A new on-chip ESI nozzle for coupling of MS with microfluidic devices

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Abstract
This paper presents a new on-chip electrospray ionisation (ESI) nozzle, which can be used as an interface for coupling microfluidic devices with mass spectrometric (MS) detection. The nozzle was micromilled in a polymer foil (polymethylmethacrylate (PMMA) 750 µm thick), normally used as a cover for microfluidic chips. The performance of this device was examined in the ESI-MS analysis of the tetrapeptide MRFA (methionine–argenine–phenylalanine–alanine). The spray quality is basically dependent on the inner diameter of the nozzle, beside the part of the organic modifier in the solution to be sprayed. Three different inner nozzle diameters (30, 50, 100 µm) and two different apex angles were investigated. Stable electrospray conditions can be generated with a relative standard deviation less than 10% of the total ion current, and down to a concentration of 0.01 µmol L−1. The production of this ESI interface is relatively simple for the purpose of a low-cost batch fabrication of miniaturized analytical instruments.