Calculation of Membrane Lipid Ratios Using Single-Pixel Time-of-Flight Secondary Ion Mass Spectrometry Analysis

Much evidence suggests that membrane domains, termed lipid rafts, which are enriched in sphingomyeline and cholesterol play important roles in the regulation of physiological and pathophysiological processes. A label-free quantitative imaging method for lipids is lacking at present. We report an algorithm which enables us to identify and calculate the percentages of the ingredients of lipid mixtures from single-pixel time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra in model systems. The algorithm is based on a linear mixing model. Discriminant analysis is used to reduce the dimension of the data space. Calculations were separately performed for positive and negative ion mass spectra. Phosphatidylcholine and sphingomyeline which have identical headgroups and cannot be easily distinguished from another by positive ion mass spectra were included in the analysis. The algorithm outlined may more generally be used to calculate the percentages of ingredients of mixtures from spectra acquired by quite different methods such as X-ray photoelectron spectroscopy.