Sheathless electrospray ionization directly from a capillary monolith for fast liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry
High-resolution Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) has emerged as one of the most efficient analytical tools for the identification of numerous different compounds in complex biological and synthetic samples only by means of the precisely determined molecular mass. 1 Especially in active areas such as proteomics or high-throughput library screening, relevant samples may contain several isobaric compounds. Their exact identification requires either a further and time-consuming in-cell fragmentation and/or a preceding chromatographic gradient elution. 2 -6 Liquid chromatography (LC) with nanobore capillaries (up to 100 µm i.d.) and volumetric flow-rates in the lower microliters per minute range meet best the optimum conditions for electrospray ionization 7 and, in addition, the inherent chromatographic focusing step in gradient elution allows low-abundance molecules to be identified even from complex mixtures. However, to guarantee highly efficient chromatographic separations, a large surface area (column adsorption capacity) in sufficiently long capillaries is also required. Capillaries packed with small-diameter porous spheres generate high back-pressure and the resulting increased analysis