. After exposure, the photoresist is developed for 45 s in a 1:1 mixture of developer (Shipley Microposit STR) and deionized water, leaving spaces corresponding to the desired electrodes clear of resist through which the Ni/Ti layer is exposed. The patterned resist layer then received a postbake (3 min, 115 °C) to improve chemical resistance to the plating baths. The gold electroplating process is robust to variations in process parameters, including temperature, current density, and plating pulse duty cycle. However. plating Rh proved significantly more difficult. While macroscale plating presented no particular problems, plating on microelectrodes was of poor quality, filled with microcracks, and the resulting Rh overcoat showed poor adhesion and peeled off easily. Even when microcracks were not readily visible, the underlayer was readily oxidized in use resulting in device failure and indicating that microcracks must have been present. The quality of the Rh plating was found to be sensitive to both the metal underlayer and the plating technique. Gold was found to be the best underlayer, and a pulsed plating technique13 was found to be essential for crack-free Rh plating. For gold plating, a cyanide-based plating bath (Orotemp-24RC, Technic Inc., Cranston, RI) was operated at 55 °C under vigorous magnetically stirred conditions. The metal seed layer on the wafer (13) Miu, W. S.; Fung, Y. S. Plat. Surf. Finish. 1985, 72 (5), 37. Miu, W. S.; Fung, Y. S. Plat. Surf. Finish. 1986, 73 (3), 58.
Figure 2. Micrograph of section of electroplated rhodium electrode with 50-µm-wide digits and 50-µm interdigit spacing. Inset shows a complete sensor with 150-µm interdigit spacing.
was connected to the positive terminal of a current-controlled pulsed power supply (model DuP10-.01-.03, Dynatronix Inc., Amery, WI), and a 10 × 10 cm platinized titanium counter electrode was connected to the negative terminal. The counter electrode and the wafer were immersed in the bath after activating the power supply. Gold was electrodeposited at 10% duty cycle (1 ms on/ 9 ms off), using an average of 1 mA of current, corresponding to an average current density of 3 mA/cm2 for the portion of the seed layer exposed through the patterned photoresist. This current is applied for 10 min, after which the wafer is removed from the plating bath and thoroughly rinsed in deionized water. The photoresist coating is removed by sonication in acetone. The Ni/Ti seed layer is removed by etching in a solution of 1:1: 10 HF/HNO3/H2O for 1 min, with the Au layer serving as a mask for the desired electrode pattern. Measurements with a stylus profilometer (Sloan Dektak II) indicated that the deposition rate is ∼0.1 µm/min; hence, the resulting electrodes are ∼1 µm thick ((10%). For the Rh sensors, Rh was electrodeposited on gold using a rhodium sulfate/sulfuric acid bath (Rhodium S-Less, Technic Inc.) at 50 °C, with a current density of 40 mA/cm2 for the exposed portion of the seed layer, and all other plating conditions being the same as those used for gold plating. Figure 2 shows that the films achieved in step 2 are of high quality. After removing the photoresist, the Au portion of the seed layer is removed by etching in a saturated solution of KI/I2 in deionized water for 1 min. The Ti portion is subsequently removed by etching in 1:1:10 HF/ HNO3/H2O for 1 min as above. Rhodium thickness was measured to be between 0.89 and 1.4 µm, with an average value of 1.1 µm and an RSD of 20%. Again, the deposition rate is ∼0.1 µm/min. Both the Au and Rh sensors were tested for adhesion by rubbing on adhesive tape (3M, Scotch brand) and lifting off the tape. Successful adhesion was indicated by failure of the tape to lift off metal. Both types of sensors passed the tape test without problems. The sensors were dried at 120 °C for 30 min and tape strips (Kapton tape, 1 mil thick, 2 mm width, Kaptontape.com, Torrance, CA) were applied to protect the bond pads, as any polymer coating on these pads will prevent subsequent wire bonding. Nafion (5% w/v solution in alcohols, Sigma) was spun onto the wafer at 4000 rpm for 30 s, resulting in a layer ∼200 nm thick. The wafer was baked for 30 min at 120 °C, and the tape was removed.
