Determination of relative molecular weights of fluorescent components in dissolved organic matter using asymmetrical flow field-flow fractionation and parallel factor analysis

Dissolved organic matter in aquatic systems is of variable structure and composition. Asymmetrical flow field-flow fractionation coupled to UV/vis diode array and fluorescence detectors (AF4–DAD–EEM) was used to assess the size and optical properties of dissolved organic matter. The results were analyzed using parallel factor analysis (PARAFAC) and statistical fractogram deconvolution to correlate fluorescing components with molecular weight fractions. This coupling, which is shown for the first time in this work, is a powerful method capable of revealing novel information about the size properties of PARAFAC components. Tyrosine/polyphenol-like fluorescence (peak B) was significantly correlated (p < 0.05) with the smallest size group (relative molecular weight = 310 ± 10 Da), microbial humic-like and terrestrial visible humic-like fluorescence (peaks M, C, A) with the intermediate size group (1600 ± 150 Da), and terrestrial fulvic-like and tryptophan/polyphenol-like fluorescence (peaks A and T) with the largest size group (4300 ± 660 Da).     

Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV-Visible diode array and excitation emission matrix fluorescence

Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000 Da. Both the number-average (M_n) and weight-average (M_w) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M_n range of 0.8–1.1 kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S_R = S_275–295:S_350–400) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40 nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition.     

Asymmetrical flow field-flow fractionation and excitation-emission matrix spectroscopy combined with parallel factor analyses of riverine dissolved organic matter isolated by tangential flow ultrafiltration

Asymmetrical flow field-flow fractionation (AF4) with sequential on-line UV/visible and fluorescence detectors was used to investigate the composition of dissolved organic matter (DOM) in permeate and retentate fractions isolated by tangential flow ultrafiltration (TFF) at various concentration factors (i.e. ratio of initial volume to the retentate volume; CF). The permeation coefficient model, which defines the log-log relationship between DOM in the permeate fractions and CFs, described the permeation behaviour of DOM with regression coefficients r ² > 0.99. The dominance of higher-molecular weight retentate chromophoric DOM (CDOM) observed in TFF was consistent with the results of AF4. The weight-averaged molecular weights (M _w) of the integral permeate and retentate at CF = 20 were determined to be 1160 and 2320 by AF4, respectively, while their molecular weight distributions (MWD) were centered at 1120 and 1600 Da. M _w, MWD, and aromaticity (i.e. ratio of absorbance at 250 and 365 nm; E2/E3) in permeate fractions were altered significantly during the early stages of TFF (CF < 9). These changes, however, were not evident in excitation-emission matrix fluorescence properties as determined using the parallel factor analysis model. The application of AF4 to TFF fractions suggests that the choice of CF may have an important impact on the size distribution and aromaticity of permeate fractions, whereas fluorescence properties appear insensitive to concentration factor. These results suggest that the choice of CF is crucial only in the study of the permeate fraction where similar CF (i.e. > 9) should be used to obtain meaningful comparison among samples.     

Accurate quantification of freely dissolved organochlorine pesticides in water in the presence of dissolved organic matter using triolein-embedded cellulose acetate membrane

A novel method is described using triolein-embedded cellulose acetate membrane (TECAM) for accurate determination of the freely dissolved fraction of organochlorine pesticides (OCPs) in waters rich in dissolved organic matter (DOM). The performance of the method was tested with an air-bridge system for extracting OCPs from aqueous solutions with and without humic acid. In addition, the partition coefficients between humic acid and water (K _docs) for 20 OCPs were determined by TECAM with negligible depletion extraction. Results show that TECAM predominantly extracts the freely dissolved compounds and its extraction efficiency decreases significantly with an increase in concentration of humic acid in water. The proposed methodology is suitable for facile laboratory K _doc measurement for moderate to high hydrophobic compounds (log K _ow > 4). The linear relationship between log K _ow and log K _doc obtained in this study agrees well with the results reported earlier. The kinetic uptake rate constants (k _us) and TECAM–water partition coefficients (K _TECAMs) for the 20 OCPs were obtained using the controlled laboratory continuous-flow and static exposure system, respectively. These calibration parameters were used in the field experiment to estimate the freely dissolved concentrations of OCPs in the water of Taihu Lake in China. Our results show that TECAM can be used successfully to determine the freely dissolved OCPs in aquatic environments containing DOM, and the method is particularly suited for long-term water sampling. Figure Schematic diagram of water sampling with a triolein-embedded cellulose acetate membrane (TECAM)     

Normal‐phase high‐performance counter‐current chromatography for the fractionation of dissolved organic matter from a freshwater source

