Anal. Chem. 1986,58,1137-1140 (3) Pradzynski, A. H.;Rhodes, J. R. “Development of Synthetic Standard Samples for Trace Analysis of Air Particulates”; American Society for Testing and Materials: Philadelphia, PA, 1976 No. 598,p 320. (4) Camp, D. C.; Van Lehn, A. L.; Rhodes, J. R.; Pradzynski, A. H. X-ray Spectfom. 1975,4 , 123. (5) Baum, R. M.; Gutnecht, W. F.; Willis, R. D.; Walter, R. L. Anal. Chem. 1975. 4 7 . 1727. (6) Stiles, A. k.; Dzyubay, T. G.; Baum, R. M.; Walter, R. L.; Willls, R. D.; Moore, L. J.; Garner, E. L.; Gramlich, J. W.; Machian, L. A. I n “Advances in X-ray Analysis”; Gould, R. W., Barrett, C. S., Newkirk, J. B., Rund, C. O., Eds.; KendalVHunt: Dubuque, IA,.1976; No. 19. pp
473-488.
(7) Giauque, R. D.; Garrett, R. 6.; Goda, L. Y. I n “X-ray Fluorescence Methods for Analysis of Environmental Samples”; Ann Arbor Science: Ann Arbor, MI, 1978;Chapter 11. (8) Dzubay, T. G.; Lamothe, P. J. I n “Advances in X-ray Analysis”; McMurdle, H. F., Barrett, G. S., Newkirk, J. B., Ruud, C. O., Eds.; Plenum Press: New York, 1977;No. 20,pp 411-421. (9) Olson. K. W.; Fassel, V. A. Proceedlngs of the 29th Pittsburgh Conference, Cleveland, OH, 1978;Paper No. 648. (10) Semmler, R. A.; Draftz, R. G. €PA Report No. 800/2-78-197,1978. (11) Pella, P. A. EPA Report No. 600/7-80-123, 1980.
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(12) Thompson, G. R. Solid State Technol. 1978 (December), pp 73-77. (13) Sleater, G. L., Crlssman, J., Pella, P. A,, manuscript in preparation. (14) Castellano, R. N.; Felnsteln, L. G. J . Appl. Phys. 1979, 5 0 , 4406. (15) Castellano, R. N.; Felnstein, L. G. J. Vac. Scl. Technol. 1979, 76, 104. (16) Christie, A. B.; Sutheriand, I.; Walls, J. M. Vacuum 1984, 34, 659.
RECEIVED for review August 22, 1985. Accepted November 5, 1985. The authors gratefully acknowledge the financial support from the U S . Environmental Protection Agency under Contract EPA IAG DW-13931193-01-0. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Fiber Optic Ammonia Gas Sensing Probe Mark A. Arnold* and Tiffany J. Ostler Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
A fiber optic ammonia gas sensing probe Is descrlbed In which the pH Indicator dye p-nitrophenol Is employed to detect pH changes of an internal electrolyte solution. The resuitlng pH change Is measured as light absorbance and Is related to the sample ammonia concentration. Ammonia measurements In aqueous soiutlons are demonstrated wlth a lower limit of detection of 5 MM. An equation Is derlved from fundamental concepts to simulate the response of this flber optic ammonla sensor.
Several approaches have been reported for the preparation of fiber optic chemical sensors (1, 2). Sensors have been developed by using colorimetric and fluorometric indicators where solution pH changes (3-9), complexation processes (10, II), and enzymatic reactions (12-14) are employed to generate a measurable optical response that is selectively related to an analyte concentration. Fiber optic sensors for ammonia vapor have been reported where colorimetric indicator dyes are immobilized on the outer surface of glass optical fibers (25, 16). Reaction between the dye and gaseous ammonia results in a complex that absorbs light passing through the fiber, and the degree of light absorption is related to the ammonia vapor concentration. These ammonia sensors are specifically designed to monitor vaporous samples and are not suitable for the determination of ammonia concentrations in aqueous samples. The purpose of this brief report is to present our initial efforts in the development of an optical fiber ammonia gas-sensing probe that is suitable €or ammonia concentration measurements in aqueous samples. Our sensor uses a colorimetric indicator dye in conjunction with a microporous membrane made of Teflon. An equation that describes the response of this ammonia probe is offered to aid in the future development of gas-sensing fiber optic probes.
