Apparatus for precision control of drop time of dropping mercury

(7) W. L. Belew, D. J. Fisher, M. T. Kelley, and J. A. Dean, .... (5) S. Wolf, Angew. Chem., 72, 449 ... at the point where it is clamped to reduce vi...
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peak height. We have obtained the most precise results and the highest sensitivity with a 0.5-second controlled drop time and 1-second filters; this in conjunction with a sweep rate of 1 volt/minute permits the recording of a polarogram in less than 30 seconds. Some polarographic work that has been done with this instrument has been reported (7, 8). EVALUATION WITH STATIONARY ELECTRODES

The performance of the instrument was checked with the hanging mercury drop electrode (HMDE) (Areas = 0.03 to 0.05 cm2), and with smooth (Area = 0.3 cm*) and platinized (Geometric Area = 3 cm2) platinum electrodes. The Pb(I1) Pb reaction in 1 M KC1 and 0.1MHC1 was chosen for the evaluation with the HMDE. The Pb(I1) concentration was either 2SmM(lMKCl) or 10-6M(0.1 MHCI). The Fe(CN)6a- $ Fe(CN)64--1M KC1 system [ O S to 6 mM K3Fe(CN)d served for the evaluation of the instrument’s performance with the platinum electrodes. The output of the control amplifier was monitored with an (7) W. L. Belew, D. J. Fisher, M. T. Kelley, and J. A. Dean, Microchem. J., 10, 301 (1966). (8) M. T. Kelley, W. L. Belew, G. V. Pierce, W. D. Shults, H. C . Jones, and D. J. Fisher, ibid., p 315.

oscilloscope during the recording of each curve. No oscillations were evident. The results obtained for regular, first-, and second-derivative curves were in excellent agreement with the expected behavior of the systems studied. In addition to these performance checks, extensive use has been made of the instrument as an analytical research tool at ORNL where a number of other types of electrodes, stationary and rotated, have been employed (9-12). No instabilities were noted. ACKNOWLEDGMENT

We wish to acknowledge the assistance of M. T. Kelley in this project, and the help of W. L. Maddox and D. R. Martin in obtaining experimental data. RECEIVED for review February 26,1968. Accepted February 6, 1969. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. (9) H. E Zittel and F J. Miller, Anal. Chim. Acta, 37, 141 (1967). (10) H. E. Zittel and T. M. Florence, ANAL.CHEM., 39, 320 (1967). (11) Zbid., p 355. (12) H. E. Zittel and T. M. Florence, Anal. Chim. Acta, 40, 27 (1968).

Apparatus for Precision Control of Drop Time of Dropping Mercury Electrode in Polarography W. L. Belew, D. J. Fisher, H. C. Jones, and M. T. Kelley Analytical Chemistry Dioision, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 An apparatus is described for the precise electromechanical control of the drop time of a dropping mercury electrode (DME) at values from 0.25 to 10 sec. A constant drop time is obtained over the entire potential range with capillaries having their orifice in the horizontal or the vertical plane. Excellent performance is most easily obtained with a Smoler 90° vertical orifice DME. Corresponding portions of controlled drop time current-time curves are essentially unchanged from uncontrolled drop time curves. The constant drop time simplifies the efficient filtering of the current fluctuations from the DME. Increased sensitivity and precision are obtained. At a 0.5-sec controlled drop time, a scan rate of 1 V/min, and with the ORNL model 4-2792 polarograph, regular or firstor second-derivative polarograms can be recorded in -30 sec with a relative standard deviation of 0.2% at the 1O-W concentration level. Satisfactory first- or second-derivative dc polarograms are obtained at concentrations as low as lO-7M.

Advantages of Constant DME Drop Time. For high precision or high sensitivity polarographic determinations, variations in the drop time ( t ) of the dropping mercury electrode (DME) are particularly troublesome. Drop time variations arising from changing capillary characteristics, surface active agents, or changes in the potential of the DME make efficient filtering of the current fluctuations corresponding to t more difficult. By precise control of the t of the DME, tuned filtering can be used more effectively. Signals from which the fluctuations corresponding to t have been removed are amenable to accurate measurement of current and are more suitable as the input to derivative com-

puters. Also, sporadic mechanical shocks cause less DME noise than with uncontrolled t DME’s. Furthermore, when t is rigidly controlled, it is possible to vary the average mercury flow rate (m)independently to match the m2I3t I i 6 valuei.e., empirical calibration factor-of different capillaries. It is also possible to select independently a low m value to eliminate or greatly reduce the probability for the occurrence of maxima. Advantages of Short Constant Drop Time. Short drop times allow use of filters having less time lag to take better advantage of the dependence of derivative signal magnitude upon scan rate and for rapid dc polarography. Obtaining a short t by increasing the mercury head (h) is not satisfactory because m increases too much. With the t controller, both short constant drop times and low m values are available. Relationships to Previously Published Work. Several investigators have published information on devices for t control. Skobets and Kavetskii (1) used a small glass spade directly beneath the mercury drop to dislodge the drop at a prefixed size. Sevzik (2) used large voltage pulses to dislodge the mercury drops to synchronize two capillaries for differential polarography. Airey and Smales (3) discussed mercury t control by both electrostatic and electromechanical methods ; they developed an apparatus for simultaneous electromechanical control of the t of two capillaries for dif(1) E. M. Skobets and N. S. Kavetskii, Zacod. Lab., 15,1299 (1949). (2) A. gevEik, Collect. Czech. Chem. Comrnun., 13, 349 (1948). (3) L. Airey and A. A. Srnales, Analyst (London), 75,287 (1950). VOL. 41, NO. 6, MAY 1969

