Sensors and Actuators B 38-39 (1997) 130-135
Miniature FlowProbeTM chemical sensor Leslie K. Moore a,*, David J. Veltkamp a, Jose L. Cortina b, Zhihao Lin ‘, Lloyd W. Burgess a a Center for Process Analytical Chemistry, Universiry of Washington, Bon 351700, Seattle, WA 9819.5, USA b Universitat Poiitecnica Catahrnya, Departument Enginyeria Qztimica, Diagonal 647, ETSE IB, Barcelona, 08028, Spain’ ’ W.R. Grace, Washington Research Center, 7500 Grace Drive, Colnmbia, MD 21044, USA
The FlowProbeT” is a new type of chemicalsensorthat integratestraditionalreagent-based chemicalanalysistechniques,fiber-optic spectroscopic detection,andmembrane samplingin acompactruggedsystem.The FlowProbeis novelrelativeto otherimmobilized-reagent fiber-opticsensors in that the reagentin the FlowProbesensoris replenished betweeneachanalysis.In a typical applicationthe probeis insertedin the samplematrix andfreshreagentis pumpedinto theprobehead.Theanalytesthendiffuseacrossthe samplingmembrane where theyreactwith the reagentto form a coloredproduct.Fiberopticsguidelight to theprobetip wherethedifferentialopticalabsorptionresulting from the productformationis measured spectroscopically. The analyzermonitorsmultiplewavelengths duringthe reaction,andadvanced mathematical calibrationtechniques(i.e., chemometrics) areemployedto providea robustcalibrationmodel,andprobediagnosticsto the user.Two field-portableFlowProbeprototypedesignshavebeenconstructed. Thefirst designis aninsertion,or dip sensorconsistingof a l/2 inch OD stainless-steel probeattachedto a casecontainingthe fluid-deliverysystem.The opticalcomponents, spectrometer,andcomputer arelocatedin a secondcase.The seconddesignalsoseparates the fluid delivery andopticalsystems,howeverthe sensorheadandfluiddelivery systemareengineered into a2 inchODunit that canbedeployedup to 250feetdownawellcasingfor in situgroundwatermonitoring. This work describesthe FlowProbeinstrumentation andfocuseson the developmentof the chemistries andmembranes for the analysisof copperin a simulatedcooling-towerrig usingtheinsertionFlowProbeprototype. Keywords: Sensors; Optical; In situ; Reagent
1. Introduction The FlowProbeTM sensoris an in situ chemical analyzer for processand environmental monitoring. The fiber-opticbasedsensorcan be adaptedfor many different types of analyses dependingon the reagent used within the sensorhead, and the type of membranesampling interface chosen.Multiple analysescan be performed with a singleprobeheadwith the uniquefeature that the sensingmechanismisregenerated. The FlowProbe sensorconcept was first developed at the University of Washington’s Center for ProcessAnalytical Chemistry (CPAC) and has beendemonstratedfor applications including the analysis of ammonia,free and total chlorine, and caustic samples[ l-31. Recent researchhasfocused on developing second-orderapplications of the FlowProbe. Generally speaking, as one increasesthe order of the data, the resulting model is able to provide diagnosticinformation and correct for more potential interferents andproblems [41. In one example, variable transport rates of analytesthrough * Corresponding author. Tel.: + 1 206 685 23 26, Fax: + 1 206 543 65
06. 0925-4005/97/$17.00 P11s0925-4005(97)00081-4
0 1997 Elsevier Science S.A. All rights reserved
the membranewere usedwith full-spectrum spectroscopy to produce a second-ordersensorfor heavy metals [5-71. Most recently, a second-ordersensorfor chlorinated hydrocarbons wasdevelopedby spectrallymonitoring the chemical reaction kinetics within the sensorover time at multiple wavelengths [S-lo]. An important aspectof this work hasbeen the application of novel mathematicaltechniquesto identify andquantitate individual analytesfrom complex spectra. In 1993, CPAC teamedwith Sandia National Laboratory (SNL) to develop aFIowProbe analyzer prototype [ 111.The goals of the project were clear: (1) to develop a generic chemical speciatingtechnology basedon the FlowProbe sensor concept for field survey and process-controlapplications; (2) to demonstratethe instrument in the field; and (3) to partner with commercial instrument manufacturers to produce a commercialproduct. SNL designedand manufactured the prototypes. CPAC provided the base technology and expertise in chemical reagentsand membranesand chemometric data analysis. Interestedmembersof CPAC’s industrial sponsorship,a community of analytical instrumentation usersand vendors, had an active role in designing and field testing the prototypes.
