Anal. Chem. 1985, 57, 2751-2752
(5) Nygren, S . HRC CC J . High Resolut . Chromatogr . Chromatogr .
DDD, 72-54-8; p,p’-DD, 50-29-3. (1)
Zlatkis, A.;
2751
LITERATURE CITED Fenimore, D. C.; Ettre, L. S.; Purcell, J. E. J . Gas Chroma-
togr. 1965, March, 75. (2) Nygren, S.; Mattsson, P. E. J . Chromatogr. 1976, 123, 101. (3) Nygren, S.J . Chromatogr. 1977, 14Z9109. (4) Poy, F. Chromatogr. Symp. Ser. 1979, I , 107.
Commun. 1979, 2, 55. (6) Littlewood, A. B. “Gas Chromatography”; Academic Press: New York, London, 1962; p 72. (7) Mattsson, P. E.;Nygren, S . J . Chromatogr. 1976, 124, 265.
RECEIVED for review March 18, 1985. Accepted July 9, 1985.
Fluorescence Detection of Polishing Alumina on Glassy Carbon Electrode Surfaces Mary L. Patterson and Craig S. Allen*’
Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Particles of polishing alumina present on the surface of glassy carbon electrodes have been observed by Kuwana to catalyze the oxidation of catechols (1). This catalytic activity was attributed to adsorption of the electroactive species to the alumina. In that study, ESCA and scanning Auger spectroscopy were performed to confirm the presence of alumina on the glassy carbon surface. Alumina-contaminated glassy carbon electrodes are considered to be a type of chemically modified electrode (2, 3); therefore, detection of alumina on the surface may be important to chemists who employ these electrodes. We have observed that polishing alumina from at least one major supplier exhibits intense fluorescence when excited at various visible laser wavelengths. We have employed this fluorescence to detect alumina on the surface of a glassy carbon electrode.
EXPERIMENTAL SECTION Excitation of the sample was performed with a Spectra-Physics Model 171 argon ion laser or a Spectra-Physics Model 375 dye laser using Rhodamine 6G dye. Diffusely scattered light from the sample was collected in a standard backscattering geometry by using an Olympus f1.2 camera lens and focused onto the entrance slit of a Spex 1403s 0.85-m double monochromator. Photons arriving at the exit slit were detected by an RCA (231034 PMT contained in a Products for Research C31034/76 rf shielded housing cooled to 20 “C.Standard photon-counting techniques were employed to detect the signal. Translation of the glassy carbon electrode was performed with an actuator (Newport850-05) mounted to one stage of an XYZ translator. An IBM personal computer with a Tecmar LabMaster board was employed to collect data and to control the actuator. Polishing alumina of sizes 20 and 5 pm (gray) and 0.3 and 0.05 pm (white) distributed by Buehler, Ltd. (Lake Bluff, IL), was investigated. The glassy carbon surface was polished with 0.05-pm alumina and rinsed with distilled water.
RESULTS AND DISCUSSION Fluorescence observed from the 0.05-pm alumina is shown in Figure 1. This fluorescence, which occurs a t 14 398 and 14429 cm-l, is due to trace amounts of Cr3+ present in the alumina matrix (4). It was observed for various grades and sizes of Buehler alumina. In addition to the Cr3+/A1203 fluorescence, the gray 20- and 5-pm sizes exhibit a broad, featureless background, which has previously been partially attributed to transition-metal impurities (5). The white 0.3and 0.05-pm sizes produce very intense Cr3+/A1,03 fluorescence on a near dark-count background. Changing the excitation wavelength confirmed that the signal was fluorescence, since its absolute frequency remained constant (see Figure 1).
Laser-excited fluorescence of transition-metal and lan‘Present address: Rohm and Haas Co., 727 Norristown Rd, Spring House, PA 19477.
Figure 1. Fluorescence from 0.05-pm polishing alumina containing trace amounts of C?’. (a)Excitation at 15803 cm-‘ (632.8 nm); laser power 25 mW; band-pass 1 cm-’. (b) Excltation at 19435 cm-‘ (514.5 nm); laser power 4.1 mW; band-pass 1 cm-’.
I
1000 750 500 250 0 Distance, microns
Figure 2. Fluorescence from 0.05-pm polishing alumina containing trace amounts of Cr3+ on the surface of a glassy carbon electrode. Laser power 5.3 mW at 16 660 cm-’ (600.2 nm). (a) Signal at 14 398 cm-’ as a function of position. A line scan across the electrode surface was performed; two spots of alumina contamination can be seen. The diameter of the laser beam was about 35 Wm. (b) Spectrum acquired from the spot in (a) marked with an arrow. thanide ions in metal oxide matrices has been demonstrated to be an extremely sensitive analytical technique, with detection limits in some cases better than those obtained with neutron activation analysis (6). A very critical parameter in the preparation of the metal-1 ion/metal-2 oxide sample is the ignition temperature of the solid coprecipitate. A high ignition temperature (1000 “C is common) is required for the observation of intense, narrow line width fluorescence. This
0003-2700/85/0357-2751$01.50/0 0 1985 American Chemical Society
2752
ANALYTICAL CHEMISTRY, VOL.
