Micellar electrokinetic chromatography–mass spectrometry: combining the supposedly incompatible

Electrophoretic extraction of low molecular weight cationic analytes from sodium dodecyl sulfate containing sample matrices for their direct electrospray ionization mass spectrometry

While the use of sodium dodecyl sulfate (SDS) in separation buffers allows efficient analysis of complex mixtures, its presence in the sample matrix is known to severely interfere with the mass-spectrometric characterization of analyte molecules. In this article, we report a microfluidic device that addresses this analytical challenge by enabling inline electrospray ionization mass spectrometry (ESI-MS) of low molecular weight cationic samples prepared in SDS containing matrices. The functionality of this device relies on the continuous extraction of analyte molecules into an SDS-free solvent stream based on the free-flow zone electrophoresis (FFZE) technique prior to their ESI-MS analysis. The reported extraction was accomplished in our current work in a glass channel with microelectrodes fabricated along its sidewalls to realize the desired electric field. Our experiments show that a key challenge to successfully operating such a device is to suppress the electroosmotically driven fluid circulations generated in its extraction channel that otherwise tend to vigorously mix the liquid streams flowing through this duct. A new coating medium, N-(2-triethoxysilylpropyl) formamide, recently demonstrated by our laboratory to nearly eliminate electroosmotic flow in glass microchannels was employed to address this issue. Applying this surface modifier, we were able to efficiently extract two different peptides, human angiotensin I and MRFA, individually from an SDS containing matrix using the FFZE method and detect them at concentrations down to 3.7 and 6.3 μg/mL, respectively, in samples containing as much as 10 mM SDS. Notice that in addition to greatly reducing the amount of SDS entering the MS instrument, the reported approach allows rapid solvent exchange for facilitating efficient analyte ionization desired in ESI-MS analysis.

Electrochemical methods in conjunction with capillary and microchip electrophoresis

Electromigrative techniques such as capillary and microchip electrophoresis (CE and MCE) are inherently associated with various electrochemical phenomena. The electrolytic processes occurring in the buffer reservoirs have to be considered for a proper design of miniaturized electrophoretic systems and a suitable selection of buffer composition. In addition, the control of the electroosmotic flow plays a crucial role for the optimization of CE/MCE separations. Electroanalytical methods have significant importance in the field of detection in conjunction with CE/MCE. At present, amperometric detection and contactless conductivity detection are the predominating electrochemical detection methods for CE/MCE. This paper reviews the most recent trends in the field of electrochemical detection coupled to CE/MCE. The emphasis is on methodical developments and new applications that have been published over the past five years. A rather new way for the implementation of electrochemical methods into CE systems is the concept of electrochemically assisted injection which involves the electrochemical conversions of analytes during the injection step. This approach is particularly attractive in hyphenation to mass spectrometry (MS) as it widens the range of CE-MS applications. An overview of recent developments of electrochemically assisted injection coupled to CE is presented.

Analysis of plasma nucleotides in rat by micellar electrokinetic capillary chromatography/electrospray ionization mass spectrometry

RATIONALE

The aim of this study was to establish a simultaneous quantitative analysis method of nine endogenous nucleotides in rat plasma using micellar electrokinetic capillary chromatography/electrospray ionization mass spectrometry (MEKC/ESI-MS).

METHODS

To select the optimum conditions for separation of the nucleotides, various pH, concentrations of running buffers and surfactants were tested. Ammonium acetate (20 mM) containing the surfactant dodecyltrimethylammonium bromide (2 mM, pH 3.5) was selected as the micellar running buffer. The plasma samples were prepared by precipitating the proteins with 2 mM EDTA in 60% ethanol. The samples were analyzed using capillary electrophoresis (CE)/MS and selected ion monitoring (SIM) mode with positive ionization. CE was performed using a silica capillary column in reversed polarity mode.

RESULTS

The limits of detection (LODs) and limits of quantification (LOQs) of the nucleotides ranged from 0.05-5 and 2.0-20 μM, respectively. The calibration curves were linear (R(2) >0.99) for all analytes, and the accuracy and precision were within ±15%. The developed method was applied to the analysis of nucleotides in rat plasma that was collected after oral administration of acetaminophen (1000 mg/kg/day) to evaluate the changes in plasma nucleotide levels under hepatotoxic conditions.

