page_title: Probe electrospray ionization
section_title: Principle of operation
hierarchical_section_title: Probe electrospray ionization / Principle of operation
caption_attribution_description: English: &#160;Conceptual diagram of the PESI ion source
context_page_description: Probe electrospray ionization is an electrospray-based ambient ionization technique which is coupled with mass spectrometry for sample analysis. Unlike traditional mass spectrometry ion sources which must be maintained in a vacuum, ambient ionization techniques permit sample ionization under ambient conditions, allowing for the high-throughput analysis of samples in their native state, often with minimal or no sample pre-treatment. The PESI ion source simply consists of a needle to which a high voltage is applied following sample pick-up, initiating electrospray directly from the solid needle.
context_section_description: The PESI ion source consists of a solid needle or wire which acts as both the sampling probe and electrospray emitter. The needle is moved up and down along a vertical axis, a process which can be either automated or manual. When the needle is lowered to the sampling stage, the tip of the needle briefly touches the surface of a typical liquid sample. During this stage, the needle is held at ground potential. The needle is then raised to be level with the mass spectrometer inlet where a high voltage of 2-3 kV is applied. Electrospray is induced at the tip of the needle, producing analyte ions which are drawn into the mass spectrometer for analysis. The mechanism by which ions are formed is believed to be identical to traditional electrospray ionization. As a result, in positive ion mode analytes are often observed as the protonated, sodiated and potentiated ions, depending on the sample and analyte type.
  Although the amount of sample picked up by the needle is largely dependent on sample viscosity, it has been estimated that just a few picolitres of the sample solution are typically used.
  Because of this, the technique can be applied to small sample sizes, particularly ideal when limited sample amounts are available. As such a small sample amount is picked up and completely exhausted during the ionization process, issues of contamination are severely reduced. Furthermore, the process of sampling and ionization takes just a few seconds, so PESI-MS is suitable for high-throughput analysis.
caption: Probe electrospray ionization (PESI) schematic
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