57, NO. 13, NOVEMBER 1985
completely dehydrates the metal oxide matrix and thermally anneals the sample, causing the metal ions to reside in their thermodynamically favored sites in the metal oxide host lattice (7). Literature from a major supplier of alumina states that the 0.3- and 0.05-~msizes of polishing alumina are calcined, or heated to high temperatures, in their processing (8). This presumably contributes to the intensity and narrow line width of the Cr3+/A1203fluorescence. Figure 2 shows the fluorescent signal obtained from alumina on the surface of a glassy carbon electrode. A scan across the surface of the sample, shown in Figure 2a, was obtained by computer-controlled translation of the electrode. Thus, spatial mapping of alumina contamination on a glassy carbon surface can be accomplished. Figure 2b shows the spectrum corresponding to the point marked with the arrow in Figure 2a. Polishing alumina containing Cr3+is also a potential spectral interference in spectroelectrochemical experiments. Its fluorescence can be deceptive since the line width is consistent with that of Raman (vibrational) signals. Examination of the absorption curve of Cr3+/A1203reveals that the fluorescence can be excited through the entire visible region of the spectrum with a red threshold of about 650 nm (9). The quantum efficiency for the fluorescence is about 0.6 for excitation with visible wavelengths a t room temperature (9),so there is little chance that this spectral interference can be avoided if alumina is present on the portion of the electrode surface being sampled by the laser beam.
In summary, laser-excited fluorescence of trace Cr3+in the alumina matrix provides the analytical chemistry with a sensitive method to trace polishing alumina on the surface of electrodes. The large cross section facilitates the use of a low-cost He-Ne laser as the excitation source. This method is simpler and less expensive than ESCA or Auger spectroscopy, which have previously been employed in the detection of alumina on glassy carbon surfaces.
ACKNOWLEDGMENT We thank James P. Reilly for his help in identification of the fluorescent signal. Registry No. Alumina, 1344-28-1; carbon, 7440-44-0.
LITERATURE CITED Zak, J.; Kuwana, T. J . A m . Chem. SOC. 1982, 104, 5514-5515. Zak, J.; Kuwana, T. J . Necfroanal. Chem. 1983, 150, 645-664. Wang, J.; Freiha, B. Anal. Chem. 1984, 5 6 , 2266-2269. Kaminskii, A. A. "Laser Crystals"; Springer-Verlag: New York, 1981; p 81. Jeziorowski, H.; Knozinger, H. Chem. Phys. Lett. 1976, 4 2 , 162-165. Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977, 4 9 , 1680-1689. Johnston, M. V.; Wright, J. C. Anal. Chem. 1982, 5 4 , 2503-2507. Beuhler Analyst. 1982 catalog, section 7, pp 22-24. Roess, 0.I n "Lasers and Their Applications"; Sona, A., Ed.; Academic Press: New York, 1970, pp 223-225.
RECEIVED for review April 19,1985. Accepted June 27, 1985.
CORRECTION Sampling Tubing Effects on Groundwater Samples Michael J. Barcelona, John A. Helfrich, and Edward E. Garske (Anal. Chem. 1985,57, 460-464). In the article, the predicted sorptive, losses of chlorinated hydrocarbons due to exposures to sampling tubes shown in Table IV, p 463, are in error. The correct values for a 400 ppb halocarbon mixture are shown below. Table IV. Predicted Percent Sorptive Loss of Chlorinated Hydrocarbons due to Tubing Exposuresa tubing diameter, in. TFE
% loss
PP
PVC
SIL
36 (36) 56 318 16 22 (25) (56) (14) (22) 112 21 29 48 74 (18) (29) (33) (81) (74) a 400 ppb mixture of chloroform, trichloroethylene, tetrachloroethane, and tetrachloroethylene calculated on the basis of initial sorption rates on passage through 1 5 m of tubing at 100 rnL.min-'. Percent loss values are tabulated for the original solution (with) and without organic carbon background. 114
11 (9)
14 (14)
PE 24
(17) 36
33 (40) 50 (61) 67
The correction does not change the conclusion that predicted sorptive losses are more dependent on tubing material than on tubing diameter. However, a t constant flow rates, the predicted losses would increase rather than decrease with larger diameter tubing. The authors apologize for the error.
