Prediction of retention time of phenols in liquid chromatography

The chromatographic behavior of phenols in reversed-phase mode liquid chromatography differs from that of non-ionic compounds such as alkyl alcohols, alkylbenzenes, halogenated benzenes, polyaromatic hydrocarbons, and aromatic acids. Therefore, the retention times of 61 phenols were measured in a system of an octadecyl bonded silica gel and acetonitrile/water mixtures. The logarithm of the capacity ratio (log k') was found to be a linear function of the hydrophobicity (log P) in acidic acetonitrile/water mixtures. This result was applied to a different octadecyl bonded silica gel. Eight phenols were selected as standard compounds, and their log k' values were measured in 0.05 M phosphoric acid in 10 to 90% acetonitrile/water mixtures. An empirical polynomial relation was obtained between the concentration of acetonitrile and the slope of the log k' vs log P curve. Finally the capacity ratio of all phenols were calculated in given eluents by the equations derived from the measurements of standard compounds and the calculated log P values. The difference between predicted capacity ratios and measured ones was within 10%.     

An approach to the prediction of retention times in liquid chromatography

The utility of Rekker's hydrophobic fragmental constant has been examined for optimization of reversed-phase mode liquid chromatographic separations. The chromatographic behavior of about 60 non-ionic compounds was measured in different acetonitrile/water mixtures and the logarithm of their capacity factors (log k) was correlated with their calculated hydrophobicities (log P). Linear relations were found in each case between log k and log P. The slope of the various lines was related to the percentage concentration of acetonitrile in the mobile phase. It was shown that, by using nine stand ard compounds and measuring their capacity factors in five eluents with different acetonitrile concentrations, the retention time could be predicted for 60 compounds. Calculation of the concentration of the organic modifier was also possible in a system of well coated octadecyl bonded packings with acetonitrile/water mixtures as eluent. Prediction of the capacity factor was accomplished to within 5% error.     

The optimized separation of aromatic carboxylic acids by HPLC

The optimization of the separation of eight aromatic carboxylic acids, using Hypersil SAS and MOS reverse phase columns, with respect to eluent (methanol : water) composition, pH, and temperature, is described. Window diagram techniques are used and the pH of the aqueous buffer and elution temperature are shown to be more critical than eluent composition.     

Regularities in the retention of isomeric aromatic hydrocarbons in liquid chromatography II. Analysis of polymethyl- and monoalkylbenzenes on silanized silica gel

Summary The regularities of the retention of polymethyl- and monoalkylbenzenes on a nonpolar adsorbent (silanized silica gel) with a polar eluent (23 isopropanol-water) have been studied. In contrast to the retention of the same compounds on a polar adsorbent (silica gel with hydroxylated surface) and with a nonpolar eluent (hexane) polymethylbenzenes are retained on silanized silica gel for a shorter time than the corresponding monoalkylbenzene isomers. The retention order of tert.-, sec.- and n-butylbenzenes is the reverse of that observed on silica gel having a hydroxylated surface. The retention order of xylenes, trimethyl- and tetramethylbenzenes is also the reverse of their retention order on silica gel having a hydroxylated surface. All the mixtures of the corresponding homologues and isomers have been separated.     

Regularities in the retention of nonsubstituted polynuclear aromatic hydrocarbons in liquid chromatography on polar adsorbents

Summary The retention of polynuclear aromatic hydrocarbons (PAH) is studied in liquid chromatography, using columns packed with silica gel with a hydroxylated surface and the same silica gel with surface-bonded amine and nitrile groups. A linear relationship was found between retention and the number of π-electrons in the molecule, the size of the molecule and the different types of carbon atoms in the molecule. The different carbon atoms in a PAH molecule contribute differently to the retention and this contribution depends on the nature of the polar functional groups on the adsorbent's surface and on the nature and concentration of a polar modifier present in a non-polar eluent.     

Study of qualitative analysis by liquid chromatography using porous polymer gel

Summary The possibility of qualitative analysis by liquid chromatography is discussed in systems of polystyrene gel or chemically bonded octadecyl packing with alcohols as standard compounds. Rekker's hydrophobic fragmental constants are used for identification purposes. Capacity ratios for alcohols can be calculated from the hydrophobicities in spite of changes in solvent mixture ratios if the capacity ratio for an internal standard compound is measured.     

Evaluation of dead time calculation in liquid chromatography using a linearization procedure for homologous series

A mathematical method for the calculation of the dead time (t_m) in HPLC was evaluated using a computer simulation approach, in which artificial perturbations were introduced to Simulated homolog retention times. The calculation was based on a modified and extended Grobler and Bálisz (GB) method. Investigated wav how the precision of the calculated t_M is affected by: statistical fluctuations in retention times and which, and how many homolog retention times are used. Based on these simulations a two-step procedure for the t_M calculation is proposed: In the first step the linearity of log t_{R, n} vs carbon number n is checked using as many homolog retention times as possible. The slope value b^o of the first linear regression in the GB method is used for the selection of homolog retention times in the final t_M calculation. In the second step the optimal selection of homologs is made and the final t_M calculation is carried out. Guidelines for homolog selection are given.     

反相高效液相色谱法快速测定大麦籽粒中13种酚酸类化合物

建立了快速测定大麦籽粒中13种酚酸类化合物(没食子酸、原儿茶酸、绿原酸、对羟基苯甲酸、咖啡酸、香草酸、丁香酸、间羟基苯甲酸、对香豆酸、阿魏酸、藜芦酸、邻香豆酸和水杨酸)的反相高效液相色谱方法.采用SUPELCOAscentis(@) C18色谱柱(150 mm×4.6 mm,5μm)分离,流动相为甲醇-0.1％甲酸水溶...     

