Table I.
Analysis of Adipate Ester Mixture
Known Component Mol? % w t . yo Unknown Unknown Dibutyl adipate .35 9 28 7 Dibutoxyethyl adipate 31 3 33 4 Diethylhexyl adipate 32 7 37 4 Column temperature, 2-13' C., detector temperature, 310" cc./min. * Column temperature, 259" C.. detector temperature, 320' cc./min. Operation in the region of 4.50' C. or higher may be possible, if thermal decomposition or rearrangement is not limiting. At these temperatures. gas chromatography may prove competitive mith molecular distillation for the resolution of natural products.
by Area Method Run 1" Run 2b 0 8
0 29 31 38 C.;
2
0 7
0 1 26 8 1 33 2 2 39 3 helium carrier, 63
6
C.; helium carrier, 63
LITERATURE CITED
(1) .Ashbury, G. K., Davles, -1.J., Drinkwater, J. W.,.4bstracts, p. 16B, 129th lIeeting, .ICs, Ilallas, Tex., April 1956. ( 2 ) cropper, F. R.,~ ~ ~.I, .\-uture ~ , 172, 1101 (1953).
(3) Zbid., 174, 1063 (1954). (4) Cropper, F. R., Heywood, -\., in "Vapoiir Phase Chromatography," D. H. Desty, ed., p. 316, Academic Press, S e w York, 1957. (5) Dijkstra, G., Keppler, J. G., Schole, J. .I.,Rec. trazi. chzm. 74, 805 (1955). (6) Dimbat, ll., Porter, P. E., Stross, F. H., , \ S A L . CHEJI. 28, 290 (1956). ( 7 ) Iieppler, J. G., Dijketra, G., Schols, J. in "1-apour Phase Chromatographr," D. H. Destv, ed.. p. 222, Academic Press, Sew Torl;, 1957. (8) Keppler, J. G., Schols, J. -I., Dijkstra, G., Rec. truz,. c k m i . 75, 965 (1956). (9) Killiams, E. F., hbstracts, p, l5B, 129th Meeting, ACS. Dallas, Teu., -\priI, 1956. RECEIVED for revien- Sovember 30, 1956. Accepted January 6, 1958. South\\-ide ~ Chemical ~ d Conference, , 18-4-dCS,Memphis. Tenn., Ilecember 6, 1956.
Trace Analyses by Gas Chromatography C. EUGENE BENNETT, STEPHEN DAL NOGARE, L. W. SAFRANSKI, and C. D. LEWIS Polychemicals Deparfmenf, Du Ponf Experimenfal Sfafion, E. 1. du Ponf de Nemours and Co., Inc., Wilmingfon, Del.
1
A gas chromatography apparatus suitable for analyses of impurities a t the parts per million level utilizes an amplifier to increase the signal from a thermistor detector. Techniques necessary for obtaining low noise and drift levels are described. Several applications of the apparatus to trace analyses are discussed.
T
determination of trace coniponents (1 to 200 p.p.ni.) in organic mixtures usually requires the development of a specific method. The successful application of gas chromatography to the study of numerous complex mixtures suggested that this technique could be extended to trace analyses. Two logical approaches to the problem were to find a more sensitive detector or to amplify the signal obtained from an available, stable detector. After a survey of the characteristics of existing detectors [radiation (1-3), gas density balance (?), hydrogen flame ( 8 ) ,gas-flojT- impedance ( d ) , surface potential (4), infrared] thermal conductivity, etc.], the combination of a thermal conductivity detector utilizing thermistors as sensing elements and an amplifier was selected as the best practical combination. This choice was based primarily on the high signal-noise ratio of thermistors and the availability of a suitable amplifier. The thermistors used were inexpensive, readily available, and usable a t temperatures u p to 150" C. HE
898
ANALYTICAL CHEMISTRY
The amplification of the bridge signal and the conditions necessary for stable operation of a chromatographic apparatus are discussed. Several analyses are presented to demonstrate the potential of this technique in organic analysis. APPARATUS
The assemdled apparatus consisted of a controlled gas source, sample inlet, thermal conductivity cells, column, flowmeter, vapor jacket, and the necessary circuitry for detwting, amplifying, and recording the detector signal. General Operation. The carrier gas,, continuously supplied from a helium cylinder, f l o w d through a preheater, reference thermistor chamber, sample inlet, U-shaped column, over the detector, and through a flo~vmeter to the atmosphere (Figure 1). The two thermistors were part of a Kheatstone bridge (Figure 2 ) , in which
the unbalance signal was constantly amplified and measured on a recording potentiometer. K i t h the carrier gas flon-ing and the bridge a t balance, the sample was injected by means of n syringe int'o the gas stream. The vaporized sample passed onto the column and the components of the mixture were carried through the column a t different rates. They arrived separately a t the exit of the column and the detecting cell. The appearance of sample vapor in the det'ecting cell produced a change in thermistor temperature and a corresponding resistance change proportional to the concentration of solute vapor in the carrier gas. The unbalance in the Wheatstone bridge resulting from the resistance change was recorded as the chromatogram. Flow Control. A cylinder of helium fitted with diaphragm-type reducing valves supplied the carrier gas a t the desired flow rate. The gas from t h e cylinder flowed through a preheater
He
1 He
Figure 1.
