Simplifying CE−MS Operation. 2. Interfacing Low-Flow Separation Techniques to Mass Spectrometry Using a Porous Tip

Abstract

A robust, reproducible, and single-step interface design between low flow rate separation techniques, such as sheathless capillary electrophoresis (CE) and nanoliquid chromatography (nLC), and mass spectrometry (MS) using electrospray ionization (ESI), is introduced. In this design, the electrical connection to the capillary outlet was achieved through a porous tip at the capillary outlet. The porous section was created by removing 1-1.5 in. of the polyimide coating of the capillary and etching this section by 49% solution of HF until it is porous. The electrical connection to the capillary outlet is achieved simply by inserting the capillary outlet containing the porous tip into the existing ESI needle (metal sheath) and filling the needle with the background electrolyte. Redox reactions of water at the ESI needle and transport of these small ions through the porous tip into the capillary provides the electrical connection for the ESI and for the CE outlet electrode. The etching process reduces the wall thickness of the etched section, including the tip of the capillary, to 5-10 microm, which for a 20-30 microm i.d. capillary results in stable electrospray at approximately 1.5 kV. The design is suitable for interfacing a wide range of capillary sizes with a wide range of flow rates to MS via ESI, but it is especially useful for interfacing narrow (<30 microm i.d.) capillaries and low flow rates (<100 nL/min). The advantages of the porous tip design include the following: (1) its fabrication is reproducible, can be automated, and does not require any mechanical tools. (2) The etching process reduces the tip outer diameter and makes the capillary porous in one step. (3) The interface can be used for both nLC-MS and CE-MS. (4) If blocked or damaged, a small section of the tip can be etched off without any loss of performance. (5) The interface design leaves the capillary inner wall intact and, therefore, does not add any dead volume to the CE-MS or nLC-MS interface. (6) Bubble formation due to redox reactions of water at the high-voltage electrode is outside of the separation capillary and does not affect separation or MS performances. The performance of this interface is demonstrated by the analyses of amino acids, peptide, and protein mixtures.

Comparing and Combining Capillary Electrophoresis Electrospray Ionization Mass Spectrometry and Nano–Liquid Chromatography Electrospray Ionization Mass Spectrometry for the Characterization of Post-translationally Modified Histones

“…A recently published concept of a sheathless interface based on a separation capillary with a porous tip acting as a nanospray emitter overcomes these disadvantages (38). The capillary tip is etched using hydrofluoric acid until the capillary wall becomes so thin and porous that an electric contact can be established.…”

Top-down proteomic analysis of protein pharmaceuticals, mixtures of protein complexes, and ribosomal protein extracts by capillary zone electrophoresis-mass spectrometry

“…The interface demonstrated attomole-level sensitivity, experimental reproducibility, robustness, and could be readily assembled [64]. Another novel, sheathless design involved a chemically-etched, porous capillary tip, which also acts as an ESI emitter [65]. A schematic of this interface is shown in Figure 1-…”

“…While the coupling of CE to MS has developed more slowly than that of LC to MS, recent strides have been made using sheath flow [232,233] and sheathless interfacing [234]. A commercially available sheathless CESI source combines high resolution separation of CE with high resolution mass spectrometry.…”

Training in metabolomics research. I. Designing the experiment, collecting and extracting samples and generating metabolomics data

“…This restricted its applicability to metabolomics. However, the introduction of the sheathless porous tip interface [90] has considerably improved the sensitivity possible with CE-MS, and a commercial form suitable for attachment to mass spectrometers of several leading mass spectrometer manufacturers has been developed by Beckman Coulter-SCIEX. While CE-MS is very effective for hydrophilic metabolites, it is also possible to perform the electrophoresis using capillaries packed with a reverse-phase packing.…”