Reversed-phase high-performance liquid chromatography of fatty acid methyl esters with particular reference to cyclopropenoic and cyclopropanoic acids

High performance liquid chromatography of saturated, monounsaturated, diunsaturated, triunsaturated, cyclopropenoic (malvalic and sterculic) and cyclopropanoic (cis-8,9-methylenehexadecanoic and dihydrosterculic) fatty acids was performed with their methyl esters. All separations were carried out with two types of reversed phase columns, the eluent consisting of an acetonitrile/water mixture. The effect of water was studied in the range 0–15%. The best separation was obtained with acetonitrile/water (85:15 v/v). Quantitative results indicated that the detection limits depended upon ultraviolet wavelength and in the present study were 4 ng of methyl sterculate and 125 ng of methyl dihydrosterculate at 195 nm.     

Regularities in the retention of isomeric aromatic hydrocarbons in liquid chromatography. Part I. Analysis of polymethyl-and monoalkyl benzenes and polynuclear aromatic hydrocarbons on hydroxylated silica gel

Summary The retention on hydroxylated silica gel surface upon elution with n-hexane increases in the series of polymethylbenzenes and decreases in the series of mono-alkylbenzenes (after toluene) All iosmers of xylenes, trimethyl- and tetramethylbenzenes are separated as well as isomers of tert.-phenyls, isomers of several dimethylnaphthalenes, anthracene, phenanthrene and poly aromatic hydrocarbons with five condensed rings. These separations are based on the influence of the corresponding molecule structures on the intermolecular interaction with both the adsorbent (strengthening or weakening the energy of hydrogen bonds) and the eluent (intermolecular interaction with n-hexane increases upon lengthening the alkyl group). The effect of orientation at the surface of the investigated hydrocarbon molecules on their retention is also discussed.     

Separation of racemic 2,4-dinitrophenyl amino acids on zirconia-immobilized quinine carbamate in reversed-phase liquid chromatography

Zirconia is known to be one of the best chromatographic support materials due to its excellent chemical, thermal, and mechanical stability. A quinine carbamate-coated zirconia was prepared as a chiral stationary phase for separation of enantiomers of DNP-amino acids in reversed-phase liquid chromatography. Retention and enantioselectivity of this phase were compared to those for quinine carbamate bonded onto silica. Most amino acids studied were separated on the quinine carbamate-zirconia CSP although retention was longer and chiral selectivity was somewhat lower than on the corresponding silica CSP. Increased retention and decreased selectivity are probably due to strong non-enantioselective Lewis acid-base interactions between the amino acid molecule and the residual Lewis acid sites on the zirconia surface.     

Investigation of the low temperature effect in reversed phase micro high performance liquid chromatography

The low temperature effect has been investigated in reversed phase micro high performance liquid chromatography with various aqueous mobile phase systems by an approach based on enthalpy-entropy compensation. The compensation temperatures, Tc , were determined for these systems, and the results show that the retention mechanism in a lower temperature column is similar to that in a normal temperature column in the reversed phase mode at mobile phase water contents above 9.8%. At lower water contents, the separation mechanism is different from that in the reversed phase mode, but otherwise very similar to that in the normal phase mode.     

Chromatographic performance of deuterated solvents in reversed phase micro high performance liquid chromatography

The chromatographic performance of the deuterated solvents, CD₃OD and D₂O, has been investigated in reversed-phase micro high performance liquid chromatography. The chromatographic performance of CD₃OD is only slightly superior to that of CH₃OH. However, the performance of D₂ is significantly superior to that of H₂O, separation of aromatics being improved by about 30%. D₂ is a particularly powerful solvent for the separation iof deuterated and non-deuterated compounds.     

Optimization of the separation of polymethoxylated flavones in reversed phase liquid chromatography

Summary An optimization procedure for the chromatographic separation of polymethoxylated flavones is described. A polyhedron working volume is drawn from a truncated tetrahedron constructed from the four solvents selected. The response function is an n'th degree polynomial the coefficients of which are calculated through the least squares method by means of B=(XX)^–1XY in which XX is the information matrix (XX)^–1 the variance covariance matrix. X is the matrix of the model (polynomial built from the selected experiments) and X the transposed matrix. Y is the quality of the chromatogram defined by the resolution of each pair of solutes. An exchange algorithm matrix yields the minimum number of runs necessary to obtain a correct B. Isoresponses curves are drawn by intersection of a cubic volume which includes the polyhedron to give a better insight. An optimal zone is then determined and provides the solvent composition. An optimized chromatogram of 8 methoxylated flavones demonstrates the application of the method.     

Retention behavior of aromatic hydrocarbons and dansylamino acids in reversed-phase liquid chromatography with cation-exchange-induced stationary phases

The retention behavior of aromatic hydrocarbons and dansylamino acids on cation-exchangers modified with alkylammonium ions has been examined by microcolumn liquid chromatography. Several parameters affected the retention of analytes, involving concentration of the modifier in the mobile phase, its alkyl chain length and mobile phase composition. Stationary phases modified with a reagent having longer alkyl chains achieved better column efficiency.     

Effect of high temperature in determination of the mathematical dead time and the constants of the n-alkane retention time curve in gas chromatography

In gas liquid chromatography, the column dead time and the constants of the n-alkane retention time curve are calculated by the multiparametric least-squares regression iterative method at high temperature between 180 and 270°C. The method was applied to two types of columns. The first group includes eight packed columns (seven OV polymethylphenylsiloxane and Apolane-87), while the second includes five glass capillary columns (four methylsilicone with different film thicknesses and Apolane-87). The calculated tM and b values were compared with those obtained by Guardino's, Grobler's, and Kaiser's methods. The influences of coating thickness and temperature thereon were investigated.