779

COIL POLE PIECES

COIL FRAMES COIL MOUNT

Bous

CELL-TOP C L A M P BoLrs ARM DME

CELL

rop

RUBBER

BOOT

CLAMP FOR CELL TOP

Figure 1. Line drawing of mechanical control unit

ferential polarography. Davis and Seaborn (4) developed an apparatus for use in differential and single-cell fast-sweep polarography. Wolf (5) investigated rapid polarography using a Metrohm-controlled f apparatus and found controlled f rapid polarography to be a desirable technique. We used the presently available Metrohm apparatus, Model E-354S, and found that careful adjustment of the apparatus at a selected cell potential is necessary to prevent substantial variations in current caused by stirring the solution at the DME. Two major problems in designing ao electromechanical f control apparatus involve: eliminating capillary vibrations which cause stirring in the solution and cause small droplets to be dislodged from the capillary as a new drop is formed; and controlling and reproducing precisely for an extended period of time the small displacement of the capillary necessary to detach the drops. Two methods of capillary movement may be envisioned: pulling the capillary vertically upward to stretch the mercury thread until it breaks, or pulling the capillary horizontally to shear the drop from the mercury thread (3). When the capillary is pulled upward and returned to its original position in the solution each time a drop is removed, a pumping action which causes stirring-mav.result.,esveciallv at short droo times. _ Several models of f controllers which displaced the caFiillary suddenly horizontally and then returned the capillary to its . . posirion . . . ~ oy a uarripeu >-~..~.-J :-- -... L . : . . ....-. original qmng r ~ i c ~ n u i i swcic ~ n built in our laboratory (6). Although some of these t controllers worked well for short periods of time, none would work reliably for long periods without readjustment. It was necessary to readjust the devices for different capillaries, and from day to day to readjust with the same capillary, to either detach the drop reliably or to prevent additional droplets from being sheared off by vibration when the capillary was returned to its original position. Devices were also built in which a capillary . . mounted in a fixed position in rubber or Teflon was struck sharply. The shock detached the mercury drop. Only marginal result!i were ~ . . ~.~~1.1 . ~AL.. ~ >s :Jl,LkVduced .*-. ooraineu w i n ~ i i e UCVILCS VCLLIU~O VI Y ~ V L ~ L N ~ O into the system from the large shock necessary to detach the drop reliably under all cell conditions. The apparatus described in this paper circumvents these difficulties by the provision of appropriate damping and by ~~~~~~~~~

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(4) H. M. Davis and J. E. Seaborn,Electron. Eng., 25,314 (1953). (5) S.Wolf, Angew. Chem., 12, 449 (1960). (6) D. J. Fisher, W. L. Belew, and M. T. Kelley, in “Polarography 1964,” G.J. Hills, Ed,, Vol. 1, Macmillan, London, 1966, pp 110-11. 780

ANALYTICAL CHEMISTRY

Figure 1. Drop time controller, stand, and cell

displacement of the capillary in one direction to shear the drop, with the capillary held in this position for a fixed period of time, and then displacement of it in the opposite direction to shear the next drov. EXPERIMENTAL

Instrumentation. The polarograph used in this work is the ORNL Model Q-2792, described elsewhere (7). The drop times were measured with an Aerometrics electronic counter, Model EC-7157. Current-time curves were observed with a Tektronix 545 oscilloscope with a Type D plug-in unit, fitted with a DuMont Type 353 oscilloscope camera having a Type 2620 Polaroid film holder. Curves were observed also with a Fabri-Tek Instruments, Inc., Model 1064 instrument computer, having XY recorder readout and numerical digital printout. Design Criteria. To be most useful, a f control apparatus for analytical applications must meet the following criteria: The apparatus should detach drops reliably without mechanical or electrical readjustment when a capillary is replaced by a similar capillary or when a different voltage range is used; it should operate for long periods of time without service or adjustment (if the device must be adjusted each time a series of polarograms is to be recorded, its value in analytical applications is in doubt); it should not cause the cell current to increase markedly by causing convective disturbances at the DME; it should include a short f with a low M . These criteria have beenmet. Description of Mechanical Part of Apparatus. Figure 1 is a drawing of the mechanical portion of the controller. Figure 2 is a photograph of the complete assembly. As (7) H. C . Jones, W. L. Belew, R. W. Stelmer, T. R. Mueller, and D. J. Fisher, ANAL.CHEM., 41, 772(1969).

