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Anal. Chem. 1989, 61, 2800-2803
(14) Ravichandran, K.; Lewis, J. J.; Yin, I . H.; Koenigbauer, M.; Powley, C. R.; Shah, P.; Rogers, L. B. J. Chromatogr. 1988, 439, 213-226. (151 ~~, Andres. J. M.: Ramera. C.: Gavilan. J. M. Fuel 1987., 66. 827-830 ._.~~ (16) Blondeau, R.; Kalinowski, E. J . Ch;omatogr.-l986, 351, 585-589. (17) Hayase, K.; Tsubota, H. J . Chromatogr 1984, 295,530-532. (18) MacCarthy, P.; Rice, J. A. I n Humic SUbStanCeS in Soil, Sediment and Water; Aiken, G. R., McKnight, D. M., Wershaw, R., Eds.; John Wiley and Sons: New York, 1985; pp 527-560. (19) Hirose. A.; Ishii, D. HRC CC. J . High Resolut. Chromatogr. Chromatogr. Commun. 1988, 9, 533-534. (20) International Humic Substances Society (IHSS), Standard and Reference Humic Substances, Golden, CO, 1986, personal communication. (21) Saleh, F. Y.; Bronnimann, C.; Frye, J., unpublished results. (22) Saleh, F . Y.; Ong, W.; Kim, I.; Mahmoud. 0.H.; Chang, D. Y. U S . ~ o l o g , c a lsurvey,~ ,Report ~ U~~~ ~ G~~~~ l No, 14~08~0001~G1146, NTIS-PB89-193429. 1989; p 332.
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(23) Jenke, D. R. J. Chromtogr. 1986, 370, 419-426. (24) Saleh, F. Y.; Chang, D. Y. J . Sci. Total Environ. 1987, 6 2 , 67-74.
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RECEIVED for review June 13,1989. Accepted October 2,1989. supported by the U.S. Geological Survey, Research was Department of Interior Assistance Award No. 14-08-0001G1146. Additional support was provided by thg University of North Texas Faculty Research Fund. The views and conclusions in this document are those of the authors and should not be interpreted as necessarily representing the official policies either expressed or implied of the U.S. Government.
CORRESPONDENCE Generation of Chemiluminescence upon Reaction of Iodine with Luminol in Reversed Micelles and Its Analytical Applicability Sir: Since iodine is often the focus of biomedical and environmental studies, improved analytical methods for the determination of trace amounts of iodine are important. Chemiluminescence (CL) methods are very sensitive for a variety of organic and inorganic analytes ( I ) . A first study of the iodine-luminol system in aqueous solutions was reported by Babko et al. (2) and the CL reaction was studied quantitatively by Seitz and Hercules (3)and then by Lutgens et al. (4). In addition, another CL reaction of iodine with a hydrogen peroxide-odium hypochlorite mixture was reported ( 5 ) . However, the analysis of iodine in aqueous solutions has some handicaps: the pH- and concentration-dependent equilibria of I, with IOH, I-, If, etc., and the irreversible loss by iodate formation in alkaline solutions (3, 4 ) . Moreover, several metal ions in aqueous solutions may interfere with the iodine determination by using the luminol CL reaction (6). If so, separation of the interferences would be needed; for example, the technique of solvent extraction or ion chromatographic separation as described in a previous paper could be used (7). Although the analytical applications of CL systems have generally included use of conventional aqueous solutions, considerable attention has been paid in recent years to use of micellar media to enhance CL signals (8, 9). Recently, reversed micelles as a medium in CL measurements have been used to amplify the CL of the H202-luminol system at mild pH in the absence of any catalyst and to determine H202(10) and glucose or glucose oxidase activity (11). In this work, we found that iodine causes CL upon mixing with the reversedmicellar solution of luminol alone: The CL intensity was proportional to iodine concentration and a detection limit (DL) of 2 X M iodine was achieved. Further, the CL generation from the iodine luminol reaction performed in the reversed micellar medium allowed us to develop a new method for iodine determination by a coupling of solvent extraction and CL detection. When solvent extraction is used first to separate from some interferences as mentioned above, a technique of subsequent separation from the solvent such as back extraction (12) or evaporation of the solvent (13) is required usually for CL detection in the conventional aqueous 0003-2700/89/0361-2800$01,50/0
1
mL min-’
Cyclohexane:
a
Reversed micellar
1 umi no1 reagent
100
Cyclohexane
.
Ill
, Detector
2.0
I
60 u1
Flgure 1. Flow diagram for the determination of iodine with CL de-
tection. solutions. On the other hand, our approach may have the advantage that such an additional separation scheme is not needed in the analysis. Accordingly, the present method was found potentially useful for the selective CL determination of iodide where oxidation of I- to I2 and solvent extraction of Iz were simultaneously carried out prior to CL analysis; the extraction procedure was effective in separating iodine from the reactants, oxidant, and acid, which could interfere with the iodine determination.