Schematic diagram of detector block, column, and vapor jacket
and then directly to the inlet of the detector block. A rotameter was attached to the exit tube to give a n approximate indication of the flow rate. A more precise flow value was obtained by measuring the displacement of a soap bubble in a calibrated glass cylinder connected to the exit end of the rotameter by means of a short length of tubing ( 5 ) . Temperature Control. T h e preheater for adjusting t h e incoming gas stream t o operating temperature consisted of 50 feet of copper tubing inch in outside diameter, coiled around the glass vapor jacket. The jacket (Figure 1) had a n internal diameter of 65 mm. and was about 28 inches long. The jacket with the attached preheater was nrapped with a double layer of l, 2-inch Fiberglas insulation. A 200-nil. round-bottomed flask, which held the reflux liquid, was heated by a heating mantle. The jacket teniperature and consequently the block and colunin temperature n c w determined by the refluxing liquids used. Suitable material. for covering the operating range of the apparatus arc: O
c.
IXchloromethane 41 11ethanol
Ethanol Benzene
64
T8 80
Table I.
Experimental Conditions Temp., Helium Flow, Ml./Nin. Analysis Liquid Phase' c. 22 Isopropyl alcohol in benzene 25% Kujol 64 62 Benzene in toluene 25Yc tetraethylene 64 glycol dimethyl ether 2 4 5 Carbowas 1540 and '2.5% Sorbitol 25'5 Carbowax 400
Cyclohevanol in toluene Methanol in water Impurities in cyclohexane Impurities in toluene a
25% S u j o l
25y0tetraethylene glycol dimethyl ether On Celite 545, graded to 80-100 mesh.
ATTENUATOR
c.
ATTENUATOR
Tricahluroethylene 87 \I-ater 100 1-Octcne
hnisole
126 155
At a recorder range of 100 p v . , the time required for equilibrium after a 50" C. temperature change w i s uhout 3 hours. The importance of good temperature control cannot' be overenipliasized. as slight temperature c,lianges are readily tlt$ected a t the 1 0 0 - ~ vrange. . For es:imple. unless the metal injection port axid other exposed metal parts outside the jacket are well insulated, a signal drift due to ambient t,enipcrature fluctuations will be observed. To minimize the amount of insulation iircessary, polyethylene tubing was used from the helium cylinder to the preheater and from the exit of the block t,o the flonmeter. The remaining exposed parts were kept to a minimum and insulated n-it,h Fiberglas. A short piecc of polyethylene tubing was placed over t'he injection port and insulation n.mpped about the tubing. The port was readily accessible and a t the same timc. protectrd from anihient teniperaturr changes. The reflus flask, placed about 1 foot w\vay from the jacket, was connected to the jacket by a glass arm. This arrangement eliminated much of tlie temperature fluctuations near tlie inlet, Column Materials. Interchangeable stainless steel columns, inch in outside diameter and 4 t o 6 feet long. were used. Colunin packings were prepared from 80- to 100-mesh Celite 545 (Johns-~Ianville Corp.), on which \vas deposited 25% of the immobile liquid phase (Table I). Detector Block and Thermistors. T h e sensing elements were matched,
n Figure 2. Schematic wiring diagram for thermal conductivity detector A.