CONCLUSIONS

Decreased level of GTP and increased level of cytosine nucleotides were found to be associated with liver toxicity, which led to the conclusion that liver toxicity is closely related to changes in nucleotide levels in plasma.

Capillary electrophoresis and small molecule drug discovery: a perfect match?

Capillary electrophoresis (CE) is an analytical technique based on the separation of the analytes within a capillary owing to their different electrophoretic mobilities. It is widely used in pharmaceutical analyses owing to its versatility and high separation power. However, its penetration into the drug discovery scene has been relatively limited until recent years. Several factors have contributed to this low implementation, including the maturity of liquid chromatography, the scarcity of experienced CE practitioners, and certain limitations intrinsic to the technique. Recently, instrumental improvements and the growing demand for analytical information have lead to a continuously expanding range of routine electrophoretic applications throughout pharmaceutical discovery and development. In this article we review CE fundamentals, review well-established CE methodologies in drug discovery of small molecules and discuss trends that, in our opinion, might emerge in the coming years.

Electrokinetic chromatography and mass spectrometric detection using latex nanoparticles as a pseudostationary phase

The utility of novel latex nanoparticles as pseudostationary phases for electrokinetic chromatography with UV and mass spectrometric detection is demonstrated. The nanoparticles are synthesized using ab initio RAFT (reversible addition-fragmentation chain transfer) in emulsion polymerization, which yields small (63 nm) particles with a narrow size distribution, a hydrophobic core, and an ionic shell. The nanoparticles are shown to provide efficient and selective separations, with retention and separation selectivity dominated by hydrophobic interactions. The nanoparticles are highly retentive, such that they are effective at relatively low concentrations. Addition of the nanoparticles to the background electrolyte at these concentrations has a minor effect on the noise with UV detection, no measurable effect on the separation current, and minor effects on analyte ionization efficiency during electrospray ionization. The nanoparticles do not cause fouling or degradation of the electrospray-mass spectrometer interface even after several weeks of use. The combination of online sample preconcentration via sweeping and selective mass spectrometric detection yields low detection limits (10-16 ppb), particularly for more hydrophobic compounds.

Microemulsion electrokinetic chromatography with on-line atmospheric pressure photoionization-mass spectrometric detection of medium polarity compounds

In this paper, we present the determination of pharmaceuticals employing microemulsion electrokinetic chromatography (MEEKC) with atmospheric pressure photoionization-mass spectrometric (APPI-MS) detection. This recent hyphenated technique allows to overcome some disadvantages of MEEKC, namely its inherent incompatibility with MS detection. Important parameters like microemulsion (ME) composition, the composition of the sheath liquid and APPI-MS detection parameters have been investigated. Using the optimized set of parameters, the eight selected substances could be detected down to concentrations between 3 mg L(-1) (phenacetin) and 41 mg L(-1) (diltiazem). Switching to the MS2 mode, the use of specific transitions for the detection of each analyte provided improved detection limits in the range of 0.6 mg L(-1) (carbamazepine) to 6 mg L(-1) (metoprolol). Calibration curves were linear over one to two orders of magnitude with correlation coefficients better than 0.98.

Microemulsion Electrokinetic Chromatography with On-Line Atmospheric Pressure Photoionization Mass Spectrometric Detection

In the present paper we report, for the first time, the successful on-line coupling of microemulsion electrokinetic chromatography (MEEKC) with mass spectrometric (MS) detection using an atmospheric pressure photoionization interface. Microemulsions (MEs) including mostly volatile ingredients and classical MEs based on nonvolatile buffer components and sodium dodecyl sulfate (SDS) as surfactant were compared with respect to their compatibility with MS detection. The investigations performed revealed that MEs with up to 3% SDS and buffers containing sodium borate can be employed without significant suppression of the MS signals. A test mixture of nine substances could be separated by MEEKC using a ME consisting of 0.8% octane, 2% SDS, 6.6% butanol, and 90.6% of 20-mmol ammonium hydrogencarbonate buffer (pH 9.5). Operating the MS instrument in the MS(2) mode provided improved signal/noise ratios for analytes leading to characteristic MS-MS transitions. Thereby, limits of detection ranging between 0.5 (carbamazepine) and 5 microg mL(-1) (phenacetin) could be obtained.