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Side View Z-cell Flow/Reaction
Fig. 1. Schematic
Stainless steel membrane support of the FlowProbe probe head.
The FlowProbe sensor head is shown in Fig. 1. The head includes: (1) a z-configuration flow channel containing 3 ~1 of the reagent; (2) a fiber-optic-based optical train to deliver and collect the spectroscopic signal; (3) a planar membrane sampling system and support attached to the end of the sensor head using four small screws; and (4) a resistive thermal detector (RTD) to monitor the temperature of the probe head. There is no attempt to control the temperature of the probe head. The z-configuration flow channel is a standard flow-cell geometry that discourages laminar fluid flow and acts to wash off the optical windows with each flush of the reagent. The probe head is made of 3 16 stainless steel and the flow channel was conventionally machined to a depth of 500 pm. The channel dimensions were calculated to allow rapid diffusionbased mixing between the reagent and the analyte once it has crossed the membrane. The optical train includes fibers to guide light from the source to the probe tip and a 50-50 beam splitter (Gould Electronics) to reference the source. Once the light reaches the probe tip it is turned through 90” with a sapphire prism, and transmitted across the diameter of the reaction/detection cell. The optical pathlength of the reaction volume is 2 mm. The light is collimated before it passes through the flow cell with a GRIN lens (1.0 mm, 0.25 pitch, NSG America, Som-
erset, NJ). To achieve good collimation, it is important to use a fiber with a small core diameter that has had the cladding modes stripped. The FlowProbe was designed to use 5011251 250 fibers, but larger fibers were installed during assembly to compensate for optical misalignments. The current prototype contains a 100/l lo/125 high-OH step-index source fiber (Polymicron Technologies, Phoenix, AZ). Once the light is transmitted across the flow cell, it is again turned through 90” and coupled into a fiber for transmission back to the diode array detector. The pick-up fiber is a 400/440/470 high-OH step-index fiber (Polymicron Technologies, Phoenix, AZ). A sampling membrane is attached to the end of the sensor head. The membrane provides a mechanism to sample analytes from the sample matrix and acts to seal the reagent within the sensor. The planar membrane geometry allows for maximum flexibility in applying commercially available membrane materials. The support systems external to the sensor head include: ( 1) a fluid-moving system to flush the reaction/detection cell and provide fresh reagent for analysis; (2) a spectrometer (American Holographic RainbowmeterTM, Littleton, MA) with a spectral range from 350 nm to 700 nm for analyzing the spectral absorbance of the reaction; (3) a mercury-xenon flash lamp (Hamamatsu, Japan) and a 40 X microscope objective to collect and focus the light onto the fiber; (4) a computer system to control the operation of the instrument, and collect and analyze the data; and (5) the necessary fluid, electrical, and optical-fiber interconnects between the sensing head and the support systems. Fig. 2 shows the fluid-handling system for the prototype. The fluid-delivery mechanism is based on the differential pressure between the reagent and the waste bellows. The bellows are fixed with an internal spring that results in a positive pressure of 20 psi when filled. For each bellow there is a septum port for filling or draining the reservoir, a manual shut-off valve, and a 12 V solenoid valve (Valcor Scientific, Springfield, NJ). The solenoid valve is operated under computer control through a D/A board (Omega, Stamford, CT). All the instrument-control software was written in-house with [email protected]
(National Instruments, Austin, TX). Assuming an analysis every hour and a flushing volume of 10 yl per analysis, the analyzer can
Reagent Bellow 1OmL = 20 psi
Fig. 2. Fluid-handling
system for the FlowProbe
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run for up to six weeks without refilling the reagent bellows. This robust fluid-delivery system is designed for single-reagent systems. Two FlowProbe prototype designs were manufactured. The first prototype is designed to be placed on a table top or along the process line. The electronics (computer, light source, spectrometer, signal processing andassociatedpower supplies) are contained in one fiberglass case ( 14”[h] X 26”[ w] X 24”[d] ) and the fluid-handling system is contained in a second case ( lO”[ h] X 23”[w] X lS”[d] ). The probe is connected to the fluid-handling system by a 3 foot umbilical, and a 10 foot interconnect cable (comprising electrical and fiber-optic cables) connects the two cases. A second design combines the sensor head and fluid-delivery system in one unit that can be deployed 250 feet down a 2 inch well casing. This work will evaluate the performance of the ‘table top’ version.
