Colors Developed in TLC of Amine Derivatives by Co(1I) Amine derivative Active group TLC color None (blank) None a Pyridine Heterocyclic, tertiary, U amine Aniline Aromatic monoamine a, b m-Phenylenediamine m-Aromatic diamine a pphenylenediamine pAromatic diamine Brown Benzidine p-Aromatic diamine Brown o-Phenylenediamine o-Aromatic diamine Deep burgundy 1,2,4,5-Tetraaminobenzene o-Aromatic diamine Deep burgundy Imidazole Imidazole Deep violet 2-Ethylbenzimidazolec 2-Subst. benzimidazole Deep blue 2,2’-Tetramethylene 2-Subst. benzimidazole Deep blue dibenzimidazoled 2,2’-Ethyl bisbenz2-Subst. benzimidazole Deep blue imidazolee a Very light blue or pink, depending on exposure to air humidity. Even though no change was noticed in the TLC, upon standing the aniline formed a pink crystalline complex with the cobaltous chloride. c 2-Phenylbenzimidazole and 2-cyclohexanebenzimidazole gave the same results. d 2,2’-Octamethylene-, 2,2’-decamethylene- and 2,2‘-trans-cyclohexane-dibenzimidazole gave the same color. 2,2’-Heptyl bisbenzimidazole and poly(2,2’-octamethylene bisbenzimidazole) developed the same color. Table I.
6 H 2 0in absolute ethanol serves admirably, both for spot tests and, especially, as a developing agent in thin layer chromatography (TLC), to differentiate between some aromatic amines, imidazoles, and benzimidazoles. The results are given in Table I. Aliphatic amines and either aliphatic or aromatic amides failed t o give positive results. Cyclic aliphatic amines yielded light green coloration in spot tests but were inconclusive in TLC. The tests are rather sensitive. A 2 % by weight of the CoC12. 6 H 2 0 in ethanol yielded easily observable TLC stains with imidazole and 2-ethylbenzimidazole in concentrations gram/cm2, obtained from solutions of down to about 5 X 0.1 by weight of the reagents in ethanol. The tests are extremely simple to perform as no other reagent or operation is needed to obtain the final results. The separation of the reactants by the eluting solvent and the sharp differences in the developed colors of the TLC affords us a very simple way to differentiate between amine derivatives and isomers. This was a n especially helpful means of following and evaluating the progress of reactions involving aromatic amines and their condensation products such as polybenzimidazoles.
RECEIVED for review April 27,1970.
Accepted June 29,1970.
Optimum Current Concept in the Operation of Electron Capture Detectors R. A. Landowne Central Research Laboratories, American Cyanamid Company, Stamford, Conn. 06904 VARIATION IN RESPONSE of the electron capture detector to the many compounds of different electron affinities is most evident only when the detector is operated t o yield maximum sensitivity. However, the proper conditions for obtaining this sensitivity are often difficult t o determine because of the many variables affecting detector response such as temperature, flow, carrier gas contamination by either inherent impurities or column bleed, cleanliness of the radioactive foil, and applied voltage. Furthermore, many of the factors affecting detector response are often predetermined by the chromatographic conditions required for the particular separation and analysis at hand, and it becomes less desirable to change these conditions at the expense of detector sensitivity. A simple means of achieving maximum sensitivity without altering chromatographic conditions is proposed. It requires only a n initial determination of the current-voltage (I-V) curve for a particular detector at any set of analytical conditions. From this curve, an “optimum current” is chosen for future use for that detector regardless of subsequent changes in operating conditions. For pulsed mode detectors, an I-V curve can be obtained at a single pulse cycle, or the maximum current can be similarly obtained from the plot of current us. pulse cycle at a fixed voltage, depending upon which of these parameters is most easily varied with the instrumentation used. EXPERIMENTAL
Equipment. The apparatus used was the Varian Aerograph Model 1527B with the concentric tube electron capture 1468
detector containing 250 mCi of tritium on foil. The only alteration to the instrument was the use of an external, variable, low voltage power supply in place of the fixed 90 V applied by the 1527B system to the detector. The voltage lead from the instrument’s power supply was disconnected and a negative dc voltage was applied to the cathode surrounding the tritiated foil by a n Electronic Measurements Model 212 power supply. The recorder was a Texas Instrument Servoriter I1 with a 1 mV per 93/4-in.chart span. The carrier gas was nitrogen. The column was 70-80 mesh silica gel packed in 3-ft by 1/4-in. 0.d. stainless steel and maintained at 150 “C. Procedures. A test mixture of 20% sulfur dioxide in air was prepared by admixing the components prior to flowing through a 250-ml glass gas sampling bulb. The bulb could be sampled by syringe through a septum on a side port. Injection of a few microliters from a 10-p1 syringe gave a suitable peak for sulfur dioxide. The operating parameters of detector flow, detector temperature, and applied voltage were varied to determine optimum conditions for achieving the highest sensitivity possible. When this was accomplished another gas sampling bulb, containing argon, had 5 pl of the 20% sulfur dioxide mixture added to it to yield a concentration of 4 ppm sulfur dioxide. One-half milliliter of this sample was used t o confirm minimum detectability. Otherwise, extrapolation of the peak from the 20% sample was used t o determine minimum detectability. A peak whose height was twice the noise level was deemed to be the smallest that could be detected. The noise level was 0.75% of full scale with the electrometer set at an attenuation of 1 x 2, or 0.5 X 10-I2A.
