Muyi He

Development and Characterizations of a Miniature Capillary Electrophoresis Mass Spectrometry System

A miniature capillary electrophoresis mass spectrometry (CE/MS) system has been developed in this work. A 100% electrical driven miniaturized CE device was integrated with a miniature MS instrument, which has a discontinuous atmospheric pressure interface (DAPI) for coupling with atmospheric pressure ionization sources. A nanoelectrospray ionization (nano-ESI) source was developed with a sheath liquid interface for coupling the miniature CE and the MS system. A systematic characterization and optimization of the analytical performance have been done. The analysis of isobaric peptides and avoiding charge competition effects in nano-ESI sources have been demonstrated.

Pulsed Direct Current Electrospray: Enabling Systematic Analysis of Small Volume Sample by Boosting Sample Economy.

We had developed pulsed direct current electrospray ionization mass spectrometry (pulsed-dc-ESI-MS) for systematically profiling and determining components in small volume sample. Pulsed-dc-ESI utilized constant high voltage to induce the generation of single polarity pulsed electrospray remotely. This method had significantly boosted the sample economy, so as to obtain several minutes MS signal duration from merely picoliter volume sample. The elongated MS signal duration enable us to collect abundant MS(2) information on interested components in a small volume sample for systematical analysis. This method had been successfully applied for single cell metabolomics analysis. We had obtained 2-D profile of metabolites (including exact mass and MS(2) data) from single plant and mammalian cell, concerning 1034 components and 656 components for Allium cepa and HeLa cells, respectively. Further identification had found 162 compounds and 28 different modification groups of 141 saccharides in a single Allium cepa cell, indicating pulsed-dc-ESI a powerful tool for small volume sample systematical analysis.

Realistic modeling of ion-neutral collisions in quadrupole ion traps.

In this study, three ion-neutral collision models have been discussed and compared, including the Langevin, the hard-sphere and the mixed collision models. With the pseudo-potential approximation, analytical expressions of ion secular motions with the hard-sphere and mixed collision models have been obtained for the first time. Through numerical simulations and theoretical calculations, it is found that the mixed collision model could be used as a general description of ion-neutral collisions under different conditions. Langevin collision model is a good description of low energy collisions between small ions and neutrals, while hard-sphere collision model could be used to describe high energy collisions and/or ions with higher masses (larger physical sizes). These analytical expressions of ion motion decay profiles enable the creation of direct relationships between time-domain image currents with ion collision cross sections.

GPU Assisted Simulation Study of Ion-Ion Reactions within Quadrupole Ion Traps.

AbstractIn this study, a gas-phase ion–ion reaction model was developed, and it was integrated into an ion trajectory simulation program. GPU parallel computation techniques were also applied to accelerate the simulation process. With this simulation tool, the dependence of ion–ion reaction rate within 3D quadrupole ion traps on both ion trap operation parameters and the characteristics of reaction pair were investigated. It was found that the m/z values and charge states of ions have significant influences on the reaction rate. Moreover, higher ion–ion reaction rate was achieved under higher trapping voltages and higher buffer gas pressures. Furthermore, secondary reaction and/or neutralization of ETD fragment ions were observed from simulation. The reaction and/or neutralization rate depends on the charge state and m/z of each fragment ion. Graphical Abstractᅟ

Electro-kinetic assisted electrospray ionization for enhanced complex sample analysis

In this work, an electro-kinetic assisted electrospray ionization (EK-ESI) source is proposed and characterized. In EK-ESI, an additional auxiliary electric field is introduced in the liquid flow of a nano-ESI. While traveling forward in the electrospray flow, charged analytes also experience a reverse electric field, which pushes them backwards. As a result, analytes could be separated preliminarily based on their electrophoretic mobility during the electrospray process. Experiments show that EK-ESI can reduce charge competition effects in the ESI source and increase biomolecule detection sensitivities. It was also found that EK-ESI effectively ionizes proteins in a relatively mild solvent condition, which does not require the addition of acids or salt buffers into the solvent. As a proof-of-concept study, a very rough separation effect was observed in this study, further experiments and theoretical study will be carried out to enhance its performances.

Dual-Polarity Ion Trap Mass Spectrometry: Dynamic Monitoring and Controlling Gas-phase Ion–Ion Reactions

AbstractA dual-polarity linear ion trap (LIT) mass spectrometer was developed in this study, and the method for simultaneously controlling and detecting cations and anions was proposed and realized in the LIT. With the application of an additional dipolar DC field on the ejection electrodes of an LIT, dual-polarity mass spectra could be obtained, which include both the mass-to-charge (m/z) ratio and charge polarity information of an ion. Compared with conventional method, the ion ejection and detection efficiency could also be improved by about one-fold. Furthermore, ion–ion reactions within the LIT could be dynamically controlled and monitored by manipulating the distributions of ions with opposite charge polarities. This method was then used to control and study the reaction kinetics of ion–ion reactions, including electron transfer dissociation (ETD) and charge inversion reactions. A dual-polarity collision-induced dissociation (CID) experiment was proposed and performed to enhance the sequence coverage of a peptide ion. Ion trajectory simulations were also carried out for concept validation and system optimization. Graphical Abstractᅟ

A two-step method for rapid characterization of electroosmotic flows in capillary electrophoresis

The measurement of electroosmotic flow (EOF) is important in a capillary electrophoresis (CE) experiment in terms of performance optimization and stability improvement. Although several methods exist, there are demanding needs to accurately characterize ultra‐low electroosmotic flow rates (EOF rates), such as in coated capillaries used in protein separations. In this work, a new method, called the two‐step method, was developed to accurately and rapidly measure EOF rates in a capillary, especially for measuring the ultra‐low EOF rates in coated capillaries. In this two‐step method, the EOF rates were calculated by measuring the migration time difference of a neutral marker in two consecutive experiments, in which a pressure driven was introduced to accelerate the migration and the DC voltage was reversed to switch the EOF direction. Uncoated capillaries were first characterized by both this two‐step method and a conventional method to confirm the validity of this new method. Then this new method was applied in the study of coated capillaries. Results show that this new method is not only fast in speed, but also better in accuracy.