ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
a correlation coefficient r2 = 0.99996. Extraction yields obtained by the flow-blending and the Soxhlet method are closely correlated as shown in Table 111. Regression of Soxhlet (y)vs. flow-blending ( x ) values yielded y = 0.8852 - 84.65 ppm. The correlation coefficient r2 of the data was 0.9886. As shown in Figure 3, efficiency of copper powder as a reagent for removing elemental.sulfur was markedly increased as alumina or carbonaceous rock was added. One gram of copper powder added to the extraction mixture will remove up to 150 mg of free sulfur.
t
1
,
0
, 2
0
,
, 4
.
, 6
.
, 6
665
*
io
&
is
lo 12 l4 Extraction t i (min)
ACKNOWLEDGMENT We express our appreciation to W. Seemann for providing the samples and C. Cornford for reading the manuscript and proposing the name "flow-blending'' method.
Figure 2. Influence of extraction time upon extraction yield
LITERATURE CITED
0 0.0
,
,
0.2
,
,
:
,
,
,
.
,
.
,
\ .
,
.
0.4 0.6 0.8 1.0 1.2 1.4 A m w n t of c m added l o thn sobtion (g)
,
1.6
.
-
Figure 3. Sulfur removal by copper powder. ( 0 )Copper only, (A) copper and alumina, (H) copper and carbonaceous rock
lacking in that the surface area of sediment particles efficiently washed by percolating solvent may decrease markedly during extractions due to the compaction of the sample or the channeling of the solvent (2). Problems arising from insufficient contact between particles and solvent have been overcome by application of the flow-blending method. The sample is finely dispersed in the circulating solvent by the action of the rotor-stator system. Particle surface area is increased rather than decreased by the impact of shear forces and cavitation as particles pass the blending cell. Kerogen degradation which may occur during extractions using high energy ultrasonics (11)and which can blur the determination of the extraction end point, was not observed using the flow-blending method. The yield of extract as a function of sample weight was determined. A least-squares fit of data from Table I1 gave a regression equation y = 5.501 X 10-3x 0.793 X g with
+
L. C. Craig and D. Craig in "Technique of Organic Chemistry", Voi 111, Part I, "Separation and Purification", A. Weissberger, Ed., Interscience Publishers, New York, N.Y., 1956,p 149. W. S. Ferguson. Bull. Am. Assoc. Pet. Geoi.. 46, 1613 (1962). J. L. Oudin, Rev. Inst. Fr. Pet., 25,(l),3 (1970). V. T. Vuchev, W. G. Howells, and A. L. Bwlingame in "Advances in Organic Geochemistry 1971",H. R. v. Gaertner and H.Wehner, Ed., Pergamon Press, New York, N.Y., 1972, p 365. H.Dembricki, Jr., W. G. Meinschein, and D. E. Hattin. Geochim. Cosmochim. Acta, 40, 203 (1976). J. M. Hunt in "Advances in Organic Geochemistry 1973".B. Tissot and F. Bienner. Ed., Editions Technip, Paris, 1974,p 593. J. W. Farrington, S. M. Henrichs, and R. Anderson, Geochim. Cosmchim. Acta, 41,289 (1977). E. D. John and G. Nickless, J . Chromatogr., 138,399 (1977). D. G.Peters, J. M. Hayes, and G. M. Hieftje in "Chemical Separations and Measurements: The Theory and Practice of Analytical Chemistry", W. B. Saunders Company, Philadelphia, Pa.. 1974, p 504. M. Vandenbroucke, Ref. 4,p 547. R. D. McIver, Geochim. Cosmochim. Acta, 26,343 (1962). M. T. J. Muphy in "Organic Geochemksw", G. Eglinton and M. T. J. Murphy, Ed., Springer Verlag, New York, N.Y., 1969, p 74. C. Golden and E. Sawicki, Anal. Lett., 9,957 (1976). J. G. Palacas and A. H. Love, U . S . Geoi. Survey Prof. Pap., 800-D,
D67 (1972). M. Blumer, Anal. Chem., 29, 1039 (1957). W. N. Tuller in "The Analytical Chemistry of Sulfur and its Compounds", Part I, J. H. Karchmer, Ed.. Interscience Publishers, New York. N.Y., 1970, P 1. P. Gearing, J. N. Gearing, T. F. Lytle, and J. S.L*, Geochim. C o s m h i m . Acta. 40, 1005 (1976). D. H. Welte, Geol. Rundsch., 55, (l),131 (1965). E. D. Evans, G. S. Kenny, W. G. Meinschein, and E. E. Bay, Anal. Chem.,
29, 1858 (1957). R. M. Cassidy, J . Chromatogr., 117,71,(1976). J. E. Baer and M. Carmack. J . Am. Chem. SOC.,71, 1215 (1949). D. Leythaeuser, Geochim. Cosmochim. Acta, 37, 113 (1973).
