The analysis of basic and acidic compounds using non-aqueous CE and non-aqueous CE-MS

Non-aqueous capillary electrophoresis for the analysis of acidic compounds using negative electrospray ionization mass spectrometry

Non-aqueous capillary electrophoresis (NACE) is an attractive CE mode, in which water solvent of the background electrolyte (BGE) is replaced by organic solvent or by a mixture of organic solvents. This substitution alters several parameters, such as the pKa, permittivity, viscosity, zeta potential, and conductivity, resulting in a modification of CE separation performance (i.e., selectivity and/or efficiency). In addition, the use of NACE is particularly well adapted to ESI-MS due to the high volatility of solvents and the low currents that are generated. Organic solvents reduce the number of side electrochemical reactions at the ESI tip, thereby allowing the stabilization of the ESI current and a decrease in background noise. All these features make NACE an interesting alternative to the aqueous capillary zone electrophoresis (CZE) mode, especially in combination with mass spectrometry (MS) detection. The aim of this work was to evaluate the use of NACE coupled to negative ESI-MS for the analysis of acidic compounds with two available CE-MS interfaces (sheath liquid and sheathless). First, NACE was compared to aqueous CZE for the analysis of several pharmaceutical acidic compounds (non-steroidal anti-inflammatory drugs, NSAIDs). Then, the separation performance and the sensitivity achieved by both interfaces were evaluated, as were the impact of the BGE and the sample composition. Finally, analyses of glucuronides in urine samples subjected to a minimal sample pre-treatment ("dilute-and-shoot") were performed by NACE-ESI-MS, and the matrix effect was evaluated. A 20- to 100-fold improvement in sensitivity was achieved using the NACE mode in combination with the sheathless interface and no matrix effect was observed regardless of the interfaces.

CE of tricyclic antidepressant clomipramine and metabolites: Electromigration and wall adsorption

CE of tricyclic antidepressants clomipramine and its metabolites demethylclomipramine, didemethylclomipramine and 8-hydroxyclomipramine resulted in partly extremely tailing peaks in bare fused-silica capillaries. Especially at high pH of the BGE this behavior was not unexpected as adsorption of the cationic analytes onto the negatively charged wall due to electrostatic attraction can be supposed. Less expected was the observation that peak tailing could not be overcome neither by using a capillary with dynamic coating with cationic CTAB added to the BGE, nor by the usage of a capillary permanently coated with polyvinyl alcohol (PVA), both operated at acidic pH. As this tailing was even more pronounced than with bare fused silica, and was suppressed upon addition of MeCN to the BGE, another source of adsorption than pure ion-ion interaction seems plausible. In the bare silica capillary the mobility, mu, of the analytes followed roughly the pH dependence of a monoacidic base, but two deviations from the sigmoid theoretical curve were evident: (i) even at low pH the mobilities were not constant; they decreased in contrary with pH over the entire range; (ii) the apparent pK(a) values of two analytes, derived at the pH with halve the mobility at low pH, are significantly smaller than the thermodynamic pK(a). Upon modifying the expression for mu = f(pH), and considering the pH dependence of the negative charge density at the wall by an additional term which takes chromatographic retention into account, an equation was derived which enables the description of the observed electromigration of the analytes as function of pH, pK(a) of analytes and surface silanol groups, actual mobility of analytes, distribution coefficient (or retention factor) due to adsorption including its pH dependence. The interplay of electrophoretic movement and residual adsorptive retention allowed to resolve the analytes finally in an uncoated capillary, namely at pH 7.65 (30 mM ionic strength), whereas at the cost of the robustness of the separation system.

Nonaqueous capillary electrophoresis-mass spectrometry

Nonaqueous background electrolytes broaden the application of capillary electrophoresis displaying altered separation selectivity and interactions between analytes and buffer additives compared to aqueous background electrolytes. In addition, nonaqueous capillary electrophoresis (NACE) appears to be ideally suited for online coupling with mass spectrometry due to the high volatility and low surface tension of many organic solvents. Despite these advantages and an increasing use of nonaqueous background electrolytes in CE, coupling of NACE to mass spectrometry has not yet been applied very often to date. The present review summarizes the applications of online NACE-MS with regard to the analysis of drugs, stereoisomers, peptides, alkaloids, polymers and others. A brief discussion of solvent effects in NACE and pH of nonaqueous background electrolyte systems is also presented.