EXPERIMENTAL SECTION Apparatus and Materials. Unless otherwise noted, all optical equipment was purchased from Oriel Corporation, Stratford, CT. Major optical components used in these studies included a Model 0003-2700/86/0358-1137$01.50/0
77504 illuminator housing, a Model 77800 fiber optic input assembly, a Model 6393 constanbvoltagetransformer, a Model 77760 detector housing, a Model 77652 collimating beam probe assembly, a Model 77761 UV-visible photomultiplier, a Mqdel77802 fiber optic input mount with X-Y-Z adjustment, and a Model 7070 photomultiplier readout. Plastic optical fibers used in these studies were type EK-20 Eska Extra from Mitsubishi Rayon America, Inc., New York, NY. These fibers have an outer diameter of 0.5 mm with a core diameter of 0.48 mm, and they have a numerical aperature of 0.47 k 0.03. Membranes made of Teflon were purchased from Gore and Associates, Elton, MD, and were microporous in nature with an average pore size of 0.02 gm. All solutions were prepared in distilled-deionized water that was purified immediately before use with a Milli-Q three house water purification unit. p-Nitrophenol was used as obtained from Sigma Chemical Co., St. Louis, MO. Other reagents were analytical grade materials purchased from common suppliers. Procedures. Figure 1shows a schematic diagram of the ammonia gas sensor that was prepared by using two plastic disposable pipet tips. Two optical fibers were inserted into the inner plastic tip and were held in place with common household epoxy. The end of the outer pipet tip was cut with a sharp razor blade to an approximate diameter of 5 mm after which the membrane made of Teflon was positioned over this opening and was held in position with an O-ring. The internal electrolyte solution, composed of 0.01 M ammonium chloride and 0.01 M p-nitrophenol, was placed in the outer tip. The inner pipet tip was then inserted into the outer tip such that the optical fibers nearly touched the membrane made of Teflon trapping a thin layer of the internal electrolyte solution at the fiber tips. In their final position, the optical fibers were immediately adjacent to one another in order to maximize the amount of light collected. The volume of internal electrolyte used in the construction of these sensors was 150 gL. Figure 2 shows the experimental arrangement used for ammonia measurements. Light from a tungsten filament lamp passed through an IR blocking filter (Oriel Model 5195) and was focused onto the end of an optical fiber that transported the light to the sensor tip. Light reaching the sensor tip was scattered by the membrane made of Teflon, and a certain fraction of this scattered light was picked up by the second fiber and was transported to the detector system. Before entering the photomultiplier tube, the light passed through a collimating lens and then through a 404.7-nm narrow band-pass filter (Oriel Model 5654). Finally, the light intensity was measured as a current from the photo@ 1986 American Chemical Society
1138
ANALYTICAL CHEMISTRY,
VOL. 58, NO. 6, MAY 1986
In-+ ti+=+? Hln NH3+ ti+@
a
NH~+
IE
YI
b
"3
&t 9
+H
' e
N$
Figure 3. Various phases and chemical equilibria involved in ammonia sensor response: a, internal electrolyte solution; b, gas-permeable membrane; and c, sample solution.
e
b-f
Figure 1. Schematic of optical fiber ammonia gas sensor: a, membrane made of Teflon: b, O-ring: c, internal electrolyte solution; d, epoxy: e, plastic optical fibers;f, outer pipet tip; and g, inner pipet tip.
