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Distribution Constant • Equilibrium constant • Also called the partition coefficient • K = [A]s/[A]m (the ratio of the concentration of a component in the stationary phase to the concentration of that component in the mobile phase)

Selectivity Factor • α (alpha) • Alpha is a measure of how well two components in a mixture will separate • α = KB/KA • By definition, component B is the component with a higher K ; therefore B prefers the stationary phase over A and will be eluted with a longer retention time. Due to this α will always be greater than 1

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Electron Ionization • Molecule + e- molecule+ + 2 e- Quadropole Detector • Four electromagnetic poles • Rapidly oscillating field • If mass of ion and frequency of oscillation are comparable then the ion will oscillate towards the detector and be measured • A certain mass range can be selected depending on the pole diameter

Secondary Ion Mass Spectrometry 20 particles 10 ion - e- Ga+ Static SIMS = + < 1 x 1013 ions/cm 2

Time-of-Flight Detection Scheme Sample +V d -V M+ Field Free Region V = stage voltage d = length of field free region V = voltage of detector MCP detector e = charge on an electron 1. 6 x 10 -19 C t = time for ion to reach detector m = mass v = velocity

Io n Gu TOF-SIMS Molecule-Specific Imaging n y total ion image Si+ Intensity x Cu+ 570 µm x 570 µm copper grid affixed to silicon substrate with silver paste Ag+ m/z, +ions molecule-specific image Cu+ Si+ Ag+

TOF-SIMS Identification of Plasma Membrane Lipids DPPC – dipalmitoylphosphatidylcholine + m/z 184 -* +2 H * m/z 552 m/z 735 (M+H)+ cholesterol m/z 369 m/z 385 (M-H)+

TOF-SIMS Allows for Chemical Identification of Lipid Domains Au PC DPPC (M+H)+ cholesterol DPPC tailgroup 370 µm x 370 µm + - m/z 184 +2 H m/z 552 cholesterol DPPC molecular ion m/z 385 molecular ion m/z 735 m/z 369 (M-OH)+ A. G. Sostarecz; C. M. Mc. Quaw et. al. J. Am. Chem. Soc. 2004.