Effects of cryogenic sample analysis on molecular depth profiles with TOF-secondary ion mass spectrometry

Although the benefits of decreased sample temperature for the molecular profiling of organic materials with time-of-flight secondary ion mass spectrometry (TOF-SIMS) have been established, the mechanism behind spectral changes observed at low temperature, particularly increased protonated molecular ion (M + H)⁺ yields, have not been examined in detail. We have developed a procedure to investigate these effects by monitoring secondary ion yields under sustained primary ion bombardment as the sample temperature is cooled from room temperature down to 80 K. Examination of biomaterials such as an amino acid (arginine), a polypeptide (Gly-Gly-Tyr-Arg), a lipid (1,2 dipalmitoyl-sn-glycero-3 phosphatidylcholine), and a drug molecule (cyclosporine A) each provide evidence of ion yield enhancement at 80 K under either 20 keV C₆₀⁺ or 20 keV Au₃⁺ bombardment. For example, arginine shows a 2-fold increase in the steady-state intensity for the (M + H)⁺ ion at 80 K compared to the steady state at 300 K. It is shown that there is a correlation between the yield enhancement and a reduction in the damage cross section, which for arginine under 20 keV Au₃ ⁺ bombardment decreases from 5.0 ± 0.4×10^-14 cm² at 300 K to 2.0 ± 0.3×10^-14 cm ² at 80 K. The role of water as the facilitator for this reduction is explored through the use of H₂O and D₂O dosing experiments at 80 K.