RESUMO
A simple and rapid method employing non-suppressed ion chromatography with electrospray ionization tandem mass spectrometry has been developed for the direct determination of trace-level haloacetic acids (HAAs) in water samples. Using 70/30 (v/v) acetonitrile/1 M aqueous methylamine as the mobile phase, three IC columns - AS16, AS18 and AS24 from Thermo-Scientific - were tested, respectively, with the AS16 column exhibiting the best overall performance with respect to resolution and retention time. To assess the effects of mobile phase composition on retention time of HAAs, the AS16 column was further tested using (i) different proportions of acetonitrile to aqueous methylamine, (ii) different proportions of acetonitrile to aqueous solution at fixed methylamine concentrations, and (iii) different concentrations of methylamine at fixed proportions of acetonitrile to aqueous solution. With a low proportion of aqueous solution, van der Waals and/or hydrogen-bonding interactions appeared to play an important role in governing HAA retention, i.e., HAAs with relatively higher apparent logKow* caused by elevated solvent sspKa exhibited longer retention times; whereas with a high proportion of aqueous solution, ionic interactions appeared to dominate retention of HAAs, with the more polarizable HAAs exhibiting longer retention times. Using 70/30 (v/v) acetonitrile/1 M aqueous methylamine, the method detection limits were in the range of 0.090-0.216 µg/L for the 11 selected chloro-, bromo- and iodoacetic acids. Finally, this method was applied to monitor HAAs yields in laboratory chlorination experiments and to determine concentrations of HAAs in tap water and wastewater effluent samples.
RESUMO
To examine the effects of different cultivation history (5 a,10 a, and 15 a) on soil microbial communities, we used Illumina sequencing to investigate the diversity and structure of soil bacterial and fungal communities from Torreya grandis 'Merrillii' fields. The results showed that bacterial Shannon index, the richness estimators Chao1 and ACE were lower in soil in 15 year-old stand than those in other cultivation histories, while Simpson index showed no significant variation. Results from bacterial community NMDS showed that cultivation history played a vital role in driving the changes of soil bacteria communitiy structure. The bacterial communities in 5 and 10 year-old stand had the similar composition. The variations of bacterial richness and diversity as well as community structure (comprised basically of Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi) were significantly correlated with soil organic matters, soil C/N, and total nitrogen. The fungi richness estimators of Chao1 and ACE were significantly decreased with increasing cultivation history. Shannon and Simpson indices were significantly higher in soil with 10 year-old stand than soils with other cultivation history. Fungal NMDS could be clustered in the same era. Fungal communities were comprosed of Ascomycota, Basidiomycota and Zygomycota. Changes in fungal richness/diversity and community structure were mainly controlled by the variation of soil organic matter. In conclusion, the predominant factors affecting soil microbial communities were the cultivation history, soil C/N, total nitrogen and organic matter, respectively.
