How does imidazole affect my quantitation of protein?
FAQ ID -132

Since imidazole absorbs UV radiation at 280 nm, an elution profile measured at 280 nm while purifying a 6xHis tagged protein by FPLC will show an increase in absorbance above the background signal allowing quantitation of your protein. The absorbance of imidazole can vary depending on its source and purity, but elution buffer containing 250 mM imidazole usually has an A280 of 0.2–0.4.

To quantitate proteins in eluates containing imidazole, we recommend the Bradford protein assay, which is based on the ability of Coomassie Brilliant Blue R250 to form strong complexes with proteins. This assay is more tolerant of higher imidazole concentrations than the Lowry and the biuret assay, which are more sensitive to imidazole because they involve the reduction of copper. For accurate measurements, always use elution buffer without protein as the reagent blank.