| Abstract
| - A robust, reproducible, and single-step interface designbetween low flow rate separation techniques, such assheathless capillary electrophoresis (CE) and nanoliquidchromatography (nLC), and mass spectrometry (MS)using electrospray ionization (ESI), is introduced. In thisdesign, the electrical connection to the capillary outlet wasachieved through a porous tip at the capillary outlet. Theporous section was created by removing 1−1.5 in. of thepolyimide coating of the capillary and etching this sectionby 49% solution of HF until it is porous. The electricalconnection to the capillary outlet is achieved simply byinserting the capillary outlet containing the porous tip intothe existing ESI needle (metal sheath) and filling theneedle with the background electrolyte. Redox reactionsof water at the ESI needle and transport of these smallions through the porous tip into the capillary provides theelectrical connection for the ESI and for the CE outletelectrode. The etching process reduces the wall thicknessof the etched section, including the tip of the capillary, to5−10 μm, which for a 20−30 μm i.d. capillary results instable electrospray at ∼1.5 kV. The design is suitable forinterfacing a wide range of capillary sizes with a wide rangeof flow rates to MS via ESI, but it is especially useful forinterfacing narrow (<30 μm i.d.) capillaries and low flowrates (<100 nL/min). The advantages of the porous tipdesign include the following: (1) its fabrication is reproducible, can be automated, and does not require anymechanical tools. (2) The etching process reduces the tipouter diameter and makes the capillary porous in onestep. (3) The interface can be used for both nLC−MS andCE−MS. (4) If blocked or damaged, a small section of thetip can be etched off without any loss of performance. (5)The interface design leaves the capillary inner wall intactand, therefore, does not add any dead volume to the CE−MS or nLC−MS interface. (6) Bubble formation due toredox reactions of water at the high-voltage electrode isoutside of the separation capillary and does not affectseparation or MS performances. The performance of thisinterface is demonstrated by the analyses of amino acids,peptide, and protein mixtures.
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