Figure 3. Polarographic drop time controller circuit shown in Figure 1, the capillary is held in a clamp on a pivot arm. The pivot arm is mounted on a shaft held in place by double-row self-aligning ball bearings so there is no excessive play. Two soft iron, rubber-padded blocks are mounted on the rear of the aluminum pivot arm to serve as armatures for two relay coils. The relay coils are mounted on aluminum blocks which can be positioned to adjust the movement of the pivot arm. Rubber pads are imbedded in the soft iron armatures, flush with the surface, and serve as stops for the pivot arm. These pads are essential to eliminate initial metal-tometal contact and thereby to reduce capillary vibration to a minimum. The capillary is wrapped with 0.02-in. thick Tygon tubing at the point where it is clamped to reduce vibration caused by resonance in the glass capillary itself. To assure complete freedom of movement of the capillary, and to maintain a sealed cell, the capillary is inserted in a rubber boot. The boot, formed from a medicine dropper bulb, is sealed to the cell cap with a RTV (room temperature vulcanizing) silicone rubber. The above combination of Tygon wrapping and boot can be replaced by a one-piece fitting molded from urethane rubber (8). A piece of Neoprene tubing approximately 4 in. long is used to connect the capillary to the glass stand tube. When correctly assembled, without the coils being energized, the pivot arm comes to rest midway between the coil pole pieces. Description of Circuit Part of Apparatus. A diagram of the circuit used to control the apparatus is shown in Figure 3. A unijunction transistor (Ql) is used in a relaxation oscillator circuit to trigger a flip-flop circuit (Q2-Q3) each time the unijunction transistor fires. This timing circuit is that of the hybrid multivibrator for generating square waves described in a GE manual (9). The output pulses from the flip-flop alternately fire the silicon-controlled rectifiers (SCR1-SCR2) which energize the relay coils. The latter circuit is essentially the latching relay or flip-flop circuit described in a G E manual (10).

The turn-on pulse from each SCR turns off the other SCR by a negative pulse through capacitor C7. To detach the drops from the capillary reliably at all cell potentials, it is necessary to apply 12 V from the power supply to move the

pivot arm with enough force. However, if the full 12 V are left on the coil, the magnetic field does not decay fast enough and thus slows down the next (oppositely directed) movement of the pivot arm. It is, therefore, necessary to shape the power pulse to the relay coils. This shaping is done with capacitors C5 and C6 and resistors R28 and R29. When the silicon-controlled rectifier is turned on, a 12-V pulse is applied to the relay coil through a large capacitor (C5 or C6). The pulse then decays in 0.1 sec to approximately 4 V owing to the drop through the 25-ohm resistor (R28 or R29). Without this pulse-shaping circuit, there is approximately 20-msec delay from the time the unijunction transistor fires until the drop is dislodged from the capillary. With the pulse-shaping capacitor and resistor installed, the drop is detached after a delay time within the range of 4 to 6 msec after the unijunction transistor fires. Any difference in the power delivered to the pivot arm by the two relay coils can result in a difference in alternate drop times. This time difference must be not greater than 1 0 . 1 msec. The delay times are matched by adjustment of resistors R31 and R32. The purpose of diodes D1 and D2 is to limit a reverse spike from the decay of coil current. Within the unit, the 20-V point shown in Figure 3 is supplied by an Acopian model 20B10 power supply (11). The relay coils are energized alternately so that the pivot arm is pulled suddenly to one side, held for the desired period of time, then pulled to the opposite side and held for the same period of time. Adjustment and Testing of Apparatus. INSTALLATION OF CELL. The cell is built so that it is tightly sealed against the atmosphere. A low dissolved oxygen content after sparging improves the signal-to-noise ratio for dc polarography at high measuring sensitivity (12). The cell cap (top) is machined from Teflon and held in a clamp, as shown in Figures 1 and 2. The large hole for the DME has clearance for the DME movement. A rubber boot sealed to the cell top allows the DME to move freely in the sealed cell. Holes are also provided in the cap for two glass sparge gas inlet tubes. One of these tubes is terminated near the

1) U. S. At. Energy Comm. Engineering Materials List (TID-

(8) W. L. Maddox, Oak Ridge National Laboratory, Oak Ridge, Tenn., personal communication, August 1968. (9) General Electric Co., Auburn, N. Y.,“General Electric SiliconControlled Rectifier Manual,” 2nd ed., Fig. 4.17 (1961). (10) Ibid., Fig. 7.7.

