Simplifying CE-MS operation. 2. Interfacing low-flow separation techniques to mass spectrometry using a porous tip

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.