Capillary electrophoresis and capillary electrophoresis-mass spectrometry in drug and metabolite analysis

Dual detection for non-aqueous capillary electrophoresis combining contactless conductivity detection and mass spectrometry

Coupling of two detectors is a powerful tool to enhance the overall analytical performance generating complementary information and overcoming the limitations of the single detectors. In this work, capacitively coupled contactless conductivity detection (C⁴D) and electrospray ionization mass spectrometry (ESI-MS) were coupled in conjunction with non-aqueous capillary electrophoresis (NACE). Non-aqueous electrolytes are highly compatible with ESI due to their volatility. Moreover, they exhibit low background conductivity, which is essential for the detection with C⁴D. A NACE-C⁴D-MS method was developed using an acetonitrile buffer containing 2 M HAc and 4 mM NH₄Ac as background electrolyte. The influence of the inner diameter of the separation capillary on the C⁴D was studied and taken into account. A capillary with 50 µm inner diameter was found to be best suited. The complementarity of the two detectors was shown by determining a sample mixture containing choline, thiamine, nitrate, and chloride as well as bromide and acetylcholine as internal standards. The C⁴D was the detector of choice for the inorganic ions, which were not detectable with the MS whereas the MS had much lower limits of detections for the organic biomolecules. The method was applied on an extract of a food supplement containing the model analytes.

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.

Experimental designs to investigate capillary electrophoresis-electrospray ionization-mass spectrometry enantioseparation with the partial-filling technique

An experimental design approach is described to evaluate the main electrophoretic parameters involved in the enantioseparation of pharmaceuticals by capillary electrophoresis (CE) coupled to electrospray ionization-mass spectrometry (ESI-MS). For all experiments, the partial-filling technique was applied to avoid the chiral selector entering in the mass spectrometer ion source with a negative effect on the electrospray performance. To carry out enantioseparation, a volatile buffer constituted of 20 mM ammonium acetate at pH 4.0, and a polyvinyl alcohol-coated capillary were used. Methadone was employed as the model compound and three different cyclodextrins (CDs), namely sulfobutyl ether-beta-CD, carboxymethylated-beta-CD and hydroxypropyl-beta-CD, were selected in order to study the countercurrent process. Two different experimental designs were chosen: (i) a full-factorial design to examine the effects and significance of the investigated factors, and (ii) a central composite face-centered design to establish the mathematical model of the selected responses in function of experimental factors. The chiral selector concentration, percentage of the capillary filled with the chiral selector, and drying gas nebulization pressure were three relevant factors taken into consideration. For each CD, the methadone enantiomeric resolution, apparent selectivity, and migration time of the second enantiomer were established as responses. The latter were systematically related to experimental parameters with the help of multiple linear regression. It is noteworthy that the behaviour was different in function of the chiral selector charge. Results revealed that the nebulization pressure involved in the electrospray process and the CD concentration had a significant effect on the enantiomeric resolution, while the effect of the separation zone length was less pronounced. Finally, response surfaces were drawn from the mathematical model and experimental conditions were selected to allow a robust determination of methadone enantiomers by CE-MS.

Potential of capillary electrophoresis, tandem mass spectrometry and coupled capillary electrophoresis–tandem mass spectrometry as diagnostic tools

Urine and blood samples from patients with known metabolic disorders have been analyzed by CE, MS-MS and CE-MS-MS. For the identification of defects in acylcarnitine metabolism, blood spots on filter paper were analyzed using an MS-MS "neonatal screening" approach. Direct CE-MS-MS analysis was used for the analysis of urine samples from patients with different metabolic disorders, including galactosemia, neuroblastoma, Zellweger syndrome, propionic acidemia and alcaptonuria. The sensitivity of the CE-MS-MS method was increased by use of multiple reaction monitoring.

Non-aqueous capillary electrophoresis

The benefits of non-aqueous capillary electrophoresis have been described in a number of recent publications. The wide selection of organic solvents, with their very different physicochemical properties, broadens our scope to manipulate separation selectivity. The lower currents present in non-aqueous solvents allow the use of high electric field strengths and wide bore capillaries, the latter in turn allowing larger sample load. In many cases detection sensitivity can also be enhanced. The potential of non-aqueous capillary electrophoresis is discussed throughout the paper, and the feasibility of capillary electrophoresis under non-aqueous media is demonstrated with reference to several applications.

