Design, optimisation, and evaluation of a sheath flow interface for automated capillary electrophoresis‐electrospray‐mass spectrometry

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Abstract
A sheath‐flow capillary electrophoresis‐mass spectrometry (CE‐MS) system utilizing a fully integrated large‐bore stainless‐steel emitter electrode tapered at the end for micro‐ionspray operation has been developed and evaluated. A separation capillary with an outer diameter of up to 360 µm was inserted into the electrode thus forming a void volume of less than 15 nL between the capillary end and the electrospray ionisation (ESI) tip. The sheath liquid, usually methanol‐water (80:20) with 0.1% formic acid for positive ion mode or methanol for negative ion mode, was delivered at 0.5–1.0 µL/min. Unlike previously reported CE‐MS interfaces, the CE‐MS probe was incorporated directly onto an Applied Biosystems/MDS SCIEX orthogonal‐spray Turbo “V” ion source for ease of use and automatic operation. This integration enables fast and facile coupling and replacement of the separation capillary without interrupting the ion source configuration, and the sheath liquid supply. The reusable electrospray electrode was precisely fabricated and aligned with the length of the nebulizing gas tube for improved reproducibility. Automation was achieved through software control of both CE and tandem MS (MS/MS) for unattended batch sample analysis. The system was evaluated for attomole‐ to low femtomole‐level profiling of model peptides and protein mixtures, bisphosphates, as well as antiviral nucleosidic drugs in cellular extracts.

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