The standard solution (100 μl) and samples (100 μl) in duplicate were added to wells and mix gently and incubated for 30 min at room temperature (20–25 °C) in the dark. After, the liquid was poured from the wells, and the microwell holder was tapped upside down (three times in a row) against absorbent paper to ensure removal of liquid from the well. The washing solution (250 μl) was added to the wells and the liquid was poured out. The microwell holder was tapped upside down (three times in a row) against absorbent paper to ensure removal of liquid from the well. The procedure was repeated two additional times.