Nonaqueous capillary electrophoresis: application possibilities and suitability of various solvents for the separation of basic analytes

Eleven organic solvents were tested for their suitability in the use of capillary electrophoresis (CE). In all cases, 25 mM ammonium acetate and 1 M acetic acid were used as electrolytes. Three basic therapeutical agents, propranolol, carteolol and imipramine, were used as analytes. Four solvents (2-propanol, ethylene glycol, propylene glycol, dichloromethane) were not suitable for use with CE under our conditions. Depending on the other solvents used, the analytes showed very different behavior. We observed that the epsilon/eta quotient alone is not sufficient for a characterization of the solvents. Further investigations with all the solvents as 1:1 mixtures with methanol were carried out. As a result some principal changes occurred compared to the pure solvents. Working with mixtures of different solvents proved to be advantageous because of the possibility to influence properties like high viscosity or low ionizing abilities by the addition of a suitable second solvent.

Application of electrospray tandem mass spectrometry to neonatal screening

For the past 30 years, neonatal screening programs have been performed largely by using the bacterial inhibition assays developed by Dr Robert Guthrie. These programs focused on a small number of diseases such as phenylketonuria and maple syrup urine disease and involved one test for each disease. During the same period many new diseases were discovered, such as organic acidemias and fatty acid oxidation defects, and they presented a diagnostic challenge to biochemical laboratories. Different mass spectrometric approaches have been the main tools for the diagnosis; however, each has its own limitation. Recently, electrospray tandem mass spectrometry (MS/MS) has provided an alternative automated high throughput, specific, and broad-spectrum approach to screening for a relatively large number of disorders, including those covered by bacterial inhibition assays tests. By using specific scan functions, a large number of amino acids and acylcarnitines in blood spots are quantified in 2 minutes analytical time. A new scan function is described here for quantification and screening for argininosuccinic acid in blood spots, which is a key metabolite in the diagnosis of argininosuccinase deficiency. We describe the results of a 3-year tandem MS/MS-based neonatal study that was performed in our newborn population. We screened 27,624 blood spots and identified 20 cases yielding a frequency of 1:1,381. No false-negative cases were identified, but several false-positive cases were eliminated by repeat analysis by MS/MS of blood or by other means. We also used MS/MS analysis of urine or blood either for confirmation of initial positive results or for follow-up of treatment, such as in glutaric acidemia, citrullinemia, argininosuccinase deficiency, and biopterin-dependent phenylketonuria.

Nonaqueous media for separation of nonionic organic compounds by capillary electrophoresis

For the separation of neutral compounds by micellar electrokinetic chromatography, separations are usually carried out in predominantly aqueous solution in order to preserve the charged micelle necessary for the separation. We now show that polycyclic aromatic hydrocarbon (PAH) compounds can be separated efficiently by capillary electrophoresis in pure methanol or in aqueous-organic mixtures containing a high percentage of methanol. Sodium tetradecyl sulfate was the preferred surfactant. The effects of pH, solvent composition, surfactant structure, and surfactant concentration on the separations were studied. Reproducible migration times and linear calibration plots were obtained.

Determination of illicit and/or abused drugs and compounds of forensic interest in biosamples by capillary electrophoretic/electrokinetic methods

The application of capillary electrophoresis (CE) methods in forensic toxicology for the determination of illicit and/or misused drugs in biological samples is reviewed in the present paper. Sample pretreatments and direct injection modes used in CE for analysis of drugs in biological fluids are briefly described. Besides, applications of separation methods based on capillary zone electrophoresis or micellar electrokinetic chromatography with UV absorbance detection to (i) analysis of drugs of abuse, (ii) analysis of other drugs and toxicants of potential forensic interest and (iii) for metabolism studies are reviewed. Also, alternative CE methods are briefly discussed, including capillary isotachophoresis and separation on mixed polymer networks. High sensitivity detection methods used for forensic drug analysis in biological samples are then presented, particularly those based on laser induced fluorescence. A glimpse of the first examples of application of CE-mass spectrometry in forensic toxicology is finally given.

