Simple Device for Transferring Thin-Layer Chromatographic Fractions for Spectroscopic Examination Colin J. Clemett Unilever Research, Port Sunlight Laboratory, Port Sunlight, Wirral, Cheshire, L.62 4 X N , U.K.
THIN-LAYER CHROMATOGRAPHY provides a relatively quick method for separating mixtures in amounts of the order of 100 pg. Such quantities are small if spectrometric examination of the fractions is required, so it is necessary to have a means of transferring the fractions as efficiently as possible from the plate into the medium relevant to the particular spectrometric technique. This can be accomplished using the simple device illustrated in Figure 1. It consists of a disposable Pasteur pipet, the tip of which has been turned in slightly and plugged with glass fiber filter sheet. The wide end is fitted with a bung carrying a capillary tube bent as shown-a Drummond 100-111 Microcap is suitable. Suction is applied a t the plugged end of the pipet (a water pump is adequate) and the mouth of the capillary applied to the spot o n the chromatographic plate to be collected. The stationary phase containing the adsorbed material is then efficiently “vacuumed” into the pipet. Alternatively, the plugged Pasteur pipet can be placed inside a side-arm tube, the mouth of which carries a bung holding the capillary. One end of the capillary is arranged to project into the mouth of the Pasteur pipet while the other end is applied to the chromatographic spot, and in this case suction is applied to the sidearm. This method circumvents the restriction caused by the plug in the Pasteur pipet and results in much more rapid pick-up. However, care is required in adjusting the degree of suction applied (very little is necessary), otherwise the material is blown out of the Pasteur pipet and may be lost. The device works extremely well for collecting granular adsorbents such as silica and alumina, but is less successful with some fibrous cellulose adsorbents. When a fraction has been collected, it is consolidated in the narrow part of the pipet, forming a miniature chromatographic column. This allows the adsorbate to be eluted in a very small volume of a suitable solvent which is important when dealing with only tenths of micrograms of material. Using this technique we have consistently recovered 80 to 90% of 100 pg amounts of a n alkyl benzene sulfonate spotted on silica and eluted with 20 p1 of water.
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Figure 1. Device for vacuum collection TLC fractions
I n many cases, the eluate from the Pasteur pipet may be used directly for spectroscopic examination. For mass spectrometry, the eluate is dropped slowly onto the probe tip and the solvent allowed to evaporate. Clean spectra have been obtained from as little as 5 pg of material. Similar success, with 20-118 amounts, has been obtained with infrared spectrometry. I n this case the eluate was dropped onto a little KBr powder which was pressed into a microdisk after evaporation of the solvent. For N M R spectrometry, it did not prove possible to transfer the eluate directly into a microcell, as it was generally contaminated with water and the solvents used to develop the chromatogram. This problem was overcome by transferring the eluate into a second Pasteur pipet, the tip of which was sealed. Bridging the neck of this pipet with a drop of solvent prior to adding the eluate confined the latter to the wide portion of the pipet, which facilitated evaporation of the solvent. After evaporation, the tip was broken off and the material remaining washed down into a microcell with the solvent of choice. I n this way we have obtained excellent spectra from 100-200 p g amounts (collected from several chromatograms) of a number of compounds, using spectrum accumulation. ACKNOWLEDGMENT
The mass spectra were obtained by D. N. Forshaw, the infrared spectra by Mrs. E. M. Joyce, and the chromatoplates were prepared by A. Rastrick.
RECEIVED for review August 24,1970. 1970.
