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contains frequency components which can be detected by the tuned amplifier and which may keep the amplifier output above the level-detector threshold for a brief time after the drop has fallen. Second, the tuned amplifier must respond to the sudden change at its input with a deliberately constrained frequency response. Several periods of oscillation at the center frequency are required to accomplish this. Third, a full period (15 to 20 ps) of the missing-pulse detection monostable must elapse between the last output transition of the level detector and the production of the trigger signal for the current-sampling circuitry. Finally, noise in the current-to-voltage converter whose power spectrum overlaps the frequency range of the tuned amplifier can delay detection of drop-fall a few additional ac cycles. The drop-fall detector operates reliably even in noisy environments provided the potentiostat employed has sufficient response a t the frequency of the ac perturbation. Although we have chosen a frequency of 100 kHz, the detector is suitable for slower potentiostats providing that a lower frequency is selected, albeit with some lengthening in drop-detection time. This modification needs to be performed only once and is accomplished by varying the passive components in the twin-T networks around amplifiers A1 and A4 (12) and the timing capacitor on MS1. A single selection of the suitable gain (two are available) for the tuned amplifier and of a threshold level appropriate to the current-to-voltage converter setting, to the electrode capacitance, and to the cell resistance generally suffices over a wide potential range (2 V or more) for a given set of cell conditions. Since no further adjustments are required, this circuit is very useful as a trigger device for automated instruments, especially those under the control of a laboratory computer. The circuit's independent nature allows the computer to perform lower priority tasks between measurements, instead of requiring that the processor continually monitor portions of the detection circuitry in order to discern drop fall (6).
PERFORMANCE SUMMARY The features of the detector include the temporary disconnection of the self-contained oscillator from the potentiostat by means of an analog switch as soon as drop fall has been detected which eliminates the need for filtering. A single adjustment generally suffices for a wide range of potentials, and the operation of the detector is unaffected by the presence of the pzc within that range. The response time, between 100 and 200 M S , is dictated by electrochemical cell characteristics, the frequency of the ac perturbation, and the time window of the missing-pulse detector. Unfavorable cell conditions are those in which large faradaic and nonfaradaic components are
present so that the output of the tuned amplifier remains substantial even after the drop has fallen. Such circumstances are encountered, for example, with a solution of 1 mM Cr3+ in 1 M NaC104 (pH 3) a t an electrode potential of -1100 mV. vs. SCE where the diffusion-controlled reduction of Cr3+ occurs, and the excess electrode charge density is large and negative (ca. -13 p C cm-2) (13). The response time was assessed by observing the current-to-voltage converter output on a Biomation Model 820 transient recorder in the pretrigger mode. Detection was considered to have occurred when sinusoidal variations at 100 kHz could no longer be distinguished in the cell current. Thirty determinations yielded an average detection time of 185 ps with a standard deviation of 36 ps. Measurements at the growing mercury drop are essentially unaffected by the ac perturbation since it is automatically disconnected within this time scale. Either individual or successive natural drop times can be conveniently determined to this accuracy, so that current-time curves may be monitored even at subsecond sampling times with high accuracy. In addition to its use as a synchronization device for rapid dc polarography (see ref. 4 for details) and determinations of the excess electrode-charge density from charge-time curves (14), we have also found it to be highly suitable for obtaining precise surface-tension data from drop-time measurements (11).
LITERATURE CITED Canterford, D. R. J . Nectroanal. Chem. 1977, 77, 113. Corbusier, P.; Gierst, L. Anal. Chim. Acta 1958, 75,254. Bond. A. M.; O'Halloran, R. J. J . Phys. Chem. 1973, 77, 915. Tyma, P. D. J . Nectrochem. SOC.1978, 725, 284C. Tyma, P. D.; Weaver, M. J.. to be published. De Blasi, M.; Gannelli, G.; Papoff, P.; Rotunno, T. Ann. Chim. 1975, 65, 315. Hahn, 8. K.; Enke, C. G. Anal. Chem. 1974, 46, 802. Yarnitzky, C. J . Nectroanal. Chem., 1974, 57. 207. Yarnitsky, C.; Wijnhorst, C. A.; Van Der Laar. B.; Reyn, H.; Sluyters, J. H. Ibd., 1977, 77, 391. Papeschi, G.; Costa, M.; Borda, S.Nectrochim. Acta 1970, 75,2015. Lawrence. J.; Mohilner, D. M. J . flecfrochem. SOC.1971, 778, 259. Peerce, P. J.; Anson, F. C. Anal. Chem. 1977, 49, 1270. Malmstadt, H. V.; Enke, C. G.; Crouch, S.R.; Horlick, G. "Optimization of Electronic Measurements"; W. A. Benjamin: Menlo Park, Calif., 1974; pp 47-53. Weaver, M. J.; Anson, F. C. J . Electroanal. Chem. 1975, 65, 711. Lauer, G.; Osteryoung, R . A. Anal. Chem. 1967, 39, 1866.