E. C. D.
Reference thermistor Detector thermistor Coarse bridge adjustment, 25-ohm Helipot Fine bridge adjustment, 5-ohm Helipot
mounted thermistors (Type A1 i I, T'ictory Engineering Corp., Union, K. J.) with 2000-ohm resistance a t 25' C. The high negative temperature coefficient of resistance (about 47, per O C.) and inherent stability of thermistors provide the high signal-noise ratio necessary in high sensitivity work. The thermistors were mounted in a cylindrical stainless steel block (63 nmi. in diameter and 50 mm. long) by means of a nicchanical pressure seal (Figure 1), nhich was obtained by placing the thermistor mount betn-een a silicone rubber gasket, placed on the !vel1 shoulder, axid a gasket of Teflon tetrafluoroethylene resin (E. I. du Pont de Seniours 8; Co., Inc.). An open-center screw retained and conipressed the awembly. The gasket of Teflon allowed the screw to slide on the thermistor mount without distorting the thermistor when pressure was applied. The effective gas seal n-as made by the silicone rubber gasket. The block n-as designed with a rliffusion well for the reference d e , so as to minimize the effect of slight flow changes. As the flow n a s more constant a t the end of the column, direct flow over the thermistor provided fast response n ith masiniuni sensitivit) .
Bridge Voltage 5 0 3 5
155
60
3.9
80
60
5.0
64
23
78
46
3.5 3 5
Bridge, Amplifier, and Recorder. The Kheatstone bridge (Figure 2). similar to the one in the Perkin-Elmer Model 154 T'apor Fractometer. consisted of a potential source, reference and detecting thermistors, tn-o fixed resistors, coarse and fine zero control, and a n attenuator. The fine adjustment control was located between a thermistor and fixed resistor to allow adjustment of the recorder pen position without changing the current through the thermistors. A bridge signal was preaniplified by a Liston-Becker Model 14 D C breaker amplifier (nominal input impedance of 50 ohms) and the increased signal was attenuated and fed t o a recording potentiometer. A Leeds 8; Sorthrup Model 9835-A amplifier \I as also suitable for this application. For stable operation, the amplifier gain n-as adjusted to produce a fullscale output signal of about 1 volt. d variable step-down attenuator (1000ohm Helipot or a fixed-resistor type attenuator similar to the one shown in Figure 2) vas then used to reduce the signal from the amplifier sufficiently to give full-scale deflection on a 10- or 100-niv. recorder. This attenuator also permitted the use of recorders of other millivolt ranges. Before a run was started, the bridge was brought to balance. With the fixed-resistor attenuator (Figure 2) a t the full signal position, the desired full scalp recorder range (usually about 100 pi-.) !vas anticipated and a corresponding test signal n as applied from the Liston-Reeker amplifier calibration dial. The gain control on the amplifier !vas adjusted to produce about 1-volt output for the test signal. The second attenuator was then adjusted so that the output test signal from the amplifier gave full-scale deflcction on the recorder. The test signal was then removed. Only signals less than about 300 pv. were linearly amplified by the amplifier. Consequently, the effectire full-scale range on the recorder 11-as selected to be less than this value. Then during or prior to a run, the attenuator between t h p bridge and amplifier n-as adjusted to keep the desired peaks on scale. Thus, by using the attenuation factor, linear results n ere obtained. VOL. 30,
NO. 5 , MAY 1 9 5 8
899
i
1\ 1 1
!
-I 1
I 0
IO
20 T I M E , MINUTES
30
p: w
a
Figure 3. Chromatogram of alcohol in benzene
70
The Liston-Becker amplifier as normally supplied does not have a feed-back circuit. Consequently, as the attenuator a t the bridge is changed, the gain of the amplifier changes, because of the change in impedance of the circuit. As a result, the attenuator factor must be calibrated for the system used. A feed-back circuit in conjunction with the amplifier is available from the manufacturer and eliminates this problem. The Leeds & Northrup amplifier has the feed-back circuit. The runs described in this paper cover the effective full-scale range between 40 and 1000 pv. However, a 25-pv. range has been used. The noise level of the bridge was about 0.1 t o 0.2 pv. and the drift about 5 to 10 p v . per hour. The drift was due mainly to temperature changes a t the thermistor block. Power Supplies. A lead storage battery was found to be very suitable as a potential source for the bridge. It was essentially free from voltage drift, which was an objectionable feature of dry cells. Two Model CV5 Transpacs (alternating current operated-direct current power pack, Electronic Research Associates, Inc., Kutley, N. J.), connected in parallel and used with a Sola constant voltage transformer, also gave a direct current source which did not drift but had an output ripple of about 3 pv. The above Transpac arrangement, however, provided a maximum of 5 volts and 30 ma. of current. When 5 volts was used with the described bridge unit, the power rating of these Transpacs would be exceeded a t block operating temperatures of about 100" C. and higher. Sampling. Liquid samples were injected through a silicone rubber diaphragm and into the column by means of an Agla micrometer syringe (Burroughs Wellcome and Co., Scarsdale Road, Tuckahoe, K. Y.) fitted with a 11/2-inch hypodermic needle (26gage). Glass wool was placed inside the sample chamber t o provide a heated surface t o promote rapid volatilization of the sample.