4100), CAPE-1651, “Polarographic Drop Time Controller,” available from Clearinghouse for Federal Scientificand Technical Information, U. S. Dept. Comm., 5285 Port Royal Rd., Springfield, Va. 22151. 2) D. J. Fisher, W. L. Belew, and M. T. Kelley, Chem. Instrum., 1,181 (1968). VOL. 41,

NO. 6, MAY 1969

781

Figure 4. Comparison of current-time curves as a function of drop time

Cell solution, 5 X lO-'M CdP+,1M KCI, 0.001M HCI Cell potential, - 0.8 V us. SCE. Smoler DME

bottom of the cell for sparging with a 10-mm diameter medium porosity borosilicate glass stick filter to reduce sparge time; the other, above the solution for sparge gas blanketing after sparging. Another hole is for a Beckman No. 39178 fiber junction SCE. A glass gas outflow tube is led out of another hole; it is dipped into a water seal, so that the cell is well sealed against the entry by back-diffusion of atmospheric oxygen. The inlet sparge gas passes through a bubbler to equilibrate the gas with the solvent. Equal readings on gas flow meters in the inlet and outlet lines indicate that the cell is gas-tight. The glass cell bottom is a high form weighing bottle, with a 40112 taper top, and is 40-mm i.d., 80 mm high. It is held up against the cell top by tension applied with an adjustable clamp underneath the cell. It is sealed against the cell top with a ring of Apiezon sealing compound Q. The solution volume is -50 ml and its upper surface is -2.5 cm below the cell top so that DME movement, gas overflow, and mercury drops collecting on the cell bottom do not cause convective disturbances at the DME. The use of a large solution volume (instead of -10 ml) also has the advantage, when precision is of interest, of minimizing concentration changes due to evaporation or addition of solvent during sparging. The counter electrode is a platinum wire, dipping directly into the solution. In highly acid solutions, the lowering of the overvoltage of hydrogen on mercury by traces of platinum is avoided by using graphite for the counter electrode and for the contact to the mercury column. The relative placement of the three electrodes in the cell is not critical in the usual low specific resistance solutions. In high specific resistance solutions (or at unusually high cell currents), the tip of a reference electrode probe is placed close above the upper surface of the drops from a Smoler 90" vertical orifice DME to minimize iRi,... (12). When high precision is needed, the cell is operated in a thermostated enclosure.

782

ANALYTICAL CHEMISTRY

INSTALLATION OF DME. Much time is saved by examining new DME's with a microscope, prior to installation, although this recommended precaution is not unique to the use of DME's with this apparatus. DME's which have cracks or spurs or other orifice defects must be rejected and bores that contain glass or other particles must be cleaned prior to installation. Half or full length Sargent 2- to 5- and 6- to 12-sec horizontal orifice DME capillaries can be used. If these horizontal orifice DME's are used, it is necessary to reduce substantially the power delivered to the coils with each pulse of the controller (even to the extent of adding series resistors at R31 and R32), while an adjustment of R31 and R32 is maintained such that the alternate delay times are matched. Also, the m value must be rather high. The corresponding parts of current-time curves (at a fixed potential on the wave) for uncontrolled and controlled drop times should coincide. Coincidence means that the apparatus is not introducing a net change in this relationship. Although excellent reproducibility of derivative peak heights can be obtainedat a controlled f of 0.5 sec, when thesecurrenttime curves do not coincide (even when alternate controlled drop delays have been matched and drop times are equal), this is an operating condition of questionable validity. Corresponding parts of controlled and uncontrolled f currenttime curves with horizontal orifice DME's do not exactly coincide. The difference can be rather large unless adjustments are carefully made. Coincidence is readily obtained when the Smoler 90" vertical orifice D M E is used. We strongly recommend the use of 90" Smoler DME's (instead of horizontal orifice DME's) and a controlled t of 0.5 sec for derivative dc polarography. The apparatus can be used, however, to control longer drop times precisely for other polarographic applications. These Smoler DME's are made by bending Sargent 2- to 5-sec DME's near the center to a right angle and breaking the capillary about 'I4 in. away from the bend (6). Three advantages result from the use of vertical instead of horizontal

Cellsolution,S X 10-'MCd2+,lMI 7C1,O.OOlMHCI Cell potential, - 0.8 V vs. SCE. Smomelr DME

orifice DME's in this app:aratus. First, sparge gas bubbles -n++nm nf thn rrnil. L..v are not trapped after sparging against the LVLLY... easier to obtain coincident corlary. Second, it is much responding portions of unc:ontrolled and controlled f current. . ~ ~ ar LOW m -values. ~-,~ ume curves Third, it facilitates the minimizatioo of iRinneFerror in high specific resistance solutions. No disadvantage fron1 the use of Smoler 90" vertical orifice DME's has been obserVed. The length and the tension of the Neoprene tube used to connect the DME to the mercury standpipe are not critical. There s some ...... i.. ...... evidenc ..-.. .e that it is possible to replace the Neoprene tubing by a flexible thin-walled length of glass tubing, if it is desired to contain the mercury entirely in glass (8). The performance of the apparatus with glass tubing in place of Neoprene tubing has not heen evaluated in detail. The capillary clamp is tightened so that there is no play between it and the wrapping and the DME. Play at this point causes to..quiver when the arm hits a stop and. creates the DME . . -. pivot . . -. .- ..convective disturbances. I h e bmoler U M D Orifice IS -1.3 cm bellow the top of the cell solution and is -4 cm helow the celI top. The Smoler DME is oriented in the capillary clamp so that the plane of its orifice is vertical and is parallel to the direction of its movement. It is imperative that the uncontrolled f of the DME for all potentials of interest be longer than the controlled value o f f to be used. The mercury head should be sufficient to minimize the effects of variations in back pressure. MECHANICAL ADJUSTMENT.With the DME installed, the coil mounts are adjusted and bolted in position so that the DME is moved equal distances from side to side by the energized pivot arm and through a total distance equal to a value within the range 0.007 to 0.011 in. If the distance moved by the capillary is too great it will cause convective disturbance. A dial indicator gauge is used for this adjustment. The clearance between each armature and coil frame will be about 0.017 in. VI