RECEIVED for review April 30,1979. Accepted August 24,1979. P.D.T. gratefully acknowledges support from an American Chemical Society Analytical Chemistry Division Fellowship which was sponsored by the Procter and Gamble Company. M.J.W. is indebted to the Air Force Office of Scientific Research for support of his research program.
Apparatus for the Dynamic Coating of Capillary Columns Eduardo 0. Murgia and Joaquin A. Lubkowitz" Instituto Tecnol6gico Venezolano del P e t d e o , Apartado 76343, Caracas 707, Venezuela
The use of glass capillary columns is expanding rapidly in the analysis of the complex substances by gas chromatography. The glass column is still costly and it is sometimes convenient for the user to control length, nature of liquid phase, and thickness of stationary phase of the columns to be used. Recently several workers (1-3) have reviewed the numerous recipes available for the preparation of columns. Thus many of the myths have disappeared. The manufacture of a column requires the careful attention of the pretreatment of the glass surface, the deactivation of its Lewis acidity, and finally the coating of the stationary phase. One of the most reproducible methods of coating the column is a variation of the dynamic
method developed by Schomburg and which has been called the mercury plug technique ( 4 ) . The method consists of introducing a short mercury plug immediately after the liquid phase has entered the column. This technique aims at achieving a thin film from a highly viscous solution which can resist drainage and non-uniformity of the coating during the evaporation stage. This method is relatively fast and has the potential that the same equipment can be used for other steps such as coating with a deactivating agent or rinsing the column. A simple apparatus has been developed which is designed to coat the columns and is capable of introducing a short plug
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979 RESTRICTOR
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L
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HELIUM
L PRESSURE
CONTROL AND NEEDLE VALVE
-TEFLON
TUBING
1 k0 '"
Ogcm
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3-WAY VALVES
EFLON FITTING
I
I
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SEQUENCE : 1.- COATING 2.- MERCURY
PLUG
v1
v2
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3.- GAS FLUSHING Figure 1. Schematic diagram of the coating apparatus
of mercury. Furthermore, the liquid phase and the mercury are contained in separated sections. The apparatus is simple to construct and operate.
EXPERIMENTAL Apparatus Construction. The assembled apparatus is shown in Figure 1. It consists of a replacement column tube (Altex Scientific, Berkeley, Calif., catalogue no. 252-00-001) and a glass-jacketed column built around this tube. Two pieces of microbore column of 2-mm i.d. (Altex, catalogue no. 251-00) were joined to the main body. Thus, all four ports use the liquid chromatography Teflon fittings which can be hand-tightened. The two lower exit ports are connected to the three-way valves (Rheodyne, Berkeley, Calif., model no. 50-31). A flow controller (Brooks, 4480, Brooks Instrument, Hatfield, Pa.) and a 2-m long,
1.59-mm o.d. stainless-steel capillary tubing are used in order to carefully control the flow so that the liquid flow through the column is adjustable from 0.2-2 m/s. The outer jacket contains about 3 mL of mercury which is introduced via the left port. The inner glass tube is capable of holding 4 mL of liquid. The connections between valves and the glass container are made with Teflon tubing (1.15-mm 0.d.). Operation. After introducing the liquid phase though the upper port, the needle valve is adjusted to give the desired flow rate through the column and its dummy. The valves VI and Vz should be in the coating sequence as shown in Figure 1. Immediately after the liquid coating phase is in the column, usually 15% of total column volume, the valves are switched to the mercury plug position as shown in Figure 1. Valve Vz is kept in this position for a short time, usually 2-3 s, since this time
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determines the length of the mercury plug. The valves are then switched to the gas flushing sequence whereby the inert gas finishes the coating and dries the column.
