procedure described. I n general, good agreement between theoretical and experimental values was obtained, percentage errors ranging from -0.7% for vanillin to +1.4% for p-ionone. Three of the compounds-carvone, p-ionone, and methyl heptenone-give low results because of reaction with S-bromosuccinimide under the experimental conditions. The interference was readily overcome by extracting the hydrolyzate with ether and titrating the carbonylfree aqueous layer in accordance with the procedure described. Most aldehydes and ketones do not react with N bromosuccinimide under the esperimental conditions and their removal from the hydrolyzate is, therefore, not
necessary. However, when dealing with unknown compounds i t is recommended that the solvent extraction procedure be followed to confirm results obtained by direct titration. Compounds which cannot be quantitatively removed from the hydrolyzate could, therefore, not be analyzed by this method. Semicarbazones are frequently prepared for the purpose of detecting and identifying aldehydes and ketones in complex mixtures. The method described should therefore prove of value for the characterization and determination of small amounts of these compounds in many natural and synthetic compositionq.
ACKNOWLEDGMENT
The authors express their appreciation to Leo Levi for his interest and support of this investigation. LITERATURE CITED
(1) Guenther, E., “The Essential Oils,” Vol. 11, p. 818, Van Nostrand, New York, 1949. (2) Radecka, C., Nigam, I. C., Can.
Pharm. J. (in press).
CZESLAKA RADECKA ISHWAR C. NIGAM Pharmaceutical Chemistry Division Food and Drug Directorate Department of National Health and Welfare Ottawa, Canada
Separation and Determination of Arsenic Trichloride and Stannic Chloride by Gas Chromatography SIR: The review by Tadmor ( 2 ) indicates partial success in the separation of a number of volatile inorganic halides by gas chromatography. Cnfortunately, severe tailing, anomalous effects, and a lack of quantitative results limit the utility of these methods. This paper describes a means for obtaining well formed chromatograms for the separation of mixtures such as GeCl,, SnClr, and AsC13, and SnClr, &c13, and TiClz. d successful quantitative technique is described for the separation of AsC13 and SnC1,. An all-glass apparatus, patterned after the one described by Dal Sogare and Safranski ( I ) , a new sampling system for rapid, repetitive sampling of reactive liquids in an inert atmosphere, and a Chromosorb W column coated with a polymer of chlorotrifluoroethylene have yielded excellent results. EXPERIMENTAL
Partition
Columns.
Chroniosorb
W, size graded t o 50- t o gO-mesh, is coated with 24.5% Halocarbon 6-00 (Halocarbon Products Co.). -124.5foot glass column (5- or 6-mm. 0.d.) is preconditioned by passing a slow stream of H e at 120” C. for 6 hours. Sampling Chamber. T h e sampling chamber permits fast repetitive sampling in a n inert atmosphere. Because t h e samples are reactive, ordinary needles will plug easily, precluding reproducible analysis. For t h a t reason a Teflon needle, having a n i.d. of 0.013 inch and attached t o a 10-pl. syringe, is used to inject the sample onto the column. A needle a t least 8 inches long is required for the sampling device described below. The sampling chamber (Figure 1 ) was made by attaching a screw-cap culture tube to a three-way Teflon stopcock, which was in turn sealed through the 1766
ANALYTICAL CHEMISTRY
,FLASHV A P O R I Z E R
/
CULTURE T U B E SCREW C A P S A M P L E HOLD DELIVERY T U ENTRANCE DELIVERY T U B E S A M P L E HOLDER
Figure 1.
Sampling chamber and attachment to sample inlet
other arms to the sample inlet of the column and to the sample holder. (The sample holder is not shown in Figure 1, which is the top view of the sample chamber.) A water-cooled jacket is incorporated in the sample inlet of the column to provide a sharp temperature change a t the point where the Teflon needle enters the flash vaporizer to prevent premature vaporization of the sample and the corresponding broadening of the peaks. Capillary tubing placed inside the culture tubes prevents the Teflon needle from buckling as it is pushed into the sample inlet. Sample holders are made by sealing the end of a 12/5 socket joint. The connection to the sample chamber is made through the corresponding ball joint which is sealed onto one of the arms of the stopcock. Kel-F 90 stopcock grease is used to grease the ball and socket joint and the stopcocks. With the arrangement described, it is possible to replace the sampling chamber with a simple screw cap with septum and then make sample injections of noncorrosive samples in the normal manner. The syringe needle must then be 4 inches long. The reproducibility
of direct injections with small bore needles is slightly better and the peaks are somewhat sharper. It is possible to take the sample chamber into a dry box to attach the sample holder, then make the connection to the sample inlet of the column as shown in Figure 1. Or, the sample holder may be connected quickly to the sample chamber under a jet of helium. I n either case a hypodermic syringe fitted with a Teflon needle may be used as described belolv to remove samples from the sample holder without exposing the sample to the atmosphere. Procedure. To take samples of the corrosive materials, a 10-pl syringe with a Teflon needle is filled to within inch of the end of the needle with CC1, and threaded through the stopcock into the sample holder. The pressure equalization valve must be opened to ensure the pressure is equal to that of the column entrance. The needle is immersed into the liquid and a measured amount of liquid drawn into the needle. The needle is withdrawn and rethreaded through the stopcock and into the flash vaporizer after the stopcock has been rotated 180’.