Accepted October 19,
Modulated Power Unit for Driving Metal Vapor Discharge Lamps F. M. Hamm, T. L. Martin, and P. B. Zeeman Department of Physics, University of Stellenbosch, South Africa METAL VAPOR DISCHARGE lamps provide convenient light sources for atomic fluorescence spectrometry (1-3) because of their high intrinsic brightness and relative narrow lines when driven a t suitable current rates, usually somewhat below the factory rated value. Normally these are driven directly from the ac mains, chokes, resistors, or leakage transformers being (1) R. M. Dagnall, T. S. West, and P. Young, Talanta, 13, 803 (1966). (2) N. Omenetto and G. Rossi, Anal. Chim. Acta, 40,195 (1968). (3) J. D. Winefordner and R. A. Staab, ANAL.CHEM.,36, 1367 (1964). 490
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
used to give the necessary current limiting. The light output from a lamp driven in this manner will be interrupted; the intervals of light emission and darkness being unequal in time, see Figure 1, a, b. This form of modulation is rather unsuitable to be used in conjunction with conventional lock-in detector systems. These systems are widely used in atomic fluorescence and atomic absorption spectrometry because flame emission and flame background can thus be suppressed to the level of their respective noise component determined by the pass band of the amplifier and detector system. I n many instruments, e.g., the Zeiss PMQ I1 spectrophotometer, the frequency of the lock-in detector is the mains frequency. For
M A I N S VOLTAGE
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Figure 2. Simple modulation circuit for vapor discharge lamps
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shorter lamp life. Second, a marked inhomogeneity of the envelope temperature leads to condensation of metal vapor at the colder sites, sometimes partly blocking the optical path. The difficulties mentioned above have been overcome by using a lamp modulator, see Figure 3, in which use is still made of suppressing one half-period of the mains voltage, thus providing the correct modulation frequency for the Zeiss instrument for example, but in the conducting phases the electrodes are switched such as to make them anode and cathode alternatively. This is demonstrated in Figure 1, a, d, e . Referring to Figure 3 a push-pull output is provided at the points A and C with reference to point B. The silicon-controlled rectifiers SCRl and SCR2 will with either contact S1 or T1 closed a t a time, only allow current to flow in say the positive half-period referring to the primary. Now contacts S1 and T1 are driven by the frequency divider circuit in such a way that SCRl and SCR2 are prepared to conduct alternately in the conduction phases. R1 and R2 are the current limiting resistors for the discharge tube. They should be adjusted for equal light output in both directions of the discharge. This can easily be monitored with a n oscilloscope at the amplifier output. R 3 and R4 have the function to damp transformer oscillations which occur when the gas discharge extinguishes towards the end of a half-period. R5 and R6 limit the current through the trigger electrodes of the SCR's. The timing of the relay contacts is not critical, as long as one is closed before the firing voltage is reached and released
Figure 3. Modulated power unit for driving vapor discharge lamps All resistors W if not indicatedotherwise. All rectifiers (RCA) 1N3196
4
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and 1 Reed relay driving coils lOOOS2jl224 V
S1 and T1 Reed Relay
contacts (HIVAC XS 2/3)
the vapor discharge lamp, this would mean that they could be driven correctly for instance, by inserting a rectifier in the circuit to suppress one half-period of the mains voltage, see Figure 2. The light output from this arrangement is shown in Figure 1, a, c. In all such arrangements the actual period of light emission is slightly shorter than the intervals of darkness. This usually does not impair the performance of a lock-in amplifier. The above method, however, has the following two serious disadvantages: When the lamps are driven from ac mains, the electrodes are alternating anode and cathode, respectively. Consequently the heat generated is equally distributed between the electrodes. In the case of the arrangement shown in Figure 2 for the same average lamp current, one electrode carries almost the total heat generated. This may lead to a
again before the firing voltage is reached in the following conducting half-period when the same applies to the other contact. These conditions are readily achieved by the circuit without further adjustment. This modulator has been used successfully in conjunction with the Zeiss PMQ I1 spectrophotometer. No difficulties should be experienced with the construction of the above circuit even by people not well versed in electronics, whereas changes due to components being at hand may well be done by the more experienced, RECEIVED for review August 12, 1970. Accepted October 20, 1970. The authors thank the South African Council for Scientific and Industrial Research for a grant in aid of this research.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 3, MARCH 1971
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