900
ANALYTICAL CHEMISTRY
p.p.m. of isopropyl
40
20
100 JJV RANGE
100 UV RANGE 0
5
1
IO
15
20
1
25
TIME, MINUTES
Figure 4. Chromatograms of cyclohexane a t 1 0-mv. and 100-pv.full-scale sensitivity ranges APPLICATIONS
The following summary of some specific applications shows the scope of high sensitivity gas chromatography. Column materials and other experimental conditions are described in Table I. In general, the results obtained with the high sensitivity unit were as satisfactory as those obtained with conventional apparatus. For example, by injecting six consecutive samples of 150 p.p.m. of isopropyl alcohol in benzene a t 200-pv. full scale sensitivity, a standard deviation of 1.4% relative was obtained for a peak height of 150. Two additional peaks with heights of 50 and 22, which were only partially resolved, gave standard deviations of 2.7 and 4.3% relative, respectively. With well resolved peaks a reproducibility to better than 2% relative was generally obtained. This error was believed to be due to the sampling technique. Isopropyl Alcohol in Benzene. Figure 3 illustrates the separation and analysis of 70 p.p.m. of isopropyl alcohol in benzene a t the 100-pv. sensitivity range; 1% of full scale represents 1 p.p.m. of isopropyl alcohol. By increasing the amplification gain or increasing the sample size, less than 1 p.p.m. could be detected. The other peaks in this chromatogram were not identified. The sample size in these runs was 10 pl. or approximately 8 X 10-3 gram. At the 1 p.p.m. level the signal resulted from only 8 X lo-$
gram of isopropyl alcohol. The baseline change occurring after the major benzene peak is probably due to adsorption on the thermistor and cell walls. After an hour or so the base line gradually returns to its normal position. This is generally observed a t high detector signal amplifications. The drift is sufficiently gradual to permit successive runs after the base line is adjusted. Benzene in Toluene. A toluene sample prepared from toluene sulfonic acid was found to contain about 5 p.p.m. of benzene. A standard sample prepared to contain 10 p.p.m. of benzene, produced 10% of full-scale deflection. The full-scale sensitivity n-as 40 pv. and the sample size was 20 pl. The sensitivity can be increased further. Cyclohexanol in Toluene. The smallest amount of cyclohexanol in toluene detected in a few preliminary runs was about 40 p.p.m. with a 10pl. sample and 100-pv. range. One factor limiting the sensitivity was that cyclohexanol appeared as a peak on the shoulder of toluene. The thermistor was less sensitive a t the operating temperature of 155OC. Methanol in Water. A 10-111. sample of a standard solution of 38 p.p.m. of methanol in water gave a 15-pv. signal a t the 100-pv. range. Consequently, less than 2 p.p.m. of methanol could be detected in such samples. A symmetrical methanol peak was obtained only after the Celite support was washed with hydrochloric acid, water,
0.8
1c
5
I I I 1
I
,
/
,
c
i F R O M SULFONIC
N.
