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judiciously choosing the region for each spectral measurement, one can minimize the number of necessary atomization steps. Registry No. Ag, 7440-22-4; Al, 7429-90-5;As, 7440-38-2;Ba, 7440-39-3; Ca, 7440-10-2; Cd, 7440-43-9; Co, 1440-48-4; Cr, 7440-47-3; Cu, 7440-50-8; Fe, 7439-89-6; Li, 7439-93-2; Mg, 7439-95-4; Mn, 7439-96-5; Na, 7440-23-5; Ni, 7440-02-0; Pb, 7439-92-1; Sb, 7440-364; Sr, 7440-244 Zn, 7440-664 bronze, 12597-70-5; water, 7732-18-5. LITERATURE CITED Fassel, V. A.; Mossotti, v. G.; Grossman, W. E. L.; Kniseiey, R. N. Spectrochim. Acta 1980, 22, 347-357. O'Haver, T. C. Analyst 1984. 709, 211-217. Marshall, J.; Ottaway, 8 . J.: Ottaway, J. M.; Littlejohn, D.Anal. Chim. Acta 1980, 180, 357-371. Harniy, J. M. Anal. Chem. 1988, 58, 933A-943A. O'Haver, T. C.; Messman, J. D. Rog. Anal. Spectrosc. 1988, 9 , 483-503. Hamly, J. M.; Miller-Mi, N. J.; O'Haver, T. C. Speclrochim. Acta 1984. 398. 305-320. (7) Harniy, J. M.; Kane, J. S. Anal. Chem. 1984, 56, 48-54. (8) Lewis. S. A,; O'Haver, T. C.; Harnly, J. M. Anal, 1985, 57, 2-5.
(9) Cochran, R. L.; Hieftje, G. M. Anal. Chem. 1977, 49, 2040-2043.
(IO) O'Haver, T. C.; Harniy, J. M.; Zander, A. T. Anal. Chem. 1978, 50, (11) (12) (13) (14) (15) (17) (18) (19)
(20) (21) (22) (23)
12 - 18-1 - 22 _ - 1. Harnly, J. M. Anal. Chem. 1984, 56, 895-899. Taimi, Y.; Simpson, R. W. Appl. Opt. 1980, 79, 1401-1414. Epsteln, M. S.; Zander, A. T. Anal. Chem. 1979, 57,915-918. Tittarelli, P.; Lancia, R.: Zerlia, T. Anal. Chem. 1985, 5 7 , 2002-2005. Shekiro. J. M.. Jr.; Skogerboe, R. K.; Taylor, H. E. Anal. Chem. 1988, 60, 2578-2582. Harnly, J, M,; O,Haver, T, c, Anal. them, 1977, 4 9 , 2187-2193, Guthrie, 8. E.; Wolf, W. R.; Velllon, C. Anal. Chem. 1978. 50, 1900-1902. O'Haver, T. C. Anal. Chem. 1979, 51, 91A-100A. O'Haver, T. C.; Harnly, J. M.; Marshall, J.; Carroll, J.; Littlejohn, D.; Ottaway, J. M. Anawst 1985, 710. 451-458. Harnly, J. M.; Holcombe, J. A. Anal. Chem. 1985, 57, 1983-1986. Miller-Ilhl, N. J.; O'Haver, T. C.; Harnly, J. M. Anal. Chem. 1982, 54, 799-803. Zander, A. T.: O'Haver, T. C.; Keliher. P. N. Anal. Chem. 1977, 49, 838-842. Miller-Ihli, N. J.; O'Haver, T. C.; Harniy. J. M. Appl. Spectrosc. 1983, 37, 429-432.
R E X E D for review January 30, 1989. Accepted May 1,1989. This research was supported by NIH 5 R01 GM38434-02.
Polymer-Coated Cylindrical Waveguide Absorption Sensor for High Acidities W.Patrick Carey* and Michael D. DeGrandpre' Chemical and Laser Science Division, G740, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Betty S . Jorgensen
Material Science and Technology Division, E549, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
The development of a chemical sendng scheme for the detection and quantltatlon of greater than l M concentration of acids, parHcularlynlMc add, b presented wlth the overall goal of uslng this chemlstry to create optlcal sensors. The detectlon chemlstry Is based on the physical entrapment of Hammetl Indicators In a polymer blend of polybenztmldazde and polytmide, which Is sllane-coupled to optlcal elements. The polymer coatkg protects the optlcal element from chemlcai attack, and Its hydrophUlc nature and low porosity make the sensor more selecthe. By use of the Indicator chromazuroi-S, nHrlc and hydrochiorlc acld concentratlons ranging from 2 to 10 M can be measured with a preclslon of 0.05 M. lndlcators that cover a range of 0.5-10.0 M acld concentratlons have also been Investlgated. Cyllndrlcal wavequkle sensors, using both a fiber optic and a sapphire rod wlth the knmob#zed polymer chemistry, were constructed based on the absorption of light by the acid Indicator, chromazuroCS. Because of the difference In the refractlve Index of the optical elements and the polymer materlal, separate detectlon techniques for the two 8811801s are presented. The flber-optic sensor uses an abwptlon measurement of a thin flkn, and for the sapphlre rod, an evanescent fleid absorption process occurs.