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MetalI-to-metal contact must not occur at this point. The c l ier ~~ n~_ h.v1m~+ i.i m~a n arrh mhher rtnn gnrl rnil nnb nCc= should be equal; it will vary between light contact and less than 0.002 in. If the clearance at this location is excessive, the pivot arm may bounce back upon striking a rubber stop. Pivot arm bounce is undesirable because it introduces convective disturbances. If the coil pole pieces and mounts have been properly machined, the rubber stops are on center relative to the pole pieces and movement of the pivot arm does not result in initial metal-to-metal contact at the pole pieces. Inspection must he made to ensure that initial metallic cnntact does not take place because it also causes convective disturbances. Misalignment can also be detected aurally by a sharp, clattering sound of the apparatus during operation. The DME must not contact the edges of the hole in the cell top or other electrodes or tubes the pivot arm. ELECTRICAL ADJUSTMENT.Electrical __ -__ auired for two Durooses. First, the timine circuit is calibrated to corresiond with the values selectedby the drop time switch. Second, other adjustments are made to obtain proper electrical actuation of the coils that operate the pivot arm (13). For calibration, the variable resistors, shown for each f value in Figure 3, are adjusted so that the pulse rates from the unijunction transistor, Q1, correspond within +0.1 Z to each choice of t. An Aerometrics electronic counter, Model EC-7157 (or an equivalent counter), is connected to the test jack shown in Figure 3. Each time Q1 fires, a 5-V pulse is supplied to the test jack. The trigger input controls on the counter are adjusted so that it responds to successive pulses.

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(13) W. L. Belew and H. C. Jones, "Polarographic Drop-Time Controller, ORNL Model Q-2942," ORNL Master Analytical Manual Method Nos. 1 003044 and 9 003044. VOL. 41, NO. 6, MAY 1969

783

For 10 or more measurements, the relative standard deviation of the times should be 5 0.01 %. The electrical adjustments required to obtain good operation of the pivot arm through appropriate coil pulses are as follows : By oscilloscopic observation of the pulses supplied to each coil, resistors R31 and R32 are initially adjusted so that the 12-V pulses decay in -0.1 sec to a steady state value of 4 V. A millimolar test solution is placed in the cell and sparged; then the potential is adjusted to a value on the limiting plateau of the wave. The vertical input of the oscilloscope is driven from the current amplifier output of the polarograph. The oscilloscope sweep is triggered by the pulses present at the test jack. Resistors R31 and R32 are then adjusted to values such that the pulse intensities are appropriate and so that the delay times between the generation of a pulse and the dislodging of alternate drops are matched. The amplitudes of the pulses supplied to the coils decrease as the amounts of series resistance in the circuit from resistors R31 and R32 are increased. Resistors can be added in series with R31 and R32 if this is necessary to reduce intensity, but the intensity ranges provided by the R31-R32 values shown in Figure 3 are usually sufficient. If the pulse amplitudes are not excessive, so that shocks at the DME when the pivot arm strikes the rubber stops are not excessive, the observed current-time traces will not include a large, sharp spike as the drop is dislodged and the subsequent portion of the trace will be smooth and will not oscillate above and below an exponential form. Resistors R31 and R32 are also concurrently adjusted so that the delay times are within the range of -4 to 6 msec and are equal to within 1 0 . 1 msec. If these times were allowed to be substantially unequal, it would cause the generation of noise that would not be fully removed by the filters in the 4-2792 polarograph. Typical current-time curves, observed with an oscilloscope, are shown in Figures 4 and 5. The current-time curves should also be observed at another potential on the wave-e.g., at the half wave potential. Also, a check should be made to show that the drops are reliably dislodged over the entire potential range of interest. Noise is generated if drops are on occasion not dislodged. After resistors R31 and R32 have been adjusted so that the above criteria are met, the adjustment does not have to be repeated unless a different type of DME is installed or the rn value is changed substantially. Note, however, that the electrochemical tests, described next, must also be satisfied before it can be concluded that the mechanical and electrical adjustments are satisfactory. ELECTROCHEMICAL TESTS. Needs and methods for electrochemical tests of polarograph performance have been described (14). The following additional electrochemical tests are used to establish that the performance of this DME t control apparatus is satisfactory. Coincidence of corresponding portions of uncontrolled and controlled t currenttime curves (at a fixed potential on the wave) is the criterion found most useful for demonstrating that current at the DME is not increased because of convective disturbances due to maladjustment of this apparatus. Criteria based upon observed polarographic diffusion coefficient values or upon the observed mathematical form of the current-time curves are of less value. The semi-empirical character of these parameters is well known. The coincidence test can be made with an oscilloscope fitted with a camera. Better tests can be made with the digital equipment listed in the Instrumentation section. The advantage of establishing that the installation of and the chosen average rn value at the DME and the mechanical and electrical adjustments have resulted in coincidence is the greater probability that highly reproducible polarograms will be obtained. Coincidence (14) D. J. Fisher, W. L. Belew, and M. T. Kelley, Chem. Instrum., 1, 225 (1968).

