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Anal.
Chem. 1988, 60, 2540-2542
the background signal is greater than that obtained with multiple fiber configurations because a single optical path is shared by the emission and excitation radiation. Hence, in addition to the requirements placed on the optical interface, the emission wavelength selector must have a high rejection ratio for stray radiation. However, background radiation from the fiber at wavelengths within the emission band-pass cannot be rejected except by time discrimination methods. When high excitation powers and wide emission bandpasses are used to increase the fluorescence signal and improve the detection limit, the double-fiber configuration yields better detectability when background signal noise is limiting (i.e., greater than dark current noise). The S / B and S / N disadvantage of the single-fiber configuration will depend on the particular optical system, fiber optics, and analyte (i.e., excitation and emission wavelengths). Background fluorescence and Raman scattering from fibers are expected to increase in intensity when excitation wavelengths shorter than that used in this study are employed. Clearly, selection of low background fluorescence fibers is more critical for the single-fiber configuration.
this case, a single-fiber probe will provide only about 80% (1.52 X 0.90 X 0.60) of the signal of a double-fiber probe utilizing the same source without the coupler, regardless of the type of source employed. To further characterize the two systems, single-point calibrations using 500 pg/mL dye and blank solutions were performed to determine the detection limits and S/B’s. The detection limit is defined as the concentration yielding an analytical signal equal to twice the blank noise. The detection limit calculated for the single-fiber system was 0.2 ng/mL with a S / B of 0.03. For the double-fiber configuration, the detection limit was 0.04 ng/mL with a S/B of 10. Thus, the double-fiber configuration provides about a factor of 300 S / B advantage and an approximate factor of 5 detection limit advantage for the specific system studied. Because the calibration slopes are approximately equal, the detection limit for the single-fiber configuration is worse primarily due to higher blank noise. Spectral scans identified the dominant background signal to be due to fiber-optic fluorescence, a common problem with single-fiber systems (13,15,19). Thus, noise in the background fluorescence signal limits detectability. The double-fiber measurements were limited by dark current noise.
LITERATURE CITED Seitz, W. R. Anal. Chem. 1984, 56. 16A-34A. Maugh, T. H. Science (Washington, D . C . ) 1982, 218, 875-876. Chabay. I. Anal. Chem. 1982, 5 4 , 1071A-1080A. Wolfbeis, 0. S.Pure Appl. Chem. 1987, 59, 663-672. Angel, S. M. Spectroscopy (SpringfieM, Oreg.) 1987, 2(4), 38-48. Arnold, M. A. (Ed.) Tabnta 1988, 35(2). Janata, J.; Bezegh. A. Anal. Chem. 1988, 6 0 , 62R-74R. Schwab, P.;McCreery, R. L. Anal. Chem. 1984, 5 6 , 2199-2204. Lieberman, S. H.; Inman, S. M.;Stromvall, E. J. Pfoc.-€lectrochem. Soc. 1987, 87-9 (Proc. Symp. Chem. Sens.), 464-475. Saari, L. A.; Seitz, W. R. Anal. Chem. 1982, 5 4 , 821-823. Kirkbrlght. G. F.; Narayanaswamy, R.; Welti, N. Analyst (London) 1984, 109, 1025. Hirschfeld, T.; Deaton. T.; Milanovich, F.; Klainer, S. M. “The Feasibility of Using Fiber Optics for Monitoring Groundwater Contaminants”; U. C.I.D.-19774; Lawrence Livermore Laboratory, 1983. Deaton, T. Thesis, University of California at Davis, 1984. Plaza, P.; Dao, N. Q.; Jouan, M.; Feurier, H.: Saisse, H. ADD/. . . Opt. . 1986, 2 5 , 3448-3454. Petrea, R. D.; Sepaniak, M. J.; Vo-Dinh. T. Talanta 1988. 35(2), 138-144 . .- . . .. Fuh, M. R. S.; Burgess, L. W.; Hirschfeld, T.; Christian, G. D.; Wang, F. Analyst (London) 1987, 112, 1159-1163. Tromberg, B. J.; Sepaniak, M. J.; Vo-Dinh, T.; Griffin, G. D. Anal. Chem. 1987, 5 9 , 1226-1230. Ealing Electro Optics Guide 1986, 222. Dakin, J. P.;King, A. J. I€€ Conf. fubl. 1983, 221 (Opt. Fibre Sens.), 195-199.