t
0
1
2
3
4
5
6
7
8
9
CCI4
B
SnCll
1 0 1 1
T I M E (MINUTES)
TIME (MINUTES1
Figure 3. Chromatographic separation of CCI4,SnC14, ASCIS,and Tic14
Figure 2. Chromatographic separation of GeCId, SnC14, and ASCI3
The sample is then injected into the carrier gas stream. h quantity of CCI4 is injected along with the sample, effectively flushing the sample into the gas stream. Better reproducibility is obtained with this procedure because small droplets of inorganic halide tend to form inside the Teflon needle, giving a 5 to 10% error in the sampling if the syringe is used in the normal may without the CCI4 ram.
A
inorganic chlorides by gas chromatography offers definite advantages over conventional methods, an attempt was made to render the chromatographic separation of XsC13 and SnC14 quantitative. The conditions of the calibration and analysis were as follows: corrected flow rate, 88 ml./minute; oven temperature, 109' C.; flash vaporizer, 225' C.; detector filament current, 100 ma. The
The conditions for the chromatogram shown in Figure 3 are identical to those of Figure 2 except the oven temperature is raised to 126' C. The components shown in Figures 2 and 3 elute in the order of their boiling points, as would be expected when the distribution between the stationary and mobile phases is normal. Because the analysis of mixtures of
RESULTS AND DISCUSSION
Chromatograms obtained using the apparatus described are practically free of the severe tailing and other anomalous effects that have plagued workers in the field of gas chromatography of inorganic chlorides. A typical chromatogram is shown in Figure 2. The conditions under which the chromatogram was taken are as follows: corrected flow rate, 82.5 mL/minute; oven temperature, 107' C.; flash vaporizer temperature, 225' C.; detector filament current, 100 ma.; inlet pressure, 25 p.s.i.; and sample size, 4 p l . The first peak appearing is GeC14 which has a retention time only 1% greater than CC1,. Thus, the flushing technique described above could not be used, and the sample was injected by displacing the sample into the column with a column of air as the syringe plunger was pushed in. Figure 3 shows the chromatographic separation of CC14, SnC14, XSC13, and Ticla. Only hsC13 and TiC14 are not well resolved into individual peaks.
Table 1.
Analysis of SnC14/AsCle Mixtures
SnC14 Sample 1
Known, mg.
Found,
mg.
Error
1.11
0.96 1.16 1.16 1.02 1.11 1.00 1.07 10.09
-0.15 +0.05 +0.05 -0.09 0.00 -0.11 -0.04
5.48 5.40 5.48 5.54 5.20 5.42 f.0.13
-0.07 -0.15 -0.07 -0.01 -0.35 -0.13
9.81 9.91 9.96 9.90 9.90 zkO.06
-0.19 -0.09 -0.04 -0.10 -0.11
Mean Std. dev. Sample 2 Mean Std. dev. Sample 3
Mean Std. dev.
5.55
10.00
Known,
mg.
9.73
5.41
1.08
ASCI$ Found,
mg.
Error
9.50 9.60 9.88 9.49 9.30 9.60 9.56 f 0 . 19
-0.23 -0.13 $0.15 -0.24 -0.43 -0.23 -0.22
5.60 5.43 5.53 5.51 5.63 5.54 & O . 08
+o. 19 +o. 02 +o. 12 +o. 10 +o. 19
0.99 1.12 0.99 0.99 1.02 f0.06
-0.09 +0.04 -0.09 -0.09 -0.06
+ O . 12
VOL. 37, N O . 13, DECEMBER 1965
1767
peak areas plotted against the sample size gave straight lines for both AsC13 and SnC14. Three solutions of AsC13 and SnC14 were prepared by mixing measured quantities of the pure liquids together in a n inert atmosphere. The results of the integration of the peak areas are shown in Table I. The errors, which never exceed 6y0,are believed to result primarily from the premature vaporization of the samples as they are being injected. By comparing the chromato-
grams of the mixtures with those of pure compounds, it was found that about 0.1 mg. of SnC14 emerged with the AsC13 peak and the amount of contamination was almost independent of sample size. Thus, greater accuracy is achieved by using larger samples, up to 15 mg. S o attempt was made to compensate for the contamination error. The accuracy and precision of the analysis indicate that gas chromatography holds considerable promise for the analysis of these and similar compounds.
