Capillary Electrophoresis-mass Spectrometry Interface Research Articles

Polydopamine-functionalized poly(ether ether ketone) tube for capillary electrophoresis-mass spectrometry

Capillary electrophoresis-mass spectrometry (CE-MS) is a hyphenated technique that combines the advantages like low sample consumption, high separation efficiency, short analytical time in CE and high sensitivity, powerful molecular structure elucidation in MS. Polyimide-coated fused silica capillary has become the most dominant capillary for CE, but it suffers from swelling and aminolysis of polyimide coating when treated with organic solvents and alkaline buffer in the CE-MS interface in which the polyimide coating at the end of the capillary is exposed to the solution, and this phenomenon can result in current instability, irregular electrospray and clogging at outlet after prolonged use. In this work, poly(ether ether ketone) (PEEK) capillary was explored as separation capillary for CE-MS. The problems like swelling and aminolysis of polyimide coating were solved due to the high thermal and chemical stability of PEEK material. After modification with polydopamine, PEEK capillary (PD-PEEK) can generate adjustable electroosmotic flow and provide good separation selectivity. The zwitterion polymer of polydopamine can provide cathodic electroosmotic flow (EOF) at high pH value (pH ≥ 5) and anodic EOF at low pH value (pH ≤ 4), and the EOF mobility can also be adjusted by controlling the modification time of polydopamine. Good separation performance was obtained in the analysis for several classes of compounds including amino acids, phenols and plant hormones at rational EOF direction. Repeatability of the PD-PEEK capillary was studied, with relative standard deviations for intra-day, inter-day runs and between tubes less than 4.94%.

Atmospheric pressure ion lens extends the stable operational region of an electrospray ion source for capillary electrophoresis-mass spectrometry

An atmospheric ion lens incorporated into an electrospray ion source for capillary electrophoresis-mass spectrometry (CE-MS) is found to extend the stable operational regions for both flow rates and electrospray ionization (ESI) voltages. The stable operating conditions for the ESI source with and without the ion lens were characterized. The results showed that the stable operation region was widest when the voltage difference between the sprayer and the ion lens ranges from 2.6 to 2.8 kV, and under these condition, the CE-MS interface can be adapted to a broader range of electroosmotic and modifier flow rates. Modeling of the electric field in the electrospray ion source with the ion lens suggests that the extension of the stable region is attributed to the flatter equipotential surfaces around the sprayer tip and higher electric field strengths in the rest of the interface region.

Mass Transport in a Micro Flow-Through Vial of a Junction-at-the-Tip Capillary Electrophoresis-Mass Spectrometry Interface

When coupling capillary electrophoresis with postcolumn detection methods, such as mass spectrometry, the presence of postcolumn band broadening must be considered. The band broadening effects introduced by junction-at-the-tip CE-MS interfaces using a postcolumn micro flow-through vial are investigated by studying the hydrodynamic flow patterns and mass transport process inside the micro vial at the end of the CE separation capillary. Simulation results obtained by solving the Navier-Stokes and mass balance equations provide insights into the velocity field and concentration distribution of the analytes in the micro vial and demonstrate that, with a low flow rate of chemical modifier solution, the laminar flow streams confine the analyte molecules to the central part of the micro vial and thus maintain major features of the peak shapes. Peaks detected by UV and MS under similar experimental conditions were compared to verify the numerical prediction that the main features of the UV peak can be retained in the MS peak. Experiments also show that band broadening can be minimized when an appropriate chemical modifier flow rate is selected.

An electrokinetic/hydrodynamic flow microfluidic CE-ESI-MS interface utilizing a hydrodynamic flow restrictor for delivery of samples under low EOF conditions

A hydrodynamic flow restrictor (HDR) that is used to combine electrokinetic and hydrodynamic flow streams has been fabricated in a microfluidic channel by laser micromachining. Combining electrokinetic and hydrodynamic flow streams is challenging in microfluidic devices, because the hydrodynamic flow often overpowers the electrokinetic flow, making it more difficult to use low electroosmotic flow in the electrokinetic portion of the system. The HDR has been incorporated into a capillary electrophoresis-mass spectrometry interface that provides continuous introduction of a make-up solution and negates the hydrodynamic backpressure in the capillary electrophoresis channel to the extent that low EOF can be utilized. Moreover, the hydrodynamic backpressure is sufficiently minimized to allow coatings that minimize EOF to be used in the electrokinetically driven channel. Such coatings are of great importance for the analysis of proteins and other biomolecules that adsorb to charged surfaces.