INTRODUCTION A variety of fiber-optic sensors have been developed for measuring pH through the immobilization of common colC u r r e n t address: D e p a r t m e n t of Chemistry, BG-10, U n i v e r s i t y of Washington, Seattle, WA 98195.
orimetric indicators (1-3). Most of these were constructed to operate in the physiological pH region. However, there are many chemical processes where the acidity is below pH 0 and outside the normal range of measurement for both fiber-optic sensors and pH electrodes. One example of a high-acid process is the separation of rareearth metals, where optimal operating conditions require maintaining a level of 7.5 M nitric acid. An on-line sensor to monitor the acidity would greatly increase the efficiency of the process and minimize operating costs. Therefore, a high-acidity in situ sensor needs to be developed for these applications. In the early 19309, Hammett and Deyrup conceived the Hammett (&) acidity function as a measure of hydrogen ion concentration in molar levels of acids. This system is based on optical absorption changes caused by the protonation of weak bases such as substituted anilines (4-6). The basicity of the amine lone pair can be adjusted with the addition of electron-donatingor -withdrawinggroups on the benzene ring. For example, acid ranges from 0.1-10.0 M can be measured with a combination of several nitro and chloronitroaniline derivatives. Similarly,more complex compounds with multiple benzene rings and amine or other weak base functionalities also exhibit characteristics of Hammett indicators. In this study, various Hammett indicator analogues such as chromazurol-S, methyl violet B, victoria blue, rhodamine B, and methyl green ZnClz were examined for their response to acid conditions between 1 and 10 M. These indicators were tested in a polybenzimidazole (PBI) and polyimide (PI) matrix that was silane-coupled to a glass microscope slide. Dynamic range and reproducibility studies were performed t o choose an indicator with a response in the 4-10 M range. The next step was to immobilize the selected indicator system t o a
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
cylindrical waveguide and to use the evanescent field penetration of visible light to monitor the absorption changes of the indicator with various acid concentrations. Evanescent field spectroscopy was developed for chemical analysis by Harrick (7) and Fahrenfort. Their technique, attenuated total reflection spectroscopy (ATR), uses planar optical waveguides with precise control of the launch angle and polarization. Using evanescent field sensing through fiber optics or cylindrical rods is a relatively recent advancement of the technique (8-10). Although the launch angle and polarization are not sustained in multimode cylindrical waveguides, they offer greater sensitivity than do conventional ATR methods because of the longer path length and increased number of reflections. In this sense, evanescent field spectroscopy is changed from a characterization tool to a practical approach for quantitative chemical analysis. The same virtues of other fiber-optic chemical sensors such as optrodes (11)apply to cylindrical waveguide sensors. These advantages include immunity to electrical interferences, remote and in situ operation, broad spectral coverage (0.3-2.0 pm for fused silica, 0.2-5.0 pm for sapphire), and the ability to be multiplexed to a single instrument. An evanescent field sensor can be made more sensitive than a reflectance type optrode, a t the expense of sensor size. Moreover, the sensor can be designed to accommodate the sensitivity required for a specific application by selection of appropriate fiber length and diameter, core and cladding (polymer coating) refractive indices, and fiber bend radius if a coiled sensor is used (10). Long lengths (>1cm) of modified commercial fiber optics for evanescent field spectroscopy are difficult to implement because of the extremely fragile and polar nature of the exposed core (typically glass or fused silica). In this work, the PBI/PI polymer coating protected the modified portion of the fiber optic from chemical attack, chemical interferences, and stresses. Indicators were incorporated into the polymer as a transducer for nitric and hydrochloric acid, neither of which have intrinsic absorbances in the wavelength region used for the analysis. EXPERIMENTAL SECTION Polymer/Indicator Immobilization. The polymers PBI (Celanese Corp.) and polyisoimide (Thermid IP 630, National Starch and Chemical Corp.) were dissolved in NJV-dimethylacetamide (DMAC) (Aldrich)to make a 10% solution and were blended in a 1:l mixture. Approximately 0.5 g of the indicator was dissolved per 100 mL of polymer solution. The polymer solution was silane-coupled to the optical elements in order to provide adhesion when exposed to highly acidic solutions. (This procedure extended the adhesion of