Measurement of film thickness by scanning electron microscopy (Hitachi S 5000) was carried out after shadowing the sensor with a very thin layer of metallic Au to render the surface conductive and thus avoid surface charging. The microscopically measured thickness was 170 nm. The film thickness was also measured for comparison purposes by ellipsometry (AutoEL IV, Rudolph Technologies, Flanders, NJ) to be 196 nm, assuming the refractive index of Nafion to be 1.46 (as stated by the manufacturer). The present mask was not optimized for the maximum number of sensors; the yield was 25 sensors/50-mm wafer. The wafer was cleaved to isolate individual sensors. These were then each affixed to a TO-5 platform with epoxy adhesive. Electrode contacts were made to the sensor bonding pads and the TO-5 posts with a hybrid thermal/ultrasonic wedge bonder (model 1204 W, MEI, Danvers, MA). The sensors were then ready for testing. Sensor Test Setup. The sensor test setup is critical in accurately determining sensor response time. Specifically, water sorption/desorption occurs on almost any surface. As such, unless the test system is properly configured, the observed response time for a fast sensor will be greatly affected by the water sorption equilibrium on the conduit. In determining the change in response from wet to dry air or vice versa, one effective option is to deliver wet air and dry air through independent conduits to the sensor surface via independent solenoid valves. Since the solenoid valves used here (Parker-Hannifin, Skinner MBD 002) have a maximum actuation time of 15 ms14 at the rated voltage, closing the valve in one line and opening the other simultaneously should allow one to measure response times that are of this order or greater. We follow essentially the same test arrangement previously used.12 Briefly, pure dry air is divided in three streams, each proceeding through a mass flow controller. One stream proceeds through a fritted bubbler to completely humidify the stream; this is mixed with a second stream of variable flow rates of dry air to attain a mixed flow of variable relative humidity (RH) (30-100%) that is delivered to the sensor via one solenoid valve. Through the other solenoid valve, driven antiparallel to the first, dry air is delivered to the sensor. The flow delivered to the sensor was within the 200-500 standard cubic centimeters per min (sccm) range; 500 sccm was used except as stated. The valve switching was controlled from a PC by custom software using a SOFTWIRE extension of Visual Basic (Measurement Computing Inc., Middleboro, MA) and a PCM-DAS16D12/AO card from the same vendor. The digital outputs from the card were connected to the solenoid valves via a power MOSFET switch (IRLI1530N, International Rectifier) to supply 12 V dc power. All experiments were conducted at ambient laboratory temperature (22-23 °C). Measurement Circuits. Both dc and square wave interrogation of the sensor was used. In the dc mode, the sensor current was converted to voltage using a operational amplifier (TL082CP, Texas Instruments)-based current-voltage converter. In the square wave interrogation mode, a square wave signal from a function generator (Precision 3011, Dynascan Corp.) was used for sensor excitation at a frequency of 5 Hz and a p-p amplitude of 2 V. The sensor current was converted to the voltage, (14) http://www.parker.com/EAD./Digital_asset_display.asp?digital_asset_ id)10389.
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Figure 3. Schematic for square wave measurements.
Figure 4. (a) New gold humidity sensors. (b) gold humidity sensor after 20 h of operation at 2 V dc; the dark electrode was the anode.
rectified, filtered, and amplified using the circuit in Figure 3. Data were recorded with the same card used for the valve switching. RESULTS AND DISCUSSION Sensor Operation at Low Voltages. When operated at relatively high (10-40 V) dc voltages, at ambient RH levels the current produced is tens of microamperes to miliamperes. Initial experience with Au-based sensors showed that under these conditions the sensor output signal observably decreases within 1 h, and eventually the sensors cease functioning. The anode was found to be visibly discolored, presumably due to oxidation. Assuming that the use of low voltages may ameliorate this problem, we operated the sensors at 2 V dc. Even in this case, in less than a day of continuous operation, the discoloration/ degradation of the anode was readily visible (see Figure 4a vs b). Rhodium is known to be more oxidation resistant;6,11 the rhodium electrode-based sensors were therefore fabricated. No degradation of the electrodes were observable over several days of 2 V dc operation. All following results are for the Rh electrode sensor, except as specifically stated. Current-Voltage Behavior at Low dc Voltages. The Rh sensor response behavior to a near-saturated air sample (∼98% 2564
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Figure 5. (a) Current-voltage behavior of the Rh sensor operated at dc voltages with logarithmic (left) and linear (right) ordinates. (b) Temporal response of the Rh sensor at (top) 2.5 and (bottom) 2 V dc. Successive plotted points are 10 ms apart.
