AIDS F O R THE ANALYST Versatile Droplet Counter Grant C. Riggle and Laurence R. Crisp, Department of Health, Education, and Welfare, National Institute, of Health, Division of Rerearch Service., Laboratory Aids Branch, lnrtrument Section, Betherdo 14, Md.
,ARCH scientists in the fields of chromatography (3) R " z d protein fractionation (e) require 8. reliable and versatile type of electronio detector to count the drops falling from a fractionating column. These requirements led to the development of a droplet counter which, after extensive testing and use, has proved reliable in operation, simple to construct, and readily adaptable to use with existing commercial apparatus. Commercial droplet counters are available, but their circuitry and optical design limit their adaptability for a variety of laboratory research problems. Most of the previously described photoelectric counters have used intense light souroes for activation of the phototube. Heat radiation from such Sources causes changes in the chemical structure of Some solvents, and partial evaporation of slow-forming drops. Another factor whioh previously caused difficulty is the necessity far exact balance of the photoelectric circuit. These aouroes of error have been eliminated by the use of a new photo detection tube and the development of a sensitive electronic circuit. Features of Developed Model. The photoconductive tube requires very low illumination; no lens system is needed, thus eliminating the focal point adjustment for the interruption of the light beem; only partial blanking of the target tube window by the falling drop is necessary to produce a sufficient signal for counting; phototube blanking time can he as short as 0.5 millisecond; droplet guide tube is designed for easy removal and cleaning to prevent contamination; line voltage fluctuations produce no spurious counts; and the counter is adaptable to existing commercial fraction collectors without modifications. Reliable operation in refrigeration rooms is achieved through the use of heaters in the phototube and the counter enclosures. It was found best to leave the counter energized a t all times while in refrigerated spaces, whether in use or not. Figure 1shows the developed circuit.
Figure 2.
Photoconductive cell housing assembly
The target lamp voltage is varied by the rheostat, R,, permitting the target lamp to be dimmed to any desired intensity level. Constant voltage transformer, T,,smoothes out line fluctuations to critical circuits. A selenium-type voltage doubler rectifier keeps the load requirements within the rating of the constant voltage transformer, reduces the chassis space requirements, and provides an inexpensive voltage divider network for supplyina
responsi;e to changes in the red-infrared portions of the spectrum.
counter RLs and relay RLa. Contacts 1-3 and 2 2 on relay RL; may be arranged to operate in a number of sequences as selected
&bed
in the relay manufacturer's literature:
The count& is
'count selector RLs to be reset t o aero, a t the 8&metime ad;aneing the fraotion collector one position. The photoconductive cell housing assembly is shown in Figure 2. Thelight source, 1, and the phototube, 2, are covered with light-excluding shields; the droplets fall through glass tube 3, held in place by clamp 4. Windows, 5, m e drilled in the housing to permit viewing when operational setups are made. RheoBtat 6 controls the voltage to the light 8ouroe lamp. Resistor X (not shown in this view) is located in the phototube art of the housing. It oonsists of 2 g e t of insulated No. 36 Nichrome wire wound in coil form. This, along with Y , a 25-watt incandescent lamp mounted in the counter enclosure, is necessary only when the counter is used in refrigerated areas. Maintenance on the units has been negligible. Some counters have been in use 4 years and only the vacuum tube required replacement. The counter has been adapted to u ~ with e a new fraction collector (I), which permits the collector to he used either as 8 timed or counbcontrolled instrument.
Figure 1. Diagram of circuit 1799
1800
ANALYTICAL CHEMISTRY ACKNOW LEDGMEh 'I
The authors wish to acknowledge the assistance of Vincent T Almasy and S. Meredith Meyers, electronic technicians, in the development of the circuit. LITERATURE CITED
Debroske, J. AI. F., Crisp, L. R., Rev. Sci. Instr. 24, 547 (1953). Heftmann, E., Johnson, D. F., A N ~ LCHEM. . 26, 519 (1954). (3) Kegeles, G., Sober, H. A , Ibid., 24, 654 (1952). (1) (2)
Feed Mechanism for Automatically Capping Fraction-Cutter Tubes J. G. Kirchner' and W . L. Stanley, Fruit and Vegetable Chemistry Laboratory, Western Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Pasadena, Calif.
automatic fraction cutters in which volatile Iin moving materials and substances sensitive to oxidation are collected banks of test tubes, it is desirable t o provide a system K OPERATIXG
for capping the tubes as they are filled. During a study of the volatile constituents of citrus juices, an automatic mechanism employing glass balls was developed for capping volatile fractions in test tubes collected from fractional distillations and elution chromatograms. A schematic diagram of the device is shown in Figure 1, and a three-dimensional diagram is shown in Figure 2.
