Anal. Chem. 1987, 59, 211-212 (11) Stolzberg, R. J. Anal. Chlm. Acta 1977, 92(1), 139. (12) Taylor, J. K.; Ziellnskl, W. L., Jr.; Maienthul, E. J.; Durst, R. A.; Burke, R. W. Net/. Bur. Stand. 1972, 21903513. (13) VOUlQarOPOUloS. A.; Vdenb, p.; Nurnberg. H. w. Ff8S8nlUS' 2.Anal. Ctmm. 1984.317,367. (14) Karadakov, B. P.; Venkova, D. I. Talanta 1970. 17, 878.
211
(15) Eskllsson, H.; Jagner, D. Anal. Chim. Act8 1982, 138, 27. (16) Jagner, D.; Aren, K. Anal. Chlm. Acta 1982, 737, 201.
R~~~~~~ for review~
~12, 1986. ~ i ~ ~l ~ ~ b~~l~ ~ 25, i t t ~ d 1986. Accepted August 15, 1986.
Luminol Chemiluminescence for Determination of Iron( I I ) in Ferrioxalate Chemical Actlnometry M a r k A. Nussbaum, Howard
L. Nekimken,' a n d Timothy A. Nieman*
Department of Chemistry, University of Illinois, 1209 West California Street, Urbana, Illinois 61801 Actinometry is a means of measuring light fluxes from an ultraviolet or visible light source. Chemical actinometry requires a solution that undergoes a chemical change (such as a redox reaction) of known quantum yield upon photoexcitation. Species used in actinometer solutions have included ferrioxalate (I,2))cobaltioxalate (3), uranyl oxalate (4), and Reineckate ion ([Cr(NH,),(NCS),]-) (5). Ferrioxalate is the most widely used actinometer solution. Its reaction sequence is shown below (1,2):
HaO*
[Fe(C20&3l3-
[Fe(C204)l++ 2(C2O4l2-
+ (C204)2 C 0 2 + Fe2+ + (C204)2-
[Fe(C204)]+-kFe2+ (C204)-
-
+ [Fe(C204)]+
Note that a single photon can potentially produce two Fe2+ ions. The Fe2+produced in this reaction has traditionally been detected by measuring the absorbance after complexation with 1,lO-phenanthroline (Arn= = 510 nm) (1). Cobaltioxalate undergoes a similar photochemical reaction in which Co2+is produced instead of Fe2+,at a somewhat lower quantum yield (3). Luminol chemiluminescence (CL) has been shown to be a very sensitive method for the determination of several metal ions, including Fe2+and Co2+(6-8). In the presence of H202, M using luminol CL Co2+can be quantitated to below (8). Because of this low detection limit for Co2+ and the observations that Co3+ (9) and Co(II1) complexes (IO) are inactive as luminol CL catalysts, it would appear that luminol CL shows promise as a means of detecting Co2+produced in cobaltioxalate actinometer solutions. We have found, however, that although luminol CL can be used to detect photogenerated Co2+, the thermal instability of cobaltioxalate (11) results in a high CL background which cannot be reduced sufficiently (via extraction of the cobaltioxalate with dithizone) to allow sensitive determinations. Luminol CL can be used to quantitate Fe2+in the absence of H202,as long as molecular oxygen is present. Seitz (6) reported an Fe2+detection limit of approximately 5 X M using such a system. The use of oxygen as the primary oxidant in the luminol CL reaction allows the selective quantitation of Fez+in the presence of Fe3+, since Fe3+ is not a catalyst for this system in the absence of H202. The sensitivity and selkctivity of Fez+detection by luminol CL indicate that such a detection scheme may be of use in ferrioxalate actinometry. The reported detection limit for Fe2+by luminol CL is 2 or 3 orders of magnitude lower than that obtained from M vs. the iron-phenanthroline absorbance method (5 X Present address: Los Alamos National Laboratory, M a i l Stop
G740, Los Alamos, NM 87545.
