  We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data
when available) for 85 submicron- to micron-sized presolar SiC grains from the
CM2 Murchison meteorite, including 60 MS, 8 AB1, 8 X, 7 AB2, and 2 Y grains.
The MS and Y grain data demonstrate that (1) C and N contamination mainly
appears as surface contamination, and sufficient presputtering is needed to
expose a clean grain surface for obtaining intrinsic C and N signals, and (2)
Mg and Al contamination appears as adjacent grains and rims, and
high-resolution imaging and the choice of small regions of interest during data
reduction together are effective in suppressing the contamination. Our results
strongly indicate that previous studies on presolar SiC grains could have
sampled differing degrees of contamination for C, N, Mg, and Al. Compared to
the literature data, our new MS and Y grains are in better agreement with
carbon star observations for both the C and N isotopic ratios. By comparing our
new, tighter distributions of 12C/13C, 14N/15N, and initial 26Al/27Al ratios
for MS and Y grains with FRUITY AGB stellar models, we provide more stringent
constraints on the occurrence of cool bottom processing and the production of
26Al in N-type carbon stars, classical asymptotic giant branch stars.
