Separation and determination of S-anions in very small samples by microbore ion chromatography

A high-throughput and cost-effective microplate reader method for measuring persulfates (peroxydisulfate and peroxymonosulfate)

Frequent use of persulfates as oxidants, for in situ chemical oxidation and advanced oxidation processes, warrants the need for developing a fast and efficient method for measuring persulfate concentrations in aqueous samples in the lab and on site. Here, we propose a modified method, based on Liang et al.'s (2008) spectrophotometric method, for measuring both peroxydisulfate (PDS) and peroxymonosulfate (PMS) in the aqueous samples. Our method involves a deep 96-well plate, multi-channel pipettes, a small orbital shaker, and a microplate reader; allowing the preparation and analysis of up to 96 samples in one run. Our proposed method shortens the time by 10 folds, consumes only ∼2% of the original reagents, and generates only ∼2% of the liquid waste compared to the Liang et al.'s method, thus, making our method high-throughput, time-efficient, and cost-effective with reduced environmental impact. The presented microplate reader method is validated in terms of linearity, LOD, LOQ, accuracy, precision, robustness, and selectivity. All the parameters satisfied the acceptance criteria, according to ICH guidelines. The linearity of calibration curves was evaluated by performing the F-test. In general, our method has linear ranges from 20 to 42,000 and 5 to 40,960 μM for PDS and PMS, respectively. Accuracy (% recovery) results suggested that the LOD and LOQ based on the standard deviation of y-intercepts of the regression lines were the most reliable. The LOD/LOQ values for PDS and PMS were 14.7/44.1 and 4.6/14.4 μM, respectively. The proposed method was also modified to work with a standard cuvette spectrophotometer and was validated. A comparison with the UHPLC analysis of PDS showed that our microplate reader method performed equivalently or even outperformed the UHPLC method, in the presence of common groundwater constituents and organic contaminants.

Determination of sulfur oxides formed during the S(IV) oxidation in the presence of iron

The reaction products (i.e., sulfate (SO4(2-)) and dithionate (S2O6(2-))) of S(IV) oxidation in the presence of iron(III) under different experimental conditions were investigated. Ion-interaction chromatography was used for the separation of sulfate and dithionate using tetrabutylammonium hydroxide (TBAOH) as an ion-pair reagent. The chromatographic method was optimized by varying the composition of the mobile phase (i.e., concentration of TBAOH, acetonitrile and Na2CO3) and by varying the flow rate of the mobile phase. The method was successfully applied to the determination of dithionate formed during the S(IV) oxidation in the presence of Fe(III). In air-saturated solutions sulfate was observed as the only product, while in N2-saturated solutions dithionate was also determined, but it is the minor reaction product and represents about 4% of the total amount of oxidized HSO3- under the studied conditions.

Ion-pair chromatography of polythionates and thiosulfate with detection based on their catalytic effects on the postcolumn azide-iodine reaction

The reduction of iodine with azide, catalyzed by polythionates (tri-, tetra-, penta- and hexathionate) and thiosulfate, has been utilized as a postcolumn reaction for chromatographic determination of these sulfur oxyanions. The method is based on the separation of polythionates and thiosulfate on an octadecylsilica column with an acetonitrile-water (20:80, v/v) mobile phase (pH 5.0) containing 3 mM tetrapropylammonium hydroxide and 6 mM acetic acid, followed by photometric measurement of the residual iodine (as triiodide) from the catalytic postcolumn azide-iodine reaction after mixing a reaction solution containing azide and iodine with the column effluent. Chromatograms obtained for the sulfur oxyanions showed negative peaks as a result of the decrease in absorbance of background. The conditions for the catalytic postcolumn reaction of the sulfur oxyanions in the column effluents were established by varying the concentrations of azide, iodine, iodide and acetic acid in the reaction solution, and varying the flow-rate, reaction temperature and length of the reaction tube. The detection limits (defined as SIN = 3) were 4.3 microM for trithionate, 0.10 microM for tetrathionate, 2.7 nM for pentathionate, 5.0 nM for hexathionate and 1.1 nM for thiosulfate. When compared with earlier methods, the proposed method gave a much higher sensitivity for the determination of two polythionates (penta- and hexathionate) and thiosulfate. This method was applied successfully to the analysis of polythionates and thiosulfate added to hot-spring water samples.

Determination of trace anions in organic solvents

Ion chromatography along with matrix elimination was used to reliably determine trace levels of anionic contaminants in organic solvents. A 5-ml sample volume was injected directly into the instrument without any sample pretreatment. High-purity deionized water was used to deliver the sample to a preconcentration column, where the anions of interest were retained while the organic matrix was rinsed to waste. A sodium carbonate eluent eluted concentrated anions from the preconcentration column and separated them on a 2-mm pellicular anion-exchange column. The separated anions were detected by suppressed conductivity. This method was used to determine the anionic contaminants of isopropanol, acetone and N-methylpyrrolidone. Method detection limits for chloride, nitrate, sulfate and phosphate were all lower than 1 microg/l.

Determination of trace anions in concentrated weak acids by ion chromatography

Ion-exclusion chromatography (ICE) followed by ion chromatography (IC) was used for the determination of trace anionic contaminants in concentrated weak acids. The ICE separation was used as a pretreatment step to isolate the contaminant anions of strong acid from the excess of matrix ions. Then a fraction containing the analyte ions was separated using IC with suppressed conductivity detection. Microbore-ion-exchange columns were chosen to address the increased purity requirements for use of these concentrated acids in semiconductor applications. The chromatographic conditions were optimized for determining trace chloride, sulfate, phosphate, and nitrate in concentrated 24.5% (v/v) hydrofluoric acid; trace chloride, sulfate, and nitrate in concentrated 85% (w/w) phosphoric acid and trace chloride and sulfate in concentrated 0.7% (v/v) glycolic acid. Method detection limits for the anions of interest were below 100 micrograms/l.