I1
Hence, an increase in sample ammonia concentration results in a larger amount of the chromophore, which is measured as a decrease in the intensity of the monitored radiation or an increase in absorbance. The relationship between the sample ammonia concentration and light absorbance at 404 nm can be derived by starting with the p-nitrophenol acid dissociation constant. Upon substitution for the hydrogen ion concentration using the ammonium/ammonia acid dissociation constant, one obtains the following relationship:
Kah = K aamm ["4+Iie[In-Iie/
I--L,
Figure 2. Optical arrangement for ammonia sensor: a, tungsten filament light source; b, IR blocking filter; c, focusing lens; d, ammonla probe; e, collimating lens: f, 404.7-nm narrow band-pass filter; g, photomultiplier tube: h, photomultiplier readout; and i, strip chart recorder. multiplier tube to which a voltage of -502 V was applied. Connections between the optical fibers and the illuminator and detector housings were accomplished by use of custom-made connectors. These connectors were prepared from a plexiglass rod that had been machined to a diameter that closely matched the opening diameter of the fiber optic input assemblies on both the source and detector housings (approximately11mm). A length of 1.5 cm was cut from this machined rod, and a hole was drilled through the center. The plastic fiber was positioned through this hole, and epoxy was applied to hold the fiber in palce. After the epoxy had dried, the fiber was cut flush with the connector on the end that was placed in the appropriate housing. Calibration curves were obtained by placing the sensor in 10 mL of a 0.1 M sodium hydroxide solution that was maintained at 25.0 f 0.2 "C using a glass jacketed cell in conjunction with a Fisher Model 90 temperature bath. To this solution, multiple microliter additions of ammonium chloride standards were made. Absorbance values were calculated as -log (&/I)where Io was the incident radiation intensity, which was taken as the initial light intensity with no ammonia in the sample solution, and Z was the steady-state intensity, which corresponded to the particular sample ammonia concentration.
RESULTS AND DISCUSSION Figure 3 shows a schematic diagram of the various phases and dynamic equilibria involved in the response of the ammonia probe. According to the ammonia concentration in the sample solution, gaseous ammonia diffuses across the gaspermeable membrane until the ammonia partial pressure is equal on both sides. Variation of the ammonia concentration in the internal electrolyte solution causes a change in the pH of this solution which alters the relative concentration ratio of the two forms of the pH indicator dye. In our system, p-nitrophenol is employed as the dye. As the sample ammonia concentration increases, the amount of ammonia in the internal electrolyte solution must increase, which results in a higher internal electrolyte pH and more of the indicator dye in the nonprotonated form. For p-nitrophenol the nonprotonated species strongly absorbs 404-nm radiation, which is monitored by the optical system (see Figures 1 and 2).
[",Iie[HInIie
(1)
where K,I" and K,arnmare the acid dissociation constants for the indicator and ammonia, respectively. [NH4f],e,[NH3],,, [In-Iie,and [HIn], are the concentrations of ammonium ions, ammonia, and the nonprotonated and protonated forms of the pH indicator dye in the internal electrolyte solution, respectively. After ammonia and p-nitrophenol mass balance expressions are considered, the following expression is obtained: KaI" = Kaam"(Carnm
- ["313[In-]ie/[NH3lie(C1,
-
Un-13 (2)
where Cam, and CI,, are the total ammonia nitrogen and the total indicator concentrations in the internal electrolyte solution, respectively. If the total ammonia concentration in the internal electrolyte solution, C,,, is assumed to remain constant over the range of sample ammonia concentrations, eq 2 can be rearranged to give
+
[In-Iie = K,I"[NH3]ieCI,/(K,"mmCa,m - Kamm[NH31ie K,"' [",Iie) (3) According to Henry's law, the partial pressures of ammonia in the sample ( p s ) and internal electrolyte (pie) solutions are related to the respective ammonia solution concentrations by the following expressions: Ps
=
(4)
U"31S
(5)
Pie = ke["31ie
where [",Is is the sample ammonia concentration and ks and hi, are Henry's law constants for the sample and internal electrolyte solutions, respectively. Under steady-state conditions, the partial pressures of ammonia on each side of the gas-permeable membrane are equal, which if k , and k,, are equivalent, reveals that the ammonia concentration in the internal electrolyte solution will be equal to that in the sample solution. Accordingly, eq 3 becomes [In-Iie =
K,'"C~n[NH3]s/(KaammC,,m - Kaamm[NH3],+ K,'"["ds)
(6)
Equation 6 can be rearranged to eq 7 after the Beer-Lambert relationship for the absorbance of the nonprotonated species of p-nitrophenol is included: A404 =
~ba,$c,'"C,n[",Is/(K,"mmCamm
K,amm["31,
+ K,"' ["3Is)
(7)
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1139
lot
c
/
i 7
0.5
o0v
I 0.5
I 1.0
I 1.5
Ammonia Concentration, (mM)
FIgure 4. Fiber optic ammonla gas sensing probe response curve.
where e is the molar absorptivity for the nonprotonated species at 404 nm and b,, is the average path length for the sensor. Analysis of the derived equation reveals that a t low sample ammonia concentrations (Le., [NH,],