Nonaqueous capillary electrophoresis in pharmaceutical analysis

This review presents different solvents and electrolytes commonly used as BGEs in NACE for the analysis of pharmaceutical compounds. Most NACE applications carried out since 1998 for the analysis of compounds of pharmaceutical interest are presented in four tables: (i) analysis of drugs and related substances, (ii) analysis of chiral substances, (iii) analysis of phytochemical extracts and (iv) analysis of drugs in biological fluids. These selected examples are used to illustrate the interest in NACE versus conventional aqueous CE.

Factorial design applied to a non-aqueous capillary electrophoresis method for the separation of β-agonists

The aim of this work was to study the effects of both chemical and instrumental parameters on the separation of beta-agonists (clenbuterol (CLE), salbutamol (SAL) and terbutaline (TER)) by non-aqueous capillary electrophoresis (NACE) method. Due to the number of parameters involved and their interactions, factorial experimental designs (EDs) at two levels was applied to investigate the influence of experimental factors (ionic strength of the background electrolyte (BGE), organic solvent, injection time, voltage and temperature) in sets of several CE responses (resolution, (RS), number of theoretical plate (N), tailing factor (TF) and migration time (tm)). As a compromise between the four responses, the optimum condition was obtained in 18 mM ammonium acetate in methanol (MeOH):acetonitrile (ACN):glacial acetic acid (66:33:1%, v/v/v) using an injection time of 4 s, the voltage and the temperature of 28 kV and 24 degrees C, respectively. The proposed NACE permitted the baseline separation of the three beta-agonists within 10.5 min with good repeatability (%RSD < 3.5%) and linearity (r2 > 0.99). The developed method was applicable for the analysis of the beta-agonists in syrup and tablets and the NACE condition was compatible with a mass spectrometer detector.

Capillary zone electrophoresis of cationic and anionic drugs in methanol

The actual mobilities and dissociation constants of acidic and basic pharmaceuticals were determined in methanol. Actual mobilities were derived from the dependence of the effective mobilities of the analytes on the pH of the methanolic background electrolyte solution (pH(MeOH)). The pKa values of the pharmaceuticals in methanol (pK(a,MeOH)) were calculated by non-linear curve fitting to the measured mobility values. It was found that the shift in pKa value (when compounds were transferred from water to methanol) increased with the acidity of the analyte. The average pKa shift for compounds exhibiting acidic properties in water was ca. 5.5 units, and the shift for basic compounds about 2 units. As was shown for a mixture of beta-blockers, the calculated actual mobilities and pKa values can be utilised in the optimisation of pH conditions for separation. The practical value of the method was illustrated by the analysis of urine samples.

Preparation of a sulfonated fused-silica capillary and its application in capillary electrophoresis and electrochromatography

In the present paper, two new methods, sol-gel and chemical bonding methods, were proposed for preparation of sulfonated fused-silica capillaries. In the sol-gel method, a fused-silica capillary was coated with the sol solution obtained by hydrolysis of 3-mercaptopropyltrimethoxysilane (MPTS) and tetramethoxysilane, and followed by age; while in the chemical bonding method, a capillary was chemically bonded directly with MPTS. Then, both the resulting capillaries were oxidized with an aqueous solution of hydrogen peroxide solution (H2O2) (30%, m/m) to obtain the sulfonated capillaries. The electroosmotic flow (EOF) for the sulfonated capillaries was found to remain almost constant within the studied pH range, and greater than that of the uncoated capillary. However, the coating efficiency of the capillary prepared by chemical bonding method was higher than that by sol-gel method, by comparing their magnitude of the EOF, the degree of disguise of the silanol and reproducibility of preparation procedure. The effects of the electrolyte's concentration and the content of methanol (MeOH) on the EOF were also studied. Especially, the study of the apparent pH (pH*) on the EOF in a water-MeOH system was reported. Finally, capillary electrophoretic separation of seven organic acids was achieved within 6.5 min under optimal condition using the chemically bonded sulfonated capillary. Moreover, separation of four alkaloids on the sulfonated capillary was compared with that on uncoated capillary in different conditions. Ion-exchange mechanism was found to play a key role for separation of these four basic analytes on the sulfonated capillary.

