Carla Vogt

Separation of ionic and neutral surfactants by capillary electrophoresis and high-performance liquid chromatography

Abstract Some of the most important anionic, cationic and non-ionic surfactants can be separated by CE; HPLC was used as a reference. A comparison of both methods with respect to separation efficiency, reproducibility and detection limits is presented. Linear alkylbenzenesulfonates were analyzed in river water, sewage sludge and laundry detergents. Chromatographic separations on RP-8 columns led to a resolution of alkyl homologues. In CE, determination of the sum was performed in a pure aqueous buffer. Using electrolytes containing acetonitrile, homologous separation could be obtained. The addition of α-cyclodextrin allowed the separation of isomers. Aliphatic anionics (alkylsulfonates and -sulfates) could be determined by HPLC with conductivity detection and CE with indirect UV detection. Various chromophores like salicylate or dodecylbenzenesulfonate and organic modifiers were compared. Sodium dodecyl sulfate could be analyzed in toothpaste. Important representatives of cationics (quaternary alkyl and benzalkyl ammonium compounds, pyridinium salts) were separated using direct and indirect UV detection. For electrophoretic separation, high content of organic solvents was necessary to obtain a sufficient peak resolution. The results of method development were applied to the analysis of cetylpyridinium chloride in mouth-was. Non-ionic surfactants of the polyoxyethylene type are separated into ethoxylate homologues by HPLC on aminosilica columns. CE separation was carried out in electrolytes containing high content of acetonitrile and SDS and is based on the formation of association complexes between the non-ionic analytes and anionic surfactants. Chromatograms and electropherograms may be considered as fingerprints of the substance for product control and to determine the average number of ethylene oxide units of unknown products.

Determination of cationic surfactants by capillary electrophoresis with indirect photometric detection

The separation of quaternary alkyl and dialkyl ammonium compounds by capillary electrophoresis has been performed using indirect UV detection. The influence of the organic modifier on the electrophoretic behaviour of the analytes is discussed. Several chromophoric electrolytes constituents, including absorbing cationic surfactants, have been investigated for indirect UV detection. The results of method development are applied to the determination of cationic surfactants in cosmetics and pharmaceuticals. A comparison to high-performance liquid chromatography with respect to efficiency, reproducibility and detection limits is presented.

Capillary electrophoretic separation of metal ions using complex forming equilibria of different stabilities

Abstract Capillary electrophoresis was investigated for the determination of metal cations in the presence of various complexing agents. Separation of most metal ions is complicated by their similar electrophoretic mobilities and their weak UV absorption. In this study, the use of ligands forming metal complexes of different stabilities was examined. These complexing agents can selectively modulate the mobility of cations by forming metal complexes with varying degrees of stability. To cover a wide range of complex stability, different agents were studied such as α-hydroxyisobutyric acid (lg β 23·5), 8-hydroxyquinoline-5-sulfonic acid (lg β 73·12) and ethylenediaminetetraacetic acid (lg β 143·20). In the experiments, a model mixture of five metal ions (Cu, Zn, Pb, Ni and Fe) was used for all ligands, although in practice complexing agents and separation parameters will be chosen according to the properties of the analyzed cations. Using computer simulation, the average complexation degree for each metal ion with the different ligands under the chosen separation conditions was calculated. Detection was carried out by direct or indirect UV absorption due to the properties of the complexing agent or the formed complexes.

