Comparison of a C30 Bonded Silica Column and Columns with Shorter Bonded Ligands in Reversed-Phase LC

The Carotenoid S is a new C₃₀ bonded silica stationary phase, intended for reversed-phase chromatographic applications, which is more hydrophobic and consequently shows stronger retention in comparison to conventionally used C₁₈ stationary phases. We compared the non-polar selectivities of the columns for homologous alkylbenzenes in acetonitrile—water and methanol–water mobile phases and polar reversed-phase selectivities employing the interaction indices and the Linear Free Energy Relationship models. Further, we investigated possibilities of separations of structurally closely related compounds in the groups of phenolic acids, flavones, phthalic acids and related compounds and of acylglycerols on the new C₃₀ column and with different types of columns for reversed-phase chromatography, including shorter alkyl C₄, C₈, C₁₈ and phenyl bonded stationary phases. The C₃₀ column has in some aspects properties similar to the non-endcapped Nova-Pak column for separation of some acylglycerols with equal equivalent carbon numbers, but enables separations of longer chain triacylglycerols in a single gradient run.

     

Capillary gas chromatographic separation of C10–C12 secondary phenylalkanes on modified cyclodextrin stationary phases

The separation of isomers and enantiomers of branched C₁₀-C₁₂ phenylalkanes by gas chromatography on fused silica capillary columns coated with some modified β- and γ-cyclodextrins was studied. It was shown that the separation of positional isomers of C₁₀-C₁₂ phynylalkanes on modified cyclodextrin capillary columns is not better than that on a column coated with modified polyethylene glycol. Differences were found in the enantioselectivity of modified β- and γ-cyclodextrins for the separation of C₁₀-C₁₂ secondary phenylalkane enantiomers. While alkylderivatives of β-CDs resolve enantiomers of 3-phenylalkanes, alkyl derivatives of γ-CD resolve enantiomers of 2-phenylalkanes. Since shape selectivity factors of modified cyclodextrins have indicated no inclusion of the considered solutes in cyclodextrin cavities, enantioselective interactions most probably occur on the outer sphere of cyclodextrins.     

Separation of Photosynthetic Pigments by High-performance Liquid Chromatography: Comparison of Column Performance,Mobile Phase,and Temperature

High-performance liquid chromatography (HPLC) has been commonly used as method of separating and identifying photosynthetic pigments such as chlorophylls and carotenoids because of such advantages as speed, high resolution and sensitivity. In this technique, high separation relies largely on the type of column material. This study compared the efficiency of five reverse-phase columns, C8, C18, C18 monolithic, π-NAP, and cholester, for separation of photosynthetic pigments at several fixed conditions of mobile phase and temperature. This investigation also analysed the parameters of Δt_R and t_R ratio for selected pigments and resolution for structural isomers, such as α- and β-carotene. Among above columns tested, cholester column is suitable for separation of pigments not only for a broad range of polarity, but also for hydrophobic pigments in a simple mobile phase. This finding can help in the selection of column and HPLC parameters in separating photosynthetic pigments.     

A Sensitive Gas Chromatograph Detector for Nitrosamines

Abstract

A GC method using a flame thermionic detector (FTD) with a RbSO₄ source to determine nitrosamines is described. The method is sensitive enough to be used for a preliminary screening of foodstuffs for the presence of volatile nitrosamines. The lowest detection limit is 0.1 ng for DMNA, DENA, or DPNA, with a linear range of 0.1 ? 100 ng determinable. The method has a favorable stability; the factors affecting its sensitivity, e.g. the H₂ flow rate, column temperature and the solvent used, have been studied.     

Modification of Selectivity in Reversed-Phase Liquid Chromatography of Polycyclic Aromatic Hydrocarbons Using Mixed Stationary Phases

Abstract

Monomeric and polymeric C₁₈ materials provide significantly different selectivities for polycyclic aromatic hydrocarbons (PAH) in reversed-phase liquid chromatography. Selectivity factors vary in a regular manner with respect to surface concentration of C₁₈ groups on different C₁₈ columns. In this study, we investigated the feasibility of “customizing” a C₁₈ column to provide an intermediate selectivity by mixing 5-μm polymeric C₁₈ material from two different lots with high and low C₁₈ surface concentrations. Polymeric C₁₈ materials from different production lots were mixed in ratios of 30/70, 50/50, and 70/30 (w/w). Selectivity factors for these columns were found to be similar to those predicted by the linear addition of the selectivities of the two individual phases. The PAH selectivities on these mixed columns were also found to be comparable to data obtained from coupled short columns of appropriate lengths each containing one of these different C₁₈ materials. These studies indicate that columns of specific selectivity can be prepared by either mixing two different C₁₈ materials or by coupling columns containing each of these different phases. The use of mixed phase columns is illustrated for the analysis of a fraction containing five condensed ring PAH isomers (molecular weight 278) isolated from an air particulate sample.     

