V O L U M E 22, N O . 2, F E B R U A R Y 1 9 5 0
100
363
-
0
v,
90-
0
3,
80-
z 4:
70-
c
5
:
60
-
50-
L
1. Figure 2.
Positioning of Cuvette
exactly the same as those for the ordinary one supplied with the Beckman spectrophotometer for holding macrocuvettes. This permits ready interchange of cuvettes, depending on whether micro or macroanalysis is to be performed. The mechanism for moving the carriage is made from parts of a mechanical stage for a microscope. A long piece of brass 0.23 inch (0.6 cm.) thick is attached to the slide mechanism, and a slot is sawed into the wall of the casing of the Beckman spectrophotometer compart>mentto connect’ LTith the exterior. Onto the outside end of the strip of brass is fastened a centimeter rule: alongside this rule and fixed in position a vernier is placed. The mechanism is easily adjustable to 0.1 mm. I n the assembly pictured (Figure 1) the screw for positioning the carriage interferes with the adjustment knob on the photocell compartment of the spectrophotometer which is used to change from one photocell to another. Rather than make the base for the carriage inconveniently long, this knob was eliminated. In addition to these changes, a diaphragm of a size t’hat will just fit into a aell made around the hole serving as the exit for the light from the spectrophotometer is substituted for the penny diaphragm. The fit is made so exact that the diaphragm requires no accessory fastening mechanism; it can be readily removed by suction or use of a pin inserted into the hole. A diaphragm whose hole is 1.4 mm. in diameter is shown in Figure 1. The data reported below were obtained using a diaphragm having 1.6-mm. hole. Incidentally, the diaphragm cuts down the light sensitivity of the instrument by decreasing the effective area of the light beam. With the 1.6-mm. diaphragm this amounts to 9%. Accordingly the diaphragm cannot be used with the very short wave lengths.
Table I. Reproducibility of Readings Made on Four Cuvettes Following Removal and Reinsertion into Cuvette Holder Cuvette NO.
1 2 3 4
100.0 99.7 100.1 99.1
Per Cent Transmittance Readings with Water, x = 500 mg 100.0 100.0 99.6 100.0 100.3 99.0 99.9 99.1 99.9 99.7 99.3 100.0 99.1 99.6 99.7 99.0 100.5 99.9 99.8 99.9
Figure 1 also shows a dummy cuvette made from brass, which can be used as a substitute for a glass cuvette in case of breakage. The position a t which readings should be taken for the four cuvettes is readily determined by direct experiment. Water or some suitable solvent is placed in the cuvettes in quantities of 50 cu. mm. or more, and the cuvettes are positioned until a maximum reading is obtained. Figure 2 shows a plot of the per cent transmittance (where the maximum is set arbitrarily a t 10070)as a function of the position of a cuvette. A maximum reading is obtained over a range of approximately 0.4 mm., the amount of light decreasing on either side of this setting because of occlusion by the cuvette wall. The cuvette should be positioned in the middle of this range when an analysis is performed, to minimize any inexactness in positioning the cuvette. In this instance the proper position of cuvette 1 is 5.55 cm. The position a t which readings should be taken for the other three cuvettes is determined in like manner. For the present apparatus these are 1.62, 2.92, and 4.23
cm. One advantage of the apparatus described is that the proper position for the cuvette can be readily checked a t any time by simply moving the carriage forward or backward to determine whether there is any further increase in the amount of light. The time required for positioning the cuvettes is not significantly greater than that required with the usual slide mechanism. Illustrative results obtained using the described modification are presented in Tables I and 11. Table I shows the variations encountered in readings as a result of repeatedly removing the cuvettes from the cuvette holder between series of readings. Fifty cubic millimeters of water Tyere used as a sample. The mavimum variation in 24 readings was about 1.5y0and the average difference in any one set of readings was 1.0%. As the reading for the first cuvette in any one set of values shown in the table was not brought back arbitrarily to loo%, the observed variations include change in output from the lamp.
Table 11. Reproducibility of Replicate Determinations of Absorption of Vitamin A in Kerosene-Xylene Cuvette Xo.
1 2
3 4
Blanks 100.0 100.0
99.8 100.3
Per Cent Transmittance, Vitamin -4 100.0 1 0 0 . 0 100.0 i3.7 72.3 73.1 73.5 72.3 73.1 73.9 i3.0 73.6
X = 328 mg
I
100.0 73.0 73.0 73.2
.
100.0 73.5 73.7 72.6
Average 73:i3 73.13 73.26
Table I1 s h o m the variation in readings encountered in making repeated measurements of the absorption of vitamin A in kerosene-xylene. Column 2 shows that the blank values between different cuvettes varied less than *0.3%. Columns 3 through 7 show that the greatest variation between samples was l.6%, and that the variation between average values for the three cuvettes was less than 0.1370. ill1 measurements were performed on 50 cu. mm. of solution. The cuvettes were cleaned with acetone between measurements but were not removed from the cuvette holder. SUMMARY
An assembly for holding and positioning the cuvettes used with the micro adaptation of the Beckman spectrophotometer is described. The system offers the following advantages: The cuvettes can readily be removed and replaced in the cuvette holder without introducing significant change in the amount of light transmitted; they can be quickly and reproducibly brought into exact alignment for taking readings. ACKNOWLEDG>lEKT
The assembly described was constructed by John Linden, J. H. Emerson Company, Cambridge, Mass., who was also responsible for much of its design. The author Tishes to thank June Twomey for technical assistance. LITERATURE CITED
(1) Lowry, 0. H., and Bessey, 0. d.,J . B i d . Chem., 163, 633 (1946).
RECEIVED January 31, 1949.
Correction I n the article on “Determination of Unsrtturation in Dehydrogenated Dichloroethylbenxene” [ Marquardt, R. P., and Luce, E. N., ANAL. CHEM., 21, 1194 (1949)], on page 1195, second column, under the heading “Reagents,” and on page 1196, first column, line 4 1.0 N sodium hydroxide should have been specified, not 0.1 N as stated.