RECEIVED for review July 26, 1977. Accepted November 4, 1977. Work supported by the Government of the Federal Republic of Germany, Energy Research and Development Program No. 3.2.1.
Computer Pnterfaceable Potentiostat Basil H. Vassos" and Guillermo Martinez Chemistry Department, University of Puerto Rico, Rio Piedras, Puerto Rico 0093 1
The present design is of a potentiostat suitable for use in automated environments, for example in computer-controlled electrochemistry. The requirements for such an instrument are: stability in operating with a variety of cell configurations, absence of manual adjustments, and a balanced correction of various errors. This last feature is important in a general purpose instrument, since various error corrections tend to 0003-2700/78/0350-0665$01 .OO/O
operate a t the expense of each other. The features we considered in this design were the following. (a) Uncompensated Resistance Correction. There is a long history (1-5) of preoccupation with this correction. Although complete computer correction of this fault is possible (6),most potentiostats use positive feedback for this purpose, which brings the system just short of self-oscillation. We 0 1978 American Chemical Society
666
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978 R2
E)e
iei
R l 22K
IK
O+E
+15V -1sv 0 Y n
7.5 K
7. 5 K
4.7M R 3
Cz
300pF
-15v +15v 100 p +
2.2 K
)-
F
-
-
OUTPUT
*
L
c
r
I
c 5
I
CELL
I
c
Figure 1. Schematic of the potentiostat. All diodes are type 1 N 4448. T,, T, are TIP 29, while T, and T, are TIP-30 transistor (Texas Instruments). All capacitors are ceramic disk. A, is a 741, all the other operational amplifiers are 40J (Analog Devices). The 2-kR variable resistor is a Helipot. The power supply used is a model giving 350 mA at & 15 V. The relay S, is a model SIGMA 191TE 1 A 1-5s
eliminated this feature for the sake of automatic use under changing conditions, but the feature can be added later. Note also that many recent application of potentiostats (1-9) still use the classical three-amplifier system (10) and no particular correction, with acceptable results. (b) Matching of Load a n d Controller. Another procedure that was considered and discarded is the electrical matching of potentiostat and cell. While this can result in spectacular performance ( 2 , I I ) we preferred to have a system that behaves satisfactorily under a wide range of conditions rather than optimally for only a particular load. (c) E/I Gain Switching. A rather serious limitation in the response time of the system is the current limitation caused by the current-measuring resistor. For example, at the 1-FA range, a 100-kR current measuring resistor is needed for a 100-mV output. With this resistor, F capacitance is of 1 V the slewing rate on charging a ~ - Fcell in 10 ms which is too slow for many purposes. In order to improve the rise time, one can use one of three methods: (1) Decrease the value of the current-measuring resistor. This means a lower output of, for example, 1 mV full scale, which causes a degradation in the S/Nratio. (2) Use of a Zener diode or a similar nonlinear device in parallel with the current-measuring resistor. The approach is acceptable except for some noise generated in the Zener region and for the reverse leakage of the diode. (3) Use of a switching system (the approach we took), to substitute a short circuit during the periods when actual current measurements are not made. In our system, the I / E function is done by amplifier A4 in Figure 1, the switching system (clamp) is done by the relay controlled by a logic signal. Upon receiving a logic “0” input,
