Separation of acidic compounds by strong anion-exchange capillary electrochromatography

Separation of the acidic compounds in the ion-exchange capillary electrochromatography (IE-CEC) with strong anion-exchange packing as the stationary phase was studied. It was observed that the electroosmotic flow (EOF) in strong anion-exchange CEC moderately changed with increase of the eluent ionic strength and decrease of the eluent pH, but the acetonitrile concentration in the eluent had almost no effect on the EOF. The EOF in strong anion-exchange CEC with eluent of low pH value was much larger than that in RP-CEC with Spherisorb-ODS as the stationary phase. The retention of acidic compounds on the strong anion-exchange packing was relatively weak due to only partial ionization of them, and both chromatographic and electrophoretic processes contributed to separation. It was observed that the retention values of acidic compounds decreased with the increase of phosphate buffer and acetonitrile concentration in the eluent as well as the decrease of the applied voltage, and even the acidic compounds could elute before the void time. These factors also made an important contribution to the separation selectivity for tested acidic compounds, which could be separated rapidly with high column efficiency of more than 220000 plates/m under the optimized separation conditions.     

Liquid Chromatography of Adenine,Caffeine, Theobromine,Theophylline, Tannic Acid,Xanthine and its Application to Tea Analysis

A polystyrenesulfonic acid resin in the hydrogen form is a useful stationary phase for chromatographic separation of adenine, caffeine, theobromine, theophylline, tannic acid, xanthine and tea components with aqueous ethanol as eluent. The results indicated that caffeine, theophylline, theobromine, adenine, xanthine and tannic acid could be separated on a column of Dowex-50W-X8(H)⁺ by elution with 25% ethanol as eluent.     

Liquid chromatography of cardiac glycosides

Summary LC was used for the separation of several cardiac glycosides with water-alcohol eluents on a silanised silica column. The influence of temperature and composition of the eluent on the retention time, the retention volumes of the glycosides, the selectivity and the capacity factor of the column were studied. Higher temperature and a higher ethanol content in the eluent reduce the retention time, the selectivity and the capacity factor but the efficiency of the column increases. The best separation of six glycosides studied was obtained at 50°C, the ethanol content in the eluent being about 30%.     

Computer simulation of ion chromatography separation: an algorithm enabling continuous monitoring of anion distribution on an ion-exchange chromatography column

A computer algorithm for the calculation of ion chromatography separation is presented. It is based on the calculation of equilibrium concentrations of present analyte in discrete column segments. The continuous column is treated as a number of discrete cells or segments where the equilibration process between the stationary phase and the eluent is simulated. The ion-exchange equilibration process is supposed to be instantaneous and quantitative. The continuous flow of the eluent is rendered by discrete transfers. The size of each transfer of the eluent corresponds to a portion of the volume contained in one column segment. The equilibrium calculations in all column segments are repeated for each transfer of the eluent, through all the stages of the chromatographic process. The distribution of the analytes between the stationary phase and the eluent can be monitored at any step and in any column segment which means that the described algorithm provides the spatial and time concentration profiles. The simulated chromatogram is acquired as a time-concentration profile in the last column segment. The obtained chromatograms are in good agreement with the experimental ones. The distribution of ions between the stationary phase and the eluent in the early stages of the ion chromatographic process can thus be studied with confidence.     

Hydrophilic interaction liquid chromatography with alcohol as a weak eluent

There has been a significant increase of interest in polar compound separation by hydrophilic interaction liquid chromatography (HILIC), in which acetonitrile is mostly used as a weak eluent. Although replacing acetonitrile with alcohols as organic modifiers has been previously reported, the separation mechanism was poorly understood. In this paper we explored the separation mechanism through the method development for the analysis of the trace amounts of polar and basic hydrazines, which were genotoxic in nature. Separation parameters such as the type and concentration of alcohol, acid modifier, and buffer in mobile phase as well as the choice of stationary phase and column temperature were studied. The data indicated that both electrostatic and hydrophilic interactions contributed to the retention and separation of the hydrazines. The results presented here provide insight into the adjustment of the retention and separation of analytes in HILIC mode with alcohol as a weak eluent. The optimized HILIC method coupled with chemiluminescent nitrogen detection (CLND) is simple and sensitive (reporting limit at 0.02%) and was applied to simultaneous analysis of hydrazine and 1,1-dimethylhydrazine in a pharmaceutical intermediate.     