High-Performance Liquid Chromatography Separations of Nitrosamines. IV. Effect of Temperature on the Separation of Nitrosamino Conformers

Abstract

The separation of the syn and anti conformers of a selected group of nitrosamines was achieved only after the temperature of the cyclodextrin bonded silica column was lowered to ?20°C. In certain cases it was apparent that a lower temperature is required for baseline separations. The mobile phases used were 90% acetonitrile/10% triethylammonium acetate, pH 5 for the nitrosamino acids, and tetrahydrofuran for the cyclic nitrosamino isomers.     

Separation of Mycotoxins,Polycyclic Aromatic Hydrocarbons,Quinones, and Heterocyclic Compounds on Cyclodextrin Bonded Phases: An Alternative LC Packing

Abstract

β-Cyclodextrin and γ-cyclodextrin chiral bonded phases were previously shown to be useful in the separation of enantiomers, diastereomers and structural isomers. In this work it is demonstrated that these stationary phases are also useful in more routine separations. As such, they provide an alternative to the popular reverse phase packings. Because the selectivity of cyclodextrin packings is often unique they can be used to compliment conventional columns, particularly when separating complex mixtures where peak overlap is a problem. The separation of several important classes of compounds is used to demonstrate the general utility of this packing.     

High Performance Liquid Chromatography Separations of Nitrosamines. I. Cyclic Nitrosamines

Abstract

The separation of cyclic nitros amines by HPLC was achieved using a cyclobond I column and a mobile phase of methanol/water. The effect of in-the-ring and on-the-ring substitutions on retention times were studied. The results show that the longer the chain of the substituent, the longer the retention time, which is consistent with the inclusion mechanism of separation. Also, the larger the ring, the longer the retention time. The results also show that the in-the-ring substitution has a dramatic effect on retention.     

Ion-Pair Liquid Chromatography of Alkylanilines

Abstract

The optimization of ion-pair liquid chromatography of structural isomers of carbon number two alkyl anilines has been studied.

The shorter alkyl cha in bonded silica gel demonstrated the stereo selectivity. the alkyl chain length of ion-pair reagent ruled the equilibration of ion-pair formation. the longer chain of ion-pair reagent formed hydrophobic ion-pair and the shorter chain could not form ion-pair, and ionized solute formed ion-pair with polar ion which was a component of eluent. Three ethylanilines and six xylidines mixture was separated on an octadecyl-bonded silica gel column using an eluent containing methanol/water mixture with sodium 1-dodecanesul-fonate.     

Effects of alkyl chain length on properties of 1-alkyl-3-methylimidazolium fluorohydrogenate ionic liquid crystals

A series of 1‐alkyl‐3‐methylimidazolium fluorohydrogenate salts (C_xMIm(FH)₂F, x=8, 10, 12, 14, 16, and 18) have been characterized by thermal analysis, polarized optical microscopy, IR spectroscopy, X‐ray diffraction, and anisotropic ionic conductivity measurements. Liquid crystalline mesophases with a smectic A interdigitated bilayer structure are observed from C₁₀ to C₁₈, showing a fan‐like or focal conic texture. The temperature range of the mesophase increases with the increase in the alkyl chain length (from 10.1 °C for C₁₀MIm(FH)₂F to 123.1 °C for C₁₈MIm(FH)₂F). The distance between the two layers in the smectic structure gradually increases with increasing alkyl chain length and decreases with increasing temperature. Conductivity parallel to the smectic layers is around 10 mS cm^?1 regardless of the alkyl chain length, whereas that perpendicular to the smectic layers decreases with increasing alkyl chain length because of the thicker insulating sheet with the longer alkyl chain.     

