All pairs of features were considered, and those pairs that (a) where feature 1 had a Pearson's correlation of greather than 0.99 with {1, 0, 0, 0} and (b) feature 2 had pearson's correlation of greater than 0.95 with {0, 0, 1, 0}, and (c) the mass difference between the two features was within 5 ppm of ethyl azidoacetate plus or minus two water molecules were examined. Derivatized features having an intensity of less than 100,000 ion counts, and underivatized features of less than 50,000 counts were excluded from the analysis. [The debris remaining after centrifugation was stored at 4 °C until DNA extraction (see below).] Remaining after solvent removal was an approximately 12 mL volume of an aqueous red-brown suspension. This was extracted with one volume of ethyl acetate, the upper organic phase removed to a separate container. The remaining material was extracted again with one volume of ethyl acetate. Ethyl acetate phases from the two extractions were pooled.