By means of UV-VIS there is unable to quantify conjugated chromophores, which have almost identical type of polysubstituents such as OH and COOH groups. The method has its own capability of quantitative analysis, but mainly of mono component or a small number of componenty.
Thus, it appears inapplicable to quantify humics.
The reference, which I have provided shows innovative concepts of quantitative treatment of mass spectrometric data. The MS method is the method of choice for environmental analysis, due to, superior characteristics, which are completely out of the range of the capability of the UV-VIS spectroscopy.