Capillary electrophoresis of inorganic anions in hydro-organic media

The capillary electrophoresis separation of four inorganic anions (NO3-, I-, Br- and SCN-) was investigated over the whole range of methanol-water mixture composition. As the separation selectivity was strongly dependent on the solvent composition, the influence of ion-pairing and solvation phenomena was examined in depth in an attempt to explain this modification. First, a series of experiments was performed in methanolic background electrolytes, with counter-ions of different size. Ion-pair formation involving electrolyte ions was assessed to allow for a correction for free electrolyte ion concentration. Ion-pair formation constants between each inorganic anion and electrolyte counter-ion were next determined from the variations of the anion mobility as a function of the free counter-ion concentration. In view of the low values obtained, ion-pair formation alone failed to explain the selectivity variations. Solvation phenomena were then investigated with the help of a theoretical quantum model, the density functional theory (DFT), coupled with a polarizable continuum model to mime non-specific solvent effects. Whereas this model proved successful at predicting the mobility order at infinite dilution in water, it failed to predict the correct order in methanol.

Dynamic coating for fast and reproducible determination of basic drugs by capillary electrophoresis with diode-array detection and mass spectrometry

The double coating principle of CEofix buffers was evaluated for the analysis of some basic drugs by capillary electrophoresis-diode-array detection (CE-DAD) and capillary electrophoresis-mass spectrometry (CE-MS). The involatile phosphate present in original low pH CEofix, was replaced with formic acid for hyphenation of CE with MS. The double coating produces a substantial and highly reproducible electroosmotic flow (EOF), even at low pH. The rinsing procedure and electrolyte composition were optimized for both CE-DAD and CE-MS. The system was evaluated with the analysis of a mixture of basic drugs and a spiked urine sample enriched by solid-phase extraction (SPE). The R.S.D. values on the migration time and peak area measured for 28 analyses with CE-DAD were below 0.25 and 2.40%, respectively. For CE-MS, the R.S.D. on the migration time was 0.85% or less and the area precision ranged from 5.65 to 14.33% (for seven injections). The LOD with the developed CE-MS method was below 50 ppb for all five drug standards tested.

Utility of nonaqueous capillary electrophoresis for the determination of lidocaine and its metabolites in human plasma: a comparison of ultraviolet and mass spectrometric detection

A nonaqueous capillary electrophoresis/electrospray mass spectrometry method for the separation of lidocaine (LID) and two of its metabolites, monoethylglycinexylidide (MEGX) and glycinexylidide (GX), has been developed. The separation medium was: 70 mM ammonium formate and 2.0 M formic acid in acetonitrile/methanol (60:40 v/v). With a sheath liquid of methanol/water (80:20 v/v) containing 2% formic acid and positive ion detection, reproducible determinations (8-11% relative standard deviation (RSD)) of lidocaine and its metabolites were performed in spiked human plasma. The limits of detection (LODs) were between 69.1 and 337 nM. The influences of sheath liquid composition, nebulizing gas pressure and drying gas temperature on the separation were examined.

Determination of dopamine and methoxycatecholamines in patient urine by liquid chromatography with electrochemical detection and by capillary electrophoresis coupled with spectrophotometry and mass spectrometry