CONCLUSIONS To predict the relative signal levels obtainable using different fluorescence probe configurations, their relative collection efficiencies (Rd),and the efficiency of the single-fiber coupler, must be determined. With the fiber optics tested, there is only about a 50% difference in the fluorescence collection efficiencies of single- and double-fiber probe configurations. For a single-fiber coupler efficiency of 70%, the two configurations would yield essentially equivalent signals. The collection efficiencies of the two basic probe configurations are expected to depend, to a degree, on the numerical apertures of the fiber optics employed. However, theoretical modeling (13, 14) has shown that the collection efficiencies of both configurations are, as a first approximation, proportional to the fiber-optic numerical aperture, and thus the ratio, Rcf, is not expected to change appreciably from the experimental value determined in this work. The experimental scheme presented in this work could be used to survey the relative collection efficiencies of probes constructed by using a variety of fiber optics. As the fluorescence signals obtained with the basic single- and double-fiber configurations would be expected to differ by no more than a factor of 2 or 3, other factors (size, cost, ruggedness, or signal-to-noise ratio (S/N)) may be important in choosing the configuration for a given application. The single-fiber configuration is, in one sense, the simplest as there are no alignment requirements a t the distal end of the system. This configuration is also easily miniaturized. However, the complexity is increased and the overall optical efficiency reduced by the need for an optical interface. Also
Jeff Louch J. D. Ingle, Jr.* Department of Chemistry Oregon State University Gilbert Hall 153 Corvallis, Oregon 97331-4003
RECEIVED for review May 11,1988. Accepted August 23,1988. Acknowledgment is made to the NSF (Grant No. CHE-8401784) for partial support of this work.
Phosphate-Selective Polymer Membrane Electrode Sir: The importance of orthophosphate concentration levels spans all areas of science and technology. A system that can continuously and selectively monitor phosphate levels in aqueous solutions will find numerous applications in fields such as pharmacology, biomedical research, clinical chemistry, 0003-2700/88/0360-2540$01.50/0
industrial process monitoring, environmental monitoring, etc. Past attempts to develop a selective membrane electrode for phosphate have, with the exception of Zolotov and co-workers ( I , 2 ) , not been successful ( 3 ) . In their work, long chain dialkyltin dinitrate species, such as dioctyltin dinitrate and D
1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
didodecyltin dinitrate, are used as electroactive species in liquid membrane electrodes for phosphate and arsenate. Although we have been unsuccessful in developing phosphate-selective polymer membrane electrodes with dialkyltin salts, we have discovered that the incorporation of dibenzyltin dichloride derivatives into a plasticized poly(viny1 chloride) (PVC) membrane gives a selective response to phosphate. The electrode described here exhibits selectivity for orthophosphate over many common anions, such as acetate, sulfate, chloride, bromide, nitrate, and iodide. A detection M and a linear range of response from 2.2 limit of 3.3 x x lo4 to 1.2 X M for dibasic orthophosphate activity are obtained when operated in a pH 7.00 buffer. Slopes of -33.0 f 0.1 mV/decade are obtained. In addition, the electrode lifetime is at least 28 days.
EXPERIMENTAL SECTION Materials. PVC (high molecular weight; Aldrich Chemical Co., Milwaukee, WI), dibutyl sebacate (Eastman Kodak Co., Rochester, NY), Nfl-dimethylformamide (Omnisolve;EM Science, Cherry Hill,NJ), and tetrahydrofuran (Gold Label; Aldrich) were used as obtained. Bis(p-chlorobenzy1)tin dichloride was synthesized according to the procedure of Kinugawa et al. (4). All other chemicals were analytical reagent grade. Apparatus. All electrode potentials were collected with a laboratory-built data acquisition circuit described previously (5). Electrode Preparation. Orthophosphate-selectiveelectrodes were constructed by formation of the active PVC membrane at the tip of a short length of Nalgene tubing. The polymer membrane was formed by dipping the electrode tip in a membrane casting solution and allowing the solvent of this solution to evaporate between successive applications. The membrane casting solution consisted of 70.5 mg of bis(p-chlorobenzy1)tin dichloride, 133.5 mg of PVC, 141.9 mg of dibutyl sebacate, 48.3 mg of N f l dimethylformamide, and 3 mL of tetrahydrofuran. Experimental Conditions. Electrode response was obtained in a pH 7.00 h 0.01 (25.0 "C) working buffer that consisted of 10 mM tris(hydroxymethy1)aminomethane (Tris) with 4.5 mM sulfuric acid. The internal reference solution WBS 0.1 M potassium chloride and a silver/silver chloride internal reference electrode was used. All electrode potentials were measured relative to a silver/silver chloride double junction reference electrode (Corning Science Products; Corning Glass Works, Medfield, MA) with 1 M lithium acetate in outer junction. The calibration curves were generated by making additions of analyte standard to an aliquot of working buffer and recording electrode potentials after each addition. All interference studies were carried out in the working buffer and the pH was continuously monitored and maintained at 7.00 f 0.01 throughout. Electrodes were conditioned prior to operation by soaking the polymer membrane in 1L of the working buffer for 20 h followed by a brief exposure to 10 mM phosphate. Ion activities were calculated based on the theory of Davies (6). RESULTS AND DISCUSSION Figure 1 shows the response curves of our electrode for phosphate, iodide, nitrate, bromide, chloride, and acetate. Each curve represents the average of eight individual electrodes which were calibrated simultaneously. A unique set of electrodes has been used for each response curve. All electrode potentials have been normalized by subtracting from each potential the value obtained in the buffer a t the beginning of the calibration. The response to orthophosphate is linear over a calculated dibasic orthophosphate activity range from 0.2 to 12.0 mM. An activity of 12.0 mM was the largest that could be generated conveniently during the calibration. The slope in the linear region was -33.0 f 0.1 mV/decade, which is slightly steeper than the theoretical slope of -29.5 mV/decade for dibasic orthophosphate. The response curve for monobasic orthophosphate (see Figure l),which is present a t a concentration almost equal to that of the dibasic form at pH 7.00, is essentially of the same curvature as that of the response curve for dibasic orthophosphate. Hence, the electrode appears to be responding to dibasic orthophosphate
2541
0.
