Improved Adaptation of Beckman Model DU Spectrophotometer for Use with Microliter Absorption Cells David Glirk and Leonard J. Greenberg, Department of Physiological Chemistry, University of Minnesota, Minneapolis, Minn. described a A microliter , . absorption( 1 ) cell and the N EARLIER REPORT
adaptation of the Beckman spectrophotometer for use with it. Subsequent experience led to improvements in certain mechanical details which facilitate use, and, in view of the considerable interest shown in the application of the Beckman instrument to microliter samples, these improvements are presented. The photomultiplier attachment to the instrument is used to obtain sufficient signal with the fine light beam employed. For use with shorter ultraviolet wave lengths, the diameters of the light beam and lumen of the capillary cuvette would have to be increased proportionately to provide a n adequate signal. PINHOLE DIAPHRAGM
A collimated light beam, 0.3 mm. in diameter, was used with the microcell in which the sample was placed in a capillary, 0.8 mm. in diameter and 1 em. long ( 1 ) . The light beam was obtained by the use of two diaphragms with matching 0.3-mm. pinholes. One was a blackened pierced brass disk fitted into one of the holes in the filter holder of the instrument; the other w a ~ a blackened sliding brass strip with a pinhole. Difficulty in alignment of the two pinholes led to the design of the adjustment device shown in Figure 1. This permits fine lateral motion of the diaphragm, J , to a total excursion of 1 mm. in either direction, by turning screw A, tlie tapered part of which, C, presses against one edge of hole L.
Figure 1 .
Figure 2. Assembled microliter cells in holder with modified precision cell-positioning device on Beckman cell compartment The sDrine. M. wressine on win H as show< mi&.& an oGtward pull on the diaphragm which counteracts the push given by screw A , thus eliminating play. .If the spring is hooked to the left of pin G, the draphragm is pushed inward and the motion due to the screw action is then reversed. Unit F is held tightly against the spectrophotometer housing, P, ,by pressure against I of removable pin N in sheath 0. Once the proper position is determined, setscrew E is tigbtmed to hold it. The other pinhole, in the brass disk, is fixed in a permanent position by fastening the filter slide to the housing with a small flat-head screw. Details of lateral adjustment device for pinhole diaphragm n
3 . 5 mm.
The diaphragm can be used conveniently with Lowry-Bessey cuvettes (B) for volumes on the order of 50 pl. or greater without the capillary microcell inserts for smaller volumes. In this case, a larger pinhole can be used to obtain stronger signals from the phototube, the pierced brass disk can be eliminated because the finer collimation is not required, and the photomultiplier attachment may not be needed. CELL POSlTlONER
I n the original design, three of the cells were precisely positioned by setscrews in movable metal bars, but inter. ference, from the rod that shifts the two phototubes, with the motion of a fourth metal bar near the housing, for the fourth cell, necessitated a different setscrew arrangement. The latter proved to be less advantageous than the other three, and therefore the cell positioner was redesigned by extending it farther from the housing to allow space for a fourth bar to be used without interference. The construction of this unit is apparent from Figure 2; i t differs in obvious details from the previously published design, Figures 2 and 3 (I). LITERATURE CITED
Gliek, D., Grunbrtum, B. W., ANA-,. CHEM.29,1243 (1957). (2) Lowry, 0. H., Bessey, 0. A., J. B i d . (1)
Chem. 163, 633 (1946).
No. LVIII in the series, Studies in Histochemistry. Work supported by research grants H2028 and RG3911 National Institutes of Health, U. S. Public Health Service. 736
ANALYTICAL CHEMISTRY