the polymer from 5 min to at least 12 weeks.) A glass slide, a fiber-optic core of a stripped fiber (200 pm core PCS, Fiberguide Industries), and a sapphire rod (1mm diameter, Saphikon) were treated with a 2% aqueous solution of (chloropropy1)trimethoxysilanefor 15 min, washed with distilled water, and heated in air (110 "C) for 30 min. The substrates were then dip-coated with the polymer mixture and dowed to cure at 175 "C for 24 h. Film thickness was dependent on the withdrawal speed of the optical materials from the polymer solution and the concentrationof the polymer/indicator mixture. Film thicknesses on the order of 1-3 pm were obtainable by this method, as estimated by using the method of interference patterns from weakly absorbing materials (12) and by scanning electron microscope cross sections of the coated fiber. Evaluation. Preliminary studies of the indicator response characteristics were performed with coated glass slides using a Hewlett-Packard 8451A diode array spectrophotometer. Standardized nitric and hydrochloric acid solutions from 0.5 to 10.5 M were used throughout the study. The fiber-optic and sapphire rod sensors were constructed in a single beam arrangement, shown in Figure 1. For the fiber-opticsensor, 15 cm of the jacket and cladding was stripped from one end and coated with the above procedure up to the unstripped cladding. The coated portion of the fiber was inserted into a flow cell consisting of low dead volume
1875
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instrumental setup involving the polymer-coated cylindrical waveguides. Flgure 1. Diagram of
Table I. Immobilized Indicator Evaluation in Nitric Acid
indicator
response range, AU
acid range, M
rhodamine B methyl green ZnCl victoria blue methyl violet B chromazurol-S
0.5 0.25 0.3 1.0 1.4
0.5-8 0.5-4 0.5-5 0.5-8.5 2-10
A,,
nm
522 590 600
630 546
T-fittings with Teflon ferrules connected to Teflon tubing and a peristaltic pump. The flow cell arrangement for the sapphire rod consisted of larger tubing and T-fittings and had an overall length of 19 cm for the 25 cm long rod. Light from a tungsten lamp was focused into the fiber optic with a 1OX objective lens (numerical aperature (NA) = 0.25) completely filling the light acceptance cone of the 0.24-NA fiber. A mode scrambler (Newport FM-1) was inserted before the sensor section to ensure that the response was independent of launch conditions. Illumination for the sapphire rod sensor used a 0.24-NA optical fiber butted to the sapphire rod endface near the entrance of the flow cell, underfilling the 0.56-NA sapphire rod. The sensor output (both fiber optic and sapphire rod) was focused into a single grating spectrograph (f/3.8, Aries Monospec 27) fitted with a photodiode array detector (Tracor-Northern 6500). A 150-groove/mm grating dispersed light from 300 to 720 nm across the active elements of the diode array. The resolution of the instrument is approximately 4 nm. AU solution refractive index measurements were performed with an Abbe refractometer. RESULTS AND DISCUSSION Polymer/Indicator Selection. The polymer system used to immobilize the organic indicators on the optical elementa was characterized on glass slides before the experimentation with cylindrical waveguide sensors. When the original Hammett indicators, such as o-nitroaniline, were used, bleeding of the organic dye from the PBI polymer occurred. Polymer blends, such as PBI mixed with PI, change the pore size of the structure, allowing transport rates and properties to be varied. A ratio of 1:l PBI/PI was determined to be optimum for both fast response time and elimination of indicator bleeding. Absorbance spectra of the polymer/indicator coatings immobilized on glass slides were taken in various acid (nitric or hydrochloric) concentrations. The PBI/PI blend has a broad-band UV absorption from 300 to 415 nm in acidic media. The absorbance of the PBI/PI mixture is sensitive to changing acid concentrations because of protonation of the secondary amine present in the structure, but this feature is not well characterized due to the high optical density of the polymer. The classical Hammett indicators absorb from 350 to 450 nm, overlapping this polymer absorbance spectrum. Because the polymer absorption ends at 415 nm, the Hammett indicators that absorb in the visible region of the spectrum can be monitored easily. The sensitivity and the wavelength of maximum absorption of various indicators that have been tested for their responses in nitric acid are listed in Table I. The indicator chromazurol-S was the most promising for monitoring a process where
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
B
A
/
1
550 nm 1754
,
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025-1
:
50
60
7 0
450
530
550
600
65C
Waveleqqth (nm)
Flgure 3. Absorbance spectra of chromazurol-S immobilized onto glass slide with PBI/PI in nitric acid solutions.