the instrument operates normally, while a “1” input shorts out the I / E resistor. The major utility of such clamps is when the signal applied involves large transitions, at high sensitivity settings. In such cases, the “rise time” can become very large in the sense that the system saturates for a long period while pumping slowly the charging current through the current measuring resistor. In Figure 2b is shown, such a case where the output of the I / E amplifier stayed at the saturation (10 V) for the whole rise time. Note that during this time, the summing point lifts off-zero and gives a false cell potential for the whole saturation period. In contrast, when the signal (c) was applied to the cell, which was switched to a high current mode (clamp) for a few milliseconds, the faradaic current emerged clearly with practically no charging current left (d). Note that the clamp need not be as long as the one used above, usually one millisecond or less will suffice. Note also the elimination of the saturation when using the clamp. This logic controlled system is useful if the potentiostat is driven by a controller such as a digital computer. More modest setups, as for instance using square wave generators to drive the potentiostat, can also be coped with. A single integrated circuit, a monostable, can provide a short clamping pulse at each transition for momentary gain switching and accommodate the steep rise times of the square wave without any difficulty. (d) Use of a Pre-Filter. Due to various limitations, for a general purpose instrument, a rise time of 10 ~s and a frequency band up to 50 kHz are both a realistic and an adequate design target. This allows for over 1000 V/s cyclic voltammetry rate (12). Note also that general purpose cells will have uncompensated RC time constants of several microseconds, indicating no real need for speeds beyond the 10
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
667
t
"t
a
b
Figure 3. (a) Example of crossover distortion. (b) The square wave response at minimum sensitivity E l I mode. The amplitude is 1.2 V and the repetition rate is 5 kHz. Most of the rounding off of the square wave comes from double layer charging. A purely resistive dummy cell gives a very clean practically square output
IV
I
b
I I I I
I
(0
Figure 2. Example of clamp. (a) Applied pulse, (b) Output of I / € showing saturation during the entire pulse and for some time afterwards, (c)clamp: a "window" type clamp, that opens to read during the time of interest and keeps the output clamped otherwise, (d) Resulting output
rise time mentioned above (13). This being the case, it is very advantageous to limit the frequency response of the signal before it is applied to the potentiostat. The reason is that the operational amplifiers, when submitted to rise times faster than they can accommodate, enter in a special form of saturation caused by the absence of the negative feedback. During this time, the amplifier simply does not respond to the input wave-form, but sails instead on the same course regardless of the input. This is followed by oscillation on reaching the top of the transition, and eventually the system settles. This type of misbehavior has been long recognized in audio engineering as transient intermodulation distortion, TIM. A large feedback capacitor does not resolve the problem too well, even though it removes some of the nonlinear behavior present by acting as an active filter. The solution we adopted is to design the system, not only the potentiostating amplifier, for the desired response. For this purpose the amplifier A,, in input follower, was given the wave-shaping function by means of the RICl circuit. It limits the waveforms to a rise time of about 5 ps. Once this wave shaping burden is removed from the potentiostating amplifier A2, the latter can be easily designed for optimal stability. At no time now is A2 limited by its slew rate; consequently the transient distortion disappears. In its place appears some frequency band limitation, but this is now done by smooth linear elements RIC1, and not by a saturated amplifier. Another consequence is that it is now possible to use more freely phase-lead compensation, in order to correct the crossover distortion (see section e below). Since phase lead increases with frequency, it is difficult to avoid instability or ps