Enantiomeric separation of D/L-carnitine using HPLC and CZE after derivatization

SummaryCarnitine is an essential component in tissues of animals, higher plants and many microorganisms. Whereas the L-carnitine enantiomer plays an important role in the metabolism of long chain fatty acids, D-carnitine has a considerable toxic influence on biochemical processes. The analytical separation of D-and L-carnitine depends upon derivatization with UV-or fluorescently active substances, e.g. FMOC and (+)/(−)-FLEC. The separation of diastereomeric (+)- and (−)-FLEC carnitine esters was performed successfully with capillary zone electrophoresis (CZE) and HPLC, after optimization of the derivatization process and of the composition and pH of the buffer, using UV- and fluorescence detection. With HPLC separation a detection limit of the carnitine esters of 5 μmol/l when using fluorescence detection was achieved. With both separation systems baseline resolution and short analysis times could be obtained. The enantiomeric FMOC derivatives could be separated using the electrophoretic system and acidic buffers with high concentrations of an osmotic flow modifier together with γ-cyclodextrine as chiral selector. The applicability of the optimized separation conditions are demonstrated in the analysis of agar culture medium inoculated withPseudomonas putida and of pharmaceutical formulations. In all samples very low amounts of D- or L-carnitine could be determined in the presence of the other enantiomeric form. Problems caused by the impurity of the carnitine standards or the derivatization agent (+)/(−)-FLEC are discussed.

Determination of surfactants by capillary electrophoresis

Capillary electrophoresis has been increasingly used during the past few years for the separation and determination of surfactants. These substances are applied in many household and industrial products such as laundry detergents, cosmetics and pharmaceuticals, often as homologous and isomeric mixtures. Product development and control as well as toxicological and environmental analyses require selective and sensitive analytical methods. This review presents capillary electrophoretic techniques to determine important representatives of cationic, anionic, and neutral surfactants. The application of different buffer additives such as organic solvents, cyclodextrins or micelles to enhance the resolution of complex mixtures is discussed. Besides direct and indirect UV and fluorescence detection, examples for conductivity and mass spectrometric detection are also given. Derivatization procedures to improve the detectability and implement charge in neutral analytes are described. The successful use of capillary electrophoresis for surfactant determinations has proven that it can serve as a routine technique in many real‐world applications. Robust, validated methods for the quantitation of single compounds, such as alkylbenzene sulfonates, sodium dodecyl sulfate and benzalkonium salts, are now available. Characteristic peak patterns (fingerprint analysis) can be used for the identification of surfactants in multicomponent formulations (e.g. ethoxylates and phosphonates).

Separation, comparison and identification of fountain pen inks by capillary electrophoresis with UV-visible and fluorescence detection and by proton-induced X-ray emission

The analysis of inks as a part of the detection of fraudulent documents is a small but important aspect of forensic science. The present study was focused on the separation of components of commercially available original fountain pen inks and on different paper substrates by capillary electrophoresis and proton-induced X-ray emission (PIXE). Electropherograms of inks from various manufacturers and countries separated in borate buffers with various methanol contents showed patterns which were in most cases distinctly different from one another. UV-Vis scans and fluorescence detection at different excitation and emission wavelengths have been used to compare similar mixtures and ink extracts from paper with the original ink composition, to identify concurrences or differences in the spectra of main and trace components and to improve the discrimination power of the technique in this way. Extraction conditions, like solvent composition, extraction time, paper carrier material, age of the ink and ink concentration have been examined with respect to signal intensity and the number of detectable components. PIXE spectra of ink samples on different paper substrates have been recorded to determine the elemental composition of the samples and to obtain additional information about the chemical composition of the dyes.

Speciation of heavy metals by capillary electrophoresis

Abstract The general abilities of capillary electrophoretic methods [capillary zone electrophoresis (CZE), isotachophoresis and isoelectric focusing] to perform species analysis are described. Several examples of the speciation of heavy metal complexes (e.g., complexes of gadolinium, platinum and iron) and metal oxoanions (arsenic and selenium) using CZE are demonstrated and the applicability to separation problems in the fields of medicine, pharmacology and ecology is discussed.

