How do I ionise in APCI and ESI modes?
To be ionised by APCI and ESI, organic molecules must possess a moiety that can accept or lose a proton (e.g.: -NH2, CO2H, SO3H, Ph-OH),
Positive ion sources:
• Amines, amides, imines, alcohols and esters
• High proton affinity
• Alkaline samples that can accept ammonium ion protons
Negative ion sources:
• Acids, polyhydroxy compounds, steroids
• High gas phase acidity
• [M-H]-, chloride, acetate, etc.
NB: negative ions are frequently more stable than positive ions.