Anal. Chem. 1980, 52, 191-192
Finally, the overlap between phenanthrene and pyrene (Figure 2, D and C, respectively) is slight. The main pyrene emission is overlapped by anthracene, and other conditions are required for their resolution. The choice of conditions need not be a matter of fortuity as Latz et al. Suggest ( I ) , but can be made from a knowledge of the component fixed excitation and emission spectra (4).
LITERATURE CITED (1) Latz, H. W.; Ullman, A . H.; Winefordner, J. D. Anal. Chem. 1978, 50, 2148.
Sir: Our Correspondence concerning synchronous luminescence measurements of multicomponent samples was intended not as a criticism of the technique which indeed has valuable applications, but rather as a concern against the sole use of the technique. The comment by Lloyd simply reinforces our original conclusions in that additional measurements are required to reveal difficulties with the analyte solution. Had the sample been “real” and the contents unknown, sample dilution could conceivably have reduced the concentration of a component below the limit of detection. We must also take exception to the comment that our conclusions were based “on errors of fact and interpretation” due to a massive inner filter effect because Lloyd makes no distinction between prefilter, post-filter effects; and self absorption effects; the extent of the former two depend entirely upon the cell geometry and methods of illumination and measurement and the latter one depends only upon the optical depth of the analyte a t the fluorescence wavelength(s). In our Correspondence, the concentrations of the PAH compounds used were selected on the basis of the sensitivity of the emission section of the instrument used with the exception of fluorene. All peaks in Figure 1 should be labeled x l except for the fluorene peak which was x 3 owing to the higher concentration of fluorene used to show the extent of the masking. A pre-filter effect (inner filter), absorption of the exciting radiation, was of concern because of the additive absorbance of the mixture at various wavelengths but was found not to be a problem with the instrumental system used by us. The sample compartment permitted translation of the 10 X 10 mm cell along the axis of excitation, and similar results were obtained from approximately 2- to 8-mm excitation depths (with the exception of increased scatter at each extreme). This is attributed to the combination of a focused xenon arc lamp and a high efficiency monochromator producing a high excitation flux. Certainly, ratio absorbance measurements alone are not sufficient to infer a significant pre-filter effect because increased absorbance of exciting radiation can be offset by an increase in the intensity of that radiation.
191
(2) Parker, C. A. “Photoluminescence of Solutions”; Elsevier Publishing Company: New York. 1968; pp 222-226. (3) Lloyd, J. B. F. J . Forensic Sci. SOC.1971, 11, 83. (4) Lloyd, J. B. F.; Evett, I. W. Anal. Chem. 1977, 49, 1710.
H~~~ Office ~~~~~~i~ science ~~b~~~~~~~ Priory House Gooch Street North Birmingham B5 6QQ, U.K.
J. B. F. Lloyd
RECEIVED for review March 5 , 1979. Accepted July 10, 1979.
The fact that a post-filter effect existed in our case, the absorption of fluorene fluorescence by pyrene, was the point intended. It is particularly significant in that a weak fluorophor was masking a strong one. Dilution of a sample to offset high absorbance due to a weakly or nonfluorescent compound could conceivably result in loss of qualitative luminescence information about either one or both of the compounds. If in the event that masking is not complete when the post-filter condition exists, then the problem of quantitation remains complex. Finally, when the sample is an unknown, the fixed excitation and emission spectra of the components are also unknown and the relative concentrations of species would indeed need to be fortuitous if error due to spectral overlap is to be avoided. The conclusion is still held that the application of synchronous luminescence to mixtures of complex and unknown composition, although excellent for fingerprinting, requires caution in the form of more conventional measurements. Lloyd is probably correct concerning the possibility of anthracene impurity in the phenanthrene since a commercial sample was used without purification (as stated). In Figure l b , the small peaks IV and I11 should be x1 not x 3 and in the caption AA should equal 5 nm as stated in the text.
LITERATURE CITED (1) Latz, H. W.; Ullman, A. H.; Winefordner, J. D. Anal. Chem., 1978, 5 0 , 2148.
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Present address, Department of Chemistry, Ohio University, Athens, Ohio 45701. ‘Present address, Procter and Gamble, Industrial Chemicals Division, Sharon Woods Technical Center, Cincinnati, Ohio 45241,
H. W. L a d A. H. Ullman2 J. D. Winefordner Department of Chemistry University of Florida Gainesville, Florida 32611 RECEIVED for review June 8, 1979. Accepted July 10, 1979.
Specificity of Low Resolution Gas Chromatography-Low Resolution Mass Spectrometry for the Detection of Tetrachlorodibenzo-p-dioxin in Environmental Samples Sir: A. diDomenico et al. ( I ) make the statement in their conclusions that low resolution gas chromatography-low resolution mass spectrometry (LR GC-LR MS) is sufficient for detection of 2,3,7,&tetrachlorodibenzo-p-dioxinat the part per trillion (ppt) level in environmental samples contrary to 0003-2700/80/0352-0191$01.OO/O
criticism by one of us ( 2 ) . Although they do not specifically so state, this conclusion gives the impression that LR GC-LR MS is specific for tetrachlorodibenzo-p-dioxinITCDD) in that type of analysis. For their study, LR GC-LR MS was suitable but in general, when the cleanup is not specific, positive D 1979 American Chemical Society
