Separation Of Positional Isomers Research Articles

Adsorption and reversed-phase high-performance liquid chromatography of p-nitrobenzyl esters of monohydroxy fatty acids

To enhance the UV detectability of hydroxy fatty acids, p-nitrobenzyl (PNB) esters of twenty-two different monohydroxy fatty acids of various chain-lengths (C 16C 22) and differing positional isomers were formed using O-( p-nitrobenzyl)-N,N-(diisopropyl)-isourea (PNBDI) as alkylating agent. Reversed-phase and adsorption high-performance liquid chromatography (HPLC) of the twenty-two monohydroxy fatty acid PNB esters were studied. The PNB group did not dominate the chromatographic properties of monohydroxy fatty acids and it did not interfere with the HPLC separation of positional isomers. PNBDI was, however, found to be less than ideal for formation of PNB derivatives of monohydroxy fatty acids because UV absorbing contaminants of PNBDI interfered with the HPLC analyses.

Ligand-exchange gas chromatographic separation of aromatic sulphides

A column packed with copper(I) stearate coated on a diatomaceous support gave a good separation of aryl alkyl sulphide under mild conditions. Good selectivity was also obtained on a column packed with copper(II)-modified Porasil E or F; the use of the latter as a packing material in a capillary column was particularly effective for the separation of positional isomers. Ammonia was effective as a mobile phase ligand, resulting in sharp peaks and suitable retention times. The length and the degree of branching of the alkyl chain of a sample as well as the position of substituents in the benzene ring were found to affect the elution order. The preparation and the use of a Porasil packed column is briefly described.

New liquid crystalline stationary phases for gas chromatography of positional and geometrical isomers having similar volatilities

New liquid crystalline compounds of the type 2-methyl-4′-R-4-( p-methoxycinnamoyloxy)azobenzene, where R = methoxy, ethoxy, ethyl or n-butyl, were synthesized. Their properties as stationary phases for the gas chromatographic separation of positional isomers of xylene, dichlorobenzene, methoxynaphthalene and geometrical isomers of insect sex pheromones of similar volatilities have been investigated. The effect of temperature on the separation ability of the liquid crystalline compounds was studied. Large separation factors, α, for isomers were obtained on a column in its supercooled nematic state. The separation of insect sex pheromones was found to depend on the structure of the liquid crystalline compound used as stationary phase rather than on its nematic range.

Cyclodextrins and methylated cyclodextrins as stationary phases in gas-solid chromatography

Cyclodextrins and their methylated derivatives were used as stationary phases in gas-solid chromatography and the effect of inclusion on the retention of compounds with various structures and geometries (alkanes, aromatic hydrocarbons, alcohols) was followed. It has been found that inclusion predominates in the gas-solid interaction of compounds with suitable geometric dimensions, while the retention of polar substances is primarily determined by the interactions with the cyclodextrin hydroxyl groups. On decreasing the cyclodextrin polarity by means of methylation, stereoselectivity was evident even in the separation of alcohols. On the other hand, the presence of methyl groups causes steric hindrance to penetration of the guest molecule into the cyclodextrin cavity. The selectivity of the inclusion process is especially important for separations of positional isomers.

Use of smectic liguid crystals for the gas-liquid chromatographic separation of positional isomers

The smectogenic compounds terephthal-bis- n-butylaniline and 4-(4- n-hexyloxybenzylideneamino)azobenzene were prepared and used as stationary phases for the gas-liquid chromatographic separation of positional isomers of dibromobenzenes, chloroacetophenones, chloronaphthalenes and methybiphenyls. These solute isomers cannot be sharply separated in the smectic B state whereas a complete separation was achieved in both the smectic A and C phases. It is suggested that the unique selectivity of the smectic A and C states relative to the smectic B state can be explained by the difference in the molecular packing in the layer between the smectic B and the smectic A or C state.

Gas chromatographic separation of positional isomers of fenvalerate.

Gas chromatographic separation of positional isomers of fenvalerate.

Gas chromatography of all C 15C 18 linear alkenes on capillary columns with very high resolution power

Conditions were studied for the separation of mixtures of all the 60 isomers of C 15C 18 n-alkenes in the presence of the corresponding n-alkanes in highly efficient glass capillary columns ( N ≈ 0.5·10 6 effective plates for n-alkenes with k ≈ 3), coated with C 87 hydrocarbon, Apiezon L or Carbowax 20M as stationary phase. The separation of cis- and trans-isomers of n-alkenes is made possible at a sufficiently high column efficiency by the different dependences of their retentions on temperature. The separation of positional isomers of n-alkenes having even carbon numbers for the pairs with the most centrally situated double bonds can be achieved only by use of highly efficient columns with polar or non-polar stationary phases ( n req ≈ 10 6 plates). Correlations between the structure and increments H, δ I, d I/d T and Δ I, derived from retention indices of n-alkenes, were used for identification as they reflect the position of the double bond, the geometrical arrangement and the length of the carbon chain in various isomers. The so-called propyl effect was confirmed for trans-4-alkenes, disturbing the regularity of all correlations between structure and retention. The influence of adsorption of n-alkanes and n-alkenes at the polar phase-carrier gas interface on the retention characteristics is discussed. Reproducibility of retention indices of n-alkene isomers is a function of their capacity ratios (thickness of the stationary phase film in the column), decreases with polarity of the stationary phase and the isomer under analysis and with the difference between a reference standard and the n-alken ein question. These effects are so significant for the columns with Carbowax 20M that a different retention sequence was found for some n-alkene isomers in columns with relatively small differences in the thickness of the stationary phase.