by researchers at the University of Colorado’s Center for Separations Using Thin Films in Boulder, CO. This membrane was prepared from a 5% (w/w) solution of 1100 EW [email protected]
polymer (Solution Technologies, Inc.). The solution was evaporated at room temperature to form a thick film. The film was then neutralized in excess 1.0 M NaOH, rinsed in water and dissolved in dimethyl formamide to make a 1% solution. The membranes were then cast onto a glass substrate and evaporated in a vacuum oven at 180°C and 300 torr for 2 h. The membrane was 20 p,m thick. The third membrane was a commercially available sulfonated styrene graft of [email protected]
PTFE (50 pm thick, Raipore 1010, RAI Research Corporation, Long Island, NY) cation-exchange membrane. All the cation-exchange membranes were converted into the sodium form by immersing them into a solution of 0.8 M sodium nitrate (NaNOa) and 0.2 M sodium hydroxide (NaOH) for IO-12 h. The membranes were then transferred into deionized (DI) water for storage. The membranes are sized for use in the sensor by punching them with a template.
The calorimetric reagent for the determination of copper was bathocuproinedisulfonic acid, disodium salt hydrate (Bcup) that selectively binds with cuprous ions (Cue’) to produce an orange-red product with an absorbance maximum at 482 nm. The reagent carrier stream consists of 1 X 10m3M Bcup and a reducing agent (2% hydroxylamine hydrochloride) in an acetic acid/acetate buffer. The buffer was prepared in 18 Ma water (Mini-Q, Millipore Corp) from acetic acid and sodium acetate and was adjusted to pH 5.5 with a few drops of 10 M sodium hydroxide solution. The reagent chemistry is sensitive and selective. Cuvette studies indicate part per billion levels of copper are easily detected [ 121. The copper standards were prepared from copper sulfate (CuS04). For the field test a formulated water known as ‘Chicago tap water’ was prepared on-site by W.R. Grace personnel. Chicago tap water consists of 184 ppm Ca, 66 ppm Mg, 49 ppm Cl, 633 ppm S04, 145 ppm Na and 384 ppm HC03. Since Chicago tap water contained high levels of calcium chloride, calcium nitrate (Ca( N03) 24H20) was added to the reagent solution ( 1.7% WI w) to match the ionic strength of the solution. Unless otherwise stated, all chemicals were purchased from Aldrich of Milwaukee, WI.
The reagent and membranes were evaluated with a FlowProbe laboratory prototype. This prototype instrument contained all the field equipment described above; however, a tungsten lamp was used instead of the xenon-mercury lamp described. The laboratory measurements were made with a 1 s integration time per scan. For the field tests, individual spectra were obtained by flashing the lamp 32 times within a 1 s integration period.
3.2. Membranes Three cation-exchange membranes were usedin this study. One was a composite membrane consisting of a 5% (w/w) solution of [email protected]
perfluorinated cation-exchange powder in a mixture of aliphatic alcohols (Aldrich, Milwaukee, WI). The polymer was solution cast onto a porous [email protected]
PTFE substrate (65 pm thick, 0.2 p,m pore size, Berghof America, Concord, CA). The composite membrane was then heated at 90-100°C for 5-10 min. A second membrane was prepared
3.4. Data analysis In laboratory evaluations, the raw signal from the spectrometer was processed by performing a point-by-point ratio of the sample signal to the reference signal. In all cases the first spectrum immediately after flushing the sensor head with fresh reagent was used as the I, scan in calculating the absorbance of the subsequent scans. In analyzing spectra from the field tests, spectra from a fixed analysis time were used to predict the copper ion concentration. The model used the average spectra calculated using the 10th to the 55th spectra collected (corresponding to a 3.5 min integration time). A one-factor partial least squares (PLS) model was used to regress these spectra, after mean centering, to the known concentrations of the calibration solutions. For the coolingtower simulation, the reference values were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) of grab samples from the tank.
