Retention behavior of ginsenosides on a poly(vinyl alcohol)-bonded stationary phase in hydrophilic interaction chromatography

The influences of the organic component of the mobile phase and the column temperature on the retention of ginsenosides on a poly(vinyl alcohol) (PVA) bonded stationary phase operated under hydrophilic interaction chromatographic mode were investigated. The retention of the ginsenosides was found to increase with increasing amount of acetonitrile (MeCN) in the mobile phase, which is typical of hydrophilic interaction chromatographic behavior. It was also found that the retention of the analytes was highly affected by the type of the organic modifier used. Aqueous MeCN (75–90%) gave the most satisfactory retention and separation of ginsenosides Rf, Rg1, Rd, Re, Rc, Rb2 and Rb1 compared with aqueous methanol, isopropyl alcohol or tetrahydrofuran at the same composition levels. The effects of the different types of organic modifiers on the retention of the analytes were attributed to their solvent strength and hydrogen-bond accepting/donating properties. The effect of temperature on the retention of ginsenoside on the PVA-bonded phase was assessed by constructing van’t Hoff plots for two temperature ranges: subambient (273–293 K) and ambient-elevated (298–333 K) temperatures. van’t Hoff plots for all analytes were linear at the two temperature intervals; however, the slopes of the lines corresponding to ginsenosides Rg1 and Re were completely different from those for the rest of the analytes especially in the subambient temperature range. Enthalpy-entropy compensation (EEC) studies were conducted to verify the difference in thermodynamics observed for ginsenosides Rg1 and Re compared with the other analytes. EEC plots showed that Rf, Rd, Rc, Rb2 and Rb1 were possibly retained by the same retention mechanism, which was completely different from that of Rg1 and Re at subambient temperatures. Retention prediction models were derived using multiple linear regression to identify solute attributes that affected the retention of the analytes on the PVA-bonded phase. The mathematical models derived revealed that the number of hydrogen-bond donors and the ovality of the molecules are important molecular properties that govern the retention of the compounds on the chromatographic system.     

Isocratic separation of erythromycin, related substances and degradation products by liquid chromatography on XTerra RP18

Summary A simple, sensitive, selective and robust isocratic LC method is described for the analysis of erythromycin on XTerra RP₁₈. The main component, erythromycin A, is separated from all known related substances and degradation products. Several unknown impurities are also separated. Acetonitrile-0.2 MK₂HPO₄pH7.0-water, (35∶5∶60, v/v) was used as a mobile phase at 1.0 mL min⁻¹. UV detection was at 215 nm. The robustness of the method was evaluated by a full-factorial experimental design.     

Preparation of anhydrotrypsin-immobilized diol silica as a selective adsorbent for high-performance affinity chromatography of peptides containing arginine or lysine at their C-termini

Summary Anhydrotrypsin (AHT), a catalytically inert derivative of trypsin in which the active site serine residue was converted to dehydroalanine residue by chemical modification, was immobilized onto diol silica through the activation with trifluoroethanesulfonyl chloride, and an AHT-diol-silica column was used for high-performance affinity chromatography separation of peptides containing arginine or lysine at their C-termini from the others. Improved separation in terms of speed was accomplished.     

Temperature effect in separation of fullerene by high-performance liquid chromatography

Summary The effect of column temperature, especially at low temperatures, on the separation of fullerenes on monomeric and polymeric octadecyl silica (ODS) bonded phases has been studied. Decreasing the column temperature induces an increase in selectivity. The best temperature for the separation of fullerenes was determined for both types of ODS phase with n-hexane eluent. The selectivity for higher fullerenes on monomeric phases becomes similar to that on polymeric phases to low temperature. It has been found that as the carbon content of monomeric phases is increased, the selectivity also becomes similar to polymeric phases.     

