Comparison of Bi(1), Bi(3) and C(60) primary ion sources for ToF-SIMS imaging of patterned protein samples.

Imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used to study protein bound to a photolithographically-patterned, commercial poly(ethylene glycol) (PEG)-based polymer film. The effect of different ion sources on the fragmentation pattern from this sample was analyzed with respect to the surface sensitivity of characteristic protein fragments and contrast in the ion images. The method demonstrates that, under similar fluence (below the static limit), Bi(3) (+) provides better surface sensitivity for low mass fragments and the best image contrast as compared to Bi(1) (+) and C(60) (+) cluster sources. Principal component analysis (PCA) was utilized to process depth profiles for this sample and shows that a primary ion fluence of approximately 20 × 10(12) ions/cm(2) is required to etch through the adsorbed protein layer.