Liquid crystals : I. Synthesis and application as stationary phases in gas-liquid chromatography

2-R 1-4′-R-4-(4- n-alkoxybenzoyloxy)azobenzenes, where R = n-butyl, methyl and methoxy, R 1 = H and methyl and n-alkoxy = methoxy, ethoxy and n-butoxy were synthesized. They have long liquid crystalline “nematic” ranges and were used as stationary phases for the separation of positional isomers of di- and trisubstituted benzene. It was observed that lateral substitution (R 1) on the middle ring has a profound influence on the relative retentions of these compounds. The separation of mixtures of free phenolic isomers such as m- and p-cresols was difficult, whereas the separation of α- and β-naphthols could be easily achieved. Similarly, the separation of mixtures of free bases such as toluidines or picolines was difficult, but naphtylamines were separated with great ease. Also, the complete separation of all positional isomers of monochlorobiphenyl was achieved by using these liquid crystalline substrates.

Glass Capillary Gas Chromatography of Fatty Acids from Lipids of Marine Organisms

High resolution gas chromatography of fatty acid methyl esters derived from marine organism lipids was carried out on wall-coated open-tubular (WCOT) glass columns of different polarity.The times required for the analysis were shortened with increasing polarity of the liquid phases, but the column efficiency decreased apparently.Effective separations of positional isomers of monounsaturated and methylene-interrupted polyunsaturated fatty acids were obtained on SILAR-5CP column. The use of the column also made possible the separation of some non-methylene interrupted fatty acids. The equivalent chain lengths of 86 fatty acid methyl esters on SILAR-5CP have been presented.Plots of carbon number vs. log relative retention time for methyl esters of normal saturated C8C22 fatty acids showed slightly positive or negative deviations from straight lines.Peak area percentage of C22 : 6 methyl ester reduced with increasing retention time, but selection of appropriate conditions permitted fair agreement with weight percentage. Fatty acid compositions of sardine, pollack, copepod, seaweed and sea urchin have been presented as examples of the analysis.

Gas chromatographic resolution of homologous monoacyl and monoalkylglycerols.

AbstractHomologous series of 1 and 2 monoacyl and monoalkyl‐sn‐glycerols with 0–6 double bonds were resolved by gas liquid chromatography as the trimethylsilyl ethers and acetates using ethylene glycolsuccinate siloxane, cyanopropylphenylsiloxane, and methylsiloxane liquid phases. The trimethylsilyl ethers gave effective separation of positional isomers and saturated and unsaturated derivatives of acylglycerols on both ethylene glycolsuccinate siloxane and cyanopropylphenylsiloxane, columns. The corresponding acetyl esters were eluted on the basis of unsaturation, but positional isomers overlapped. The ethylene glycol‐succinate siloxane and cyanopropylphenylsiloxane columns resolved the positional isomers and saturated and unsaturated species of the alkylglycerols when chromatographed as the acetates, while the corresponding trimethylsilyl ethers were separated on the basis of unsaturation only. On methylsiloxane columns essentially complete overlapping was observed among the positional isomers within each acyl and alkylglycerol series when chromatographed as the acetates. There was an effective resolution of the positional isomers of the acylglycerols as the trimethylsilyl ethers on methylsiloxane. From methylsiloxane, the alkenylglycerols were eluted earlier than the alkylglycerols of corresponding carbon numbers. These species were not separated on conventional ethylene glycol‐succinate siloxane or cyanopropylphenylsiloxane columns.

Use of Liquid Crystals for the Separation of Position Isomers of Disubstituted Benzenes

Liquid crystals used as stationary phases for GC columns have been reported for the separation of a number of meta- and para-position isomers of disubstituted benzenes. Few of the ortho isomers have been reported and little information concerning the effect of the substituent type upon the order of elution of the isomers has been published. This paper summarizes work undertaken to add some of these data to the knowledge concerning the behavior of liquid crystal materials when used as stationary phases in gas chromatography. Three liquid crystal materials were studied: 4,4′-azoxydianisole, 4,4′-azoxydiphenetole, and 4,4′-biphenylylene bis[p-(heptyloxy)benzoate]. The behavior of the ortho, meta, and para isomers of twenty-five disubstituted benzenes was determined with one or more of these liquid crystals. Data are tabulated.

Chromatographic behaviour of isomeric long-chain aliphatic compounds : II. Argentation thin-layer chromatography of isomeric octadecenoates

A novel and improved procedure for separating positionally isomeric cis- and trans-octadecenoates on silver nitrate impregnated thin-layer plates is described. The method consists of triple development with toluene at −25° of layers having a 30 % level of silver nitrate impregnation. Substantial variation in the mobilities of a wide range of cis- and trans-octadecenoates is effected and the migration behaviour of all of the positionally isomeric cis- and trans-octadecenoates is predicted. Some examples are shown of the practical utility of this procedure for separation of positional isomers in natural and synthetic methyl ester mixtures.

Separation of positional isomers of long-chain unsatu rated fatty acid methyl esters by thin-layer chromatography and presence of mono- trans, di- cis isomers in commercial linolenic acid

Separation of positional isomers of long-chain unsatu rated fatty acid methyl esters by thin-layer chromatography and presence of mono- trans, di- cis isomers in commercial linolenic acid

Separation and identification of tryptamine-related indole bases by gas chromatographic methods

The possibility of separating and identifying a number of indole bases related to tryptamine by gas chromatographic techniques has been investigated. Methods were found for the separation of positional isomers of phenolic amines and related compounds in this series. The major indole base of epená, a South American snuff with reported hallucinogenic properties, was found to be 5-methoxy- N,N-dimethyltryptamine.