acid conditions in the 4-10 M range are frequently encountered. This indicator has the highest sensitivity and covers the concentration region needed for our application. All of the other indicators, such as methyl violet B in Figure 2, are amine derivatives that respond in the 1-6 M acid range. Chromazurol-S responds in the 4-10 M acid region because of its weakly basic carbonyl functionality, and the structure was more stable in the acidic environment than that of the amines. The absorbance spectra of chromazurol-S in the PBI/PI polymer mixture at different levels of nitric acid are presented in Figure 3. The indicator has absorbance maxima a t 470 and 546 nm and an isosbestic point at 500 nm. The 470-nm band (base form) decreases with increasing acid concentration, and the 546-nm band (acid form) simultaneously increases. This type of response aids in calibration because acid concentrations can be based on pattern identification or peak intensity ratios only, without the need for intensity reproducibility over long periods of time. The longevity of the coated slides with immobilized chromazurol-S and methyl violet B was tested. They withstood nitric acid for an average period of 8 weeks with partial functionality intact. When the slides were soaked in 8.8 M nitric acid, the signal intensity of the 546-nm band of chromazurol-S degraded approximately 9% after 1week, whereas the 630-nm band of methyl violet B degraded 51% in the same period of time in 4.4 M nitric acid. These results were similar for hydrochloric acid at the same concentrations. The responses of chromazurol-S and methyl violet B were tested for possible interferences from the nitrate salts of sodium, calcium, and thorium. No interferences were detected from salt concentrations ranging from 0.1 to 1.5 M in 4 M nitric acid. Whether the aqueous metal complexes can penetrate the porous polymer is unknown, but no interaction would be expected to take place between the metal ions and the indicators at these acid concentrations. The sensitivity and reproducibility of the polymer-immobilized chromazurol-S, to both nitric and hydrochloric acid, are presented in Figure 4. The sensitivity to hydrochloric acid is much greater than to nitric acid for this indicator
~
90
:
I 100
:
110
Acid Concentration (M)
Flgure 4. Signal response at chloric and nitric acids.
4C0
80
550
nm for chromazurol-S for hydro-
because of the relative degree of dissociation of the two acids. The degree of dissociation of nitric acid drops off rapidly at approximately 4 M concentration (13). The random error of the spectrophotometer response at all acid levels for chromazurol-S in each acid is approximatelythe same at 0.012 AU. For the indicator chromazurol-S,the relative error in the signal at the mean of the response curves in Figure 4 is 1.2% for hydrochloric acid and 1.9% for nitric acid. Fiber-optic Absorption Sensor. Because the refractive index (RI) of the PBI/PI coating (RI = 1.67 in distilled water) was greater than that of the fused silica fiber optic (RI = 1.46), it was possible to operate the fiber-opticsensor in two different ways, depending on polymer coating thickness. If the coating thickness is on the order of the wavelength propagated, the guided light will be refracted into the polymer coating and either be totally guided in the polymer or traverse the core and polymer. This inverted waveguide structure has been used by other researchers as a gas-phase chemical sensor (14). There are disadvantages to using the inverted waveguide sensor in liquid media. Penetration of the thin polymer film by the liquid phase will often change the optical properties and dimensions (swelling) of the coating, which can result in the appearance of intensity maxima and minima during ratio calculations in a broad-band transmission. This effect was evident with PBI/PI coatings of 0.5-3 pm, which exhibited spectral shifts and intensity changes that completely masked the indicator response. A study of the PBI/PI films on glass slides (no indicator), using the interference fringe method described in ref 12, indicated that the polymer blend swelled nearly 8% when transferred from an environment of distilled water to 8 M nitric acid. These unwanted effects may be circumvented by making the coating much thinner than 1 pm (nearly a monolayer). Under these conditions, no interference can be observed and total internal reflection occurs at the solution/polymer interface (15). Since the polymer film is much thinner than the wavelength of light being propagated, light at the polymer/ fused silca interface is only being refracted and not reflected back into the polymer in the same manner as an inverted waveguide. The results of a fiber-optic sensor operating by an absorption interaction are shown in Figure 5. The reference spectrum was taken from transmission through the coated fiber when it was in 2.0 M nitric acid. The absorbance band of the protonated form of chromazurol-S appears at about 550 nm. The large offsets at the nonabsorbing wavelengths (beyond 600 nm) are due to the RI changes of the nitric acid solutions, indicating that the waveguide is evanescently coupled to the sensing medium. Figure 6 shows that the nitric acid solutions fall on the same curve as variable RI sucrose solutions measured by this sensor at 700 nm. This result supports the assumption that the polymer film is thin enough to avoid interferences from swelling in the acid solution.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
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