In this technical article you will learn about the adduction of alkali metal ions (Na, K), to analyte molecules with the term “metal adduct ion” being used to refer to these species.
The most commonly observed metal adduct ions in positive-ion electrospray analyses are single-charge sodium and potassium adducts, symbolized [M+Na]+ and [M+K]+ respectively. However, ions in electrospray can form with many other adducting species, e.g., ammonium ions [M+NH4]+; metal ions abstracted from the electrospray needle at high voltages, for example, [M+Fe]+; adducted solvent molecules such as [M+H+H2O]+; and, in negative-ion analysis, negative-charged adducting species like [M+Cl]–. Adduct species other than protonated molecules may also occur in atmospheric pressure chemical ionization (APCI) interfaces. Adduct ions in APCI must be volatile which means the ammonium, chloride, and water adducts described above can occur in APCI but the metal adduct ions cannot.
By reading the full article you will learn how important it is to understand the source of metal adduct ions and to know how to control their formation, particularly for quantitative applications, using two strategies that work reliably for dealing with these ions.