Desorption Electrospray Ionization - Ionization Efficiency

Ionization Efficiency

The ionization efficiency of DESI is complex and depends on several parameters such as, surface effects, electrospray parameters, chemical parameters and geometric parameters. Surface effects include chemical composition, temperature and electric potential applied. Electrospray parameters include electrospray voltage, gas and liquid flow rates. Chemical parameters refers to the sprayed solvent composition, e.g. addition of NaCl. Geometric parameters are α, β, d₁ and d₂ (see figure on the right).

Furthermore, α and d₁ affect the ionization efficiency, while β and d₂ affect the collection efficiency. Results of a test performed on a variety of molecules to determine optimal α and d1 values show that there are two sets of molecules: high molecular weight (proteins, peptides, oligosaccharide etc.) and low molecular weight (dizazo dye, stereoids, caffeine, nitroaromatics etc.). The optimal conditions for the high molecular weight group are high incident angles (70-90°) and short d₁ distances (1–3 mm). The optimal conditions for the low molecular weight group are the opposite, low incident angles (35-50°) and long d₁ distances (7–10 mm). These test results indicate that each group of molecules has a different ionization mechanism; described in detail in the Principle of operation section.

The sprayer tip and the surface holder are both attached to a 3D moving stage which allow to select specific values for the four geometric parameters: α, β, d₁ and d₂.