oscillations if steep waveforms are encountered; as it is, the system is stable with only 100 pF of phase lag compensation, capacitor C5, and the phase lead discussed below. Additional stability elements are provided by R3 and Rd. I t has been shown (14, 15) that instability occurs in some systems at high frequency and high load resistance. Consequently we provided for a minimum load of 100 i?, which was found to improve the transient behavior, even though it is a bit wasteful of power. It can be left out without ill effects. The resistor R3 serves to limit the effective open loop gain of A2 to about 500. It might appear unwise to cut so low the gain (of 200000) of the amplifier used (405). In reality, because of gain-bandwidth considerations, this amplifier maintains the full gain only up to about 10 Hz, after which it drops continuously. By the addition of R3, the response becomes flat (at a lower level) from dc into the kilohertz region. This smooth frequency response makes for a much cleaner transient behavior. Of course an error is introduced, proportional to the reciprocal of the new gain, but l/m is quite an acceptable precision (16). It must be also mentioned that, for really stable operation, one must select very carefully the amplifier types used. We found the ubiquitous 741 and the Analog Devices type 405 to be very satisfactory. Many other amplifiers showed various dynamic idiosyncrasies. In any case it is very advisable to always use the same type of amplifier for A2, AB,and A,. (e) Crossover Distortion. This type of error is produced by the finite time needed to switch back and forth from the NPN to the P N P transistors in the power stage, and generates a hesitation on a sine wave on crossing zero, visible as a bright dot on the scope. At higher frequencies and very small amplitudes, around zero, the effect can be so strong as to make the response unrecognizable. This form of distortion did not receive too much attention in potentiostat design, since it is visible only in the particular conditions mentioned above (Figure 3a). The crossover distortion is best fought by some phase-lead compensation. In our case, C3 and C4 prove to be adequate. An alternate approach would be to increase the current into diodes D1 and D2 which will ensure a continuous biasing on of the transistors T1,and T2,and to decrease the value of their emitter resistor. This approach though, we found out, could lead into self-destruction of T, and T,. Some potentiostat designs used one single power transistor, with elimination of the cross over between P N P and NPN (17), a t the expense of versatility. Use and Performance. The potentiostat can be used manually a t a fixed voltage by selecting with switch S1,the internal voltage source, the 2-K helipot. In the same time switch S2 must be connected to "OV" to remove the clamp.
668
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
For automatic use, the control of the relay is done remotely, and a control voltage is introduced through SI. The data a t the output is attenuated to 1 mV for recording purposes; this can be changed to suit. The potentiostat was used under various conditions and showed exemplary behavior a t small amplitude around zero. For larger transitions, the clamp could cope easily will 1-V steps on 5-pA scale. An example of transient behavior, in the absence of clamping, is shown in Figure 3b where the response of the system is shown a t the highest current range. The cell solution used was 0.01 M K2S04 and 0.01 M H2S04and the working electrode was of platinum wire. At 1.2-V square wave produces about 10 ps of rise time with moderate overshoot and no instability. Large transitions a t low current setting are accommodated by clamping. The relay used allows about 1 ms of reliable operation which is satisfactory for most applications. In conclusion, the present design, a t low signal amplitude and with small current measuring resistors is able to operate within about 3% dynamic error to about 30 kHz sine and 50-ps rise times. Useful response extends to 60 kHz sine waves and 10-ps rise times. Low frequency operation is error free. When using large voltage steps in dilute solutions and a high current measuring resistor, the effective rise time is maintained to about a millisecond by clamping. The speed of the clamping relay (0.2 ms) is here the limiting factor. FET switching was experimented with and shown to give faster response, but we prefer the relay for reasons of simplicity and reliability.
