Pre-selection follows these steps: a the absorption spectra of the individual complexes are recorded; b the differences between each pair of spectra are calculated; c first order derivatives of these differences are then obtained; d finally, wavelengths for which the derivative does not exceed 10% of the absolute value of the derivative maximum in all pairs
are eliminated. This criterion was met by 61 wavelengths, from 470 to 530 nm. Fig. 2a shows the spectral profiles of the individual chelates in this range.