c.f.h.; column temperature] 177' and 190' C.; detector temperature] 250' C.; injector temperature, 280' C.; nitrogen carrier gas flowrate, 71.5 cc./minute. To compare the area response per microgram of Lebaycid with both the electron affinity and flame ionization detectors, the electron affinity detector's senistivity was optimum when operated at 12 volts a t a 1 x 10-9 ampere gain setting. With this detector, the 6-foot column was packed with 5y0 SE-30 (General Electric's methyl silicone gum rubber) on 80/90 mesh -4nakrom AS. This liquid phase was used in view of the high background and noise level resulting from the QF-1 phase. At a column temperature of 177' C., the nitrogen carrier gas flowrate was increased to 150 cc./minute in order to obtain equivalent retention times with both liquid stationary phases. All other operating parameters were maintained similar to those noted above for the flame detector with the exception of the detector temperature which was held at 210' C.; this was limited by the detector's tritium source. One-microliter Hamilton microsyringe injections of varying Lebaycid concentrations dissolved in benzene as solvent were made directly onto the coated column solid support; thereby elimi-
nating the possibility of catalytic degradation a t metallic surfaces. DISCUSSION
OF
RESULTS
Using the conditions cited in the experimental section for the flame detector, typical chromatograms obtained for Lebaycid at 177' and 190' C. are shown in Figure 1. Chromatogram A compares the peaks obtained a t two different sensitivity and column temperature settings for known amounts of the phosphorothioate compound injected, whereas Chromatogram l3 shows superimposed gas chromatographic peaks resulting from different concentrations of Lebaycid a t 177" C. and 1 x ampere. By plotting peak-heights us. concentration of Lebaycid a t gain settings and 1 X 10-10 ampere, of 1 x nearly linear relationships are obtained as shown in Figure 2. A t 1 x 10-lo ampere and a column temperature of 177' C., as little as 22 nanograms of this compound is easily detected; thus showing the gas chromatographic procedure to be about 500 times more sensitive than the spectrophotometric analysis.
This study has also shown that the area response in square millimeters per microgram of compound at 1 x 10-9 ampere with the flame detector (1517 mm*) is approximately 1.53 times greater than that obtained with the electron affinity detector (995 mm.2).
S
//
This indicates that the -P-0-group-
l
ing in Lebaycid has a poor affinity for electrons and confirms and is consistent with the findings of Cook et al. ( I ) and Gudzinowicz et al. (3. LITERATURE CITED
( 1 ) Cook, C. E., Stanley, C. W., Barpey,
J. E. 11, Abstracts, Fifteenth Pittsburgh Conference o n Analytical Chemistry and Applied Spectroscopy, X a r c h
1964, p. 53. (2) Emerson, E., J . Org. Chem. 8, 417 (1943 ). (3) Gudzinowicz, B. J., Clark, S. J., J . Gas Chromatog. 2, 335 (1964). ( 4 ) Hirano, Y., Tamura, T., ANAL. CHEM.36, 800 (1964).
Jarrell-Ash Co. 590 Lincoln Street Waltham, Maas.
B. J. GUDZINOWICZ
Quartz Window Cuvette Prepared by Sealing Quartz to Borosilicate Glass Wm. T, Roubal, U. S. Bureau of Commercial Fisheries and The Department of Food Science and Technology, University of California, Davis, Calif.
of commercially automated liquid flow analytical systems, many laborious and time-consuming analyses can now be run a t the rate of 60 per hour. The modular approach, extensively used for these analyses, also allows the operator to incorporate a wide variety of other laboratory instruments into an analytical scheme. The relatively low liquid flow rate of such systems coupled with the fact that air bubbles often find their way into optical systems, either by accident or intentionally] places severe limitations on existing small volume flow cuvettes. A variety of commercial flow cuvettes designated as microcuvettes were tried as cells for a commercial spectrophotometer. These cells ranged from rectangular quartz cells and small diameter cylindrically shaped cells to the recently available microaperture flow cell. At low flow rates (several milliliters per hour), bubbles either I T H T H E ADVENT
Mi available
440
e
ANALYTICAL CHEMISTRY
lodged within the optical path of the cell or peak distortion was observed. At higher flow rates (several milliliters per minute), channeling was a problem in cells with large cross sectional area. This paper describes in detail the fabrication of a streamlined, liquid flow, small volume, deep path, UV transmitting cuvette for the Beckman DB or similar spectrophotometer. Air bubbles do not lodge within the cell and peak distortion is no problem a t low or high flow rates.
MORGANITE FORM
a,
3
b.
PROCEDURE
Prepare a heat resistant form, the dimensions of which correspond to the desired internal width and depth of the cell, from Morganite (Morganite] Inc., Canoga Park, Calif.). A form with dimensions of 5/u inch x '/a inch x 11/4inch gives a finished cell with an active volume of about 0.3 ml. Push the form into the middle of a 12- to 14-inch length of borosilicate glass
C.