> E
-18.
Y
w
0
2z
-36.
0
-I
a
Y
I-
-54.
W
IO
a. -72.
-90.0
-6.00
-5.00
.,
-4.00
-2.00
-3.00
-1.00
LOG ACTIVITY A N I O N ( M )
Figure 1. Electrode response curves for several anions: 0 , dibasic orthophosphate; +, monobasic orthophosphate; A, iodide; 0,nitrate: A, bromide; acetate; and X, chloride. Data at log activity -6.00 were collected in Tris buffer only. The maximum standard deviation
of potential change (of any curve) for the lowest interferent ion activity is 0.9 mV. The maximum standard deviation (of any curve) at the highest interferent ion activity is 3.8 mV. Table I. Selectivity Data for Phosphate-Selective Membrane Electrode interferent interferent activity, M iodide nitrate bromide chloride acetate
1.21 X 3.39 X 4.37 X 10" 8.49 X 1.03 X
limit of detection of dibasic phosphate activity, M
selectivity coefficientn
2.036 (f0.006) X lo4 1.20 (f0.02) X lo4 4.22 (f0.04) X 6.6 (kO.l) X 8.6 ( f O . l ) X
138.8 (k0.4) 10.5 (10.2) 2.20 (f0.02) 0.92 (kO.01) 0.80 (fO.01)
Measured by fixed interference method (see ref 7 for details). under these conditions. The detection limit for dibasic orthophosphate, as defined by conventional IUPAC recommendations (7), is 0.033 f 0.007 mM. Potentiometric selectivity coefficients, obtained by the fixed interference method, for the tested anions and other selectivity related information are summarized in Table I. Values were calculated according to the following equation: KPOt
a*HPOI HPO42-/X-
-2
=
where a * H P O I s corresponds to the limit of detection for dibasic orthophosphate in the presence of ax- of the tested interferent. The selectivity coefficients calculated by this method appear to be larger (indicating low selectivity for dibasic orthophosphate) than would be expected given the excellent responses of the electrode to dibasic orthophosphate in the presence of activities of interferents listed in the table. It is believed that the selectivity coefficients calculated by this method are not a valid indication of the electrode's selectivity due to the sub-Nernstian responses of the electrode to the interferents at the specified activities. For this reason, Table I1 lists several ratios of activities of interferent to dibasic orthophosphate. The activities of the ions shown are those at which the electrode exhibited the same potential relative
Anal. Chem. 1988. 60. 2542-2544
2542
Table 11. Ion Activities and Their Ratios Potentials potential change, interferent
mV
interferent activity, M
iodide nitrate bromide chloride acetate
-50.0 -30.0 -14.0
4.67 x 10-3 1.14 X 4.08 X
-14.0
-7.0
1.34 X
4.57
X
at
Equivalent
dibasic orthophosphate activity, M 1.19 x 10-3 2.59 X 5.22 X 5.22 X 1.51 X
activity ratioo 3.92 44.0 78.1
257 302
Ratio = (interferent activity)/(dibasic orthophosphate activi-
Our bis(p-chlorobenzy1)tin dichloride based membrane electrode possesses selectivity for dibasic orthophosphate that is clearly superior to previous anion-selective polymer membrane electrodes. Based on the excellent selectivity, low detection limits, and favorable lifetimes of this membrane electrode, development of practical continuous monitor systems for orthophosphate may soon be possible.
ACKNOWLEDGMENT We wish to thank Professor Louis Messerle of the Department of Chemistry at the University of Iowa for his assistance in the synthesis and utilization of organotin compounds.
ty).
