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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.