0003-2700/87/0359-0211$01.50/0
lo-' M) (1,6). Improved detection limits would lead to the ability to detect lower light levels. In addition, the CL method offers a potential advantage in that no complexation step is required; the ferrioxalate actinometer solution could be analyzed directly for Fe2+immediately after irradiation. Finally, the use of a technique that does not depend on complexation by phenanthroline would obviously circumvent the errors reported to arise from photodegradation of competitive complexation of phenanthroline (12). The purpose of the work reported here was to investigate the potential advantages offered by luminol CL for quantitation of Fe2+ generated photochemically from solutions of ferrioxalate. EXPERIMENTAL SECTION Reagents. Solutions containing 6 mM ferrioxalate (K3Fe(C204)3-3Hz0; Pfaltz and Bauer) in 0.05 M H 8 0 4were prepared on the day of use from a 60 mM stock solution. Standard solutions of Fez+,also in 0.05 M HzS04,were prepared from FeS04.7H20 (Mallinckrodt). Iron-phenanthroline solutions were prepared by using 1.0 mL of 0.1% 1,lO-phenanthroline monohydrate (GFS), 3.0 mL of buffer (pH 4.4) containing 0.18 M HzS04and 0.6 M sodium acetate (Mallinckrodt), and 5.0 mL of the appropriate iron solution, diluted to 10.0 mL with water. The Fez+ CL measurements made use of 5 mM luminol in 0.4 M borate buffer (pH 10.8). Carbonate or ammonia would provide better buffering capacity at the pH used, but each has been found to adversely affect luminol CL measurements (13). The luminol concentration and pH were selected based on the optimum values found previously for determination of Fez+(6) All water used was purified by a Continential/Millipore Milli-Q system. Apparatus. Iron-phenanthroline absorbance measurements were made at 510 nm with a Hewlett-Packard 8450 diode array spectrophotometer and 1-cm cuvettes. A few of the CL measurements were made with a home-built stopped-flow instrument previously described by Stieg and Nieman (14). For the remaining CL measurements, a home-built flow-injectionanalyzer was used (Figure 1). The instrument made use of a Rainin Rabbit peristaltic pump, Rheodyne Model 5020 injection valve (70-pL sample loop), and Teflon tubing (0.8 mm i.d.). One channel contained 0.05 M HzSO4, and the other contained the buffered luminol solution. The observation cell volume was approximately 180 pL. The large cell volume was used so that changes in reaction rate with Fe2+concentration would not cause the maximum CL intensity to occur outside of the cell. Flow rates were 0.7-0.8 mL/min in each channel. Chemiluminescence wm measured with a 1P28 photomultiplier tube and Pacific Precision Model 126 photometer linked to a strip-chart recorder. A tungsten light source was used to irradiate the actinometer solutions. The optics of the source were such that the lamp image was brought to a focus 2 cm beyond the source housing. Procedure. Except for weighing the solid starting material, which was done under dim room light, all solutions containing ferrioxalate were prepared in a darkroom under safelights. Flasks containing stock ferrioxalate solutions were wrapped in aluminum foil and stored in a darkroom refrigerator. 0 1986 American Chemlcal Society
212
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
U Figure 1. CL
I
PMT
1
W 0
z
flow system.
Working curves for Fe2+in 6 mM ferrioxalate were obtained by both the iron-phenanthroline absorbance and luminol CL methods. The iron-phenanthroline solutions were prepared as described previously and allowed to stand 30 min or longer prior to measurement. Three blanks (i.e., solutions containing 6 mM ferrioxalate only) were prepared in order to take into account the imprecision involved in sample preparation. All absorbance measurements were done under dim room light to prevent unnecessary photoreaction of the ferrioxalate. Water was used in the reference cell. For the CL measurements, samples of Fez+ in 6 mM ferrioxalate were taken without additional treatment and injected directly into the FIA instrument. The syringe and FIA tubing were wrapped with black tape to prevent photoreaction of the ferrioxalate. For the irradiation experiments, an 80-mm-diameter X 40mm-high glass dish filled with approximately 180 mL of 6 mM ferrioxalate was illuminated from above by the tungsten source. The bottom of the dish was located 17 cm below the focus position of the source lamp. The solution was stirred continuously and insulated with Teflon from heat generated by the magnetic stirrer. Irradiation for a given interval was performed by opening and closing the shutter of the source. For the CL studies, the solution was initially stirred for 20 min prior to any sampling to ensure oxygen saturation. Samples were taken by syringe and injected into the FIA instrument after each irradiation interval. For the absorbance measurments, iron-phenanthroline solutions were prepared as described above, using 5-mL aliquots of the ferrioxalate solution taken after each irradiation interval. The ironphenanthroliie solutions were allowed to stand for at least 30 min prior to measurement.
