My TLC Plates? Quantum LQD Timesavers, of course.
p e r m a n e n t record is by sketching the chromatogram in the notebook and recording the RF values and the solvent and adsorbent used. Two modern methods of recordkeeping use photography (27) and the photocopier. T h e photocopier copies chromatograms t h a t absorb UV radiation at 366 nm but is not suitable for compounds which can only be detected under short UV (254 nm). T h e T L C plate and photocopy paper are placed on the machine and exposed. All visible chromatograms, whether charred or sprayed with a color developer, can be copied. An exception is when the spray reagent (indicator) absorbs at 366 nm. Copies are made only in blue. Compounds detected under short UV (254 nm) can be made copyable by t r e a t m e n t with iodine vapor or charring. Photocopying is as fast as photography b u t considerably cheaper, costing less than 10 cents/copy, which is less than Polaroid's M P 4 system unit cost. Also, expertise is not necessary. T h e copying sheets are 20 X 20 cm. Of course, a Xerox copy of plates is also possible for visible compounds. Quantitative TLC (22)
Equal or better resolution in 1/10 the time Unique preadsorbent sample dispensing area saves people-time where it matters: in sample prep aration and application As much as 90%. in fact. As the solvent migrates through the preadsorbent area, it collects the material to be separated at the solvent front and applies it to the silica gel as a concentrated band. The result: sample application and preparation need not be precise, but the resolution is still equal to or better than can be achieved with conventional plates. The LQD Timesaver can handle crude, dilute, organic or aqueous spotting solutions, and comes in several configurations, with or without separated channels. Save important people-time in your laboratory. Writeforcomplete information on LQD Timesavers and other TLC supplies.
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Quantitation of in situ spots on T L C plates is an established technique. T h e methods used are visual and spectrometric. T h e visual technique is carried out by spotting the unknown sample either alongside a standard or between two standards and then comparing the intensity of the spots by fluorescence, fluorescence quenching, or color intensity. T h e error in this method is ±15-50%. T h e spectrometric technique may be divided into three different modes. Reflectance or Transmittance: Colorless compounds are made visible by charring or by treating with a reagent to produce color. Fluorescence: Certain compounds, such as aflatoxins, fluoresce under UV radiation. T h e area under the peak is proportional to concentration [caution here: some compounds deteriorate under UV radiation (28)]. Compounds which do not possess natural fluorescence may be made to fluoresce by spraying with a reagent. Fluorescence Quenching: Spots appear as dark circles on the fluorescent plate. This method usually gives a nonlinear relationship between integrated area and sample concentration. Fluorescence is more sensitive than absorption; also, the measured signal is a linear function of the concentration. Errors in spectrometric measurements are between 5-10%. T h e sources of error are sample application, variation in spot size, uniformity of layer thickness, and variation from plate to plate. Errors can also be attributed to
CIRCLE 175 O N READER SERVICE CARD
92 A · ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
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