Comparison of provitamin A determination by normal-phase gravity-flow column chromatography and reversed-phase high performance liquid chromatography

Summary The provitamin A content of some food samples was determined by methods involving MgO: Hyflosupercel gravityflow column chromatography (GFCC) and reversed phase high performance liquid chromatography (HPLC), the quantitation being done by external standardization (HPLC-ES) or internal standardization (HPLC-IS) with Sudan. The results obtained with - and -carotene in carrots, -carotene and -cryptoxanthin in papaya and -carotene in tomato and kale agreed well, showing that any of the these techniques can be used, provided the analysis is done under optimum conditions. Good separation of the different provitamins using GFCC depends on the analyst's skill and visual acuity. HPLC-ES required a constant supply of provitamin standards, thus the varying purity of commercially available standards and the high instability of these compounds could pose grave problems. Due to the stability of Sudan, HPLC-IS appeared to be the method of choice although passage of the extract through a MgO: Hyflosupercel minicolumn was required prior to injection to separate chlorophylls, dihydroxy- and polyoxycarotenoids which would otherwise elute with Sudan. Nonconformity of the Sudan structure to those of the provitamins did not effect the quantitative results. The chromatographic separation, identity and quantification of the provitamins could be more easily established by using HPLC-IS, complemented with GFCC.     

Determination of metoprolol in human plasma by column switching high-performance liquid chromatography

Summary Retention characteristics of metoprolol have been studied in reversed phase mode on RP2, RP8 and CN columns. The plots of retention time as a function of the acetonitrile content and of the ionic strength of the mobile phase permitted the choice of the best conditions to separate metoprolol from plasma components by switching of these three types of columns.Human plasma (0.5–1 ml) diluted with water is first injected on a RP2 column (25–40 m particle diameter, prepared by dry packing) and rinsed with water. The sample is then back eluted with acetonitrile-0.022 M acetate buffer (7525, v:v) and switched to a CN column (10 cm long, 5 m particle diameter). The heart cut of the eluate is selected and loaded on a RP8 analytical column (25 cm long, 5 m particle diameter) with acetonitrile-0.088 M acetate buffer (7525, v:v) as mobile phase.Auto-sampler and switching valves are actuated automatically by a computing integrator based on a fixed time schedule. The duration of one cycle is about 30 min, but the last analytical step is about 15 min and represents the time interval between two injections. Metoprolol, its alpha-hydroxy metabolite and the internal standard are detected by fluorescence (_ex= 225 nm; _em > 320 nm).Presented at the 14th International Symposium on Chromatography London, September, 1982     

Optimized separation of pesticide enantiomers by chiral high-performance liquid chromatography

Summary A computer-assisted method is described for optimization of multi-component, mobile phase selection for separating enantiomers of four pesticides in normal-phase HPLC. The method is based on the triangle, solvent-selection concept using a statistical scanning method. The optimization of the separation over the experimental region is based on a special polynomial estimation from seven experimental runs, and resolution (R_s) is used as the selection criterion. Excellent agreement was obtained between predicted and experimental data.     

Influence of injection parameters on column performance in nanoscale high-performance liquid chromatography

Summary The influence of the injection volume and the sample solvent on column efficiency has been evaluated in packed nano liquid chromatography using columns 150μ i.d. Evaluation of column performance was by means of reduced plate height (h) versus reduced velocity (v) for four polyaromatic hydrocarbon test compounds (PAHs). When compounds are dissolved in a weak solvent (such as MeCN: H₂O, 30∶70), and whatever the injection volume −60 or 200 nL-a gain in efficiency can be observed due to the well-known on-column focusing phenomenon, but keeping constant solute retention factors. Under optimized conditions (flow rate: 150 nLmin⁻¹, solvent sample MeCN: H₂O, 30∶70, injection volume 200 nL), a reduced plate height of 1.83 has been obtained for a 15 μm C₁₈ packing corresponding to 36000 plates m⁻¹, which illustrates the absence of any extracolumn band broadening under nano LC conditions.     

