Identification and Quantification

The forte of TLC is qualitative analysis. It is possible to identify unknown alkaloids owing to the large amount of RF data available from the literature and the ability to perform a chemical reaction using a wide spectrum of different reagents in situ. For some alkaloid drugs, a compilation of TLC data has been elaborated and stored in computer-based information systems.

Many authors make an identification based on Rf values in a number of chromatographic systems. One scheme has been described in which the analysis of a series of alkaloids by eight TLC systems, combined with observations under UV light (A = 366 nm) and colour reactions with iodoplatinate reagent (Table 6) is used.

For precise identification, UV or infrared spectra after elution have become indispensable. Together with the melting point and optical rotation values, they are sufficient for the identification and comparison of isolated pure substances. Other spectral methods such as nuclear magnetic resonance or mass spectrometry have frequently been used to identify alkaloids.

Although quantitative determination in TLC is more difficult and requires more effort, it is becoming increasingly important nowadays. There exist two forms of quantitative analysis: direct and indirect. The first method is based on the elution of spots with a suitable solvent and determination in solution, followed by spectrophotometric, fluorometric or acid-base potentiometric titration. The second possibility utilizes adsorption of UV and visible radiation or luminescence of alkaloids, and is performed by the means of photodensitometry, densitometry and fluorimetry in situ. This latter technique requires the use of an optical scanner, which is a relatively expensive piece of equipment.

Solar Panel Basics

Solar Panel Basics

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