Gas cooling secondary ions emitted by gas cluster ion beam at the travelling wave ion guide of a Q-ToF-SIMS system

The work was aimed at using gas to cool the secondary ions emitted from gas cluster ion beam (GCIB) bombardment of biosamples for lowering the ion energy distribution and thus enhancing the detection sensitivity for secondary ion mass spectrometry (SIMS). An Ar-cluster ion source at energy in an order of keV was installed with the Q-ToF (Quadrupole Time-of-Flight) premier at the Quantum Science and Engineering Center, Kyoto University. The main component of the Q-ToF premier consisted of a travelling-wave ion guide, a mass filter quadrupole lens, a travelling-wave collision cell, and a ToF analyzer. A 1,2-Distearoyl-sn-glycero-3-phosphocholine (DSPC) sample was used in the study on cooling the secondary ions at the travelling-wave ion guide by using helium and molecular nitrogen gas. The DSPC sample was impinged with primary Ar-cluster ions at energy 10 keV and sputtered for secondary ions in an energy range from zero to several hundred eV. Sputtering emitted secondary ions were extracted to the Q-ToF mass spectrometer. The experiment was carried out to measure DSPC secondary ion mass spectra in two modes at varied pressures of helium or molecular nitrogen gas. The first mode was the MS mode and the other was the MS/MS mode which defined the m/x = 790.6 Da for the protonated molecule at the mass filter quadrupole lens. The experiment found that the secondary ion yields (SIYs) in the MS mode reached the maximum at 2.0 and 0.35 Pa for helium and molecular nitrogen, respectively. In the MS/MS mode, the SIYs reached the maximum at 2.5 and 0.35 Pa for helium and molecular nitrogen, respectively, and however, some fragments could not be eliminated by the cooling molecular nitrogen gas. The cooling of the secondary ions for lowering the energy distribution and the transverse direction of the secondary ions by using helium was more effective and stable than by using molecular nitrogen.