Determination of linear alkylbenzenesulfonates in industrial and environmental samples by capillary electrophoresis

Linear alkylbenzenesulfonates (LAS) are applied extensively as surfactants in consumer formulations, e.g., laundry detergents, as complex mixtures of homologues and isomers. For product control and environmental analysis it is important to know the total content of LAS in addition to the homologous and isomeric distribution. Capillary electrophoresis (CE) was used for the determination of the sum, homologues and isomers of LAS in household products and waste-water samples. A technical LAS mixture was used for method development. In buffers without organic solvent, all homologues and isomers produce only one peak; these conditions were applied to the analysis of waste water after preconcentration. A recovery of 101.8%, a limit of detection of 1 mg l–1 and a reproducibility of 3.5% (RSD) were achieved. The determination of homologous distribution was carried out by adding acetonitrile to a borate or phosphate buffer. An application of this method is the analysis of sewage sludge for LAS after methanolic extraction, with an average recovery of 75.5% and a limit of detection of 4 mg l–1. Isomeric separation was achieved using electrolytes with high contents of sodium dodecyl sulfate (SDS) and acetonitrile as organic modifier. The identification of components in household products was possible by the standard additions method.

Separation of d/l-carnitine enatiomers by capillary electrophoresis

Abstract Carnitine is an essential component of the tissue of animals, higher plants and many microorganisms. Whereas the l -carnitine enantiomer plays an important role during the metabolism of long chained fatty acids, d -carnitine has a considerably toxic influence on biochemical processes. The analytical separation of d/l -carnitine is possible only after derivatization with UV- or fluorescence active substances, e.g. FMOC (9-fluorenylmethyl chloroformate) and (+)/(−)-FLEC ((+)-1-(9-fluorenylethyl)chloroformate). The capillary electrophoretic separation of enantiomeric FMOC derivatives was performed using acidic buffers and different cyclodextrins as chiral selectors. Baseline separation as well as detection limits in the micromolar range could be achieved using UV detection. The migration order of both enantiomeric derivatives was reversed by the application of substituted cyclodextrins (CDs), thus making the analysis of low amounts of d - or l -carnitine in the micromolar range could be achieved using UV detection. The migration order of both enantiomeric derivatives was reversed by the application of substituted cyclodextrins (CDs), thus making the analysis of low amounts of d - or l -carnitine in high concentrations of the other enantiomeric form possible. Different kinds of coated capillaries are compared with respect to separation time and resolution. Examples for the analysis of pharmaceuticals with high l - and low d -carnitine contents and of serum samples spiked with d/l -carnitine are shown.

Investigation of Ball Point Pen Inks by Capillary Electrophoresis (CE) with UV/Vis Absorbance and Laser Induced Fluorescence Detection and Particle Induced X-Ray Emission (PIXE)

In the process of examining fraudulent documents ink analysis is a small but important part of the operation of forensic laboratories. Systematic approaches to ink comparison and identification have been performed by optical methods and various chromatographic techniques. Capillary electrophoresis (CE), a relatively new separation technique with very high resolution power, and Particle Induced X-Ray Emission (PIXE) were used for the analysis of ball point pen inks. In comparison to water soluble fountain pen inks, ball point inks are less soluble or insoluble in water and these inks contain only few components. The study focused on the optimization of the separation of ink extracts from paper material of commercially available inks with respect to resolution and analysis time. During the method development process different buffers, organic modifiers, and surfactants were tested. Good results were obtained with a 50 mM borate buffer pH 9.0 containing 50% acetonitrile. Reproducible extraction procedures as well as separations enables one to perform the quantification of the ink peaks within 1–8% standard deviation for parallel extractions of the same ink. Electropherograms of 20 inks of various origin showed patterns which were in most cases distinctly different from each other. PIXE measurements with an external proton beam were used to determine the metal composition. The ratio of the peak areas for copper and zinc as well as differences in the elemental composition could be used to distinguished between the samples. No coincidence was observed between samples hardly distinguishable by electrophoretic separations and by PIXE-measurements. Samples with nearly identical metal composition showed different peak pattern in the electropherograms, and nearly identical electrophoretic behavior of two or more samples was accompanied by quite different copper/zinc-ratios or supplementary metals identified by PIXE.