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ANALYTICAL CHEMISTRY

means that the controller is terminating t at an earlier, controlled time without a substantial change to the net currenttime relationship. As a final and specific test for reproducibility, triplicate regular and first- and second-derivative dc polarograms are recorded with a millimolar solution of cadmium in a 1M KC1-0.001M HC1 supporting electrolyte with 0.001 % Triton X-100 as a maximum suppressor. With a 0.5-sec controlled t and a scan rate of 0.1 V/min, polarograms should reproduce within a pen width on the XY recorder (7). This is a sensitive test; this excellent degree of reproducibility is much easier to obtain at faster scan rates. The signal-to-noise ratio of recorded derivative polarograms is highest at a controlled t of 0.5 sec and at a scan rate of 1 V/min. To test for sensitivity-Le., signal-to-noise ratio-first- and second-derivative dc polarograms are recorded under the above conditions, except that a scan rate of 1 V/min and a concentration of 10-6M cadmium are used. In this case, the quality of the first-derivative polarograms should be at least equal to that of those which have been published elsewhere (7). With 1-sec filters and a 0.5-sec controlled t and with the Q-2792 polarograph, the signal-to-noise ratio is excellent. The quality of the second-derivative polarograms should equal or exceed that of those which have been published elsewhere as Figure 6 (12). These were obtained with the ORNL Model Q-1988-FES polarograph and without controlled t. Satisfactory derivative polarograms have also been obtained with lO-7M cadmium test solutions with this apparatus and the Q-2792 polarograph.

RESULTS AND DISCUSSION Timing Precision. To predict the limit of t reproducibility that could be expected from the completed t controller, the timing precision of the unijunction transistor circuit was measured. The measurements were made with a sevenplace electronic counter by timing the intervals between the pulses from the unijunction transistor circuit. The short term relative standard deviation of the timing circuit was less than 0.01% in all cases. For data collected over a period of 20 days, the relative standard deviation was 0.03% for times of 0.5 sec or longer, and 0.05% for times as short as 0.125 sec. Therefore, for any series of polarograms, the precision attainable for t should be better than O.Ol%, and for extended periods, a precision of 0.05% or better can be expected. Current-Time Curves. Current-time curves were recorded with an oscilloscope fitted with a camera to determine the effect of controlling the t. The accuracy of the results was limited by the accuracy of the scope trace; however, any major disturbances can be seen in this way. More precise observations of current-time curves have also been made with the digital equipment listed in the Instrumentation section. Indeed, models of t controllers developed earlier at ORNL were plagued with vibrations transmitted to the mercury drop and cell solution which caused considerable distortion of the current-time curves. The vibration damping and capillary mounting techniques used in this apparatus make it possible to obtain current-time curves as shown in Figure 4. In each of the four photographs, the line 2 cm below the center line designates 0 cell current. Photograph 1 is the first 90% of a current-time curve for a 1-sec t . The other three photographs have curves for various drop times superimposed as indicated. Four photographs showing the shape of the current-time curve immediately prior to and after the fall of the drop are shown in Figure 5 . The oscilloscope sweep was triggered from the unijunction transistor in the t controller circuit.

E , DME us.

SCE, V 0.0 -1.0

-1.5

-2.0

Table I. Precision of Mercury Drop Times ( t ) Cell solution: 0.1M KCI, sparged with argon Smoler 90" vertical orifice DME Horizontal orifice DME Uncontrolled Controlled Uncontrolled Controlled t, sec S, Z (4 t, sec S, Z (4 i, sec S, Z (n) t, sec S, Z (n) 0.008(10) 1.2515 0.27(20) 0.50046 0.008(20) 5.2002 0.25( 19) 5.0094 0.01l(20) 2.0020 0.007( 10) 0.50049 0.003(10) 1.2821 0.18(20) 0.50048 0. 007(20) 5.3587 0.33(19) 5.0096 0,005(20) 2.0018 0.005(10) 0.50047 1.0153 0.46(20) 0.50051 0.010(20) 4.3230 0.19(19) 2.0019 0.004(20) 0.008(10) 0.50051 2.6563 0.3419) 0.007(20) 0.6631 1.40(20) 0.50043 0.033(20) 2.0017 0 .08(10) 0.50059