RH at 23 °C) was studied in an applied voltage range of 1-5 V dc. The measured sensor current is shown in Figure 5a as a function of voltage, both as a semilogarithmic plot and a linear plot. At voltages below ∼2 V, the current is low and increases in
an exponential fashion with the applied voltage. In this region, the applied voltage is not sufficient to induce electrolysis of the sorbed water and the sensor behaves essentially as a resistive element; Traversa3 showed that a logarithmic response should be observed in this case. When the voltage is raised above a certain threshold value (in the present case, 2-2.5 V), electrolysis begins to take place and the sensor current begins to increase linearly with the applied voltage. The precise value of this electrolytic threshold voltage is dependent on the level of sorbed water and the interelectrode distance. The iR drop across the sensor film between the electrodes increases with decreasing moisture content and thus controls the actual voltage available for electrode reactions. Temporal Response of Sensors. The response time of the Rh electrode sensors was measured by switching the sample between dry air and humidified air at 40, 60, 80, and ∼98% RH with an applied voltage in the range of 2-5 V and a data acquisition rate of 100 Hz. The response of the sensor at applied voltages of 2 and 2.5 V are shown in Figure 5b as the sample is switched from dry air to nearly water-saturated air and back. At 2 V, sample switching from 0 to 98% RH results in a sharp signal increase; the maximum value is achieved in ∼40 ms, followed by slight decrease in the signal to reach a stable plateau. At 2.5 V, the signal attains 80% of its maximum value within 50 ms and then attains the plateau value at a slower rate. For 2.5-5 V applied voltages, the same temporal behavior as at 2.5 V was observed, although the plateau current increased with increasing applied voltage. At 2 V applied across the electrodes, the sensor behaves primarily as a resistive device, the voltage applied is not sufficient for the electrolysis of the sorbed water. Note that Figure 5b shows that plateau response is attained within 30-50 ms as long as there is adequate surface flow; water partitioning in the moisture-sensing layer must reach equilibrium within this period. Factors other than diffusion in the polymer contributes to this. The diffusion coefficient of water in Nafion at 25 °C is known15 to be 2.65 × 10-6 cm2 s-1; for a 200-nm-thick film, one computes a characteristic diffusion time of only 0.15 ms. The slight overshoot of the equilibrium plateau probably occurs due to electrothermal reasons; the current for example may cause a minor change in film temperature that causes a new equilibrium to be established with slightly less sorbed water at the higher effective temperature. When 2.5 V is applied, the potential is sufficient for electrolysis to take place and the current is composed of both ohmic and electrolytic components. It will be observed that a 25% increase in the applied voltage caused a 250% increase in the plateau current. The difference in the absolute magnitude of the plateau currents at 2.5 versus 2 V provides an indication of the relative magnitude of the two components. As the sorbed water is electrolyzed, new water is sorbed to take its place and the time taken to reach a steady-state governs the 80% f plateau response time. Note that the current-voltage characteristics shown in Figure 5a also indicate that some mechanistic change takes place at a voltage of ∼2 V, corresponding to the transition between the two processes. As the air switches from wet to dry, the fall time is observed to be the same in both the 2 and 2.5 V cases; the 90 (15) Yeager, H. L. ACS Symp. Ser. 1982, No. 180, 48.
Figure 6. Humidity response of Rh sensor. (a) Response shown graphically; (b) the response is exponential with RH, specifically, log(response) is linearly related to the cube root of humidity. 2 V dc excitation.
