rt-m/z plot of cluster 19. Marker candidates associated with prototype 19 as big red dots in the retention time vs. mass-to-charge ratio (rt-m/z) plane. The markers represented by the big blue dots are COOH-22:0, OH-22:0, OH-24:0 and OH-26:0 (see also table 2) and the formate adducts of the latter three hydroxy fatty acids These formate adducts are characterized by identical rt values and a mass shift of m/z 46. The remaining marker candidates of the experiment are represented by small black dots. On the right hand side the average intensity profile associated with prototype 19 is shown.