4
ACID
w
K 4
1.5
I
a
.o-
I
0.5
I
I
1 \
-3 m P
Y
" 1
REAG.GRADE
c
J
= 2.0
l
I
a
Y
n
I
1
I
6.0
3.0 VOLTS
20
80 FLOW (rnl./rnin)
Figure 6. Variation of peak height and peak area with bridge voltage, helium flow, and temperature
1 1 1
0.8L
L
I
,
0
I REAG. GRADE
-
1
I
1
10
1
20
TIME, MINUTES
Figure
5. Chromatograms of four toluene samples
1;M sodium hydroxide, and again with n-ater. After drying a t 100" C. under vacuum, Carbowax 400, adjusted to an apparent p H of 7.8 with methanolic potassium hydroxide, was added. This treatment of Celite is recommended for polar materials that tend to form hydrogen bonds (6). Impurities in Cyclohexane. A good illustration of the potential of gas chromatography for trace analyses is shown in Figure 4, where chromatograms of cyclohexane a t 10-mv. and 100-pv. ranges are presented. Chromatograms I and I1 are typical runs that can be obtained with conventional gas chromatography apparatus. Runs I11 and I V were made a t a 100-fold increase in sensitivity. The impurity peaks, which are barely detectable in the first two chromatograms, are large and in some cases off-scale at the increased sensitivity. The total concentration of impurities, as determined by area measurements, was about 0.3%. Although no attempt was made to identify all the peaks, the impurity occurring after the cyclohexane peak was methylcyclohexane. The importance of this technique to trace analyses can be better appreciated when one realizes that no impurities Rere detected in the cyclohexane sample by infrared analysis.
Impurities in Toluene. In Figure 5, chromatograms of toluene conforming to ACS specifications from three sources are compared with a chromatogram of toluene made from toluenesulfonic acid. The results were somewhat surprising in the number and fairly large quantity (0.1 to 0.27, in Reagent C) of impurities that are present in the reagent grade material. The chromatograms shown in Figure 5 were made a t 1-mv. full-scale range. The impurity peaks n-ere off scale a t 100-pv. range. DISCUSSION
The results of a general study on the response characteristics of Type A l l 1 thermistors a t various voltages, temperatures, and flow values are summarized in Figure 6. Each point on the voltage-response curves is an average of duplicate values obtained with a 10p1. sample of n-hexane a t a constant carrier gas flow of 43 ml. per minute. The values on the flowresponse curves were obtained in a similar manner a t the optimum voltage setting for each temperature. The results show that 5 to 5.5 volts is a good compromise value for the bridge circuit (Figure 2) over the block temperature range of 41 O to 155" C.; a t constant flow, the peak area
decreases n ith increasing temperature n-liile the peak height goes through a maximum; the peak area is inversely proportional to the flow; and the peak height decreases slightly with increased flow over the range of 20 to 90 ml. per minute. Quantitative analyses are often made by comparing the height or area of peaks of the unknown to that of a standard. The present study emphasizes the need for well chosen operating conditions. For example, a t 41 O C. and 3.5 volts, a variation of 0.05 volt causes approximately a 5% change in both peak height and peak area; the same voltage variation a t 5 volts has a negligible effect. Likewise, well-controlled flow conditions are extremely important for comparison of peak areas, since peak areas are inversely proportional to flow. Thus, a t 20 ml. per minute, a variation of 1 ml. per minute in flow causes a 5% change in peak area; a t 50 ml. per minute a variation of 1 ml. per minute causes 2% change in peak area. The use of an internal standard, however, eliminates much of the need for such close control. Although the above study was made without an amplifier, the data are applicable to high sensitivity. For esample, the peak height obtained from 100 p.p.m. of isopropyl alcohol was doubled by merely changing the bridge voltage from 3.5 to 5.0 volts. The data in Figure 6 illustrate that the magnitude of the change depends upon the operating temperature. The optimum temperature varies for different compounds. Consequently, the optimum sensitivity can be obtained only by careful selection of bridge voltage, column temperature, and rate of carrier gas flow. Selection of Optimum Operating Conditions for Thermistors. The opVOL. 30, NO. 5, MAY 1958
901
tinium bridge voltage valves for obtaining maximum response from Type A l l l thermistors as shonn in Figure 6 are not directly applicable to a different bridge design or to different thermistors. I n order to establish optimum operating conditions for any type of thermistor in any giren bridge design, the following simple method can be used. The method involves plotting a bridge voltage-response curve or a bridge current-response curve obtained by injecting repetitive samples into the gas chromatography apparatus. Air is frequently a good standard to use. 4 curve having a maximum is obtained (similar to the ones in Figure 6 a t 41’ and 80’ C.) if sufficiently high current or voltage values are used. The curves obtained in Figure 6 a t 126’ and 155’C. did not shoIv this maximum, because of the limited bridge voltage applied. The maximum becomes less pronounced a t higher temperatures. The optimum operating bridge current or voltage value for maximum signal-noise ratio is at the top of the response curve. This value shifts to increased voltages at increased temperatures. -4s the absolute bridge voltage or current value for maximum signal from a given thermistor varies with the bridge design, a universal method for evaluating optimum thermistor response is needed. One such method involves the plotting of power dissipation in the thermistor against response in a manner similar to the bridge voltage response curves. Again a maximum is obtained corresponding to the optimum operating condition for the thermistor. The power dissipation is equal to the product of the current through the thermistor and the voltage drop across the thermistor. These current and voltage values are easily determined by placing a milliammeter in series with the thermistor and a high impedance voltmeter across the thermistor. A constant power dissipation value for maximum signal is obtained for the same thermistor in any bridge dfsign. The value also remains approximately constant a t various operating temperatures. I n addition, t h r optiniuni power dissipation value remains constant for other thermistors having similar temperature coefficients and physical size (similar dissipation constants). Different optimuiii values
902
rn
ANALYTICAL CHEMISTRY
v)
W
I
resistance thermistors n ithout as much amplification. Unfortunately, the noise level of the thermistor increases proportionally to the increased signal. Hon ever, a t higher temperatures the higher resistance thermistors are useful, as the high amplification required t o obtain parts per million sensitivity for low resistance thermistors causes noise due to soldered connections, etc., t o become significant.