f 10% fall time was ∼30 ms. Within the limits of our measurement, at 2 and 2.5 V applied voltage, the rise and fall times did not change with the change in the wet sample RH in the 40100% range. Response to Varying Humidity. It has been previously observed12 that, for similar sensors, the current in the 10-100% RH range is exponentially related to the RH level. Figure 6a shows the response over the intermediate 30-80% RH at an applied voltage of 2V dc that can be conveniently shown in a linear scale. Figure 6b shows the exponential current voltage behavior and specifically that the logarithm of the response is linearly proportional to the cube root of the relative humidity as proposed in a model invoking spherical water clusters in Nafion.5 Long-Term Stability with dc Voltage. Compared to an applied voltage of 5 V dc, at 2 V dc an Au electrode sensor remains usable considerably longer. Nevertheless, even at 2 V dc, the upper limit of this usability in continuous use is limited to ∼4 h, after which the sensor stops responding. Even within this period, if quantitative accuracy is of interest, recalibration must be performed to account for the decrease in sensor response (∼10%/h). Unlike the Au electrode sensors, the Rh electrode sensors show no change in response (or in electrode appearance) in prolonged continuous operation at 2 V dc. Figure 7 shows data for ∼170 h of continuous operation with the sensors being alternately exposed to the 0 and 100% RH periods lasting 20 s each. The inset shows the repeatability of the response at 0-80% RH swing. Interestingly, we have found that, if instead of dc excitation, a low-voltage pulsed waveform is used, even a Au electrode sensor can be used over long periods, albeit at the expense of response time. This is described in more detail in the next section, and Table 1 shows the comparative performance of the two types of sensors under different conditions. Low-Amplitude Pulsed Waveforms for Increased Longevity of Gold Electrode Sensors. Because gold is considerably less expensive than rhodium, we sought means to allow the Au electrode sensors to be used over prolonged periods. Lowamplitude ((1 V) square wave excitation and signal processing by the circuit of Figure 3 successfully achieved these objectives. Analytical Chemistry, Vol. 76, No. 9, May 1, 2004
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Figure 7. Comparison of the stability of the gold and rhodium sensors operated at 2 V dc. The inset shows the repeatability of the response at 0-80% RH swing for the Rh sensor under these conditions (RSD (n ) 7) was 0.59%). Table 1. Peformance Comparison Based on Electrode Type and Operating Modea excitation 2 V dc 2.5 V dc 10 V dce 1 V pulsed dc 2 V dc 2.5 V dc
sensor type Au Au Au Au Rh Rh
response timeb
sensitivityc
longevityd
30-50 ms 30-50 ms 20-100 ms 2-3 s 30-50 ms 30-50 ms
0-10 nA 0-200 nA 0-200 µA 0-10 nA 0-10 nA 0-200 nA
3 monthsf
a 50-µm electrodes, 50 µm apart. b Typical range, 10-80%, 80-10%. Current range between 0 and 80% RH. d Period over which sensor maintains usable response, best case results. e From ref 12. f May be indefinite, based on the study period.
c
Figure 8. Top (top and left axes): stability of the response for Au sensor operated with a (1 V 5 Hz square wave over 3 months. Bottom (bottom and right axes): within-day repeatability of 0-100% RH swings of same sensor over a 12-h period.
Figure 8 shows the stability of response of a gold sensor operated in this manner. The RH of the air supplied to the sensor was changed every 24 s from ∼0 to ∼100%. The sensor output with 2566 Analytical Chemistry, Vol. 76, No. 9, May 1, 2004
Figure 9. Following breathing with the Rh sensor operated at 2 V dc. Subject was asked to breathe at different rates ranging from slow at rest breathing (18/min’ the numbers on each group of traces represent approximate breaths per minute) to very fast deliberate panting. An expanded view of 2-s segment of the later is shown in the inset.
the dry air input was essentially constant and near zero throughout, the output at ∼100% RH is shown over a 3-month-long period in Figure 8. Because of the extreme data density, a 10-min average is shown in the plot. The sensor output decreased over the first ∼100 h and then became stable; over the rest of the period, the relative standard deviation was 5%. The typical repeatability within a 12-h period was better than 2% in RSD, as shown in the inset. In evaluating this performance, it should be noted that the entire arrangement was operated without temperature control. While Au electrode sensors can be operated in this mode over long periods, it should be noted that it is not possible to attain subsecond response times in this mode. Typical 10-90% response time is 2-3 s. Breath Monitoring. The very fast temporal response (3.5 Hz). At the present time, the most common technique used to monitor breathing (e.g., for a patient in critical care) involves monitoring CO2 levels in the exhaled breath via inserted nasal cannulas. Clearly, the present Rh sensor offers a robust, simpler, even disposable, option. CONCLUSIONS Microfabricated Rh electrode moisture sensors based on a Nafion film of submicrometer thickness provide robust fast sensors that can be used as a exhaled breath counter. Unlike Au electrodes, low dc voltages can be applied to Rh electrode-based sensors over prolonged periods. For applications that can be conducted with several-second response times, Au electrode
sensors can be operated with stable response over long periods using low-amplitude square wave excitation.
of Washington, Seattle, and by the Paul Whitfield Horn Professor funds at Texas Tech University.
ACKNOWLEDGMENT This work was supported in part by NSF Grant CTS-0088198, by the Center for Process Analytical Chemistry at the University
Received for review December 31, 2003. Accepted March 4, 2004. AC0355451
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