Desorption Electrospray Ionisation (DESI): a technique that uses ‘charged water mist’ to propel molecules into the mass spectrometer at atmospheric pressure
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Desorption Electrospray Ionisation (DESI) is an ambient ionisation technique that enables direct chemical analysis of sample surfaces under atmospheric pressure, without the need to place the sample in a vacuum.
Basic Principles
DESI directs a stream of charged solvent mist (charged droplets) towards the sample surface from a distance of a few millimetres. Upon impact with the surface, the droplets spread out and dissolve the analytes; the desorbed and charged analytes are then carried away from the surface in the form of splash droplets, passing through the air into the ambient pressure port of the mass spectrometer for detection.
It is essentially a combination of electrospray ionisation (ESI) and desorption (DI): electrospray ionisation is responsible for generating charged droplets, whilst desorption is responsible for ‘flushing’ molecules from the surface and charging them.
Key Facts
Date of proposal and research team: Proposed in 2004 by Zoltán Takáts, Justin Wiseman and Bogdan Gologan in the R. Graham Cooks research group (Purdue University, USA) (Takáts et al., Science 2004, DOI:10.1126/science.1104404).
Operating conditions: Operates at atmospheric pressure; samples generally require no complex pre-treatment and the method is minimally destructive.
Typical parameters (source: Wikipedia “Desorption electrospray ionisation”): The atomisation gas is usually nitrogen, at a pressure of approximately 160 psi; the solvent is commonly methanol/water, sometimes with 0.5% acetic acid added, and the flow rate is approximately 10 μL/min.
There are two types of ionisation mechanisms: high-molecular-weight molecules (proteins, peptides) form multicharged ions, similar to electrospray; low-molecular-weight molecules are ionised via charge transfer (proton or electron transfer).
Applications
DESI is used in surface analysis, forensic testing, drug and metabolite research, and plant and food analysis, amongst others. It is also a mainstream ionisation method for mass spectrometry imaging, with a resolution of approximately 50 μm, making it suitable for scanning larger sample areas.
Relation to Neo Source products
Neo Source INSTRUMENT’s MSI DPI imaging ion source utilises DESI as the primary desorption ionisation method, combined with a post-ionisation (PI) step to further enhance the detection capability for non-polar molecules.
References
Wikipedia: Desorption electrospray ionisation. https://en.wikipedia.org/wiki/Desorption_electrospray_ionization
Takáts Z, Wiseman JM, Cooks RG. Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionisation. Science. 2004;306(5695):471–473. DOI:10.1126/science.1104404
Ifa DR, Wu C, Ouyang Z, Cooks RG. Desorption electrospray ionisation and other ambient ionisation methods: current progress and preview. The Analyst. 2010;135(4):669–681.