Figure 1 . Steps in the fabrication of a quartz window flow cell ( a ) Cell body molded over Morganite form ( b ) Shoulders drawn down to form Inlet and exit tubes. (c) Quartz windows are sealed onto narrow faces after borosilicate glass portion has been cut away
hide-composition, wet c u h f f wheel (Figure IC). U s care to keep the faces parallel. Finish by grinding on a glass plate with a slurry of 300- to 400-mesh carbornndum powder in water followed with 6Wmesh powder and finally jewelers' rouge. Figwe 2A shows the. cell at this point. This particular cell has been platinized prior to receiving quartz windows by the second sealing method described below. Windows are alvaged from broken rectangular quartz cuvettes and are cut to shape with the wet cut-off wheel. The inlet tube has been bent so that the assembly will fit into the l-em. wide cell compare :nt of the DB. Windons can be mounted onto 5 borosilicate glass body hy one of o methods. The first method, which the easiest, uses Helix R-363 epoxy iin (Carl H. I3iggs Co., Inc., Santa ..-mica, Calif.) as the bonding agent. The ground surfaces of the borosilicate glass body are given a thin coating of the prepared resin and the windows are carefully put in place. When the resin has cured, a fillet of Helix R-318 is flowed in around the outside of the cell where the quartz meets brosilicate glass. 130th R-363 and R-318 show excellent resistance to aqueous salt solutions, acids, and bases of moderate strength and a wide variety of organic solvents. Helix-363, unlike many epoxy resins shows a slight amount of elasticity in the cured state, hence the glue line does not fail when glasses of differing coefficients of expansion are waled b-
3CIAL SIDE ARM W A L CELL D. I Icm LONG
\ L FLOWING THROUGH EACH C E L L yf the type described in this paper are :ells at a low flow rate
VOL. 37, NO. 3, MARCH 1965
-
gether as in this example. Finally, the window areas are masked with narrow strips of Scotch tape, the lower inlet tube is plugged and the assembly is dipped repeatedly into a thin acetone slurry of Eccobond 51 black epoxy resin (Emerson and Cuming, Inc., Canton, Mass.) until a black deposit has been built up. After cure, the masks are removed and small rectangular Plexiglas spacers are sealed onto the broad borosilicate glass faces of the cell to make the finished cell wide enough to fit correctly into the spectrophotometer cell compartment. If solutions of thiosulfate, cyanide, and ammonium hydroxide, which attack silver salts, are not to be encountered, the quartz windows can be sealed to the borosilicate glass in a more elegant fashion. First, the ground surfaces of the borosilicate glass body are coated with Liquid Bright Platinum No. 05 (Hanovia Liquid Gold Division, Engelhard Industries, Inc., East Newark, K. J.) and fired at 1025’ F. in a furnace
or lehr. The quartz windows are masked with tape and coated on the masked side with the platinizing solution. After the coat has dried, the masks are carefully removed and the windows are fired a t 1200’ F . A thin layer of a thick slurry of silver chloride in glycerine-water (50/50) is applied to the platinized borosilicate glass body. The windows, platinized side toward the borosilicate glass body, are then carefully put in place. In place of the silver chloride slurry, a “gasket” of silver chloride can be cut from sheet silver chloride (A. D. Mackay, Inc., New York 38, N. Y.). The assembly is then heated in the oven a t 840’ F. for 10 minutes and the cell is then allowed to cool slowly to room temperature. Masking of the outer surface of the windows and blackening of the cell is conducted as in the first method. The cell with the longer (straight) exit tube shown in Figure 2B is a finished assembly; windows have been
epoxy bonded. The shorter cell, assembled by the silver chloride technique, is complete with the exception of plastic spacers. Flow characteristics for two commercial cells are compared in Figure 3 with that obtained for cells described in this paper. The silver chloride-platinized glass technique finds general use for sealing together glasses with the same or differing coefficients of expansion ( I ) ; seals are vacuumtight (1 X 10-lo Torr). Sickel, nichrome, copper, alumel, and tungsten wires can be sealed directly into holes drilled in glass with silver chloride as sealant ( 2 ) . LITERATURE CITED
(1) Eichenbaum, A. L., Norman., F. H., Sobol, H., Rev. Sci. Znst. 35, 1056 (1964). (2) McCandles, H. E., private communi-
cation, RCA Laboratories, Princeton, N. J., 1964. Trade names referred to in this paper do not imply endorsement of commercial products.
A Scanning Potentiostat K. I. Wood, Division of Protein Chemistry, C.S.I.R.O., Wool Research Laboratories, Parkville N2, Melbourne, Victoria, Australia HE PRESENT apparatus was deT s i g n e d to provide controlled electrical conditions for studying the electrolytic reduction of groups such as disulfides in peptides and proteins ( 2 ) . Reduction could be effected either directly a t the mercury cathode surface or indirectly in the presence of a trace amount of carrier-e.g., thiol-which is continuously regenerated to the fully reduced state by electrolysis. The apparatus has a scanning feature for automatically producing a currentvoltage curve. This enables progress of preparative reduction to be quickly checked by pushing a button, rather than by taking an aliquot from the potentiostat cell and using a separate polarograph for estimations. The cell used, Figure 1, was similar to that of ikfeites (3) having a conical 500-ml. cathode compartment containing mercury of about 40 sq. cm. surface area, A glass stirring propeller is used to agitate the mercury cathode surface as vigorously as possible without causing detached droplets to be formed. This rapid and efficient stirring keeps the thickness of the diffusion layer to a minimum, permitting increased currents to flow, which expedites the electrolytic process. As a result of this stirring, the internal resistance of the cell is not steady and its value fluctuates in phase with the stirring motion. Potentiostats described in the literature have the disadvantage of being slow in response where a reversible
442 *
ANALYTICAL CHEMISTRY
ANODE-7 MOTOR
a
S. E.
- n
b
Figure 1 . Cell and basic circuit
CELL POTENTIAL POTENTIOMETER
DC.AMP: TO
motor has been used to control the power output from a rheostat or Variac, or inefficient in supplying large currents if entirely electron tube operated. Those where the output is controlled by power transistors ( I , 4, 6)
appear to offer the best solution to the requirements of high current handling capacity and an adequately rapid rate of correction. The present potentiostat is entirely line operated and transistorized through-