氯甲基苯乙烯包夹硅基单柱强阴离子色谱柱的研究

采用氯甲基苯乙烯包夹硅胶基质制备出一种新型强阴离子色谱柱填料，该填料具有良好的色谱柱性能，可以较地的分离常见的６种无机阴离子，６种低碳链有机酸以及甲酸，乙酸和一些无机阴离子的混合物。     

Double gradient ion chromatography using short monolithic columns modified with a long chained zwitterionic carboxybetaine surfactant

The rapid separation of inorganic anions on short monolithic columns permanently coated with a long chained zwitterionic carboxybetaine-type surfactant is shown. The surfactant, N-dodecyl-N,N-(dimethylammonio)undecanoate (DDMAU), was used to coat 2.5, 5.0 and 10 cm long reversed-phase silica monoliths, resulting in a permanent zwitterionic exchange surface when used with aqueous based eluents. The unique structure of the surfactant results in a charge double layer structure on the surface of the stationary phase, with strong internal anionic and weak external cationic exchange groups. The dissociation of the weak external carboxylic acid group acts to shield the inner anionic exchange site, resulting in substantial effective capacity changes with eluent pH. Utilising this effect with the application of an eluent pH gradient, simultaneously combined with eluent flow-rate gradients, very rapid simultaneous separations of both weakly retained anions and strongly retained polarisable anions was possible, with up to 10-fold decreases in overall run times. Coating stability and retention times under isocratic and isofluentic eluent conditions were shown to be reproducible over >450 repeat injections, with peak efficiency values averaging 29,000 N/m for the 2.5 cm column and 42,000 N/m for the 10 cm monolithic column, again under isocratic elution conditions.     

Ion chromatographic investigation of the ion-exchange properties of microdisperse sintered nanodiamonds

The possibility of using sintered diamonds as a stationary phase in ion chromatography has been evaluated. Bare sintered synthetic nanodiamonds demonstrated the properties of a weak cation-exchanger. The observed ion-exchange selectivity is similar to carboxylic type cation-exchangers. The regularities of retention of alkali, alkaline-earth and transition metal ions on a column packed with sintered nanodiamonds in dilute nitric acid were studied and the occurrence of chelating properties was noted. For the first time chromatographic separations of model mixtures of cations on diamonds have been obtained.     

Ion-chromatographic behavior of alkali metal cations and ammonium ion on zirconium-adsorbing silica gel

The preparation and evaluation of zirconium-adsorbing silica gel (Zr-Silica) as an ion-exchange stationary phase in ion chromatography for inorganic anions and cations was carried out. The Zr-Silica was prepared by the reaction of silanol groups on the surface of the silica gel with zirconium butoxide (Zr(OCH2CH2CH2CH3)4) in ethanol. The ion-exchange characteristics of the Zr-Silica were evaluated using 10 mM tartaric acid at pH 2.5 as eluent. The Zr-Silica was found to act as a cation-exchanger under the eluent conditions. The retention behavior of alkali and alkaline earth metal cations was then investigated. The Zr-Silica column was proved to be suitable for the simultaneous separation of alkali metal cations and ammonium ion. Excellent separation of the cations on a 15 cm Zr-Silica column was achieved in 25 min using 10 mM tartaric acid as eluent.     

