V O L U M E 2 8 , NO. 11, N O V E M B E R 1 9 5 6 fluoride flux. The relative variability (coefficient of variation) of the method employing the mixed-fluoride flux is 0.7%, compared with 4.7% when sodium fluoride is used as the flux. I n order to illustrate the increase in sensitivity afforded by measuring the fluorescence of the buttons in a polished holder, calibration curves mere prepared by measuring the fluorescence of a series of mixed-flux buttons containing known amounts of uranium. These buttons were measured once in a polished aluminum receptacle, and again in a nonreflectant, anodized aluminum holder. 4 s can be seen from the calibration curves shown in Figure 10, a higher sensitivity is obtained with the highly reflectant background. The accuracy of the fluorometric method employing the mixedfluoride flux is illustrated by analyses of five standard ore samples sholm in Table 111. All analyses were performed in such a manner that each fluoride button contained approximately 0.1 y of uranium. I n all cases the observed UB08 content ~1-m within the limits of uncertaintv of the certified U,O, content. LITERATURE CITED
Adams, J. 4.S.,Naeck, W. J., A4r;a~. CHEx 26, 1635 (1954). Di Giovanni, H. J., Graveson, R. T., Dwork, B., U. S. Atomic Energy Commis%ion,NYO-4508 (1954). Draganic, I., Rec. trav. inst. recherche8 structure mati& 1, 89 (1952). Florida, C. D., Davey, C. N., J. Sci. Instr. 30, 409 (1953). Fortner, L. R., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Paper 99 (1956). Galvanek, P., Jr., hIorrison, T. J., Jr., U. S. Atomic Energy Commission, ACCO-47 (1954). Grimaldi, F. S., Levine, H., Ibid., AECD-2824 (1950). Grimaldi, F. S.,May, I., Fletcher, 31. H., U. S. Geol. Survey, Circ. 199 (1952). Grimaldi, F. S., May, I., Fletcher, 11. H., Titcomb, I., U. S. Geol. Survey, Bull. 1006 (1954). Hassialiu, XI. D., lIusa, R. C., International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, Paper 116 (1955).
1657 (11) Kelley, RI. T., Hemphill, H. L., Collier, D. AI., U. S. At,omic Energy Commission, ORNL-1445 (1954). (12) Kennedy, R. H., Kaufman, D., Ibid., MITG-A60 (1949). (13) Kinser, C. A., U. S. Geol. Survey, Circ. 330 (1954). (14) Koskela, U., Kaufman, D., U. S. Atomic Energy Commission, MITG-A65 (1949). (15) Kosta, L., Bull. sci. consei2 acad. RPF Yougoslav. 1, KO.2, 41 (1953). (16) Lynch, F. J., Baumgardner, J. R., Rev. Sci. Instr. 26, 435 (1955). (17) Michelson, C. E., U. S.Atomic Energy Commission, HW-36831 (1955). (18) JIorrison, T. J., Jr., Galvanek, P., Jr. (to American Cyanamid Co.), U. S. Patent 2,710,924 (June 14, 1955). (19) Nietzel, 0. A., De Sesa, M. A., International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, Paper 532 (1955). CHEM.25, (20) Price, G. R., Ferretti, R. J., Schwartz, S., ;INAL. 322 (1953). (21) Rodden, C. J., Ibid., 25, 1598 (1953). (22) Rodden, C. J., ed., “Analytical Chemistry of the Manhattan Project,” XIcGraw-Hill, New York, 1950. (23) Rodden, C. J., International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, Paper 952 (1955). (24) Rodden, C. J., Tregoning, J. J., “>Ianual of -4ualytical Methods for the Determination of Uranium and Thorium in Their Ores,” U. S. Government Printing Office, Washington, D. C., 1955. (25) Sentementes, T. J., De Sesa, 31. il., Chemist-Analyst 44, 54 (1955). (26) Sill, C. W., Peterson, H. E., r l s . 4 ~ CHEM. . 24, 1175 (1952). (27) Stevens, R. E., Wood, TV. H., Goetz, K. G., Horr, C. A , Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Paper 98 (1956). (28) Zimmerman, J. B., Rabbitts, F. T., Kornelsen, E. D., Dept. hIines Technical Surveys, Ottawa, Canada, Tech. Paper 6 (1953). RECEIT-ED for k v i e w March 8, 1956. Accepted August 15, 1956. The R a w Materials Development Laboratory is operated b y the National Lead Co., Ino., for the U. S. Atomic Energy Commission. Work carried o u t under Contract No. AT(49-6)-924.
Multiple Thickness Cell Assemblies Application to Ultraviolet Spectrophotometry D. D. TUNNICLIFF Shell Development
Co., Emeryv;lle, Calif.
The use of a single absorption cell with a fixed length is often inefficient for making absorption measurements. Much time can be saved by using a multiple thickness cell assembly consisting of a group of individual cells of different thickness all filled with the same sample. The absorbances are then measured using the most appropriate cell thickness for the particular spectral region. This arrangement eliminates the need for preparing a different dilution of the sample for each region. Three cell assemblies of this type are described. These include a gas cell for the Beckman DU spectrophotometer and liquid cells for the Cary and the Beckman spectrophotometers.
S
PECTROPHOTORIETERS are useful for measuring absorbances through only a limited range of values. If the absorbance falls outside this range it becomes necessary to adjust either the cell length or the sample concentration. Most ultraviolet absorption measurements are made using I-cm. cells, with the sample concentration adjusted accordingly. Hirt and King
( 1 ) have pointed out that in many cases it is advantageous t o vary the cell length rather than the sample concentration, and they have described the use of variable-length cells for this application. Another approach which has been useful in this laboratory is the use of a so-called multiple cell. This consists of a group of cells, each with a different length. The cells are all mounted together as a unit and arranged to fill in series so that all cells contain the same solution. The proper absorbance value is then obtained by merely selecting the appropriate cell for the measurement. One important advantage of this arrangement over variable-length cells is the higher accuracy that can be obtained in determining the thickness of the thinner cells. Also, one sample can be replaced with another sample somewhat more conveniently. On the other hand, the variable-length cell has the advantage of having only one cell blank, whereas the multiple cell has a separate cell blank for each cell. Three different variations of multiple cells which have been used extensively in this laboratory are described below. CELL CONSTRUCTION
Gas Cell for Beckman DU. The first multiple cell was designed for measuring the absorbance of dienes in the vapor state using
ANALYTICAL CHEMISTRY
1658
Unmounted
Mounted
COYrn/8" ~
Figure 1. Triple gas cell assembly for Beoki
B E C I M A N INSna"MENT6
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sample passesin through thisspherical joint;thraug
