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 P M Q I1 spectrophotometer, the frequency of the lock-in detector is the mains frequency. For