LITERATURE CITED
(1) Dal Nogare, Stephen, Safranski, L. W., ANAL.CHEM.30,894 (1958). (2) .Tadmor, J., "Chromatographic Reviews, Michael Lederer, ed., p. 223, Elsevier, London, 1963.
JAMES E. DENNISON~ HARRY FREUND Department of Chemistry Oregon State University Corvallis, Ore.
Present address, Western Electric Go., P. 0. Box 900, Princeton, N . J.
Simplified Use of Transfer Functions in Analysis of Operational Amplifier Electroanalytical Instrumentation SIR: Recently, Booman and Holbrook ( I , 2 ) have demonstrated the importance of a detailed analysis of the gain-frequency characteristics of operational amplifier circuits for controlled-potential electroanalytical instrumentation. It was shown how to obtain optimum performance for any cell-amplifier circuit after analyzing the circuit parameters by suitable combination of cell and amplifier transfer functions. The quantitative application of these concepts is relatively complex and instruments such as xave analyzers are essential for measuring cell transfer functions properly. The use of these same concepts for the qualitative characterization of the circuits is a valuable aid in the preliminary selection of circuit configuration and in the specification of necessary amplifier characteristics. The approach involves formulating expressions for the potentials a t various points in the circuit in terms of simplified transfer functions, without explicitly defining the frequency dependence of the components. These are then used to relate the applied potential to the cell potential and to evaluate the performance of the circuit in compensating for IR drop in the cell and in the current measuring device, etc. The rapid development of solid state operational amplifiers has made it feasible to use many of the possible circuit configurations described previously (3). I n many cases, chopper stabilization no longer is required, and both inputs of the operational amplifier may be active. Such circuits can be evaluated conveniently using this simplified approach. Although obvious expressions are obtained for most of the single amplifier circuits, the interrelations between the components are not so straightforward with multiamplifier circuits. The use of the transfer function approach frequently reveals aspects of the circuit performance which are not 1768
ANALYTICAL CHEMISTRY
drop in the current measuring device (load resistor) is properly compensated, and the control amplifier can be stabilized if required. From the properties of an operational amplifier, eo =
I Figure 1 .
J
Single amplifier potentiostat
GI control amplifier inverting input to amplifler non-inverting input to omplifler amplifier output counter electrode reference electrode W working electrode RL load resistor or current measuring device V signal generator el
e2 eo C R
FORMULATION OF TRANSFER FUNCTION EXPRESSIONS
Using a simple example, the circuit of Figure 1 can be analyzed. As discussed previously (S), this is a particularly useful circuit because the signal generator can be grounded, the IR
(1)
where eo is the amplifier output potential, el and e2 are the potentials a t the inverting and noninverting inputs, respectively, and G1 is the transfer function of the operational amplifier used as the controller. For these purposes, GI can be considered as the gain of the amplifier. Next, the input potentials el and e2 must be defined in terms of the other circuit parameters.
E
obvious from inspection of the circuit diagram. Analysis of all of the circuits published previously (3) indicated that five of them were not suitable for the intended purpose. This paper shows how such errors can be detected easily. This general approach to the analysis of multiamplifier circuits permits a qualitative evaluation of their characteristics in applications where the high frequency ax. performance is of importance. The expressions obtained include terms for frequency-dependent transfer functions of the electrolysis cell and of each amplifier. From a qualitative knowledge of how each of these components responds at any particular frequency, the expressions can be used to estimate the circuit performance when rapid response is required.
- (el - e2)G1
=
el=E+V
(2)
ez = 0
(3)
(eo
- iRL - V ) A
(4) Here E is the cell potential (between the reference and working electrodes) , V is the potential of the signal generator, i is the cell current, RL is the load resistor, and A is the cell transfer function. The cell transfer function is the ratio of the reference electrode potential to the counter electrode potential, both with respect to the potential of the working electrode (2)-i.e., the ratio of the cell potential E to the total potential across the cell. Equations 1 4 can be combined to obtain E(&+
1) =
-v
(1 + -kl) - -i;
(5)
Whenever the control amplifier gain Gl is high, Equation 5 will reduce to E=-V (6) and this circuit configuration will function as a controlled potential instrument. At higher frequencies the gain of the amplifier decreases, and the terms containing 1/G1 cannot be neglected.