Normal‐phase high‐performance counter‐current chromatography (HPCCC) is used to obtain a preliminary fractionation of components in dissolved organic matter (DOM) from a freshwater source. The HPCCC solvent system involved a normal‐phase approach with water/methanol (1:1) as the lower stationary phase and hexane/ethyl acetate (1:1) as the upper mobile phase. The critical experiment parameters were optimised: revolution speed 1800 rpm and flow rate 0.15 mL/min. Under these conditions 50 μL of a 0.50 mg/mL DOM solution was loaded. The detection wavelength was monitored at 330 nm in order to isolate the main portion of DOM, which includes substances such as carboxyl‐rich alicyclic molecules. By optimising this system it was possible to isolate materials that, according to GC–MS, can be related to molecules with an analogous structural background. Where fraction analysis was not suitable for GC–MS, RP‐HPLC with UV absorbance detection was used, showing unique chromatograms for each fraction at both 210 and 330 nm.     

Determination of phenol in the presence of its principal degradation products in water during a TiO2-photocatalytic degradation process by three-dimensional excitation-emission matrix fluorescence and parallel factor analysis

This paper describes a simple and rapid way of monitoring a photocatalytic degradation of phenol in aqueous suspensions of TiO₂. A three-way analytical methodology based on fluorescence excitation-emission matrix (EEM) and parallel factor analysis (PARAFAC) was developed to resolve the species present in the reaction mixture and quantify the concentration of phenol and its principal degradation products throughout the degradation. Parameters such as core consistency, fit% and correlation coefficients between recovered and pure spectra were used to determine the appropriate number of factors for the PARAFAC model. The accuracy of the model was evaluated by the root mean square error of prediction (RMSEP). Using a four-factors PARAFAC model, phenol, hydroquinone, resorcinol and catechol, were satisfactorily determined. The proposed method is an interesting alternative to the traditional techniques normally used for monitoring degradation reactions.     

Characterization of chromophoric dissolved organic matter (CDOM) in the East China Sea in autumn using excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC)

Samples of chromophoric dissolved organic matter(CDOM)in the East China Sea in autumn(October in 2011)were analyzed by excitation emission matrix(EEM)fluorescence spectroscopy combined with parallel factor analysis(PARAFAC).Three terrestrial humic-like components(C1,C2 and C3)and one protein-like component(C4)were identified.Based on spatial distributions,as well as relationships with salinity,the following assignments were made.The three humic-like components(C1,C2 and C3)showed conservative mixing behavior and came mainly from riverine input.The protein-like component(C4)was considered a combination of autochthonous production and terrestrial inputs and a biologically labile component.Path analysis of samples from the middle and bottom layers revealed that the causal effects on C1 were 78.46%for salinity,and 21.54%for apparent oxygen utilization(AOU);those on C2 were 76.43%for salinity,and 23.57%for AOU;those on C3 were70.49%for salinity,7.01%for Chl-a,and 22.50%for AOU;those on C4 were 55.54%for salinity,14.6%for Chl-a,and29.86%for AOU in middle layer;and those on C4 were 57.37%for salinity,29.02%for Chl-a,and 13.61%for AOU in bottom layer.Results indicated that CDOM in the East China Sea was mainly affected by terrestrial inputs,and microbial activities also played a key role in biogeochemical processes of CDOM.The application of the EEM-PARAFAC model presented a unique opportunity to observe compositional changes in CDOM in the East China Sea.In addition,the humification index(HIX)suggested that CDOM from the East China Sea was less stable and stayed shorter in the environment.     

Use of a photocatalytic membrane reactor for the removal of natural organic matter in water: Effect of photoinduced desorption and ferrihydrite adsorption

The photocatalytic degradation of natural organic matter (NOM) would be an attractive option in the treatment of drinking water. The performance of a submerged photocatalytic membrane reactor (PMR) was investigated with regard to the removal of NOM and the control of membrane fouling. In particular, this work focused on the adsorption and desorption of humic acids (HA) and lake water NOM at the surface of TiO₂ photocatalyts and ferrihydrite (FH) adsorbents in the PMR for water treatment. The addition of FH particles with a large sorption capacity helped remove the NOM released from TiO₂ particles, but FH suspended in water affected the photocatalysis of lake water NOM with a low specific UV absorbance (SUVA) value. To prevent the UV light being scattered by FH without any photocatalytic activity, FH particles were attached to a submerged microfiltration (MF) membrane, which contributed to a greater removal of NOM during long-term PMR operation. The further removal of NOM from aqueous solution was achieved due to the synergistic effect of TiO₂ photocatalysis and FH adsorption in PMR while minimizing the influence of photoinduced desorption of NOM. No significant membrane fouling occurred when the submerged PMR was operated even at high flux levels (>25 L/m² h), as long as photocatalytic decomposition took place.