Question

A visible light spectrophotometric determination of penicillin makes use of color-forming reaction of the penicillin. If this compound absorbs at 475 nm with an ε = 1560 M^-1cm^-1, what would be calculated as the limit of detection with an instrument, using a 1.00 cm cuvette, with its smallest absorbance measurement at 0.005?

Answer 1

Ans. Beer-Lambert’s Law, A =e C L                 - equation 1,               where,

                                    A = Absorbance

                                    e = molar absorptivity at specified wavelength

                                    L = path length (in cm)

                                    C = Molar concentration of the solute

Given,

                e = 1560 M^-1cm^-1

            L = 1.0 cm

                A = 0.005              - smallest absorbance, limiting detection

Putting the values in equation 1

                0.005 = 1560 M^-1cm^-1 x C x 1.0 cm

            Or, C = (0.005) / (1560 M^-1) = 3.20 x 10^-6 M

That the as small as 3.20 x 10^-6 M concentation of penicillin can be detected using the specified VIS-spectrophotometer.

Thus, detection limit = 3.20 x 10^-6 M = 3.20 x 10^-6 moles /L