Investigation of partial concentration and absolute concentration detection limits. Six antibody mixes were added to FCS solutions to produce a 10-fold and 100-fold increase in total protein concentration and a corresponding decrease in partial concentration. The high protein solution was diluted 10-fold, so that the total protein concentration was equivalent between the mixes. The images at the top of the figure present a detail from one of the arrays in each set. The median log Cy5/Cy3 fluorescence ratio is plotted as a function of cognate antibody for the three indicated antigens. The dashed line represents the log of the true ratio of antigen concentrations in the Cy5-labeled and Cy3-labeled solutions.