Ultrafast capillary electrophoresis/mass spectrometry with adjustable porous tip for a rapid analysis of protein digest in about a minute

Abstract

This paper introduces a novel ultrafast capillary electrophoresis (UFCE) mass spectrometry (MS) method comprised of a short (< 20 cm) and narrow (≤ 5 µm) capillary, with an integrated porous segment at the terminus of the capillary, for sheathless interfacing of CE to MS. The sheathless nature of the interface minimizes dead volume which is particularly important for ultrafast CE because of the small diameters of the capillaries. The separation voltage across the capillary was set to produce an electric field of ≥1000 V/cm to provide an analysis time of approximately 1 min. For the analysis of peptides and protein digests, a background electrolyte containing 0.1% cationic polymer (polybrene or Poly-E) in 0.1% acetic acid in water was used to prevent analyte-wall interaction. Uniquely, the porous tip of the capillary is adjustable. This allows the porous tip to be pulled inside the sheath liquid for sample stacking during injection, or sample focusing in isoelectric focusing (IEF), or, alternatively, pushed outside of the sheath liquid for electrospray ionization (ESI). The adjustable interface preserves electrical conductivity during electrokinetic injection with reverse electro-osmotic flow. Therefore, pre-concentration of analytes at the capillary inlet is achieved without the need to disassemble the interface, improving the throughput. As a proof of concept, the application of UFCE-MS was demonstrated by the analysis of a peptide mixture and a protein digest in ~1 min.