Determination of association constants of dansyl-amino acids and beta-cyclodextrin in N-methylformamide by capillary electrophoresis

The use of nonaqueous background electrolytes in capillary electrophoresis (CE) is a promising new trend which should widen the scope of this technique. We demonstrate the chiral separation of dansyl-amino acids (Dns-AAs) in N-methylformamide (NMF) using beta-cyclodextrin (beta-CD) as chiral selector. The solubility of beta-CD is much better in NMF than in water, allowing high concentration of the chiral selector and successful enantioseparation despite the weak host-guest interaction between the Dns-AAs and beta-CD. The association constants for the complexation between Dns-AAs and beta-CD could be calculated from the electrophoretic mobilities, with attention paid to the change in viscosity of the electrolyte upon addition of beta-CD. The association constants ranged between 2 and 13 M(-1).

In vitro studies on the metabolic fate of mifentidine, a novel histamine H2-receptor antagonist

The in vitro metabolism of mifentidine and several of its metabolites was studied using hepatic microsomes from seven animal species. The effects of potential enzyme inducers, inhibitors and activators were also studied. Mifentidine metabolites identified and characterised were: 4-imidazolylphenylamine (amine), 4-imidazolylphenyl-formamide (formamide), the urea derivative of mifentidine (urea) and the imidazole-hydroxylated derivative of the amine (i-OH-amine), along with three unidentified metabolites, M1, M2 and M3. Evidence for the presence of the amine, formamide, urea and i-OH-amine was obtained by comparison with authentic reference compounds: (i) HPLC retention times; (ii) UV spectra; and (iii) MS spectra of metabolites. The postulated intermediates are: carbinolimine (for formamide, amine, i-OH-amine and urea formation); formamide (for amine and i-OH-amine formation); amine (for i-OH-amine formation), and nitrone (for urea formation). One 'metabonate' of mifentidine was also identified, namely the nitro analogue of the amine. A possible prerequisite for the formation of this nitro is the corresponding hydroxylamine or nitroso compound. Cytochromes P450I and P450II were shown to be involved in the in vitro microsomal biotransformation of mifentidine, but the involvement of the flavin monooxygenase system was not proven.

Capillary electrophoresis in pharmaceutical analysis

Capillary electrophoresis (CE) is a separation technique particularly suited to the analysis of pharmaceutical compounds. This review offers a detailed discussion of the four common modes of detection coupled to CE-UV absorption, fluorescence, electrochemical, and mass spectrometry-and gives examples of the use of these methods in pharmaceutical analyses. Sample preparation and pretreatment techniques used for CE separations are described, as well as methods of preconcentration including hydrophobic retention, affinity concentration, sample stacking, and isotachophoresis. The use of affinity CE, chiral CE, and capillary gel electrophoresis for analysis of pharmaceuticals is covered in detail, and recent advances in capillary electrochromatography and CE on a chip are also discussed.

Application of capillary electrophoresis and related techniques to drug metabolism studies

The use of capillary electrophoresis (CE) for the separation of small organic molecules such as pharmaceutical agents and drug/xenobiotic metabolites has become increasingly popular. This has arisen, at least in part, from the complimentary mode of separation afforded by CE when compared to the more mature technique of HPLC. Other qualities of CE include relative ease of method of development, rapid analysis, and low solvent consumption. The recent introduction of a variety of detector systems (including UV diode array, laser-induced fluorescence, conductivity) and the demonstrated coupling of CE to MS have also aided acceptance of this technology. In the present report, we review the role of CE coupled to various detector systems including a mass spectrometer for the characterization of both in vitro and in vivo derived drug metabolite mixtures. Attributes of CE for this application are demonstrated by discussion of metabolism studies of the neuroleptic agent haloperidol. Various aspects of the development and use of CE and CE-MS for the characterization of haloperidol metabolites, including criteria for selection of parameters such as pH, ionic strength, extent of organic modification, and the use of nonaqueous capillary zone electrophoresis are discussed. We also consider potential limitations of CE and CE-MS for drug metabolism research and describe the introduction of membrane preconcentration-CE (mPC-CE) and mPC-CE-MS as a solution that overcomes the rather poor concentration limits of detection of CE methods without compromising the resolution of analytes or separation efficiency of this technique.