Functional electrospray emitters

Electrospray ionisation (ESI) is a soft technique used in mass spectrometry (MS) analysis able to introduce a wide variety of analytes in the gas phase. Briefly, it consists in the application of a high-voltage to a solution to spray it through a small orifice. Hence, charged droplets and ions produced in the gas phase are directed towards a counter electrode. Based on the electrospray process itself, it is possible to carry out diverse experiments by adding functions at three different levels. The first one, taking advantage of the microfluidic nature, incorporates operation units such as mixers, reactors or chromatographic elements into the emitter. In this part, biphasic electrospray ionisation (BESI) sources will also be presented. The electrochemical properties of ESI provide the second level of functionalities. Electrochemistry, being inherent to the ESI process, has proved to be useful for different on-line purposes such as protein electrolysis or electrochemical-induced chemical derivatisation. Finally, the third level is the aerosol zone where a fine mist of charged droplets is formed. In this zone, desorption and extraction reactions can happen for different substrates and vapours. Hence, desorption and extractive electrospray ionisation (DESI and EESI, respectively) will be presented with an emphasis on the advantages brought by these methods. The present review does not intend to focus on the electrospray ionisation technique in itself but rather on the introduction of functional emitters for ESI-MS. The first part covers basic concepts required for developing the other sections. Then, the different options available at the three different levels are reviewed in order to highlight the potential of functional ESI in the growing field of mass spectrometry.