The Atmospheric Pressure Open-Type Ionisation Technology Family

Ambient ionisation is a general term referring to a class of techniques in which samples are ionised directly under ambient pressure with virtually no pretreatment. It challenges the traditional perception that ‘mass spectrometry requires a vacuum and complex sample preparation’, enabling mass spectrometers to act as ‘probes’ that target the surface of real objects to acquire spectra; it is therefore often referred to as an ‘in situ, real-time, direct’ ionisation method (see Atmospheric Pressure Mass Spectrometry Imaging).

This family comprises numerous members, which can be categorised according to their mechanisms of action. Desorption-type techniques, represented by DESI (see Desorption Electrospray Ionisation ), rely on charged solvent droplets impacting the surface to remove molecules; Laser-based methods, exemplified by LDI, MALDI and LDPI, use a laser to ‘invite’ molecules from the surface into the gas phase before ionisation; plasma-based methods, exemplified by DART, rely on excited-state species in a helium plasma to ionise the exposed sample; extraction-based methods include various variants involving solvent extraction followed by ionisation. Their common characteristics are that samples are analysed at atmospheric pressure with their morphology largely unchanged, and that analysis is rapid.

Both of New Source’s imaging ion sources belong to this family. The MSI DPI is a dual-mode source combining DESI (Desorption Electrospray Ionisation) with photochemical ionisation (PI); it first uses DESI to desorb molecules from the tissue surface, then employs vacuum ultraviolet photons to perform secondary ionisation on the released neutral molecules. This retains the ambient-pressure, in-situ advantages of DESI whilst eliminating polarity discrimination through post-ionisation (see Polarity Discrimination and Non-polarity Discrimination); MSI LDPI is transmission laser desorption coupled with photochemical ionisation, which also performs matrix-free imaging at atmospheric pressure. Both are compatible with mass spectrometers from Agilent, AB SCIEX and Thermo.

The value of open-access ionisation lies in transforming mass spectrometry from a ‘precision instrument in the laboratory’ into an ‘analytical tool that can be taken to the sample’. For scenarios requiring on-site screening, process monitoring, and in vivo/in situ analysis, such technologies are irreplaceable; the trade-off is that ionisation conditions are more susceptible to environmental factors (humidity, surface condition), and quantification requires the use of internal standards and calibration (see Isotopes and the Isotope Internal Standard Method).

Viewing the family of open-system ionisation techniques as a whole, they share the common characteristics of ‘atmospheric pressure, in situ and minimal sample preparation’; the only differences lie in ‘how the molecules are introduced into the gas phase’: DESI relies on droplet impingement, LDPI on laser desorption, and DART on helium plasma. Xin Ceyuan’s approach involves uniformly incorporating a photochemical ionisation (PI) step following the desorption/ionisation stage (see Photoionisation Sources and Photochemical Ionisation), enabling different front-ends to be connected to the same ‘soft ionisation, non-polarity-discriminating’ back-end. This is also the design basis for the compatibility of MSI DPI and MSI LDPI with instruments from the three brands: Agilent, AB SCIEX and Thermo. For users, this means there is no need to rewrite methods for each brand; simply changing the source allows existing workflows to continue.