Solvent strength, thin layer chromatography

In reversed-phase thin-layer chromatography (RP-TLC), the choice of solvents for the mobile phase is carried out in a reversed order of strength, comparing with the classical TLC, which determines a reversed order of values of compounds. The reversed order of separation assumes that water is the main component of the mobile phase. Aqueous mixmres of some organic solvents (diethyl ether, methanol, acetone, acetonitrile, dioxane, i-propanol, etc.) are used with good results. 

The following table contains the common solvents used in thin-layer chromatography, with a measure of their strengths on silica gel and alumina. The solvent strength parameter, s°, is defined as the relative energy of adsorption per unit area of standard adsorbent.13 It is defined as zero on alumina when pentane is used as the solvent. This series is what was called the eluotropic series in the older literature. For convenience, the solvent viscosity is also provided. Note that the viscosity is tabulated in cP for the convenience of most users. This is equivalent to mPasec in the SI convention. Additional data on these solvents may be found in the tables on high-performance liquid chromatography. 

As the relative elution power depends not only on the adsorbent, but in many cases on the compound types being separated, there exists no universal eluotropic series of solvent strengths. This series was given by L. R. Snyder". For another eluotropic solvent series, see J. C. Touchstone Practice of Thin-Layer Chromatography, 2 " ed., Wiley, Chichester, 1983. 

Thin-layer chromatography is a separation technique in which the components of a lipid mixture are differently distributed between a solid stationary phase, spread as a thin layer on a plate made of inert material, and a solvent mobile phase. Depending on their type, the components are retained with different strengths on the layer to give distinctive spots or bands. The migration of a band is presented quantitatively by the corresponding Rf value. The stronger the retention, the lower is the Rf value. 

In practice, the selection of the solvent of the right strength can be made either by tedious trial and error method or by using the rapid open bed separation (thin layer chromatography). For strongly adsorbed compounds, it is usually best to use a polar solvent such as an alcohol, pyridine, or an ester, whereas with weakly adsorbed solutes the solvent is normally petroleum ether, carbon tetrachloride or cyclohexane. Mixtures of two or three solvents of different polarity often give better separation than unmixed solvents. 

Thin-layer chromatography is widely used to optimize separation conditions for silica gel flash chromatography. For isocratic separations, a mobile phase that provides a Rf 0.35 for the zone of interest is chosen. If several zones are to be separated, then the solvent strength is adjusted such that the center zone has a Rp 0.35. If all zones of interest are well separated from each other and from impurities (ARp > 0.2), then the solvent strength is adjusted so that the most retained zone of interest has a Rp 0.35. For fractionation and large sample loads, it is critical that the most selective solvent composition for the separation is used. This can be quickly... 

The best mobile phase strength for a specific separation problem can be determined by thin-layer chromatography (TLC). It is represented by that solvent or solvent mixture which gives an value of about 0.3 in the thin-layer chromatogram. TLC results are readily transferable to HPLC and k values can be predicted if the stationary phase in both methods is the same product, albeit with a difference in particle size. A developing distance of 5 cm is sufficient for this test, so it can be carried out on small TLC plates in a very short time. Small sealable jam-jars make good developing tanks. 

In extraction techniques (LLC, SPE, etc.), normal-phase liquid chromatography (NPLC), and thin-layer chromatography, aliphatic hydrocarbons (e.g., -hexane, -heptane) are usually used. The elution strength of these solvents is often modified by addition of more polar solvents. The fundamental problem with the eluents in NPLC is dissolved water and trace amounts of olefins. These contaminations can induce a change in the wavelength cut-off values (UV detection, spectrophotometry), baseline perturbation, and poor reproducibility of retention data. Halogenated solvents such as dichloromethane can react with some organic solvents (e.g., acetonitrile) to form crystalline products. 

Polyclonal antibody Pu e-and-trap Quiescent solution Radioimmimoassay Relative affinity Reversed phase Solid-phase extraction Solid-phase microextraction Solubility Solvent extraction Solvent strength Stationary phase Thin-layer chromatography Titer... 

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