Separation and purification of fructooligosaccharides on a zeolite fixed‐bed column

Fructooligosaccharides (FOS), a well‐known prebiotic product, are obtained by enzymatic synthesis and consist of a mixture of mono‐ and disaccharides. In this work, a methodology for their separation and purification was developed using a zeolite fixed‐bed column. The effects of column temperature (40–60°C), eluent flow rate (0.10–0.14 mL/min), injected to bed volume percent ratio (2.6–5.1%), and ethanol concentration in the eluent (40–60%, v/v) were investigated using a fractionary factorial design (2^4–1), having the separation efficiency and purity as target responses. Additional experiments were performed as well, where the temperature and ethanol concentration were studied in a central composite design (2²). In this work, the zeolite fixed‐bed column was shown to be a good alternative for FOS purification, allowing a FOS purity of 90% and separation efficiency of 6.86 between FOS and glucose, using an eluent at 45°C with 60% ethanol concentration.     

Heterogeneity of adsorption mechanisms in chiral normal-phase liquid chromatography. 2-Propanol and ethyl acetate adsorption equilibria

The adsorption equilibria of two commonly employed strong mobile phase modifiers, ethyl acetate and 2-propanol, on a polysaccharide-based chiral stationary phase have been studied by modeling nonlinear perturbation peaks measured after equilibration of the column with hexane (the weak component of the binary mixture). The investigation of adsorption processes from dilute solutions for species that are strongly retained on the stationary phase could be performed by this approach. On the opposite, limits of traditional linear perturbation technique for isotherm determination, in presence of strong interactions, have been evidenced. Alcohol adsorption has been modeled by a single Langmuir isotherm, while the ester has required a BiLangmuir model. These findings have found to be in a semi-quantitative agreement with available spectroscopic data about 2-propanol and ethyl acetate adsorption on thin silica sol-gel films in contact with a weak solvent. Experimental features observed for racemic separation on polysaccharide-based chiral stationary phases, such as the dependence of the separation factor on the amount and type of the employed additive, have been explained in light of these measurements.     

聚马来酸包夹硅胶基质单柱弱阳离子色谱柱填料

用马来酸包夹硅胶基质制备出一种新型弱阳离子色谱柱填料，该填料具有良好的色谱性能，可以较好地分离碱金属离子、碱土金属离子及一价胺离子。另外该填料可同时分离一价、二价金属阳离子。本文还考察了流动相的ｐＨ值和浓度对溶质保留的影响。     

Micro-HPLC 1: Linear “breakthrough-gradients” using the discontinuous interface between eluents in a packed gradient-generator column

A new approach is described for micro-HPLC for generating nearly linear gradients in the 40 to 550 μl volume range. These “linear” gradients have a straight line middle portion with a gentle curved onset of the gradient andd rapid attainment of the final 100% composition. Such “breakthrough-gradients” are produced by the interface between weak and strong eluent when a small (3 × 0.46 cm) packed “gradient-generator” column is switched abruptly from weak to strong eluent. The model system described here, without an analytical column, shows gradients from weak eluent (water) to strong eluent (modele with water plus acetone) using a 12-port high pressure valve to 1) vent the “flush” eluent, and simultaneously 2) begin the gradient and 3) make the sample injection. The volume of the gradient can be changed in two ways. If the breakthrough-generator is packed with very large (300 to 1600 μm nonporous particles, the gradient volume is nearly independent of the flush flow rate. The flush flow is used to quickly move the breakthrough-interface to the head of the micro-HPLC column. However, the gradient volume can be increased by using larger solid particles in the gradient-generator column because of increased eddy diffusion. If the gradient-generator is packed with porous particles (75–150 μm), the gradient volume can be changed over a broad range by simply changing the flush flow rate. This simple control over the gradient volume is due to the increased mass transfer of weak eluent out of the porous particles into the strong eluent at higher flush flows. The breakthrough-interface gradient-generator column approach provides the following advantages for micro-HPLC:

• – Gradient volumes can be readily varied over the micro-HPLC range 40–550 μl.
• – Gradient are linear with smooth and rapid onset of intial and final concentrations.
• – Gradient are produced inexpensively with a single high-pressure precision pump.
• – Gradient can be automated with a timer and single 12-port valve that provides both column regeneration and sample injection.

