Identification of a lipid-related peak set to enhance the interpretation of TOF-SIMS data from model and cellular membranes

Principal component analysis (PCA) of time-of-flight secondary ion mass spectrometry (TOF-SIMS) data enables differentiating structurally similar molecules according to linear combinations of multiple peaks in their spectra. However, in order to use PCA to correctly identify variations in lipid composition between samples, the discrimination achieved must be based on chemical differences that are related to the lipid species, and not sample-associated contamination. Here, we identify the positive-ion TOF-SIMS peaks that are related to phosphatidylcholine lipid headgroups and tail groups by PCA of spectra acquired from lipid isotopologs. We demonstrate that restricting PCA to a contaminant-free lipid-related peak set reduces the variability in the spectra acquired from lipid samples that is due to contaminants, which enhanced differentiating different lipid standards, but adversely affected the contrast in PC scores images of phase-separated lipid membranes. We also show that PCA of a restricted data set consisting of the peaks related to lipids and amino acids increases the likelihood that the discrimination of TOF-SIMS data acquired from intact cells is based on differences in the lipids and proteins on the cell surface, and not sample-specific contamination without compromising sample discrimination. We expect that the lipid-related peak database established herein will facilitate interpreting the TOF-SIMS data and PCA results from studies of both model and cellular membranes, and enhance identifying the origins of the peaks that contribute to discriminating different types of cells. Copyright (C) 2011 John Wiley & Sons, Ltd.