1c
RESERVOIR
nism. h simple coneshaped reservoir cannot be used, because the marbles pile up a t the mouth of the cone and block off the opening. However, if a tube is used for the reservoir, it must be bent with an offset, so as to limit to three the number of marbles resting directly on the spring-loaded suspension points. having a diaFigure 2. Three-dimensional meter of 0.75 i 0.01 diagram inch are handled satisfactorily by this feeding mechanism. For marbles of this a17e the space hetneeri the suspension points should be 21/32 inch, as indicated in Figure 1. Minor variations in the height and diameter of the test tubes do not affect the operation of the feed mechanism. hbsence of tubes in the moving bank of collection tubes will not result in the complete discharge of marbles from the reservoir. hfter the dropping mechanism had been perfected, the next step was to test the efficiency of differently treated marblecapped tubes to use for maximum retention of lox-boiling solvents over evtended periods of time.
This was accomplished by setting up two sets each of five untreated tubes, one set of five tubes with greased lips, one set with beveled lips, and one set with beveled-greased lips. Beveling consisted of grinding the tube lips with water and a medium weight Carborundum powder, using one of the glass marbles The marble was carefully rotated so as to obtain a narrow bevel approximately 1 mm. wide around the inside of the tube lip. As long as the bevel around the lip is unbroken, small variations in tube roundness are insignificant. The beveled rims of one set of tubes were coated with grease. For h drocarbon solvents, a v-ater-soluble grease similar to that descriged by Meloche and Frederick [ J .Am. Chem. SOC.54,3265 (1932)l is satisfactory. Fifteen-milliliter portions of petroleum ether (boiling range 35" to 60" C.) were next pipetted into all tubes. One set of untreated tubes was sto pered with corks, the remaining tubes were capped nith marbes, and all tubes were immediately weighed. The corks were removed from the one set of tubes, and these open tubes and the capped tubes were allowed to stand a t room temperature (25' to 27' C.) for 26 hours. After this period the open tubes were again stoppered, and all tubes were reweighed. Efficiency of solvent retention was calculated by subtracting from 100 the product of 100 times the average weight loss from each set of five open tubes.
'Table I.
Figure 1.
Schematic diagram
The device consists of a storage reservoir connected from above to a chamber in which a marble is held on suspension points by t\yo spring-loaded, pivoted metal jaws. The pivoting jaws are opened and a marble is released when a test tube in the moving rack passes between the lower extensions of the jaws. As the marble is being released, blocking suspension points a t the opposite end of the j a m retain the marbles in the reservoir until the capped test tube has moved on. The jaws are then closed by the loading springs, causing the suspension points a t the bottom of the chamber to open and release another ball from the reservoir into the feed chamber.
A reservoir for coin-operated candy vending machines, or a long tube, can be used to supply marbles to the feeding niecha1
Present address, Tenco, Inc.,720 West Edgar Road, Linden, N. J .
Efficiency of Solvent Retention by Ball-Capped Test Tubes
Treatment of Tube Lip Pl-one Beveled Greased Beveled-greased
.4verage Weight Loss, Grams Capped tube. Open tube, .4 B 5 47
5 47 5 07 5 07
3.48 2.92 1 01 0.12
Retention Efficiency. 100 - 100 -B ..I
37 47 80 98
Table I indicates maximum solvent retention when beveledgreased test tubes were used. Similar results were obtained with other organic solvents. ACKNOW LEDGMEKT
The authors' appreciation is extended to L. F. htkinson of this laboratory, who contributed to the design and constructed this apparatus.
