positieveionmodus

Positieve ionmodus, in mass spectrometry often referred to as the positive ion mode, is the operational mode in which analyte molecules are ionized to produce predominantly positively charged ions. In this mode the mass spectrometer detects ions with a positive charge, such as protonated molecules [M+H]+, sodiated adducts [M+Na]+, or other protonated or cation-adducted species. The choice of positive mode depends on the chemistry of the analyte and the ionization method used.

Ionization sources commonly employed to favor the positive mode include electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and, in some cases, matrix-assisted laser desorption/ionization (MALDI) when operated to generate positive ions. In LC-MS workflows, the positive mode is often achieved by acidifying the mobile phase with formic or acetic acid to promote protonation.

Typical analytes detected in the positive mode are basic or neutral compounds that readily form cations, such as amines, peptides and proteins (producing [M+H]+ and related adducts), and many small molecules. Adduct formation with salts or solvents, such as [M+Na]+ or [M+K]+, is common and can influence spectral interpretation and sensitivity.

Compared with the negative ion mode, which detects deprotonated or anionized species, the positive mode offers higher sensitivity for many basic compounds but may require consideration of adduct complexity and potential in-source fragmentation. The mode is widely used in proteomics, metabolomics, and pharmacology for its broad applicability to protonatable species.