Solvent systems for TLC separation

Bota et al. [84] used the PCA method to select the optimum solvent system for TLC separation of seven polycyclic aromatic hydrocarbons. Each solute is treated as a point in a space defined by its retention coordinates along the different solvent composition axes. The PCA method enables the selection of a restricted set of nine available mobile phase systems, and it is a useful graphical tool because scatterplots of loading on planes described by the most important axes will have the effect of separating solvent systems from one other most efficiently. 

Gb 6en and Samuelsson [61] quote solvent systems for TLC separation of all known prostaglandins, both as fatty acids and as methyl esters. Bygdemait and Samuelsson [20] have published work on the quantitative determination of prostaglandins in pooled human semen. 

Numerous solvent systems have been described for the TLC of carbohydrates. For comprehensive listings, consult Ghebregzabher et al. (1976) or Scott (1969). Churms (1981) has provided tabular data showing various combinations of layers and solvent systems for TLC separations of carbohydrates. 

TLC separation of DNS-amines is usually made on layers of silica gel with solvents covering a range of polarity, e.g., chloroform, ethyl acetate, diisopropyl ether and methanol. Seiler and Wiechmann [97] developed 30 solvent systems for the separation of DNS derivatives of over 100 biogenic amines on TLC plates of silica gel. The selective reaction of DNS-C1 with the amino group of catecholamines has been examined [98]. The drugs dopamine, norepinephrine and epinephrine are adsorbed on alumina which protects their hydroxyl groups from dansylation. The N-dansylated compounds are separated with benzene-dioxane-acetic acid (90 25 4) on layers of silica gel. 

Bhushan and Ali (1987) tested amino acid separations on silica gel layers impregnated with various metal salts. Bhushan and Reddy (1989) reported the separation of phenylthiohydantoin (PTH) amino acids on silica gel with new mobile phases. Laskar and Basak (1988) de.scribed a new ninhydrin-based procedure that produced different colors and good sensitivity for amino acid detection. Bhushan and Reddy (1987) reviewed the TLC of PTH amino acids. Gankina et al. (1989) described a unidimensional multistep silica gel HPTLC method for the separation and identification of PTH and dansylamino acids. Bhushan et al. (1987) developed numerous solvent systems for effective separations of 2,4-dini-trophenyl-(DNP) amino acids. Bhushan (1988) reviewed the TLC resolution of enantiomeric amino acids and their derivatives. Kuhn et al. (1989) reported the amino acid enantiomer separation by TLC on cellulose of d- and L-tryptophan and methyltryptophan. Guenther (1988) determined TLC-separated enantiomers by densitometry. 

Suitable solvent systems for the separation of aglycones on silica layers include petroleum ether-ethyl acetate-formic acid (75 25 1) (58) and petroleum ether-ethyl formate-formic acid (75 25 5). The mobile phase in Ref. 60 offers no advantages compared with earlier systems. Finally, a method developed by Ebel and Kaal involves direct hydrolysis of the glycosides on the TLC plate (61). 

Table D1.6.1 Solvent Systems for Lipid Class Separation on an Iatroscan TLC-FID...

Separation of a number of keto acid hydrazones may be accomplished as their free hydrazones [37], as sodium salts [38] or as ammonium salts [39]. For TLC separation of the sodium salts a plate (20 X 20 cm) is coated with a 0.25-mm layer of a mixture of silica gel and 0.1 N sodium bicarbonate (1 2 w/v). The plate is activated by heating at 110 °C for 40 min and is then cooled and kept in a desiccator until required. The solvent systems are ethyl acetate (saturated with 0.1 N sodium bicarbonate)-methanol (5 1) and butanol-ethanol-0.1 N sodium bicarbonate (10 3 10) (upper layer). The plates are developed for 2.5 h at room temperature. For quantitation, the spots may be removed from the plate and dissolved in 2.07V sodium hydroxide for color development and determination in solution. Treatment of the plate directly with base (as a spray) should also be possible for quantitation in situ. The wavelengths of the absorption maxima of a number of DNPH-keto acids in aqueous base are listed in Table 4.6... 

Pure solvents of Analab grade should be used in TLC. The various solvents possess different eluting properties. Further, because of the differences in polarity of the organic insecticides and variations in their water solubility, several solvent systems are tested for separation on the TLC plate. The large number of solvent systems listed in Table II for organophosphorus-organochlorine and carbamate insecticides provide satisfactory alternative systems for the separation of the group of insecticides under consideration. 

TLC. Analytical TLC separations on Cl8 and silica were used to determine the best HPLC solvent systems for chromatography of the p-damascenone precursor. Figure 5 shows the results from Cl8 TLC separation plotted as charm vs Rf. The best... 

Even threefold developments with the same solvent mixture are described in the hter-ature. For example, Kennedy describes a solvent mixture for the separation of hexoses, deoxyhexoses and some disaccharides on cellulose [65]. The threefold development over the same migration distance is performed with a mixture of ethyl acetate, pyridine and water (100 + 35 + 25 ml). Madaus uses this type of threefold development for the sugars obtained by hydrolysis of mucilages. The separation is performed on TLC silica gel 60 using acetonitrile + glacial acetic acid + water (85 + 14 + 1 v/v) as the solvent system [65a]. 

For the separation of substances having small Rf values, it is advisable to run the TLC plate more than once, drying the layer before it is developed again. If the solvents used for subsequent runs are the same as in the first run it is called multiple or repeated development, but when the solvents are different in the first and later runs, the technique is called stepwise development. These techniques are specially useful for mixtures which contain polar as well as non-polar substances. Polar solvent systems are usually employed for the first run and a non-polar solvent system for the later runs. 

A glass cartridge is filled at the tapered end with a small amount of quartz wool, followed by about 50 mg powdered drug and about 50 mg starch. The cartridge is sealed with a damp and placed in the oven block of the TAS apparatus, which is heated to about 220 C. The open end of the cartridge points directly to the TLC plate. Volatile compounds at the given temperature then distil onto the starting zone of the TI,C plate in about 1.5 min. Immediately afterwards, the plate can be placed in a solvent system sufficient for TLC separation,... 

General instructions for TLC are given in Kates (1972) and Christie (1982). Useful solvent systems for use with silica gel G plates are listed by Jensen and Pitas (1976). For assay of polar contaminants, the acylglycerols should be separated with petroleum ether (35 5 °C fraction)-diethyl ether-acetic acid... 

Any of these reversed-phase plates can be useful in developing solvent systems for similar reversed-phase HPLC work, or can help determine what may not be moving through such a column. It should be remembered that only a TLC plate shows what is left at the origin with a given solvent system and hence would remain at the top of any HPLC column with this same solvent system. It is these remains of samples that drastically affect HPLC column separations and column life. 

Table 5 Different solvent systems used as developers for TLC separation for simple phenolic and related compounds such as cinnamic acid, p-coumaric acid, ferulic acid, and tannic acid.

As mentioned in Section III, separation techniques for amino acids designed originally for PC are generally applicable for TLC amino acid studies on cellulose. Solvent systems for the TLC of amino acids generally employ mixtures of alcohols, acids or bases, and water. TLC is advantageous compared to PC for amino acid analysis in that it is faster and provides more compact spots, leading to better sensitivity and resolution of compounds. Experiment 1 provides a simple introduction to the TLC analysis of amino acid standards on silica gel. Experiment 2 provides a similar experience with cellulose and amino acid standards. Experiment 3 uses a reversed-phase layer to separate amino acids. For TLC exper-... 

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