The authors have more detailed construction details available for persons interested in duplicating the construction, as well as some details about preliminary work to program completely automatically the gain of the I / E section.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)
(IO) (1 1) (12) (13) (14) (15) (16) (17)
F. Haber, Z . Phys. Chem., 32, 193 (1900). G. L. Booman and W. B. Holbrock, Anal. Chem., 37, 795 (1965). R . R. Schoeder and I. Shain, Chem. Instrum., 1, 233 (1969). J. E. Mumby and S. P. Pernne, Chem. Instrum., 3, 191 (1971). C. Lamy and C. C. Herrmann, J . Electroanal. Chem., 59, 113 (1975). C. Yanitzki and Y. Friedman, Anal. Chem., 47, 880 (1975). K . S. Stuiick and V. Hora, J . Electroanal. Chem., 70, 253 (1976). J. P. Van Dieren, B. G. W. Kaars, J. M. Los, and B. J. C. Wetsema, J . Electroanal. Chem., 68, 129 (1976). J. Deroo, J. P. Dard, J. Guitton, and B. Le Gorrec, J. Electroanal. Chem., 67, 263 (1976). M. T. Kelley, H. C. Jones, and D. J. Fisher, Anal. Chem., 32, 1263 (1960). See ref. 4 . J. L. Anderson, Chern. Instrum., 7, 25 (1976). K. B. Oidham, J . Electroanal. Chern., 11, 171 (1966). D. K . Roe, Chem. Instrum., 4, 15 (1972). J. E. Davis and E. C. Toren. Anal. Chem.. 46, 647 11974) B. H Vassos and G. W. Ewing, "Analog and Digital Electronics for Scientists", J. Wiley, New York, N.Y., 1972, p 163. T. S. Randhawa and R L. Snthwell. Analyst(London), 100, 726 (1975).
RECEIVED for review September 7, 1977. Accepted December 7 , 1977. Acknowledgement is made to the Office of Coordination of Research and to the Center for Energy and Environmental Research of the University of Puerto Rico for financial support.
Technique for the Prevention of Column Contamination in Pyrolysis Gas-Liquid Chromatography Annabel Mitchell and Manuel Needleman" Victorian College of Pharmacy, 38 1 Royal Parade, Parkville, Victoria, Australia, 3052
There has been a n increasing interest in the past decade in the application of pyrolysis gas-liquid chromatography to the identification of microorganisms. It is surprising, therefore, to observe that the problem of column contamination has been mentioned only recently ( I , 2). The contamination, which evidences itself as a tarry deposit on the column after approximately 100 h of column use: results in a loss of resolution, with a concomitant decrease in long-term reproducibility (2), and is most marked when capillary columns are used. Attempts have been made to solve the problem by repacking (3)or removing ( I ) the first section of the column. Alternative proposals have been to use long precolumns ( I ) and backflushing of the column after each chromatographic run (2). Experiments using the latter idea showed that high boiling contaminants still built up insidiously on the column, and a significant fraction, once deposited, was resistant to backflushing. Therefore, the final successful arrangement, shown in Figure 1,incorporated a short disposable precolumn and effectively amalgamated the above-mentioned proposals.
EXPERIMENTAL All apparatus and procedures, except for those detailed below, have been previously described (2). The glass precolumn shown in Figure 1was obtained by cutting a 1-m length from a column similar to the main SCOT column. The columns were connected with glass-lined steel tubing, which was also used for the ancillary plumbing. The diagram shows how closing the valve ( M N W needle valve, Scientific Glass Engineering, Melbourne) changes the gas flow 0003-2700/78/0350-0668$01.OO/O
IO cmymi:
DETECTOR
INLET
VALVE 5cm3/mi; INLET
GAS FLOW DURING ELL'TIOS
L
5 c m min
INLET
INLET
Figure 1.
I
t
VALVE CLOSED
GAS FLOW DURING BACKFLUSHING
Backflush plumbing arrangement
pattern from the elution to the backflush mode. The sequence adopted was to raise the temperature to 180 "C, after elution was complete, then to remove the pyroprobe from inlet A, leaving it open, and to backflush for 1 h. RESULTS AND DISCUSSION This technique has been in operation for over 500 h of column use and no contamination of the main column has been observed. After about 200 h of use, the first 5 cm of precolumn showed some discoloration but, because of the disposable nature of the precolumn, this has not proved t o 1978 American Chemical Society
