Enhancing the Sensitivity of Molecular Secondary Ion Mass Spectrometry with C60+-O2+ Cosputtering

In the past decade, the C-60-based ion gun has been widely utilized in the secondary ion mass spectrometry (SIMS) analysis of organic and biological materials because molecular secondary-ions of high masses could be generated by cluster-ion bombardment. This technique furthers the development of SIMS in bioanalysis by eliminating the need for either heteroatom or isotope labeling. However, the intensity of high-mass parent ions was usually low and limited the sensitivity of the analysis, thus requiring an enhancement in the intensity of these molecular ions to widen the application of SIMS. In this work, the aim was to preserve samples in their original state while using a low kinetic energy O-2(+) beam cosputtered with high-energy C-60(+) to enhance the ion intensity through the depth-profile. Although O-2(+) is generally used to enhance ion intensities in positive SIMS, it is known to alter the chemical structure and primarily provide elemental information; hence, it is not suitable for profiling organic and biological specimens. Nevertheless, owing to its high sputtering yield, cluster C-60(+) ion removes and masks the structural damage, hence O-2(+) may be used to enhance the ion intensity. The characteristic molecular ions of polyethylene terephthalate (PET), trehalose, and a peptide (papain inhibitor) are enhanced by 35X, 12X, and 3.5X with the use of the auxiliary O-2(+) beam, respectively. This significant enhancement in ionization yield is attributed to the oxidation of molecules and formation of a hydroxyl group that serves as a proton donator. In addition to enhancing molecular SIMS signals, C-60(+)-O-2(+) cosputtering could also alleviate several problems, including sputtering rate decay, carbon deposition, and surface roughening, that are associated with C-60(+) bombardment and produced better depth profiles.