Determination of ephedra alkaloids by high-performance liquid chromatography

Summary Two simple methods were developed for the simultaneous determination of six alkaloids (ephedrine, pseudoephedrine, norephedrine, norpseudoephedrine, methylephedrine and methyl-pseudoephedrine) inEphedrae Herba by high-performance liquid chromatography. The first method was carried out by using a Cosmosil 5C₁₈-MS column with a gradient solvent system consisting of a phosphate buffer and acetonitrile, and detection at 210 nm. The contents of alkaloids in non-pretreated ephedra herb extracts could be determined easily in 50 min. Alternatively, the alkaloids could be determined within 35 minutes by using a Cosmosil 5C₁₈-MS column with an isocratic solvent system of a sodium dodecyl sulfate-acetonitrile solution. The two methods are compared and discussed.     

High-performance liquid chromatographic analysis of ginsenosides inPanax ginseng extracts using glass-ODS column

Summary Octadecyl-porous glass was prepared and used as the packing for reversed-phase high-performance liquid chromatography. A mixture of ginsenosides, saponins of ginseng was analyzed with detection at 203 nm. Ginsenosides Rf, Rg₂, Rb₁, Rc, Rb₂, and Rd were separated with acetonitrile-water (27.5:72.5) as the mobile phase. A well-resoluted chromatogram of ginsenosides Ro, Rg₁ and Re was also obtained with acetonitrile-water (16.5:83.5). The whole separation was achieved in 12 min with a flowrate of 1 ml/min. Calibration curves of ginsenosides Rb₁, Rc, Rb₂, Rd, Rg₁ and Re were linear up to 5 μg. It can be concluded that the rapid and accurate analysis of ginsenosides is possible by the described method.     

Isocratic high-performance liquid chromatographic determination of plasma adenosine

Summary Due to manifold physiological and cardioprotective actions of adenosine, the demand for a simple but accurate method to determine its concentration in plasma is increasing. The aim of this study was firstly to develop a simple isocratic method instead of the gradient elution or peak-shifting techniques used earlier and secondly to check conflicting data on the composition of stop-solution, added to the sample in order to prevent changes in adenosine concentration. Isocratic elution improved signal to noise ratio and concentrations of 100 mol L^–1 dipyridamole and 2.5 mol L^–1 erythro-9(2-hydroxy-3-nonyl)adenine in the blood sample effectively prevented both adenosine formation and degradation, even without the use of a 5-ecto-nucleotidase inhibitor. Lowering the concentration of dipyridamole to 25 mol L^–1 caused more than a tenfold increase of adenosine concentration in two out of five cases and even 100 mol L^–1 dipyridamole alone is not sufficient to inhibit adenosine deaminase in blood samples.     

Size-exclusion and high-performance liquid chromatography separation of peptides from peptic haemoglobin hydrolysate obtained by ultrafiltration

Summary Gel filtration (size-exclusion) and high-performance liquid chromatography have been used to separate peptic peptides from haemoglobin hydrolysate. Elution profiles on Sephadex G-25 displayed nine fractions with molecular weights lower than 6500 daltons. Each fraction was analysed for total amino acid content and showed less than 1% free amino acids. Reversed phase HPLC, using ammonium acetate buffer and acetonitrile as solvent, was applied to each fraction in order to obtain pure peptide peaks. The importance of acquiring a better knowledge of such an hydrolysate is discussed. Various potential applications of this type of hydrolysate, some of them already being undertaken, are envisaged.     

Effect of subambient temperature on RP-HPLC of β-carotene isomers

Summary (E)-β-carotene and three mono-(Z)-isomers (9(Z)-, 13(Z)-, 15(Z)-) were eluted isocratically on a VYDAC 201TP54 column, using column temperatures between +30°C and −7°C. In this temperature range, their elution order changed dramatically with decreasing temperature. Nearly baseline separation of the four isomers within one hour was achieved at −7°C. For laboratory routine, temperatures of 9±2°C can be recommended. Thus, a good resolution of β-carotene- and lycopene-isomers (e.g. in tomato products) within 30 minutes is possible.     

