a) Please resolve the following acronyms:
- API: Atmospheric pressure ionization
- APCI: Atmospheric pressure chemical ionization
- APPI: Atmospheric pressure photoionization
b) Estimate the factor in pressure by which ionization processes in classical chemical ionization differs from that in APCI.
Classical chemical ionization (CI) is run at a pressure of about 0.1-1 mbar inside the ion volume while APCI is run at atmospheric pressure, i.e., at 1000 mbar. The factor in pressure and thus particle density between these two is about 1000 to 10.000.
c) What effect does the higher pressure in APCI have in terms of softness of ionization as compared to CI?
APCI is considerably softer than CI because there is a tremendously large number of very soft collisions that effect almost instantaneous thermalization of the freshly formed analyte ions. Even though the gas may be at 150-200°C in the reaction zone, this corresponds to a ion internal energy of just 0.1-0.2 eV, i.e., ten times lower than the minimum required to cleave a covalent bond of the ion.