of rhodamine are very close together as was true with fluorescein, making filter selection difficult. However, the corrected equal-energy excitation maximum for rhodamine B is very close to 546 mp where there is a powerful mercury line. Because of the energy present in this line and because a line source permits much more freedom in choosing both primary and secondary filters to give high transmittance without encountering serious problems from scattered radiation due to overlap, a mercury light source is indicated. Morin and I t s Beryllium and Thorium Complexes. Fluorometric procedures for the determination of both beryllium and thorium have been developed in this laboratory using Rforin as reagent. Spectra of Norin and its complexes with both metals are shown in Figure 5. Although there appears to be a slight concentration shift still present at t h e concentration used for making the corrections, the agreement between the corrected values of the excitation curves and the absorption curves is acceptable particularly since the corrections are large.
The exitation and emission oveilap as usual but are more 11idely separated than was the case with fluorescein and rhodamine B. Uranium(VI) Phosphate Complex. T h e spectra of sexivalent uranium in phosphoric acid are particularly interesting because of their structure. Because the peaks are so sharp, no a t t e m p t \vas made to correct them for source distribution or detector response nor would i t have made much difference. However, i t is clear from Figure 6 that for every peak or shoulder on the absorption curve there is a corresponding peak or shoulder on the excitation spectrum a t almost identically the same mave length. Bplow about 330 mp> the intensity of the tungsten lamp and the response of the 1P28 phototube decrease more or less rapidly to zero. However, the absorption curve indicates that excitation of the fluorescence will be very high at the shorter wave lengths and indeed the mercury resonance radiation a t 253T A. is known to excite the fluorescence more efficiently than the mercury lines a t 405 or 436 mp.
ACKNOWLEDGMENT
The author is indebted to P. R. Boren of the Instrument and Development Branch of this Division for constructive suggestions in the design of the excitation accessory and for the machine work required in its construction. LITERATURE CITED
(1) Lerniond, C. A, Rogers, L. B., Anal. Chirrz. -4cta 13, 387 (1955). ( 2 ) Parker, C. A,, Suture 182, 1002
(1958).
13) Parker, C. A, Barnes, IT. J., Analus1 ‘ 82,606 (1957).
(4)Parker, C. A,, Rees, K. T., Ibid, 85,587 (1960). (5) Sill, C. W.,A a a ~ .CHEV. 33, 1584 (1961). ( A S ) (6) Keber, G., Teale, F. W. J., Trans. Faraday Soc. 54, 640 (1958). ( 7 ) Khite, C. E., Ho, M., Keimer, E. Q., Speclrochzin. Acta 16, 236 (1960). (8) Khite, C. E., Ho, AI., Weimer, E. Q., h S A L . CHEM.
32,438 (1960).
( 9 ; White, C. E., Hoffman, D E., Magee, J. S.. Jr.. Spectrochzrn. .Acta 9, 105
RECEIVED for revierv January 3 , 1961. Acrepted June 15, 1961.
Transmittance Spectra of Color Filters CLAUDE W. SILL Health and Safety Division,
U. S. Atomic Energy Commission, Idaho Falls, Idaho
b The color filters used in colorimetric or fluorimetric methods of analysis have a very important effect on the sensitivity, linearity, and selectivity of the procedure. Their selection should be given at least as much attention as selection of the proper chemical conditions. While investigating such effects on several fluorometric methods being developed in this laboratory, transmittance spectra were obtained of 55 glass and 35 gelatin color fillers individually and of 98 two-filter combinations of the glass filters. As many spectra as practicable were recorded on the same chart to facilitate intercomparison. After having determined the absorption and emission spectra of the fluorescent species, selection of the optimum combination of filters for isolation of both the exciting radiation and the emitted fluorescence can be made quickly and simply from the transmittance spectra of the filters presented.
I
N THE DEVELOPMEST of fluorometric methods of analysis, selection of the optimum combination of color filters for isolation of both the primary ex-
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ANALYTICAL CHEMISTRY
citing radiation and the secondary fluorescent radiation is generally as important as selection of the proper chemical conditions. Sensitivity, selectivity, linearity, the ratio of net fluorescence to blank fluorescence, and stray light ale all dependent on the relationship between the transmittance of the filters and the excitation and emission spectra of the fluorescent species and on how much the transmittance spectra of the primary and secondary filters overlap. Since the e\citntion spectruni is as characteristic of a given fluorescent compound as its absorption spectrum, the production of fluorescence can be made relatively selective by using a primary filter having a narrow transmittance band. The secondary filter is then selected to transmit a large fraction of the fluorescent radiation from the compound of interest M hile discriminating as much as possible against unwanted fluorescence that may be present either from excess reagent or from other species. Both filters should be opaque to radiation a t wave lengths both longer and shorter than the transmitted band for greatest selectivity and should not overlap significantly. I n most cases, a two-filter
coiiibination will be required. Because the excitation and emission spectra of a given fluorescent species always overlap and frequently are very close together ($), filters may be required having a cut-off point nithin a few millimicrons of some designated wave length if optimum excitation and transmittance of the fluorescence to the detector is t o be obtained without significant overlap and the resultant problems caused by scattered radiation. Similarly, 111 colorimetric methods of analysis, sensitivity, conformance to Beer’s law, and freedom from unnecessary interference due to impurities or excess reagent depend to a significant extent on the transmittance characteristics of the color filter used. A large number of colored filters are available commercially, but the glass filters available from Corning Glass Works and the Wratten gelatin filters from Eastman Kodak Co. are probably the ones most widely used in the United States. Transmittance spectra are available from the manufacturers that are representative of each single filter type ( 1 , Z), but the curves are presented on many different charts and are so reduced in size that intercomparison
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of various filters id verj difficult. Furthcrniore, transmittance curves are not available for the many combinations that can be made from the single filters and which are frequently the ones of greatest use in practical analytical work. Although the spectra of the combinations can he calculated from those of the individual filters, the process is tedious for large numbers of filter