Effect of Alkyl Chain Length of Bonded Silica Phases on Separation,Resolution and Efficiency in High Performance Liquid Chromatography

Abstract

A comparative study of alkyl bonded phases was carried out under optimum solvent conditions for each phase. Three columns, RP-2, RP-8 and RP-18, were tested for their efficiency and resolving power using three groups of compounds in three binary organic-water mobile phases. The organic solvents were acetonitrile, methanol and tetrahydrofuran which are widely used as solvent modifiers.

The results indicate that each of the three factors, i.e. solvent, solute and bonded alkyl chain length, play an important role, with the solvent being the most significant. When tetrahydrofuran-water was used as the mobile phase, the ratio of THF/H₂O did not vary by much when an RP-2, RP-8 or RP-18 column was used to separate naphthalene from biphenyl, dimethylphthalate from diethylphthalate or anthraquinone from methyl, anthraquinone and ethyl anthraquinone. When acetonitrile-water and methanol-water were used the ratio of organic modefier to water changed so as to accomodate the hydrophobic properties of the columns. The efficiency of the columns, expressed as theoretical plates per meter (TPM) was highest when acetonitrile-water was used as the mobile phase. Although there were variations in TPM and resolution from column to column, the three columns gave good separation of the components of the three groups of compounds.     

Synthesis of the first per(3-deoxy)-cyclooligosaccharide: hepta(manno-3-deoxy-6-O-t-butyldimethylsilyl)-β-cyclodextrin

Reduction of hepta(manno-2,3-anhydro-6-O-t-butyldimethylsilyl)-β-cyclodextrin with lithium triethylborohydride gives hepta(manno-3-deoxy-6-O-t-butyldimethylsilyl)-β-cyclodextrin. This compound plus the hepta(2-O-methyl)- and hepta(2-O-benzyl)-derivatives all have the ⁴C₁ conformation. Capillary GC columns manufactured with hepta(manno-2,3-anhydro-, hepta(manno-3-deoxy-2-O-methyl- and hepta(manno-2-O-benzyl-6-O-t-butyldimethylsilyl)-β-cyclodextrin stationary phases were evaluated for enantio-discrimination with 39 non-polar racemic analytes. The GC column coated with the benzyl derivative showed enantioselectivity comparable to, and in some cases superior to, a commercial per(methyl)-β-cyclodextrin column. The other columns showed little or no enantio-discrimination. A thermodynamics study established a linear enthalpy–entropy compensation effect for two series of analytes on the commercial permethyl-β-cyclodextrin column, but not for the column coated with the benzyl derivative.     

Development and optimization of a method for the separation of platycosides in Platycodi Radix by comprehensive two-dimensional liquid chromatography with mass spectrometric detection

Comprehensive two-dimensional chromatography (LC × LC) using combinations of two columns (C₁₈ × CN and C₁₈ × NH₂) was employed with electrospray (ESI) mass spectrometry to analyze platycosides from root extract. Based on the capability of the C₁₈, CN and NH₂ columns to separate the platycosides, the orthogonality in two-dimensional space according to each combination of columns was predicted from the correlation coefficients between the retention times of the 17 compounds separated by the independent CN and C₁₈ columns, and NH₂ and C₁₈ columns. The expected distribution of the peaks was also compared with the two-dimensional plots obtained by practical separation in an LC × LC system. The increased peak capacities using C₁₈ × NH₂ allowed three minor components and five isomers of the platycosides to be newly separated, which were not identified with 1D-LC using the individual C₁₈ column, whereas the combination of C₁₈ × CN did not result in any improvement of the separation performance.     

HPLC Identification and Separation of Phenolic Compounds Diazotized With Sulphanilic Acid. Structural Effects on Retention Times

Abstract

A HPLC method for the separation of phenol derivatives has been developed either on a RP Phenyl-bonded or an alkyl C₁₈ -bonded columns. Derivatization with diazotized sulphanilic acid allows

a reversed-phase chromatography with spectrophotometric detection at 370 nm. The method sensitivity permits detection down to 0.08- 0.15 ng of phenols. In particular structure-retention time relationships are discussed.     

A Sensitive Analysis for Nitrosamines

Abstract

A number of nitrosamines have been reduced to hydrazines and condensed with 9-anthraldehyde and 9-phenanthraldehyde to give highly fluorescent hydrazones which can be separated by thin layer chromatography and identified using R_f, ultroviolet and mass spectra.     

