The standard solution (100 ml) and samples (100 ml) in duplicate
were added to wells and mix gently and incubated for 30 min at
room temperature (20e25 C) in the dark. After, the liquid was
poured from the wells, and the microwell holderwas tapped upside
down (three times in a row) against absorbent paper to ensure
removal of liquid from the well. The washing solution (250 ml) 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.