We have a compound that is a mixture of different substances. The OECD 101 indicates that the concentration of the compound to be measured is given in mol/l, but what happens if you cannot give that value? is there any alternative?
If I understand oecd 101 correctly, then it is for diluted solutions for which Beer's approximation is usually valid. For a mixture, you have to factor in that this approximation is generally not valid, not even for (thermodynamically) ideal mixtures. If you nevertheless want to determine compositions, select comparably weak bands and use volume fractions. Take a lot of measurements to establish your model and be aware of the fact that errors can nevertheless be large.