Detection limit for secondary ions of organic molecules under MeV ion bombardment

In the present work we have investigated the chemical sensitivity of a mass spectrometry imaging method MeV - SIMS. Primary ion beam within the MeV energy range domain was employed to bombard samples of two organic compounds; amino acid arginine, and peptide hormone angiotensin II (human), with average molecular weights of 174.2 u and 1046.2 u respectively. Secondary ion yield was measured as a function of number of molecules per area unit, and the detection limit was determined. For both molecular compounds and two different energies of primary 35Cl ion beam, the secondary ion yield exhibited a significant decrease below the area density of 1015 molecules/cm2, while the density of 1013 molecules/cm2 resulted in molecular peak / background ratio being lesser than 3, which is below the commonly used sensitivity threshold in other techniques. Other experiment related drawbacks to sensitivity were also discussed.