EXPERIMENTAL SECTION Reagents and Solutions. All reagents were of reagent grade and used without further purification. All aqueous solutions were prepared with water from an Advantec Toyo (Tokyo, Japan) Model GSU-901 water purification system. A 6:5 (v/v) chloroform-cyclohexane (both Wako HPLC grade) mixture containing 0.260 M hexadecyltrimethylammonium chloride, CTAC (Tokyo Kasei), was used as a reversed micellar bulk solvent to prepare reversed-micellar solutions of luminol (Aldrich)according to the literature (IO),but the reagent conditions were optimized for the iodine determination: A 2.5 X M aqueous solution of luminol, prepared daily in a buffer solution of 0.2 M sodium carbonate (pH 11.5),was used (the luminol concentration calculated on a final volume total solution basis was 1.00 x M) and a water:surfactant molar ratio, R = [H,O]/[CTAC], of 10.1 was chosen as optimal.
C
1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 24,DECEMBER 15, 1989 2801
10
.-*
CI
u)
E Q
-S J
0
Time (s) Flgure 2. Peak shape for a CL signal observed from the iodine-luminol reaction system in a CTAC reversed micellar medium under the op-
.-Q5
5
-m CI
Q
a
timum conditions for 100 ng mL-' iodine.
0 0
2
4 Time
6 (min)
8
Figure 4. CL-time responses for various concentrations of iodine obtained under the optimum conditions: (a) the blank, 0; (b) 50; (c) 100 pg mL-' iodine.
Model LF-800 photometer system (Tokyo, Japan) used to detect CL signals. A strip-chart recorder was used to observe peak height and shape. The flow rate of 2 mL m i d for both the streams was determined to be optimal. PTFE tubing (0.5-mm id.) was used between all components in the flow system.
__
- so0 J'
.... . ' ...-.....,
', ,
'.
I. .__.._--_ .----.-. 500 70 Wavelength (nm)
Figure 3. Absorption spectra of iodine species in various media: (a) CTAC reversed micellar solution (R = 10.1)of 0.2M sodium carbonate buffer ([I,] = 1 X lo-' M); (b) 6:5 (v/v) chloroform-cyclohexane mixture ([I2] = 1 X M); (c) aqueous solution of 2 X M potassium iodide ([ 12] = 1 X M).
A 1000 pg mL-' stock solution of iodine was prepared daily by dissolving solid iodine in cyclohexane. Working solutions of iodine were prepared in cyclohexane daily by serial dilutions from tne 1000 pg mL-' 1, solution. All glass sample vessels used for iodine solutions were wrapped with aluminum foil to prevent the solutions from exposure to sunlight or to avoid any photochemical reactions. Apparatus and Procedures. A Hitachi Model F-2000 fluorescence spectrometer (Tokyo, Japan) with a 1-cm cell was used to obtain the CL intensity as a function of time. With a dispensing syringe, 0.5 mL of the I, cyclohexane solution was injected through a rubber septum into the cell in which 0.5 mL of the reversed micellar luminol solution was initially placed. The injection needle had to dip into the reaction mixture near the bottom of the cell, and the I, solution had to be injected quickly for optimal mixing of the reactants. UV-vis absorption measurements for I, in the CTAC reversed micellar medium of the carbonate buffer alone, in the chloroform-cyclohexane mixture, and in a normal aqueous solution of M potassium iodide for comparison, were made in the 2X conventional manner on a Hitachi Model 228 A spectrophotometer (Tokyo, Japan) using a 5-cm cell. Figure 1shows a manifold of the flow system used for the iodine determination. A Hitachi Model K-1000 flow injection analyzer (Tokyo, Japan), equipped with a 16-port, rotary injection valve which serves as introduction loops of the 1, sample and the reversed micellar luminol reagent, was used. The programmed automatic injection valve was used to insert slugs of the 1, sample and the luminol reagent into the respective flow lines of cyclohexane driven continuously by two pumps of the device. After an optimization study of their sizes, the 60- and 100-pL injection loops of the sample and the reagent were chosen, respectively. In this flow system, the sample is mixed with the luminol solution just before entering a spiral flow cell (70 pL) mounted directly in front of the photomultiplier tube in conjunction with a Niti-on
RESULTS AND DISCUSSION When a cyclohexane solution of I2 was mixed with a reversed-micellar solution of luminol at room temperature, CL was observed for a few seconds, quickly decreasing in intensity as shown in Figure 2. Also, the mixture was found to change in color from violet to brownish yellow immediately upon mixing. As presented in Figure 3, absorption measurements for I2 dissolved in the three different media show that the visible absorption band a t 517 nm for Iz in the chloroformcyclohexane mixture alone does not appear on the spectrum in the reversed-micellar solution of the carbonate buffer (pH 11.5), while in the latter medium there is an alternative absorption peak at 352 nm, quite analogous to that in the normal aqueous KI solution, indicating that upon mixing the 1,- ions were generated under the conditions of the reversed-micellar solution. These results demonstrate that the following reactions upon mixing occur in CTAC reversed micelles containing the basic buffer (or OH-):
I2 + OH-
2
IOH
+ I-,
I-
+ I2 2 I,-
As a result of the reactions, 1, should enter almost completely into aqueous cores or water pools of the reversed micellar system to form IO- and I