     

亲水作用色谱柱与阳离子交换树脂柱结合分离无机离子（英文）

A combination of hydrophilic interaction chromatographic(HILIC) column and a weakly acidic cation-exchange resin(WCX) column was used for simultaneous separation of inorganic anions and cations by ion chromatography(IC).Firstly,the capability of HILIC column for the separation of analyte ions was evaluated under acidic eluent conditions.The columns used were SeQuant ZIC-HILIC(ZIC-HILIC) with a sulfobetaine-zwitterion stationary phase(ZIC-HILIC) and Acclaim HILIC-10 with a diol stationary phase(HILIC-10).When using tartaric acid as the eluent,the HILIC columns indicated strong retentions for anions,based on ion-pair interaction.Especially,HILIC-10 could strongly retain anions compared with ZIC-HILIC.The selectivity for analyte anions of HILIC-10 with 5 mmol/L tartaric acid eluent was in the order of I-> NO-3 > Br-> Cl-> H2PO-4.However,since HILIC-10 could not separate analyte cations,a WCX column(TSKgel Super IC-A/C) was connected after the HILIC column in series.The combination column system of HILIC and WCX columns could successfully separate ten ions(Na+,NH+4,K+,Mg2+,Ca2+,H2PO-4,Cl-,Br-,NO-3 and I-) with elution of 4 mmol/L tartaric acid plus 8 mmol/L 18-crown-6.The relative standard deviations(RSDs) of analyte ions by the system were in the ranges of 0.02%-0.05% in retention times and 0.18%-5.3% in peak areas through three-time successive injections.The limits of detection at signal-to-noise ratio of 3 were 0.24-0.30 μmol/L for the cations and 0.31-1.2 μmol/L for the anions.This system was applied for the simultaneous determination of the cations and the anions in a vegetable juice sample with satisfactory results.     

HPLC semi‐preparative separation of diclazuril enantiomers and racemization in solution

Diclazuril has been widely used in poultry feed for prevention and treatment of coccidiosis, and its chiral separation is rarely reported. Herein, semi‐preparative separation method of diclazuril enantiomers has been developed through normal‐phase high‐performance liquid chromatography. Effects of chiral stationary phases, alcoholic modifiers, and column temperature on separation of diclazuril were discussed in detail. Both the single‐urea‐bound 4‐chlorophenylcarbamoylated β‐cyclodextrin and amylose tris(3,5‐dimethylphenylcarbamate)‐coated chiral stationary phases showed strong ability in separation of diclazuril by using n‐hexane–trifluoroacetic acid–ethanol. Then, semi‐preparative separation of diclazuril was carried out through stacked injection, and the "enantiomeric excess" purities of two fractions were over 98%. Next, the electronic circular dichroism profiles of these two fractions in ethanol solution displayed the mirror image of each other in the range 360–200 nm. Moreover, effects of acidic/basic additive, time, and temperature on racemization of diclazuril enantiomers in ethanol solution have been studied in detail through normal‐phase high‐performance liquid chromatography. Racemization of diclazuril enantiomers was remarkably accelerated through adding triethylamine at high temperature. We envision that this systematic investigation of diclazuril at an enantiomeric level would provide valuable information in future studies involving enantioselective bioactive, metabolic, and toxicological activities.     

Liquid Chromatography of Aspirin,Salicylic Acid and Acetic Acid on Cation Exchange Resin

A 8% cross-linked sulfonated polystyrene cation exchanger in the hydrogen or sodium form was used as stationary phase for the chromatography of aspirin, salicylic acid and acetic acid. The eluent is 0.02 M mineral acids or their sodium salts in aqueous ethanol. Phosphoric acid, nitric acid or sulfuric acid added in aqueous ethanol eluent made successful separation of aspirin, salicylic acid and acetic acid on hydrogen form cation exchanger.     

