The objective of the present study was to explore the minimum epitope distance of the analyte in the sandwich immunoassay for small molecules. For this purpose, two hapten epitopes, MEL and NIA, were selected and separated by linear chain linkers with varied lengths to mimic epitope distance. Therefore, a library of model analytes, including nine compounds with linkers of 1-14 methylenes with the MEL and NIA epitopes on each end, was designed and named MN1-MN9 with increasing length (Fig. ).