4. Results and discussion The sensor transduction has three steps: transport, reduction, and complexation.
Cu+‘( sample) + Cu4’(membrane) +cu+2
Cuf2 + NH,OH + 1/2N,(aq)
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-’ + [Cu(Bcup),]
Metal transport of copper is based on chemically facilitated Donnan dialysis. As the ion permeates the membrane it reacts with reagent to form a negatively charged species. Fig. 3 shows theresponse of the FlowProbe sensor to 50ppm copper solution. The sensor signal increases with time as copper ions migrate across the samplingmembraneand react to form the detectable analyte-reagent complex. Fig. 4 is a calibration curve for the responseof the FlowProbe sensorto aqueous copper standards.The responsetime for theseanalyses was 10 min, resulting in a limit of detection of 25 ppb copper. The blank produced a significant offset at all wavelengths. This is attributed to changesin the refractive index of the reagent solution aswater migrates through the membraneduring the stop flow analysis. Three cation-exchange membraneswere comparedfor use in the sensorhead. The preparation of the membraneswas describedin Section 3. Fig. 5 showsthe relative responseof the FlowProbe sensorwith the three membranesin a 100ppb aqueouscopper solution. Sequential analyseswere run in a stop flow mode with a sampletime of 3 min and a flush time of 1 min betweenruns. Each data point representsthe absorbance at 480 run. The porous Raipore membraneshowedthe best overall transport of copper and was usedfor all subsequent FlowProbe analyses. Fig. 5 alsoshowsthat it takesseveral runs for the transport of copper through the membraneto become steady. When a fresh membrane is used, the initial responseof the probe is slow as copper ions replace sodium ions at the cationexchange sitesof the membrane.In this study it took at least 30 min for the sensor response to stabilize when a new membranewas used.To improve responsetime calcium was addedto the reagent solution. The divalent calcium ions coma71
Copper Fig. 4. FlowProbe
for the analysis
ppb of copper
Fig. 5. Comparison of the performance of three cation-exchange membranes to 100 ppb copper in water using the FlowProbe sensor. Each data point represents the relative response of the sensor at 485 nm after a 3 min integration period: A, 50 pm thick Raipore 1010 (0.02 Frn pore size); 0, 20 pm thick Nation film; 0, composite membrane of 5 wt.% N&on solution cast on a 65 pm thick PTFE support (0.02 pm pore size).
pete with the copper ions for the transfer sites within the cation-exchange membrane,allowing the copper to crossthe membrane,more quickly, by diffusion alone. After adding calcium to the reagent, the sensor response with a fresh membranewas stablewithin 10 min of contact with the copper ions.
5. Field tests
0.6 0.5 8
0.1 . O-
Wavelength Fig. 3. FlowProbe
sensor response to 50 ppm Cu over a 10 min time period.
The FlowProbe field prototype was tested at W.R. Grace to monitor copper in water asan indicator of corrosion. The test platform was a simulated cooling-tower rig through which ‘Chicago tap water’ was circulated. To calibrate the FlowProbe sensor, 12 standard solutions were prepared by spiking aliquots of Cu( N03)2 stock solution into formulated Chicago tap water. The standardsranged from 0 to 10 ppm CUf2. Before each measurementthe probe was flushed with fresh reagent for 30 s, and for each analysis spectra were collected continuously for 10 min, resulting in 150 spectra per sampling. For each analysis, an average spectrum was calculated using the 10th to the 55th collected spectra (cor-
Fig. 6. One-factor PLS model fit to calibration copper in a formulated ‘Chicago tap water’.
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(ppm) data set for the analysis of
respondingto a 3.5 min integration time). Calibration of the averaged spectra to the known copper concentration was obtainedusing PLS regression.The PLS modelfit to the mean centered calibration data is shown in Fig. 6 for a one-factor model.For the averagedspectra,the calculateddetectionlimit is near 300 ppb with an average relative prediction error of 15% over the range from 0 to 10 ppm. After calibration, the FlowProbe sensorwas tested in a simulatedcooling-tower rig. The test involved monitoring a formulated Chicago tap water at several different copper ion concentrations. The predicted copper concentrationsdetermined by the FlowProbe sensorwere comparedto reference values obtained from ICP-AES analysis of grab samples. Theseresultsare shown in Fig. 7. Better than 80% agreement between the probe predictions and the ICP-AES resultswas achieved when the samplepH wasrelatively constant.However, when the tank pH decreasedby one pH unit, the predicted concentrationsdeviated by more than50% (datapoints at 49 and 53 h) . The pH dependencyis believed to be caused by the formation of Ct1(0H)~-r and CUE-’ ions in solution. These negatively charged ions cannot passthrough the cation-exchange membrane,so they are not detectedby theprobe. The FlowProbe calibration wasbasedon thecopper ion flux at the high pH value. 4.0 3.51 .s-
Because the formation of the Cu( OH) 3- ’ and CUE-’ ions is pH dependent,the FlowProbe response to copper ions will vary asthe tank pH changes.This effect is largely independentof the pH of the buffer solution within the reaction/ detection cell. Becausethere is no changein the spectral responsewith changing sample pH, the effect of sample pH on the membranetransport cannot be inferred from a single FlowProbe measurementor included in the calibration model as an interferent. However, the pH effect can be compensatedfor by determination of the copper ion flux at different pH levels and either applying a correction to the predicted values or by multiple calibration models for different pH regimes.Both thesesolutionswould require an independent measurementof the samplepH. Alternatively, one could usea neutral membranewhich would transport all the cation andanion complexesof the copper ions. The selectivity of the copper reagentalong with the multivariate calibration should still provide results which are relatively free from interferences. The simulatedcooling-tower rig test ran for the planned 86 h with no problemsin the operation of the FlowProbe prototype hardware.
