TLC on Silica Gel Layers

The hi /-values of many alcohols are about the same when chromatographed with benzene-ethyl acetate (80 + 20) [151a] or chloroform on silica gel layers [183, 251]. Subdivision only into 10 and Cx5 alcohols can be recognised (Table 28). The alcohols which are not found in the normal region, are mentioned specially. 

It is surprising that many stereoisomers can be comparatively well separated, such as the menthols quoted in Table 29 [79, 105,196, 260]. 

The 4 isomeric thujyl alcohols also have different hi /-values [177]. The sequence found, using chamber saturation and triple development with benzene-chloroform (50 + 50), was isothujyl alcohol (h.Rf 47), thujyl alcohol (50), neothujyl alcohol (57) and neoisothujyl alcohol (67) [264]. 

The stereoisomeric farnesols have been similarly investigated [300]. These possess juvenile hormone activity [225, 226] and could be detected thin-layer chromatographically as the sexual attractant of the male bumble bee [269, 270]. The chromatography was carried out with benzene-ethyl acetate (95 + 5) on silica gel V- and szial gel 47-layers (Firm 114) [303, 309]. The trans4rans-ia,mesol (hi / 27) then lay below the as- mns-farnesol (hi / 36). This sequence remains also when pure benzene is used for development [304] and is observed with the farnesol esters too [307—309]. Seikel and Rowe [237] have studied the TLC of the eudesmols. They separated milligram amounts of a- and p-eudesmol with benzene-petroleum ether (50 + 50) on alumina layers, 500 (xm thick. The use of ascending technique is said to augment the differences in the hi /-values. 

Gutzwiller and co-workers [84] have purified and checked two furanoid sesquiterpene alcohols by chromatography also on silica gel layers, using mixtures of chloroform and methanol. 

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Dinitrobenzenediazonium tetrafluoroborate can be used to form colored derivatives of activated methylene compounds such as malononitrile and related species. The products may be separated by TLC on silica gel layers [71]. 

Methylcarbamate insecticides have been recently labeled with DNS-C1 [145]. The procedure involves the hydrolysis of the carbamates with 0.1 M sodium carbonate to form a phenol and methylamine [166]. The two hydrolysis products are labeled with DNS-C1 and subsequently detected and determined quantitatively by TLC on silica gel layers by scanning spectrofluorimetry in situ. The reaction conditions were examined, and optimum conditions for hydrolysis and labeling were established [167]. The overall reaction scheme is shown in Fig. 4.62. The phenol derivatives of a number of N-methylcarbamates have been separated by one- and two-dimensional TLC [168], and the fluorescence behaviour and stability of the derivatives have been examined [169]. Most of the DNS derivatives fluoresce at similar wavelengths (excitation, ca. 365 nm emission, ca. 520 nm). The fluorescence spectrum of a typical DNS derivative is shown in Fig. 4.63. The method has been applied successfully to the analysis of low concentrations of carbamates in water and in soil samples with little or no clean-up being required [170,171]. Amounts as low as 1 ng of insecticide can be detected instrumentally. Visual limits of detection are ca. 5-10 ng per spot. 

Surface active substances, such as the commercially available preparations of sucrose palmitate and other esters, have been separated into their individual components by Mima and Kitamori [63] and by Gee [33], using TLC on silica gel layers. Solvents were benzene-ethanol (75 + 25) or toluene-ethyl acetate-95 % ethanol (50 -f 25 + 25) (see also Linow [57]). 

The organo-tin compounds are associated closely with the plastics they are especially important as stabilisers in the PVC (polyvinyl chloride) industry. They may be satisfactorily separated by TLC on silica gel layers [9, 31, 56, 87] solvents so far used for development are ... 

Solvents at serial no. 1-8 two-dimensional TLCon polyamide layers. Solvents at serial no. 9-14 one-dimensional TLC on silica gel layers. 