I n photograph 1, the last 4 msec of one current-time curve and the first 40 msec of the next current-time curve for a 1-sec drop are shown. The oscilloscope is triggered and the trace begins when the unijunction transistor fires; the drop falls 4 msec later, thus showing that it takes approximately 4 msec for the pivot arm to move from one position to the stop in the other position. Two curves are superimposed in photograph 1 showing the reproducibility of the t and the lack of vibration as the drop is detached. The current increases slightly just as the drop is ready to fall; this is probably caused by the distortion of the shape of the drop as it is stretched at the point of attachment to the mercury thread just before its fall and by movement through the solution. This same increase can be seen when t is not controlled, but is not quite so pronounced. Otherwise the drop pattern for the controlled t drop is identical to that for an uncontrolled t drop. In photograph 2, a double exposure is made representing a 1-sec and a 0.5-sec t. The curves superimpose extremely well, the only difference being the slight decrease in limiting current at the end of the drop life caused by the shorter t . In photograph 3, a third t (0.25 sec) is added to the 1-sec and 0.5-sec current-time curves. At the 0.25-sec t , the delay from the time the unijunction transistor fires until the drop falls is increased to 4.5 msec and the start of the current-time curve is not so reproducible as with the two longer drop times. When the t is shortened to 0.125 sec, there is a significant change in the shape of the curve as can be seen in photograph 4; the delay time increases to 7.5 msec and a large current spike appears before the drop is detached. Because the drop is held longer on the capillary before it is detached, it may be postulated that at this small drop size, considerable distortion of the drop takes place before it is detached from the mercury thread. This may cause an increase in the surface area of the electrode and perhaps stirring of the solution at the diffusion layer. These photographs (Figures 4 and 5) indicate that the t controller is performing satisfactorily at drop times of 0.25 sec and longer. Although shorter drop times can be reproduced and used with the apparatus (in fact, drop times as short as '/sa second can be reliably obtained), it is doubtful that the usual polarographic equations for diffusion-controlled conditions can be applied to these short drop times. Although the shorter drop times may be useful for amperometric titrations and for other situations where it may be desirable to monitor the current in stirred solutions, drop times shorter than 0.25 sec were eliminated from the final design of this

apparatus for normal polarographic work. The photographs presented here were taken with a Smoler 90" vertical orifice DME. Precision of Controlled Drop Times. Several measurements were taken to determine whether the precision of the t had been increased by using the t controller. The electronic counter was connected to the current amplifier output of the polarograph. The counter was turned on and off automatically by sets of two successive current pulses. However, the drop times were sampled randomly, so the data do not represent alternate drop times only. The cell was filled with 0.1M KCl and sparged until oxygen-free. The data in Table I show the precision of the t , uncontrolled and controlled, for both the horizontal orifice DME and the Smoler 90" vertical orifice DME. The relative standard deviation of the drop times was improved by a factor of 20 to 40 by the t controller. A significant advantage, the constancy of t with varying electrode potential, is clearly shown in the table. Summary of Performance. Superior precision and sensitivity are obtained with this DME t control apparatus in conjunction with the ORNL Model 4-2792 polarograph (7). At a controlled t of 0.5 sec and with scan rates of 1 or 3 V/min, regular or first- or second-derivative dc polarograms can be recorded in -30 or 10 sec. At the millimolar concentration level, triplicate regular or derivative polarograms superimpose within the pen width of the XY recorder; the relative standard deviation of wave and peak heights is 0 . 2 x . In contrast, the precision attainable with first derivative dc polarography with uncontrolled t DME's is 0.6 (6). Useful first- or second-derivative dc polarograms are obtained at concentrations as low as 1 X lO-'M. With uncontrolled t DME's and with the ORNL Model Q-1988-FES polarograph, data were reported for the relationship between concentration and second derivative peak height over the range of 5 X lo-' to 1 X 10+M (12). The drop times are independent of potential. Short drop times are available at low m values. Longer drop times are available for other applications. Another important advantage of the use of this apparatus is that good results are obtained even in the presence of sporadic mechanical shocks and vibration at the cell. Construction Information. Detailed circuit and mechanical drawings, specifications, and parts lists for the construction of this apparatus (4-2942 series) are available (11). Also, check-out and test procedures and operating instructions have been prepared (13). ORNAL Master Analytical Manual Method nos. 1 003044 and 9 003044. Copies of this VOL. 41, NO. 6, MAY 1969

785

method in limited numbers can be obtained on request from Director, Analytical Chemistry Division, Oak Ridge National Laboratory, P. 0. Box X, Oak Ridge, Tenn. 37830. Mechanical drawings for the construction of this apparatus were prepared on the basis of the prototype. Subsequently, two units were fabricated at ORNL from these drawings. Also, a 4-2792 polarograph and this 4-2942 apparatus were built from the ORNL drawings in Australia and perform very well (15). The satisfactory operation of these units shows

A substantial portion of the work to establish the conditions that result in optimum performance of this apparatus was done by W. L. Maddox, Oak Ridge National Laboratory. R. W. Stelzner and T. R. Mueller, ORNL, have also contributed to the project.

w. L. Belew, Oak Ridge National Laboratory, Oak Ridge, Tenn. (temporary assignment, Australian Atomic Energy Commission Research Establishment, Lucas Heights, N.S.W., Australia), personal communication, July 15, 1968.