The applicability of capillary electrophoresis (CE) with UV and mass spectrometric (MS) detection for the determination of dopamine and methoxycatecholamines in urine was evaluated in comparison with the liquid chromatography-electrochemical detection (LC-EC) method widely used in catecholamine analysis. The catecholamines in urine were deconjugated with acid or enzyme hydrolysis, purified by cation exchange (CEX) or solid-phase extraction (SPE) with a copolymer of N-divinylpyrrolidone and divinylbenzene and analyzed by LC-EC, CE-UV, and CE-MS. Acid hydrolysis was more effective in the deconjugation than enzymatic hydrolysis with Helix pomatia. However, the recoveries of HMBA, DA and NMN from spiked samples were less than 30% after acid hydrolysis and SPE purification. The CEX purification was more efficient than SPE in removing matrix compounds from the urine samples. The limits of detection were lower in LC-EC analysis than in CE-UV or CE-MS. Many factors in the analytical procedure caused deviations in the concentrations measured for urinary dopamine and methoxycatecholamines. The recovery of HMBA, which was used as the internal standard, was poor after acid hydrolysis and SPE purification. The purification methods were validated in conjunction with the analytical methods and therefore cross analysis was unsuccessful. The LC-EC method was the most sensitive, but CE-UV and CE-MS were sensitive enough for the determination of dopamine and methoxycatecholamines even in healthy patient urine. The EC and MS detections were superior to the UV detection in specificity since, after acid hydrolysis, some matrix compounds were migrating close to I.S., DA and 3MT.

Analysis of catecholamines by capillary electrophoresis and capillary electrophoresis-nanospray mass spectrometry. Use of aqueous and non-aqueous solutions compared with physical parameters

Catecholamines were analysed in aqueous and alcoholic non-aqueous solutions by capillary electrophoresis and capillary electrophoresis-mass spectrometry using sheathless nanospray coupling. Decreases in the electrophoretic mobilities of the catecholamines and in the electroosmotic mobilities were observed from water to 1-propanol. Separations were more efficient in all non-aqueous media than in water. The diffusion coefficients of the catecholamines in the different media were determined. The solvent had little effect on the sensitivity of the UV or MS detection. Both methods were successfully applied to the analysis of urine samples.

Recent advances in pharmacokinetic applications of capillary electrophoresis

This article reviews recent capillary electrophoresis (CE)-based assays which were published for pharmacokinetic studies. Both the advantages and disadvantages of these CE-based assays are discussed based on their feasibility and the significance towards the better understanding of pharmacokinetics. In addition, as a future outlook, novel assays such as immunoaffinity CE and chip-based CE for analyzing drugs in biological fluids are summarized in view of their potential for pharmacokinetic applications.

Analyses of quaternary ammonium drugs in horse urine by capillary electrophoresis-mass spectrometry

A capillary electrophoresis-mass spectrometry (CE-MS) method for the analysis of quaternary ammonium drugs in equine urine was developed. Quaternary ammonium drugs were first extracted from equine urine by ion-pair extraction and then analysed by CE-MS in the positive electrospray ionization (ESI) mode. Within 12 min, eight quaternary ammonium drugs, each at 1 ng/mL in horse urine, could be detected. The confirmation of these drugs in urine samples was achieved by capillary electrophoresis tandem mass spectrometry (CE-MS/MS). A direct comparison of this method was made with existing liquid chromatography/mass spectrometry (LC-MS) methods in the detection and confirmation of glycopyrrolate and ipratropium bromide in horse urine. While the two drugs could be detected within the same CE-MS run at 1 ng/mL in urine, they could only be detected in separate LC-MS runs at 5 ng/mL in urine. In addition, CE-MS consumed a much smaller volume of extract; the analyte peak widths, in some cases, were much narrower; and as the quaternary ammonium ions were well separated electrophoretically from the mainly neutral urine matrix, a much cleaner background in the CE-MS total ion trace was observed.

Nonaqueous capillary electrophoresis: a versatile completion of electrophoretic separation techniques

Nonaqueous capillary electrophoresis (NACE) is the application of a conductive electrolyte dissolved in either one organic solvent or a mixture of several organic solvents to carry out zone electrophoresis or related techniques in fused-silica capillaries. A complete review on the fundamentals, the optimization of analytical methods, practical considerations, and applications is given. To explain the differences to CE in aqueous media, a brief summary on solvent properties and molecular interactions in solutions introduces the reader into these fields. The use of additives to tune separation selectivity by means beyond a pure zone-electrophoretic mechanism is discussed in detail for organic media. Special detection techniques providing high potential for NACE are presented. Data on the precision of NACE methods and a list of relevant applications are included. More specialized applications like the determination of physicochemical constants in NACE or the setup of a semipreparative mode are described.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (6 Weeks journals - Search completed at 7th. June 2000)