Ambient ionization

[8] Desorption atmospheric pressure photoionization (DAPPI) is a solid-liquid extraction ambient ionization method that enables the direct analysis of samples deposited on surfaces by means of a jet of hot solvent vapour and ultraviolet light.

[9] Plasma-based ambient ionization is based on an electrical discharge in a flowing gas that produces metastable atoms and molecules and reactive ions.

[10][11] Besides protonated water clusters, other positively charged reagent ions, such as NO+, O2+, NO2+ and CO2+, may be formed in the afterglow region.

[10][11][12][13] These additional reagent ions are capable of ionizing compounds via charge-transfer processes and, thus, offer alternative routes of ionization besides proton transfer, leading to a broader range of suitable analytes.

Nevertheless, these ionization mechanisms may also lead to the formation of adducts and oxidation of the original analyte compounds.

[11][13] Measurements in the negative ion mode are especially favorable when the analyte molecules exhibit a high gas-phase acidity, as it is the case e.g. for carboxylic acids.One of the most used plasma-based techniques for ambient ionization is probably Direct analysis in real time (DART), since it is commercially available.

UV laser ablation can be combined with a flowing afterglow plasma for mass spectrometry imaging of small molecules.

[32] In two-step non-laser methods, the material removal from the sample and the ionization steps are separate.

SESI is commonly used for the analysis of trace concentrations of vapors being able to detect low volatility species in the gas phase with molecular masses of up to 700 Da.