High temperature normal phase liquid chromatography of aromatic hydrocarbons on bare zirconia

The normal phase HPLC behavior of a bare zirconia column was studied at temperatures up to 200 °C using a hexane mobile phase. The use of elevated column temperatures significantly decreased the retention of twenty five aromatic model compounds according to the van't Hoff equation (>30-fold decrease for some compounds). Large improvements in peak shape, efficiency (>2.2-fold), aromatic group-type selectivity, and column re-equilibration times (>5-fold) were obtained at elevated temperatures. The thermal decomposition of two polar nitrogen compounds (indole and carbazole) was observed in a hexane/dichloromethane mobile phase at temperatures greater than 100 °C. The first order decomposition of carbazole was studied in further detail.     

The use of microparticulate guard columns in reverse-phase high-performance liquid chromatography

Summary The use of mciroparticulate guard columns in conjunction with high efficiency reverse-phase columns has been evaluated in terms of system efficiency and peak symmetry using three standard test mixtures. The effects of linking tube dimensions, particle size of guard column packing and the use of low or zero dead volume couplings have been investigated. Both valve and stopped-flow injection techniques have been used. Recommendations are given for the most efficient use of guard columns.     

Computer-assisted transposition of conditions in high-performance thin-layer chromatography to high-performance liquid chromatography for the separation of pesticides

Summary A computer-assisted method is presented for mobile phase selection for the optimal separation of pesticides by HPTLC and HPLC. The system is based on a plot of solute retention value and separation criterion vs. binary mobile phase composition for graphic optimization. The result of HPTLC can be transposed to HPLC for optimal separation. The transposition equation is given.     

Implications of column peak capacity on the separation of complex peptide mixtures in single- and two-dimensional high-performance liquid chromatography

Column peak capacity was utilized as a measure of column efficiency for gradient elution conditions. Peak capacity was evaluated experimentally for reversed-phase (RP) and cation-exchange high-performance liquid chromatography (HPLC) columns, and compared to the values predicted from RP-HPLC gradient theory. The model was found to be useful for the prediction of peak capacity and productivity in single- and two-dimensional (2D) chromatography. Both theoretical prediction and experimental data suggest that the number of peaks separated in HPLC reaches an upper limit, despite using highly efficient columns or very shallow gradients. The practical peak capacity value is about several hundred for state-of-the-art RP-HPLC columns. Doubling the column length (efficiency) improves the peak capacity by only 40%, and proportionally increases both the separation time and the backpressure. Similarly, extremely shallow gradients have a positive effect on the peak capacity, but analysis becomes unacceptably long. The model predicts that a 2D-HPLC peak capacity of 15,000 can be achieved in 8 h using multiple fraction collection in the first dimension followed by fast RP-HPLC gradients employing short, but efficient columns in the second dimension.     

Separation of alkylnaphthalenes by preparative liquid chromatography using column switching systems

Summary The paper describes the separation of the mixture of alkynaphthalenes from distillation cuts of a pyrolysis oil, by preparative liquid chromatography on silica. The design of the system permits the connection of the columns to form multicolumn systems.The samples were first separated on a single column. The mixtures were further separated using two-column chromatography systems.The obtained fractions were analyzed by capillary gas chromatography. In most cases a substantial increase in the concentrations of the individual components was achieved. In several cases, pure compounds have been obtained. Separation efficiency increases in the following order: single column, two directly coupled collumns, two-step switching chromatography, heartcutting.     

Optimization of St John's Wort flavonoid separation by reversed phase liquid chromatography on a silica-based monolithic column

Summary An optimization procedure for the separation of flavonoids from St John's Wort by reversed-phase high-performance liquid chromatography on monolithic stationary phase is described. Three-component mobile phase systems are studied using experimental design methodology. The starting experimental domain is a triangle, each vertex of which is a pure component. From preliminary isocratic experiments, truncations in the domain are performed leading to a quadrilateral shape working domain with no symmetry. To cope with both separation and analysis time, desirability functions are used. Optimal conditions are finally given by binary systems and the four flavonoids are separated in less than seven minutes.