Rapid separation of inorganic anions by capillary ion chromatography using monolithic silica columns modified with dilauryldimethylammonium ion.

The rapid separation of inorganic anions was determined by capillary ion chromatography using monolithic silica capillary columns modified with dilauryldimethylammonium bromide. The stability of the modified stationary phase was satisfactory owing to a strong hydrophobic interaction between the lauryl groups of the reagent, even if the eluent did not contain dilauryldimethylammonium ion. Bromide in seawater samples could be determined by the present system. The repeatability of a retention time of bromide for six successive measurements was around 1.8% when a 500 mM sodium chloride aqueous solution was used as the eluent. Seawater samples were directly injected onto the prepared column without any interference of matrix ions, because an aqueous solution of high-concentration sodium chloride could be used as the eluent. Bromide in seawater samples could be determined within 2 min.     

Ion chromatographic determination of hydroxide ion on monolithic reversed-phase silica gel columns coated with nonionic and cationic surfactants

The determination of hydroxide by ion chromatography (IC) is demonstrated using a monolithic octadecylsilyl (ODS)-silica gel column coated first with a nonionic surfactant (polyoxyethylene (POE)) and then with a cationic surfactant (cetyltrimethylammonium bromide (CTAB)). This stationary phase, when used in conjunction with a 10 mmol/l sodium sulfate eluent at pH 8.2, was found to be suitable for the rapid and efficient separation of hydroxide from some other anions, based on a conventional ion-exchange mechanism. The peak directions and detection responses for these ions were in agreement with their known limiting equivalent ionic conductance values. Under these conditions, a linear calibration plot was obtained for hydroxide ion over the range 16 micromol/l to 15 mmol/l, and the detection limit determined at a signal-to-noise ratio of 3 was 6.4 micromol/l. The double-coated stationary phase described above was shown to be superior to a single coating of cetyltrimethylammonium bromide alone, in terms of separation efficiency and stability of the stationary phase. A range of samples comprising solutions of some strong and weak bases was analyzed by the proposed method and the results obtained were in good agreement with those obtained by conventional potentiometric pH measurement.     

Peak shapes in open tubular ion-exchange capillary electrochromatography of inorganic anions

An experimental study of parameters influencing peak shapes in ion-exchange open tubular (OT) capillary electrochromatography (CEC) was conducted using adsorbed quaternary aminated latex particles as the stationary phase. The combination of separation mechanisms from both capillary electrophoresis and ion-exchange chromatography results in peak broadening in OT-CEC arising from both these techniques. The sources of peak broadening that were considered included the relative electrophoretic mobilities of the eluent co-ion and analyte, and resistance to mass transfer in both the mobile and stationary phases. The parameters investigated were the mobility of the eluent co-ion, column diameter, separation temperature and secondary interactions between the analyte and the stationary phase. The electromigration dispersion was found to influence peak shapes to a minor extent, indicating that chromatographic retention was the dominant source of dispersion. Improving the resistance to mass transfer in the mobile phase by decreasing the capillary diameter improved peak shapes, with symmetrical peaks being obtained in a 25 microm I.D. column. However, an increase in temperature from 25 degrees C to 55 degrees C failed to show any significant improvement. The addition of p-cyanophenol to the mobile phase to suppress secondary interactions with the stationary phase did not result in the expected improvement in efficiency.     

土壤淋溶柱色谱中同系物、氯代苯保留值与流动相组成、沸点的交点规律

基于土壤淋溶柱色谱符合logk'-ψ的线性保留方程,在同系物（正构烷基苯、甲基苯）和氯代苯中,同类溶质与固定相（土壤）和淋洗剂（甲醇-水）有相似的作用方式,所以溶质保留值（logk'）与淋洗剂有机调节剂浓度间存在浓度交点,与沸点间存在沸点交点规律,并用实验进行了验证。