R~~~~~~~for review February 26, 1968, Accepted March 5 , 1969. Research sponsored by the U S . Atomic Energy Commission under contract with Union Carbide Corp.

(15)

that the drawings are adequate for the duplication of this apparatus. ACKNOWLEDGMENT

Preparative Thin-Layer Chromatography and High Resolution Mass Spectrometry of Crude Oil Carboxylic Acids Wolfgang K. Seifert Chevron Oil Field Research Co., P.O. Box 1627, Richmond, Calif. 94802 Richard M. Teeter Chevron Research Co., Richmond, Calv. 94802

A carboxylic acid fraction of high interfacial activity isolated from a California crude oil was subjected to preparative thin-layer chromatography (TLC). The complexity of this fraction was reduced by TLC to about 1500 compounds, many of which belong to homologous series. Proof for absence of contamination was obtained. High resolution mass spectrometry studies of these carboxylic acids and derived trihydroperfluoroheptyl esters were performed. Based on exact masses of parent ions and fragments and on relative ion abundances, the presence of terpenoid, polynuclear saturated and mono- and polynuclear aromatic as well as naphtheno-aromatic ring structures is indicated. Comparison of acids and esters shows that the most abundant species contain 2, 3,4, and 5 saturated rings and fused polynuclear structures. The presence of many compound classes of carboxylic acids not discovered previously in petroleum is postulated.

A PREVIOUS paper ( I ) describes the isolation and separation of carboxylic acids from Midway Sunset 31E, California crude oil by countercurrent extraction, ion exchange, and silica gel chromatography. These techniques succeeded in separating phenols from carboxylic acids. However, the purest fractions were still too complex for direct identification by high resolution mass spectrometry. The thin-layer chromatographic (TLC) separation of petroleum products (2) in general, and, more specifically, of carboxylic acids (3) on silica gel has been described. This communication deals with preparative separation of the best fraction by thin-layer chromatography followed by high resolution mass spectrometry of carboxylic acids and derived fluoroalcohol esters (4). (1) W. K. Seifert and W. G. Howells, ANAL.CHEM., 41,554 (1969). (2) F. C . A. Killer and R. Amos, J. Inst. Petr., 52, 315 (1966). (3) J. C. Kirchner, “Techniques of Organic Chemistry, Vol. XII, Thin-Layer Chromatography,” E. S. Perry and A. Weissberger, Eds., Interscience, New York, pp 17, 243. (4) R. M. Teeter, ANAL.CHEM., 39, 1742 (1967). 786

ANALYTICAL CHEMISTRY

The fraction analyzed amounts to 5% of all carboxylic acids present in this crude oil and is representative of a larger percentage because of overlap of compound classes in various fractions. Because of the relative position (nonpolar portion) of the fraction in the total separation scheme, most of the structures postulated in this carboxylic acid fraction are free of heteroatoms. Besides the observation of a great number of compound classes of carboxylic acids not discovered previously in petroleum, the present study represents the first semiquantitative attempt to analyze a fraction of the carboxylic acids relative to a total virgin crude oil. EXPERIMENTAL Preparative Thin-Layer Chromatography. Reagents. The silica gel was Grade SG-DF-5 produced by the Camag Co. It contains 5 % calcium sulfate as a binder and an inorganic fluorescent indicator (3). The development solvent was a mixture of 0.5% distilled acetic acid (Baker and Adamson), 1% methanol (Mallinckrodt Chemical Co.), and 98.5 benzene (Baker and Adamson). The latter two were purified before use by passage through 28-200 mesh silica gel (Grace/ Davison). The ether (J. T. Baker Co.) was purified similarly with alumina (Merck Reagent Grade). All solvents were free of residue on evaporation. Separation. A slurry of 40 grams of silica gel in 84 ml of water was applied to eight glass plates (20.3 cm square), with a thin-layer spreader (Research Specialties Co.) producing a gel layer of 250-micron thickness. The plates were allowed to stand for 10 minutes at room temperature and thereafter for 1 hour at 105 “C and then in a desiccator for 2 hours. Carboxylic acid Fraction D-4 (53.9 mg), whose isolation from Midway Sunset 31E, California crude oil has been described in a previous paper ( I ) and whose separation is shown again in Figure 1, was dissolved in 0.5 ml of benzene and charged to the plates by the spot method. Development of the plates with the above-described solvent mixture was carried out by the ascending method and led to the formation of three separate zones. Zone 3 (Table I and