U.S. Department of Energy

Pacific Northwest National Laboratory

Effective coupling of CE with nanoESI MS via a true sheathless metal-coated emitter interface for robust and high sensitivity sample quantification

Introduction 

Capillary electrophoresis (CE) coupled with mass spectrometry (MS) is a promising alternative to conventional liquid chromatography (LC) MS in chemical and biological sample analysis due to its high resolving power and fast separation speed. Reproducibility and ruggedness problems, suffered to a certain degree by almost all the CE-MS interfaces, limit its broad applications. We present the development of a new sheathless CE-MS interface aiming at overcoming these problems and pushing CE-MS suitable to routine sample analysis with high sensitivity. A systematic evaluation of the new interface was performed using a hybrid capillary isotachophoresis (CITP) and capillary zone electrophoresis (CZE) separation coupled with electrospray ionization (ESI) MS for its achievable sensitivity and reproducibility in sample quantification.

 

Methods 

The new CITP-MS interface utilized a commercially available capillary with an integrated metal coated ESI emitter. The electric contact between the ESI voltage power supply and the CITP/CZE separation liquid was accomplished by using an electrically conductive liquid that came in contact with the metal coated outer surface of the ESI emitter. Stable electrospray was established by avoiding the formation of gas bubbles from electrochemical reaction at the emitter tip and inside of the separation capillary. This new CITP/CZE interface was directly coupled with a nanoflow ESI (nanoESI) triple quadrupole mass spectrometer operating in selected reaction monitoring (SRM) mode to evaluate its achievable limit of quantitation (LOQ) using targeted peptides spiked into a complex biomatrix.

 

Preliminary data 

The initial experimental evaluation of the new sheathless CITP/CZE-MS interface demonstrated its unique capabilities in achieving reproducible and high sensitivity CITP/CZE-nanoESI-SRM MS sample analyses. The new interface also allowed stable and dilution free nanoESI operation at flow rates as low as 5.7 nL/min, which improved both the CITP/CZE separation quality and MS detection sensitivity. Preliminary data from analyzing a nine peptide mixture spiked in a BSA digest sample matrix at different concentrations demonstrated a LOQ as low as 5 attomole by using the new sheathless CITP/CZE-ESI-SRM MS. Linear dynamic range of four orders of magnitude was also demonstrated with excellent analysis reproducibility by the new sheathless CITP/CZE-nanoESI-SRM MS instrument.

 

Our repeated sample quantification also showed that even after more than a hundred sample analyses, the stability of new sheathless CITP/CZE-SRM MS still remained without a noticeable loss of the metal coating on the emitter surface. The constant inner diameter and tapered outer diameter of the integrated separation/emitter capillary used in the new sheathless CITP/CZE interface also avoided the emitter clogging problem. All these unique features made the new interface suitable to high sensitivity and robust quantitative CE-MS sample analyses.

 

Novel Aspect

CITP/CZE-nanoESI-SRM MS using a metal coated emitter sheathless interface demonstrates increased robustness and sensitivity in complex sample analyses

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