RESULTS AND DISCUSSION Before irradiation experiments were attempted, it was important to determine whether luminol CL could be used to quantitate small amounts of Fez+in the presence of a large amount of ferrioxalate. Solutions of Fez+ a t 0-40 pM in a matrix of 6 mM ferrioxalate were used to prepare working curves for Fez+ by both luminol CL and absorbance. Both working curves are monotonic and nearly linear over this range and indicate Fez+ detection limits of somewhat under 0.5 pM. Irradiation experiments were then performed as described in the Experimental Section. Irradiation times ranged from 0 to 720 s. The results are shown in Figure 2. For comparison, an Fe2+concentration of 1.0 pM would produce a CL signal of approximately 0.03 nA above the blank or an absorbance of approximately 0.006 above the blank. These working curves than represent about 0-2 p M Fez+. The CL method is comparable to somewhat superior to the absorbance approach in detection limits and precision. The detection limits (at a signal-to-noise ratio of 3) were 200 of irradiation for the absorbance method and 120 s for the CL method. The CL intensity for a given Fez+ concentration can be increased by using lower concentrations of HzSO4 so that the pH after mixing remains closer to the optimum (6). This was verified by one attempt a t using 0.01 M HzS04instead of 0.05 M HzS04;however, the precision obtained in this case deteriorated so that the detection limit was not improved. The precision obtained by the CL method for a given determination at the lowest Fez+concentrations was generally superior to that of the absorbance method and was largely limited by pulse noise from the peristaltic pump. It is likely that the precision (and consequently the detection limit) for the CL method could be improved by using a different means of solution delivery. Both the absorbance and the CL method
- .01$
x
m 4
- ,005
c 0
200
400
600
o
800
IRRADIATION TIME (SEC)
Figure 2. CL (0)and absorbance (0)working curves for irradiation of ferrioxalate. Error bars represent 1 standard devlation. Error bars for the absorbance curve include only the imprecision of the actual absorbance measurement, except for those of the blank, which include imprecision from the required sample treatment as well. The error bars for the blank are therefore more representative than those for the remaining points on the absorbance curve.
yield improved relative precisions for measurements done with longer irradiation times (to yield higher Fez+concentrations). It should be noted that day-to-day variations in CL signal and precision were more significant than those observed for the absorbance method, and the CL response shows slight upward curvature in this concentration region (Figure 2). Thus, one would need to construct a CL working curve on a daily basis for the most accurate results. These disadvantages are compensated, however, by superior speed, simplicity, and sample size requirements. For the absorbance method a sample of 5 mL was withdrawn from the actinometer solution, mixed with the complexing reagents, and allowed to stand to reach equilibrium. In contrast, the CL method required less than 100 pL of solution; this sample was injected directly into the CL flow system and the measurement completed about 1min later. Thus, the CL method may speed actinometry quantitation to yield either higher throughput for many samples or nearly real-time monitoring of a few samples. Although this work was performed on a CL flow system of in-house construction, the CL quantitation could also be performed using any of several inexpensive luminescence photometers available commercially. Registry No. Fe, 7439-89-6;luminol, 521-31-3;ferrioxalate, 15321-61-6.
LITERATURE CITED Parker, C. A. Photoluminescence of Solutions : Elsevier: Amsterdam, 1968; pp 208-214. Balzoni, V.; Carasslli, V. Photochemistry of Coordination Compounds : Academlc Press: London, 1970; pp 167-172. Porter, C. B.; Doering, J. G. W.; Karanka, S . J. Am. Chem. SOC. 1982, 8 4 , 4027-4029. Adamson, A. W.; Fleischauer, P. D. Concepts of Inorganic Photochemistry; Robert E. Krieger: Maiibar, FL, 1964. Wagner, C. E.; Adamson, A. W. J. Am. Chem. SOC. 1966, 8 8 , 394-404. Seitz, W. R.; Hercules, D. M. Anal. Chem. 1972, 4 4 , 2143-2149. Nau, V.; Nieman, T. A. Anal. Chem. 1979, 5 1 , 424-428. Carter, T. J. N.; Kricka, L. J. I n Clinical and Blochemical Luminescence: Krlcka, L. J., Carter. T. J. N., Eds.; Marcel Dekker: New York. 1982; pp 135-151. Nekimken, H. Ph.D. Dissertation, University of Illinois, Urbana, IL, 1986. Sheehan, T. L.; Hercules, D. M. Anal. Chem. 1977, 4 9 , 446-450. Copestake. T. B.; Uri. N. R o c . R. SOC.London, A 1955, A228, 252. Kirk, A. D.; Namaslvayam, C. Anal. Chem. 1983, 55, 2428-2429. Klopf. L. L.; Nieman, T. A. Anal. Chem. 1963, 55, 1080-1083. Stieg, S.;Nieman, T. A. Anal. Chem. 1977, 4 9 , 1322-1325.
RECEIVED for review March 24, 1986. Accepted September 4,1986. This work was supported, in part, by National Science Foundation Grant CHE81-08816.
