Figure 28.

(a) The histogram of thickness increments of 25 000 spots incubated at 10 ng∕ml after secondary antibody incubation. Based on the mean values and standard deviations of the distributions, the detection limit of one pair of antibody and reference spots is estimated to be 1.69 ng∕ml in 2 mg∕ml background concentration. (b) The scaling PSA detection limits of the sandwich assay are shown. The scaling detection limit is fit by a power law with an exponent −0.44. A 20 pg∕ml detection limit for the PSA sandwich assay is achieved based on 11 520 antibody spots or 250 pg∕ml based on 45 spots.