L. Blomberg

Direct and indirect chiral separation of amino acids by capillary electrophoresis

Abstract Two approaches to the chiral separation of racemic mixtures of amino acids by means of capillary electrophoresis have been evaluated. These were indirect separation of diastereomers formed by derivatization with (+)- or (−)-1-(9-fluorenyl)ethyl chloroformate and direct chiral separation after derivatization with 9-fluorenylmethyl chloroformate. Separation conditions were optimized by the application of a full factorial design. For indirect separation, concentration of sodium dodecyl sulphate (SDS) and pH were the most important separation variables, and for direct separation, concentration of isopropanol (IPA), β-cyclodextrin and SDS were the most prominent factors affecting separation. The presence of IPA was a prerequisite for chiral recognition. With regard to selectivity, efficiency, analysis time and ease of method development the best results were provided by the indirect method. It should be noted, however, that the success of this approach is based on the availability of a derivatization reagent in high optical purity.

Deactivation of glass capillary columns for gas chromatography

Abstract The adsorption of some polar volatile compounds on glass capillary columns has been investigated. In general, silanization and etching with hydrogen fluoride or chloride reduces the column activity only slightly. Coating of such capillaries with some polar phases deactivates AR-glass but not Pyrex. As initially described by Aue for packed columns, a thin layer of non-extractable Carbowax 20M provides a well deactivated surface for further coating with polar or non-polar stationary phases. Such deactivated non-polar capillary columns are valuable for the separation of samples, e.g., tobacco smoke, containing both polar and non-polar compounds.

In situ synthesis of highly thermostable, non-extractable, methylsilicone gum phases for glass capillary gas chromatography

Abstract The high thermal stability of crosslinked poly(methylsiloxane) gums, as compared with linear polysiloxanes, makes them especially suitable as stationary phase for glass capillary columns. Because such gums have a very limited solubility in commonly used solvents, it is necessary to develop a method for their formation within the column. To further increase the stability of the gum films, conditions have been chosen which facilitate chemical bonding to the glass surface. On temperature programming to 300°, such columns show good efficiency, low bleeding and good durability. The columns have been successfully used for the routine analysis of polyaromatic hydrocarbons.

Peroxide-initiated in situ curing of silicone gums for capillary column gas chromatography

Abstract The combination of leaching, silanization with cyclic siloxanes and peroxide-initiated in situ vulcanization of silicone stationary phases for gas chromatography gives capillary columns that show very high efficiencies, very low adsorptivity as reflected by stringent tests and very high efficiencies, very low adsorptivity as reflected by stringent tests and very high stability, e.g., low bleeding rates. Such columns have been prepared using AR-glass and fused silica as the supports and SE-30, SE-52, SE-54 and polar OV-215 as stationary phases. The versatility of these columns is demonstrated by the analysis of underivatized drugs, barbiturates and tricyclic antidepressants. The separation of aqueous solutions is also described.

Some factors affecting the properties of thin layers of Carbowax 20M intended for deactivation of glass capillary columns

Abstract The adsorptivity of glass capillary columns has been suppressed by application of a thin non-extractable layer of Carbowax 20M to the glass surface. The properties of such polymer surface were found to differ greatly with the etching technique, the type of glass and the procedure of Carbowax application. Further, the temperature stability of columns deactivated by coating with SP-2100 was greatly affected by the deactivation procedure. Columns prepared from etched Pyrex glass showed the highest stability, and such columns have been used regularly for the separation of polycyclic aromatic hydrocarbons at temperatures up to 300°.

Direct chiral separation of amino acids derivatized with 2-(9-anthryl)ethyl chloroformate by capillary electrophoresis using cyclodextrins as chiral selectors Effect of organic modifiers on resolution and enantiomeric elution order

Abstract Direct chiral separation of amino acids derivatized by 2-(9-anthryl)ethyl chloroformate (AEOC) has been performed by capillary electrophoresis in capillary zone electrophoretic (CZE) mode and micellar electrokinetic (MEKC) mode using cyclodextrins (CDs) as chiral selectors. The concentrations of chiral selector, surfactant and organic modifier were optimized. Application of β - and γ -CD as chiral selectors in MEKC, resulted in d/l separation of 12 and 13 amino acids, respectively. In CZE, chiral separation was obtained only with γ -CD, 8 amino acids were chirally separated in this mode. Separation in the MEKC mode resulted in higher resolutions of the enantiomers than in the CZE mode. Addition of 2-propanol to the buffer caused reversal of the enantiomeric elution order for some amino acids in MEKC mode.

Modifier effects on packed and capillary columns in supercritical fluid chromatography

SummaryThe separation of polar thermally labile solutes is one of the potentially most rewarding fields of SFC application. A presupposition for such applications is, however, mobile phases having relatively high solvent strengths. A promising approach to achieve this is the use of mobile phases consisting of carbon dioxide with a polar additive. In this work, the chromatographic effects of different concentrations of an additive, isopropanol, in carbon dioxide have been studied on capillary and packed columns. A series of antibiotics was used as test substances. Best results were obtained with carbon dioxide/8% isopropanol as mobile phase on a capillary column coated with a cyanopropyl-substituted polysiloxane stationary phase.

Cyanosilicones as stationary phases in gas chromatography : II. Gum and rubber phases on fused silica

Abstract The presence of cyano groups in the stationary phase may provide useful selective interactions during separation. In capillary gas chromatography, the full utility of such interactions will be evident when the columns show high efficiency, good deactivation, high thermal stability and reproducibility in retention characteristics. Columns that fulfil these requirements were prepared. Fused silica capillaries were treated with hydrochloric acid, silanized at a high temperature with cyclic cyanopropylsiloxane and statically coated with cyanopropylsilicone gum, which was finally immobilized by in situ cross-linking. Vulcanization is normally inhibited by cyanopropyl groups, but a certain percentage of tolyl groups in the gum made cross-linking possible. The usefulness of the cyanosilicone columns is demonstrated by the separation of several samples such as fatty acid methyl esters and pheromones.

Glass capillary columns for gas chromatography coated with non-extractable films of cyanosilicone rubbers

Abstract A procedure for the preparation of glass capillary columns using a bonded cyanosilicone rubber as stationary phase is presented. Two types of cyanosilicone pre-polymer were synthesized, with 25% and 100% cyano-substitution, respectively. Glass capillaries that had been leached with HCl or treated with a sodium chloride sol were coated with the pre-polymer, and a rubber was formed situ. Such columns show very good thermal stability up to 300°C; further, activity is low and the efficiency good. The utility of these thermally stable polar columns is demonstrated by the analysis of aldononitrile derivatives prepared from sugars and also polynuclear aromatics (PNAs), especially aza-PNAs.

Glass capillary columns coated with non-soluble methyl silicone gums

Abstract When used as stationary phases in glass capillary columns, films formed from gums are generally more stable than liquid phases. The stability indicates that a high column efficiency can be maintained at elevated temperatures, column bleeding is low and the column lifetime is long. In this paper we present a method for increasing further the gum film stability by using gums with a higher degree of cross-linking. Such gums are not soluble in commonly used solvents and are therefore synthesized in situ . In addition, measures have been taken to promote chemical bonding between glass and polymer. The columns have been used successfully for the routine analysis of polyaromatic hydrocarbons.