Data Analysis. Images were analyzed using ImageJ (National Institutes of Health, USA) with the OpenMIMS plugin (National Resource for Mass Spectrometry, Harvard University USA). Planes were summed and regions of interest were manually selected in order to exclude any debris on the sample. Total counts within each region of interest were determined via the "Tomography" tab. These counts were then used to determine the ratios of interest (typically 2 H -/ 13 C -, C - /( 13 C -+ 12 C -) and 2 H -/( 13 C -+ 12 C -)). Calculating these ratios allows for the size of the analyzed patches to be accounted for and allows for further quantification via external calibration standards.