6. Conclusions A FlowProbe instrumentprototype was built and testedat an industrial site. In a laboratory evaluation the probe demonstrated a limit of detection (LOD) of 25 ppb CU+~ in DI water for a 10 min analysis.Field testsshoweda LOD of 300 ppb CU+~ for a 2 min analysis of Chicago tap water. Better than 80% agreementbetween the probe predictions and the ICP-AES resultswere achieved when the sensorwas tested in a simulated cooling-tower rig. The samplepH was found to affect the sensorresponseby altering the amount of free copper available for transport through the cation-exchange membrane. This paper describesone of the four FlowProbe field tests performed at industrial and government sites. These shortterm testsdemonstratedthe potential of the technology. As a result, the FlowProbe technology packagehas been licensed by the University of Washington’s Office of Technology Transfer to Life SciencesInc., of St. Petersburg,FL, USA.
ICP-AES Measurements x
Sample Time (Hours-->) Fig. 7. Results from the corrosion test-rig trials showing the copper concentrations predicted from PlowProbe measurements and off-line ICP-AES reference analysis. The two high-value FlowProbe predictions at approximately 50 h are due to changes in sample pH (see text).
The authorsacknowledgethe designandfabricationexpertise of Frank Peter and George Laguna at Sandia National Laboratories. We thank Debbie Peru at W.R. Grace. This project wasfinanced by the Department of Energy’s CMSTCP program, theNational ScienceFoundation, andthe Center for ProcessAnalytical Chemistry. The FlowProbe trademark belongs the University of Washington’s Office of Technology Transfer.
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[ 1 l] 
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Biographies Leslie K. Moore is a research associate at the Center for Process Analytical Chemistry located at the University of Washington in Seattle, Washington. Her research involves the development of reagent-based optical sensors for on-line
chemical analysis. Dr Moore is also employed by Life Sciences, Inc., St. Petersburg, FL, where she works to commercialize sensor technologies such as the FlowProbeTM chemical analyzer. David J. Veltkamp is a senior research scientist at the Center for Process Analytical Chemistry, an Industry/University cooperative research center located at the University of Washington in Seattle. His research involves the development and application of chemometrics methods for calibrating and characterizing chemical sensing systems for industrial and environmental monitoring applications. Dr Veltkamp wrote the FlowProbeTM operating software and performed most of the data analysis related to the field testing. Jose L. Cortina is an associate professor in analytical chemistry in the Department of Chemical Engineering at the Universitat Politecnica de Catalunya, Barcelona, Spain. His research work is directed toward the development of metal separation processes based on solvent extraction and ionexchange techniques, and most recently includes the development of analytical probes for process control and environmental pollution control based on fiber-optic chemical sensors. Dr Cortina was a visiting scientist at the Center for Process Analytical Chemistry in 1994 and 1995. Zhihao Lin was a graduate student and research assistant from 1990 to 1994 at the Center for Process AnalyticalChemistry at the University of Washington. He researched secondorder fiber-optic chemical sensors under the guidance of Dr Lloyd W. Burgess and Professor Bruce R. Kowalski. He is currently with W.R. Grace & Co., as a research chemist at the company’s Washington Research Center in Maryland. Lloyd W. Burgess is a senior research scientist at the Center for Process Analytical Chemistry, an Industry/University cooperative research center located at the University of Washington in Seattle. He directs the Optical Waveguide Sensor group, which is investigating new concepts in the development of integrated multicomponent analysis systems for use in industrial and environmental monitoring applications.