A high-temperature combustion (HTC) total organic carbon analyzer, which has significant design improvements over existing systems, was developed. The new injection system directly connects a loop-type autoinjector to the head of the HTC column. This connection facilitates the coupling of an autosampler to the injection system. The entire injection process is closed to the atmosphere, thereby improving the precision and eliminating potential contamination during injection. Injections can be made every 3-5 min, depending on the injection and inorganic carbon sparging modes used. The HTC column was designed without a "cold" zone or dead space at the top. These improvements eliminated the memory (or carryover) effect, which is a potential problem in some HTC column designs. The HTC column is packed with pure quartz beads instead of a relatively expensive Pt-based catalyst, without loss in the oxidation efficiency, as indicated by 100% recovery for various compounds with different refractory properties and by intercomparison with Pt-based HTC systems. The precision for seawater is ∼(0.6% (RSD) at the 80 µM C level. Typically, greater than 5000 injections of seawater can be made without significant deterioration of column performance. The effects of column temperature and carrier gas flow rate on the oxidation efficiency, sensitivity, and reproducibility are reported. Finally, evidence is presented that suggests that there is a relationship between the refractory nature of pure compounds and the peak width. This potential relationship may be a useful tool for quantifying the refractory nature of organic carbon in natural waters.

The hydroxyl radical ( • OH) is the most reactive oxidant produced in natural waters. Photoproduction by chromophoric dissolved organic matter (CDOM) is one of its main sources, but the structures responsible for this production remain unknown. Here, a series of substituted phenol model compounds are examined to test whether these structures could act as a source of • OH. We find that many of these compounds do produce • OH with quantum yields (Φ) ranging from ∼10 −4 to ∼10 −2 . In particular, two compounds that have hydroxy groups and carboxyl groups in a para relationship (4-hydroxybenzoic acid and 2,4-dihydroxybenzoic acid) exhibit relatively high Φ values, ∼10 −2 . For 2,4-dihydroxybenzoic acid, the formation of • OH was confirmed through the use of competition kinetics and reaction with methane. We conclude that these types of structures, which may derive from polyphenolic source materials such as lignins, tannins, and humic substances, could be an important source of • OH in natural waters.

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Predicting the viscosity of ionic liquids (ILs) is crucial for their applications in chemical and related industries. In this study, a large data set of experimental viscosity data of ILs with a wide range of viscosity (7.83–142 000 cP), pressure (1–3000 bar), and temperature (258.15–395.32 K) are employed to build predictive models. The structures of cations and anions for 89 ILs are optimized, and the S σ‑profiles descriptors are calculated using the quantum chemistry method.Two new models are developed by using extreme learning machine (ELM) intelligence algorithm with the temperature, pressure, and a number of S σ‑profiles descriptors as input parameters. The coefficient of determination (R 2) and average absolute relative deviation (AARD %) of the total sets of the two predictive models are 0.982, 2.21% and 0.951, 4.10%, respectively. The results show that the two ELM models are reliable for predicting the viscosity of ILs.