If drug residue is present in the sample the solution turns blue (λmax = 605 nm, ε = 19,200M-1cm-1). A test for drug use involves addition of 5.00mL of colorless reagent to 5.00 mL urine. When you measure the sample using the test described above and get an Abs of 0.52 at 605 nm (b = 1 cm). Do you conclude that this person has been using illegal drugs? Because other things in urine may give slightly positive results, only tests that measure a minimum concentration of 5.0 x 10-5M drug residue in the original urine sample are considered positive for illegal drugs.