Modification of the Photovolt Densitometer to Permit Monitoring Ultraviolet Absorption of Column Effluents W. D. Conway and E. J. Lethco, Environmental Cancer Section, National Cancer Institute, Bethesda 14, Md. common laboratory practice to I use ultraviolet light absorption as a T 15
locating device for aromatic substances eluted from chromatographic columns. Several commercial devices have recently been offered to monitor the column effluent automatically. This paper describes a flow-cell holder and a simple voltage attenuator and event marker which permit use of the Photovolt Model 525 Densitometer for this purpose. The device has been mentioned earlier but has not been described in detail [Conway, W. D., Lethco, E. J., Ax.4~.CHEM. 32, 838 ( 1960) 1. Construction of tlie flow-cell holder is illustrated in Figure 1. Balsa wood was used to construct the present model, which was then painted flat black. The holder accommodates a standard flow cell (Pyrocell Co., outside dimensions 1.2 x 1.2 X 4.0 em., volume 1.5 ml., light path 1.0 cm.). Standard Beckman cells will also fit. A more complicated model having an adjustable entrance slit which produced a collimated light beam was also constructed, but its performance was not INPUT
1
l TO
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EVENT M A R K E R SWITCH 0
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OUTPUT
Figure 2. Voltage attenuator and event marker
Figure 1 .
superior to the simple model described here. The voltage attenuator and eventmarker circuit is shown in Figure 2. The components are conveniently mounted in a 6 X 4 X 4 inch box. The input leads are connected in parallel with the microammeter and its series resistor in the Photovolt Densitometer, which is equivalent to attaching one lead to each cathode of the 6K6 output tubes.
A 1-volt input to the attenuator can be adjusted with the 50,000-ohm potentiometer to produce an output of 2.3 to 116 mv., which is suitable for driving most laboratory recorders. The output resistance of the circuit can be a maximum of 51,000 ohms and is usually much less in actual use. This is suitable for the Varian G-10 graphic recorder with a 10-mv. span, n hich we used, but may have t o be changed to suit other recorders. The event-marker circuit imposes a small d.c. voltage from a flashlight cell on the output voltage of the attenuator. This results in a vertical line on the chart. The event-marker voltage, and therefore the length of the line produced, can be varied from 0 to 116 mv. with the 1000-ohm potentiometer. The polarity of the voltage can be re-
Flow-cell holder
versed using the double-pole doublethrow toggle switch, permitting the event mark to be made in either direction from the recorder trace. The spring return switch is used to test the size of the event mark. The event
Figure 3. Linearity of instrument in ultraviolet and visible range Benzophenone in 9570 alcohol, ultraviolet source and 5265 filter - - - - Citrus Red No. 2 in 9 5 % alcohol, visible source and 525-mp glass filter
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marker is actuated by a single-pole switch placed in the fraction collector in such a position that i t is depressed by a cam in the motor-drive assembly when the collection tube is changed. A 110-volt relay attached to the motor terminals could also be used. It is felt that the 6-volt circuit employed here minimizes any shock hazard which might arise at connection points along the extension to the event-marker sm-it ch .
For use, the unit containing the light source and photocell is laid on its side and the flow-cell holder and cell are inserted. I t is convenient t o attach a
light spring between the base and the arm holding the photocell, t o hold the cell holder tightly in place. With solvent in the cell and the light shutter closed, 0% transmittance is set first on the transmittance density unit and then on the recorder. Then, lOO%T is adjusted first on the transmittance density unit, using the iris diaphragm in front of the photocell, and then on the recorder using the voltage attenuator. The germicidal lamp ultraviolet light source provided with the Photovolt emits most of its energy as the 2537-A. mercury line. When used in conjunction with the Photovolt 5265 filter, this
provides a fairly monochromatic light source. A semilog plot of %T us. concentration of benzophenone in 95% alcohol (absorption masinium 253 mk) is linear down to about 25% T . h p parently enough longer wavelength light passes through the filter to cause deviation from linearity a t higher concentrations of benzophenone. This is not serious if the unit is used only as a locating device. -4 similar plot for Citrus Red KO. 2 in 95% alcohol (absorption maximum 515 mp) obtained with a visible light source and a 525-mp glass filter is linear to less than 20% T. The results are shown in Figure 3.
A Simple Micro-Mull Technique for Obtaining Infrared Spectra C. Szonyi and J. D. Craske, Unilever Australia Pty. Ltd., Balmain, Australia
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of paraffin mulls of solid compounds may give in some cases more detailed information than those of solutions. Normally, 10 to 20 mg. of sample is required if the mull is prepared by mixing the solid compound with paraffin oil in a small mortar until a smooth paste is obtained. This can be reduced t o 5 to 7 mg. by careful work. Lohr and Kaier ( 2 ) described a special micro-mulling technique whereby the required sample amount was reduced to about 3 to 5 mg. Techniques for spectroscopic investigation of 0.1 mg. and even 5 p g . of solids have been published (1, 3 ) . but these, however, require a beam condenser and/or a microscope attachment on the spectrophotometer. A micro-mull method has been developed in this laboratory, which requires only 0.3 to 0.5 mg. of sample and n hich is directly applicable to industrial infrared spectrophotometers tvithout the need to construct any special apparatus. I n the present iyork, a Perkin-Elmer Infracord Model 137 spectrophotometer vias used. The size of the rock salt 11indon s usrd is 6 X 22 mm. -4 droplet of paraffin oil (about 0.005 ml.) is placed on one of the rock salt plates from a microsyringe. The solid compound is dissolved in a volatile solvent to yield an approximatrly 1% solution-e.g., 0.5 mg. in 0.05 ml. Using a microsyringe, about 0.05 nil. of the solution is then transferred drop$& to the oil droplet, allowing time for the solvent to evaporate after application of each drop, so that a mixture of the solid and paraffin oil remains on the plate. This mixture is then homogenized by pressing a second rock salt plate on top of NFRARED SPECTRA
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ANALYTICAL CHEMISTRY
Micro-mull method (0.5 mg,)
Figure 1. Infrared spectra of n-decanal 2,4-dinitrophenylhydrazone
Macro-mull method
I I I I 6 7 0 9 Wavelength (Microns)
the first. The spectra obtained by this method are equivalent to those of paraffin mulls prepared by the normal macro-technique. See Figure 1. ACKNOWLEDGMENT
The authors thank the Directors of
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Cnilever dustralia Pty. Ltd. for permission to publish this note. LITERATURE CITED
(1) Clark, D. A, Boer, A. P., Spectrochinz. Acta 12, 276 (1958). ( 2 ) Lohr, L. J., Kaier, R. J., ANAL.CHEW 32, 301 (1960). ( : 3 ) Star-art, J . E., Ihid., 31, 1287 (1959).
