Prospect of increasing secondary ion yields in ToF‐SIMS using water cluster primary ion beams

Low ionization yields in time of flight secondary ion mass spectrometry (ToF‐SIMS) particularly from single cells and tissues are proving to be a significant limitation in allowing this technique to reach its full potential. A number of approaches including embedding the sample in water or spraying water above sample surface has shown great prospective for increasing the ionization yield by a factor of 10 to 100 through ‘proton mediated’ reaction. Based on this hypothesis, a water cluster primary ion source has been developed in collaboration with Ionoptika Ltd to generate giant water cluster ions (H 2 O) n + ( n  = 1−10 000) using a similar supersonic jet expansion methodology as for argon cluster beams. The ion yields of arginine, cholesterol, angiotensin II and a lipid mix have been measured under static and high ion dose conditions using (H 2 O) 5000 + , (H 2 O) 3000 + , Ar 3000 + and C 60 + primary ion beams at 20 keV. An enhancement in yields up to a factor of around 4 is observed under water cluster impact, in comparison with C 60 + at 1 × 10 11  ions/cm 2 ion dose, whereas this increases by around 10–50 times at high ion dose conditions. Copyright © 2014 John Wiley & Sons, Ltd.