The lipid (up to 50 mg) is dissolved in acetic anhydride in pyridine (2 mL, 5 1. v/v), and is left at room temperature overnight. The reagents are then removed in a stream of nitrogen with gentle warming and the acetylated lipid is purified, if necessary, by preparative TLC on silica gel layers, generally with hexane-diethyl ether (80 20, v/v) as the mobile phase."... 

Aqueous solutions of dyes ean also be employed instead of water. In the ease of hydrophilic dyes such as methylene blue or patent fast blue the transparent background of the TLC/HPTLC plate is stained blue. Pale spots occur where there are nonwetted zones. Dauble [89] detected anion-active detergents in this way on silica gel layers as pale zones on a blue background with palatine fast blue GGN... 

These can usually be easily separated by thin layer chromatography (TLC) on silica gel. 

To a stirred suspension of 2.3 mmol LDA in 20 mL of THF arc added at — 80 X under a nitrogen atmosphere 253 mg (1 mmol) of methyl 2-(2-hydroxy-2,6,6-trimethylbicyclo[3.1.1]hept-3-ylideneamino)propanoate and the mixture is stirred for a further 30 min. After the addition of 114 mg (1 mmol) of ethyl ( )-2-butenoate the mixture is stirred at — 80 °C until the reaction is complete (followed by TLC on silica gel). The mixture is poured into 70 mL of sat. aq NH4C1 and subsequently extracted three times with diethyl ether. The combined ether layers arc dried over Na2S04 and the solvent is evaporated. The crude adduct is purified by chromatography (silica gel. diethyl ether/hexane 66 34) yield 330 mg (90%). 

Separation by adsorption chromatography takes place preferentially as a result of hydrogen bonding or dipole-dipole interactions. Hence, separation of mixtures of substances on silica gel layers by lipophilic solvents primarily takes place according to polarity differences. Further separation within a polarity group can then be achieved either two-dimensionally or off-line by partition chromatography on anotho TLC plate (Fig. 4). 

A review on TLC and PLC of amino adds, peptides, and proteins is presented in the works by Bhushan [24,25]. Chromatographic behavior of 24 amino acids on silica gel layers impregnated tiraryl phosphate and tri-n-butylamine in a two-component mobile phase (propanol water) of varying ratios has been studied by Sharma and coworkers [26], The effect of impregnation, mobile phase composition, and the effect of solubility on hRf of amino acids were discussed. The mechanism of migration was explained in terms of adsorption on impregnated silica gel G and the polarity of the mobile phase used. 

Another interesting TLC method for the isolation and determination of bromate ion in flour dough and breads has been developed [59]. It involves extraction of BrOj from foodstuff, purification on alumina column, TLC separation on silica gel layer developed with water -1- -butanol + n-propanol (1 1 3), and quantification by densitometry. Bromate ion down to 0.1 pg in bread (1.0 g) was detected with tohdin-FIQ reagent. 

The oldest method of resolving enantiomers by TLC takes advantage of the natural chiral properties of cellulose and triacetylcellulose resulting from the helical structure of the polymers (98). Amino acid derivatives have been resolved on silica gel layers impregnated with chiral acids or bases, for example. 

Methylacrylate and ethylenediamine were obtained from the Merck. Jeffamine T-3000 was purchased from Texaco Chemical Company. Other chemicals were used as obtained from the Fluka without further purification unless otherwise noted. Solvents were dried and distilled according to literature procedures prior to use. Reactions were controlled by thin layer chromatography (TLC) on silica gel 60 F254 and spots were detected either by UV-visible light or by charging with vapor. 