301
u 20,
E t-
I
.
$?
W
I
ACKNOWLEDGMENT
The authors wish to acknowledge the extensive contribution made by Richard A. Parkinson, who performed a large part of the experimental Ivork described. LITERATURE CITED
IO 20 30 POWER DISSIPATION, MILLIWATTS
Figure 7. Power dissipation-response curves for 2000-, 8000-, and 80,000ohm thermistors are obtained with thermistors having different physical size. Power dissipation-response curves for 2000-, 8000-, and 80,000-ohm thermistors (Types A l l l . IX1039, and ilX1040, respectively, obtained from T7ictory Engineering Gorp.) are shon n in Figure 7 . The points on the curves nere obtained from repetitive injections of nhexane in the gas chromatography apparatus a t 64’ C. The bridge design used with each thermistor was similar to the one described in Figure 2, except that the fived resistors were changed to provide a n approximately equal arm bridge a t the operating temperature. I n addition, the attenuator n-as eliminated and the signals measured on a 500- to 1600-mv. variable range recorder. The same optimum power dissipation value n-as obtained for each type of thermistor as shown in Figure 7 . This constant value holds true only as long as the thermistors have similar dissipation constants. The signal from thermistors having similar characteristics increases approximately as the square root of the resistance. The data in Figure 7 show this to be true, as the signals obtained from 2000-, 8000-, and 80,000-ohm thermistors are in the ratio of 1:2:6. This increased signal from the higher resistance thermistors is helpful in obtaining equivalent sensitivity of Ion er
Boer, H., “Comparison of Detecting Nethods for Gas Chromatography Including Detection by Beta Ray Ionization,” Symposium on Vapor Phase Chromatography, Hydrocarbon Research Group, Institute of Petroleum, London, June 1966. Deal, C H., Otvos, J. W., Smith, Y. S . , Zucco, P. S.,A\.IL. CHEX.28, 1958 (1956). Evans, J. B., Killard, J. E., J . -4m. Chetn. SOC.78, 2908 (1956). Griffithe, J., James, D., Phillips, C., .Inalyst 77, 897 (1952). James. A . T.. 1Iartin. A. J. P.. Bio&em. J . 50, 679 (1952). ( 6 ) Kirkland, J. J., Grasselli Department, E. I. du Pont de Semours 8: Co., Inc.,,Ki!mington, Del., private communication. ( i )AIartin, .4. J. P., Brit. M e d . Bull. 10, 170 (1954). (8) Scott, R. P. W., .Ynlure 176,793(1965). RECEIVEDfor review March 20, 1957. Accepted December 7 , 1957. 9th Annual Delan-are Chemical Symposium, X’eTyark, Del.. Februarv 16. 1957. and Division of Anaiytical Chem’istry, ’ 131st Meeting ACS, AIiami, Fla., April 1957.
Alternating Current Polarography. Determination of Transfer Coefficient of Electrochemical Processes-Correction Tn o n-ords have been transposed in footnotesd and e to Figure 1 of the paper on “Alternating Current Polarography” [AXAL.CHEN.30, 312 (195S)l. The footnote. should read: dmplificntion control (rarbon potrntiometcr. ctc.) e Frcqwnc>- control (ganged c a r h i , etc.) P. J. ELYIXG
