“Blue shift” of emission maximum and fluorescence quantum yield as qualitative spectral characteristics of dissolved humic substances

Humic substances (HS) and especially their water-soluble fraction play very important role in environmental biogeochemistry. Their spectra could be used for rapid quantification and characterization. Fluorescence spectra of natural HS excited with the UV light exhibit blue broadband emission, so called “humic-type fluorescence”, with maximum position depending on the excitation wavelength. With excitation around 310 nm the position of emission maximum undergoes a “blue shift” to shorter wavelengths. We explain the existence of “blue shift” by heterogeneity of natural HS. The presence of several components in humic-type fluorescence is caused by fluorophores of different origin with fluorescence quantum yield depending on excitation wavelength. As a result of numerous biological and geochemical processes the contribution of different fluorophores to “humic-type” fluorescence changes affecting values of “blue shift”.

The target of this work was to study in details the effect of “blue shift” and its correspondence with the origin of the sample and other spectral properties like optical density and fluorescence quantum yield. We summarize the results on absorption and fluorescence study performed for natural and commercial HS of the following types: (i) coloured dissolved organic matter (CDOM) in natural water samples from Baikal Lake region, Moscow region and the White Sea coast; (ii) soil aqueous extractions; (iii) 6 samples of commercial HS dissolved in water. In addition natural water samples were separated into low molecular weight fraction and colloidal fraction using ultrafiltration. Absorption spectra in the UV and visible range and fluorescence spectra with excitation wavelength varying from 270 to 355 nm were measured in laboratory conditions. Fluorescence quantum yield was estimated using quinine sulphate solution as a standard.

Fluorescence “blue shift” was quantified with two values: Δ1(ex270-ex310) and Δ2(ex355-ex310) showing the shift of fluorescence emission maximum upon changing excitation wavelength from 270 to 310 nm and from 355 to 310 nm correspondingly. For CDOM in natural water shifts Δ1 and Δ2 are about 25 nm. Natural soil HS in extractions have the shift Δ1= 30…40 nm prevailing shift Δ2 =15…20 nm. Commercially available HS did not exhibit noticeable “blue shift” (Δ1=Δ2<5 nm). Their fluorescence quantum yield (QY355=0.008) is smaller compared to that of CDOM in water (0.028), but bigger than that for soil HS (0.003) with excitation at 355 nm. For commercial HS fluorescence quantum yield keeps practically constant under UV excitation. In contrast, fluorescence quantum yield for all natural HS, like CDOM, its molecular fractions, natural soil HS, is rising with increasing excitation wavelength. On the basis of our spectral observations we may suppose that commercial HS are less heterogeneous compared to soil HS and CDOM in natural water. Fluorescence quantum yield and “blue shift” values could be used in practice for rapid characterization of humic substances of different type.