LITERATURE CITED
to the potential in Tris buffer. Ratios that are greater than one clearly indicate that the electrode is responding to a larger extent to dibasic orthophosphate under the specified conditions than to the interferent. Additional ratios of this type can easily be derived for other activities by inspection of the responses in Figure 1. Although the electrode's selectivity for orthophosphate over sulfate was not measured directly, it can be reasoned that this selectivity is considerable given the low limit of detection for dibasic orthophosphate found by calibration in the sulfate-containing Tris buffer. Usable calibration curves for dibasic orthophosphate are obtained over a 28-day period when electrodes are stored in the working buffer a t room temperature between measurements. After approximately 2 weeks of use, however, the detection limit begins to gradually deteriorate and slightly shorter linear ranges are observed. Detection limits below the millimolar activity level are observed even after 28 days.
Shkinev, V. M.; Shpigun, L. K.; Spivakov, B. Y.; Trepalina, V. M.; Zarinskii, V. A.; Zolotov, Y. A. 2%. Anal. Khlm. 1980, 35, 2137. Shkinev, V. M.; Shplgun, L. K.; Spivakov, V. A.; Zolotov, Y. A. Zh. Anal. Khlm. 1980. 35, 2143. Midgley, D. Ion-Sel. Nectrw'e Rev. 1988, 8, 3. Kinugawa, Z.; Sisido, K.; Takeda, Y. J. Am. Chem. Soc. 1961, 83,
538. Arnold, M. A.; Glarier, S. A. Talsnta 1988, 35, 215. Butler, J. N. Ionic Equilibrium A Mathematical Approach ; Wesley: Reading, MA, 1964 Chapter 12. Pure Appl. Chem. 1978, 48, 127.
Scott A. Glazier Mark A. Arnold* Department of Chemistry The University of Iowa Iowa City, Iowa 52242 RECEIVED for review March 29, 1988. Accepted August 23, 1988. Support for this work from the National Institute of Dental Research (DE07996) is greatly appreciated.
TECHNICAL NOTES Laminar-Flow Torch for Helium Inductively Coupled Plasma Spectrometry Hsiaoming Tan, Shi-Kit Chan, and Akbar Montaser*
Department of Chemistry, T h e George Washington University, Washington, D.C. 20052 Helium inductively coupled plasmas (He ICPs), operated at atmospheric pressure ( 1 4 , possess two advantages compared to Ar ICPs for atomic emission spectrometry (AES) and mass spectrometry (MS). First, for the elements tested so far, the detection powers for the He ICPs are superior to those for an Ar discharge. Second, the emission background spectra of the He ICPs are quite simple in the red and the near-infrared regions, thus reducing the spectral interference problems encountered with the determination of halogens and other nonmetals. Relatedly, certain mass spectral interferences noted in the detection of monoisotopic elements are eliminated when helium is used as the plasma gas instead of argon. For the most recent studies of He ICPs (2-6), we used a tangential-flow torch to form an annular plasma at forward power of 1500 W with a total helium gas flow of 8 L/min. The present study is concerned with the formation and preliminary characterization of a He ICP using a laminar-flow torch. The total helium gas flow for this torch is less than 2 L/min. Studies of plasmas formed in laminar-flow torches are important because of the possibility to reduce one major source of noise resulting from the rotation of the plasma gas in
tangential-flow torches. Previous studies on Ar ICP discharges have documented the advantages of laminar- versus tangential-flow torches for AES (7-11).
EXPERIMENTAL SECTION 1. Instrumentation and Operating Conditions. Except for
the laminar-flow torch, the ICP-AES system and the operating conditions for the spectrometer are described elsewhere (12,13). Briefly, most experimental data were acquired with an intensified photodiode array spectrometer using a slit width of 50 pm. However, measurements of rotational temperature (Trot)and electron number density (ne)required the use of a photomultiplier tube to utilize the maximum resolution of our spectrometer. The slit widths were 5 and 10 pm for measurements of Trotand ne, respectively. The aperture of the imaging optics was set at 25 mm diameter, and a red filter (catalog no. CS2-63,2424, Corning Class Works, Corning, NY) was placed in front of the entrance slit of the spectrometer to eliminate possible spectral interference from the higher order spectra (3). Pure helium (99.997%,MG Industries, Valley Forge, PA) was used to form and sustain the He ICP. To introduce sample into the plasma, the injector gas was replaced with a gas mixture containing 105 pL/L of SFB,99 &/L of CC12F2,and 96 pL/L of CRrF3in helium (Certified gas mixture, Matheson Gas Products,
0003-2700/S8/0360-2542$01.50/0 0 1988 American Chemical Society
