Physical properties of the nanopore ion source

Mass spectrometry (MS) is a vital technique for biological and chemical analysis due in large part to the availability of soft ionization techniques such as electrospray ionization (ESI), which can transfer molecular ions into the gas phase without damaging them. However, the ESI process utilizes a background gas which degrades ion transmission, resulting in analyte ion transmission efficiencies of around 1%, limiting the sensitivity of MS. We have developed a nanopore ion source which can deliver ions directly into high vacuum from aqueous solutions with high efficiency. The heart of the ion source is a pulled quartz capillary with a tip diameter of less than 100 nm. Surface tension at the nanoscale liquid-vacuum interface can maintain a stable meniscus with high charge density, enabling electric fields on the order of 1 V/nm to be reached. At such high fields, ions can be emitted from the meniscus by ion evaporation without the emission of liquid droplets as in conventional electrospray. We discuss potential applications for the nanopore ion source, notably in increasing MS sensitivity and enabling single molecule protein sequencing.