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20 A P P L I E D VOLTAGE, -DCV
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Figure 1. Change in voltage-current curve with changes in detector temperature and flow 0-0 50 "C, 80 ml/min O--O 50 "C, 40 ml/min a-U 125 "C, 80 ml/min H-w 125 "C, 100 ml/min x-x 200 "C, 80 ml/min
optimum voltages, as determined from response curves RESULTS AND DISCUSSION
The response of various vapors to the electron capture detector has been shown to increase with increasing detector temperature and decreasing detector flow. Of course, the response is also voltage or pulse dependent (I, 2). In the course of determining the optimum conditions for achieving high sensitivity for sulfur dioxide, several sets of conditions of temperature and flow were used and the voltage was varied as well. Each set of conditions had its own unique voltagecurrent curve as seen in Figure 1, and its own optimum voltage, different in each case as determined from a set of response curves. The arrows in Figure 1 indicate the voltage at which maximum sensitivity was obtained. However, the same minimum detectable value of sulfur dioxide of 2.5 x 10-12 mole/sec was obtained from each response curve. In Figure 1, the optimum voltage for each set of conditions listed, as marked by the arrows, varies from 10 to 25 V. In common practice the optimum voltage occurs at the knee of the curve, or just below the plateau of current saturation if the knee area is not easily delineated. Similar curves are obtained in pulsed voltage operation, as well. The optimum voltages for each set of conditions depicted in Figure 1 correspond to the same detector current to within 5 % . (It is not necessary to know this value in amperes.) This current can be defined as the optimum current for that detector system. In effect, it would have been necessary to determine this only once under any set of conditions, and when operating conditions were changed, this same current could be immediately obtained by altering the applied voltage or pulse. Thus the (1) R. A. Landowne and S. R. Lipsky, ANAL,CHEM., 34,726 (1962). (2) S . J. Clark, Residue Rev., 5 , 32 (1964).
need for constructing a new curve in order to find the optimum every time the detector flow or temperature was changed is eliminated, and for all detector conditions, the optimum current is all that need be determined and obtained for optimum sensitivity operation. The only occasional check that may be additionally required would be the maximum or saturation current for the detector system. If this is constant, then the optimum current would be constant also. Actually the saturation current is probably more rapidly determined in practice by merely applying a voltage many times higher than is normally used for detector operation. A rule of thumb could then be applied whereby the optimum current could be chosen as approximately 85 of the detector saturation current. There is no reason to believe that this rapid method of setting up an electron capture detector for achieving optimum sensitivity is not generally applicable. The only possible exceptions might be in those few cases where complex dissociative electron capture occurs with a dependency on electron energy ( 3 , 4 ) . However, the concept of optimum operating current points out the need for variable voltage supplies and even variable pulse generators for electron capture detectors if optimum results are to be achieved. With this type of instrumentation, the determination and employment of the optimum current to the detector is easily accomplished.
RECEIVED for review March 6 , 1970. Accepted July 24, 1970. (3) W. E. Wentworth, E. Chen, and J. E. Lovelock, J . Pkys. Chem., 70, 445 (1966). (4) W. E. Wentworth, R. S. Becker, and R. Tung, ibid., 71, 1652 (1967).
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