RESIJLTS AND DISCUSSION The system designed has been tested repeatedly and offers several advantages over other reported apparatus. It depends on valves rather than on using a moving capillary to introduce the mercury plug. The coating phase is physically separated from the mercury and, thus, the mercury supply remains essentially noncontaminated. The mercury is isolated from the liquid phase container. Only the delivery Teflon pathway toward the column presents a contact of liquid and mercury. The apparatus handles mercury safely making it difficult to spill mercury during such operations as loading, cleaning, and pressurizing the assembly. The same apparatus can be used to rinse the column or for the passage of deactivating solutions. It is possible to expand the unit by attaching an additional glass cylinder tu the unused ports of one valve. Thus the second glass container can be
used exclusively for washing or deactivation treatments. This would reduce further the possibilities of contamination. The additional cylinder can be made of plastic and used for etching the columns with liquid solutions ( 5 ) or by attaching a gas generator. Finally, the assembled unit as shown in Figure 1 has components which are found in most separation laboratories and has a cost of less than $150. We have repeatedly used this system in coating columns obtaining theoretical plates number of 2000-3000 for k = 14-20 (C-14) and internal column diameters of 0.25 mm.
LITERATURE CITED (1) Jennings, W. "Gas Chromatography with Glass Capillary Columns"; Academic Press: New York, 1978; p 31 (2) Sandra, P.; Verzele, M. Chromatographia 1977, 10, 419-425. (3) Onuska, F. I.; Comba, M. E.; Bistriki, T.; Wilkinson. R. J. J . Chromatcgr. 1977, 142, 117-125. (4) Schomburg, G.; Husmann, H. Chromatographia 1975, 8,517-519. (5) Heckman, R . A . ;G-een, R.; Best, F Anal. Chem. 1978, 50, 2157-2158.
RECEIVED for review April 23,1979. Accepted August 6,1979.
Surge Control Unit for Mercury Manometers Jesse S. Ard Eastern Regional Research Center, Agricultural Research, Science and Education Administration, 600 E. Mermaid Lane, Philadelphia, Pennsylvania 19 1 !8
The reading of a mercury meiiiscus can be critical to obtaining correct pressure and volume measurements. Sometimes correction9 have to be applied for local gravity, temperature, capillary depression, the height of a cylinder equivalent to the volume of a curved section, glass refraction, static electricity, and dryness of the gas exposed ( I , 2). These factors can produce errors, but these can be reduced or eliminated by the use of wide-diameter glass tubing where the meniscus is read. Such tubing is also desirable for obtaining large displncernent volumes for McCleod gauges, hydrogenation apparatus, Toepler pumps, and other gas handling systems. However, glass wnlls of large chambers cannot well survive the liquid hammering action of large masses of surging mercury, whicli often occurs through careless handling of valves or other accidents. The usual provision for surge control has been a constriction in the lower connections, but this, if small enough, niay rrtard normal flow and may result in long delays in reading the menisccls. A simple glass unit (Figure 1)that provides division and reunion of a liquid stream has been designed tu overcome these difficulties. Diameters of the channels can be chosen to fit particular circumstances. Normally, the unit would be placed in the lower part of' a glass structure. Connecting units in tandem may be used t o enhance the dampening effect. Glass tubing may he bent to form a unit. A somewhat less efficient unit may be made more easily by blowing a bulb, flattening it, and pushing in each side with carbon to form an island-like structure encircled by channels The glass units should be annealed well to overcome local stresses that are likely to occur if one branch contracts more than the other. The unit does not trap bubbles, avoids the need for narrow constrictions, and offers resistance to flow that is an increasing function of the mercury velocity. Thus, surging iu prevented without appreciable effect on normal flow rates. Resistance to mercury motion approaches zero as the velocity approaches zero, and the meniscus settles to the correct level with negligible hysteresis. Surges upward and downward are dampened alike. For example, on upward flow, collision of the mercury streams produces a resistance to flow a t the top
Figure 1. Glass unit for mercury surge control by divergent-convergent actions
juncture. A t the same time, the velocity is attenuated a t the bottom juncture because of the division of the mercury into two streams. This attenuation not only minimizes the forces from the moving mercury but also nearly balances the lateral forces through opposing symmetry. In actual operation, no tendency for the unit to jump could be observed, and the small glass structure withstood the internal forces well. These units have served for manometers, hydrogenation apparatus, and large fixed McCleod gauges. For example, a surge control unit measuring 40 mm across the squared part and containing channels 2.7 mm in diameter for the divided stream proved satisfactory in controlling mercury hammer in a 200-mL volume bulb and overshoot in the comparison column of a McCleod gauge.
LITERATURE CITED (1) "International Critical Tables," H. H. Kimball, Ed., No. 1, Maple Press CO.. York, Pa., 1926, p 68. (2) W. Cawood and H. S. Patterson, Trans. Faraday Soc.,29, 514 (1933).
RECEIVED for review July 19, 1979. Accepted August 23, 1979.
This article riot subject to U S . Copyright. Published 1979 by the American Chemical Society