Procedures. Chromatographic Purification of Ozonization Products. Ozonization products from ethyl 10-undecenoate and 1-octene were chromatographed on silica gel columns (Baker) and eluted with 15 or 25% ether in petroleum ether (b.p., 30°-60°). Fractions were examined by thin-layer chromatography (TLC) on silica gel G Chroma-gram sheet eluted with 40% ether in petroleum ether. For development of ozonide and peroxide spots, 3% KI in 1% aqueous acetic acid spray was better than iodine. The spots (of iodine) faded, but a permanent record was made by Xerox copying. Color of die spots varied from light brown (ozonide) to purple-brown (hydroperoxide), and the rate of development of this color was related to structure (diperoxide > hydroperoxide > ozonide). 2,4-Dinitrophenylhydrazine spray revealed aldehyde spots and also reacted with ozonides and hydroperoxides. Fractions were evaporated at room temperature or below in a rotary evaporator. 

The pure benzoyl isocyanide derivative is Anally obtained as red crystals (1.40-2.23 g, 50-80% yield, mp 78 °C) following thin-layer chromatography (TLC) on silica gel plates (40 x 20 cm) in a large developing tank (43 x 23 x 29cm) using 700 mL of (C2H5)20-petroleum ether (1 4) as eluent. The desired product is contained in the red band on the TLC plate. 

Thin-layer chromatography (TLC) on silica gel is well known for its separation power for lipids and related compounds. The flame ionization detector (FID) is a universal analytical instrument that offers high sensitivity and linearity for carbon-containing organic compounds. The combination of TLC and FID led to the wide use of the Iatroscan TLC-FID for the analysis of lipid classes. The adoption of the Iatroscan TLC-FID in both academia and industry has generated sufficient data to indicate that TLC-FID is currently one of the most efficient tools for the quantitation of lipids classes (Ackman et al., 1990 Hammond, 1993). 

The contents of the test-tube are mixed and warmed at 55 °C for 1-5 h. The mixture is cooled and an aliquot portion is spotted on to a TLC plate for separation. Two-dimensional chromatography is carried out on silica gel layers with cyclohexane-ethyl acetate (1 1) and light petroleum-chloroform-diethyl ether-acetic acid (33 33 33 1). Chromatography on polyamide layers is accomplished with heptane-ethyl acetate-butanol (8 1 1). The Rp values of six NBD-amines in these systems [99] are given in Table 4.15. Amounts of less than 15 ng of NBD-amine per spot can be detected. HPLC of some NBD-amines has been carried out using Zipax coated with 0.5% 0,/3 -oxydipropionitrile and 1% tetra-hydrofuran in hexane as the mobile phase (see Section 4.2.4.2.2). 

Figure 1 shows the scheme for the preparation of purified lipid A from endotoxin. S. typhimurium G30/C21 was extracted by the method of Galanos t aK (24) and submitted to one of two different conditions of hydrolysis (a) 0.1 N HC1 [in methanol-water (1 1, v/v)], 100 °C, 45 min, to yield the crude monophosphoryl lipid A (nontoxic), and (b) 0.02 M sodium acetate, pH 4.5, 100 °C for 30 min (two cycles) to yield the crude diphosphoryl lipid A (toxic). The 0.1 N HC1 hydrolysis product was fractionated on a Sephadex LH-20 column (23). Each of these fractions was then separated by preparative thin layer chromatography (TLC) on silica gel H (500 ym), with the solvent system chloroform-methanol-waterconcentrated ammonium hydroxide (50 25 4 2, v/v) as previously described (23) to yield TLC fractions 1-7 and 1-9 respectively. 

A variety of /3-sultams such as 65 with a poorly migratory substituent at C-3 have been treated with ethylaluminium dichloride to afford /ra r-l,2,3-oxathiazolidine-2-oxides 66 as 70 30 mixtures of isomers separable by preparative thin-layer chromatography (TLC) on silica gel. However, cA-aziridines 67 are obtained as the major products when the reaction is carried out in refluxing dichloromethane (Scheme 10) <1998T8941>. 

Co(II) (BDHC) perchlorate was prepared by hydrogenation of the corresponding Co(II) (TDHC) perchlorate and purified by preparative thin-layer chromatography (TLC) on silica gel (16). 

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