Complexation of natural and methylated β-cyclodextrins with long-chain Carboxylic acids in aqueous solutions. Calorimetric studies at 298 K

The formation of complexes between β-cyclodextrin or methyl-β-cyclodextrin and monocarboxylic acids from C₅ to C₁₂ has been studied calorimetrically at 298 K in aqueous buffer phosphate, pH 11.3. When a complex forms, calorimetry enables the calculation of both the enthalpy and the association constant, from which the Gibbs free energy and the entropy of the process can be obtained. The forces involved in the association process are then discussed in the light of the signs and values of the thermodynamic parameters obtained. For β-cyclodextrin, 1:1 inclusion complexes form, characterized by an irregular variation in the values of enthalpies, entropies and association constants at increasing alkyl chain length. A model is proposed to explain this unusual behaviour for acids longer than ten carbon atoms. The association involving methyl-β-cyclodextrin shows, instead, a regular variation of the thermodynamic parameters up to the C₁₂ term. An elongation of the cavity effect is discussed: the cavity behaves as it were deeper than that of the natural cyclodextrins. Association is characterized prevailingly by hydrophobic interactions; for the longer terms, the high and almost invariant entropic contribution determines the large association constants, notwithstanding the positive, unfavorable enthalpic contribution. This is an indication that the relaxation of water molecules from the hydrophobic hydration shells of the external methyl groups is the contribution that mainly determines the association process.     

Isolation of Gram Quantities of Configurational Isomers of Cyclic Nitrosamines by Preparative Liquid Chromatography

Abstract

Substituted cyclic nitrosamines, e.g., 2,6-dimethyl-N-nitrosomorpholine and 3,5-dimethyl-N-nitrosopiperidine are separable into their component configurational isomers by preparative hplc. Gram quantities of the individual isomers sufficient for animal testing were obtained. The identification of contaminants in commercial 2,6-dimethylmorpholine of purported 99% is discussed.     

Preparation and Characterization of Amine Modified Porous Polymer for Rplc

Abstract

The retention behaviour and selectivity of various aromatic compounds, including halobenzenes and a IkyIbenzenes, were examined on the stationary phase of chloromethylated polystyrene(CMS) beads, of which the chloride group was substituted by the alkylamines: n-hexyl(C₆), n-octyl(C₈), n-decyl (C₁₀),n-dodecyl (C₁₂), n-tetradecyl (C₁₄) or n-hexadecy1 (C₁₆). Changes in the distribution coefficients of benzene derivatives are demonstrated not only by the changes of the eluent compositions but also by the changes of chain length in the stationary phase. In general, a longer alkyl chain in the stationary phase gives longer retention time and different selectivity. The effects of MeOH/H₂O ratios on retention and selectivity was also demonstrated on the C₁₄ stationary phase. Good separations of halobenzenes and alkyIbenzenes were obtained with C₁₄ stationary phase. Moreover, the retention time for samples was shortened by about a half compared to polystyrene stationary phase. These results show the chemical modification of CMS gel with long chain alkyl amines decreases tailing and provides new supports with different selectivities.     

Mixed Packings in High Performance Liquid Chromatography. II. Mixed Packings vs. Mixed Ligands

Abstract

The use of mixed packing of different selectivities for the separation of antidepressants and anticonvulsants was studied. The results show that columns packed with mixed ligand supports (C₈ and cation exchange), gave the better resolution and peak shapes than the physically mixed C,₈/cation exchange and serially connected columns. Also, the retention times obtained on the mixed ligands column, the physically mixed supports column and the two columns in series were different in each case.     

Simultaneous enantioselective separation of chlordanes,a nonachlor compound,and o,p′–DDT in environmental samples using tandem capillary columns

Two capillary columns, one coated with 90 % biscyanopropyl 10 % phenylcyanopropylpolysiloxane (RT_x 2330) the other with dimethyl-t-butyl-silylated β-cyclodextrin dissolved in 85 % methyl 15 % phenyl polysiloxane (PS-086), have, when employed in tandem, enabled the enantioselective separation of all the chiral chlordanes present in the fish oil reference material SRM 1588, without any overlap between enantiomers of different isomers. This capillary column combination is, furthermore, able to separate the enantiomers of other chiral polychlorinated pesticides such as o,p′–DDT, oxy-chlordane, a chiral nonachlor, and heptachlor exo-epoxide. It can be used to determine the enantiomer ratio of these compounds present in the low pg/g range in extracts of marine biota and ambient air.