Kang Tian

Determination of oxidative stability of oils and fats.

In a new approach to evaluating the oxidative stability of oils and fats, the consumption of oxygen by a sample confined in a reactor of adjustable temperature is monitored with a gas-phase flow injection analysis (FIA) system. Temperature-dependent data are collected in a low-oxygen-content atmosphere. For a variety of samples, log(oxygen consumption) is linearly related to the reciprocal of the absolute temperature (minimum linear r(2) > 0.99). This makes it possible to extrapolate the temperature-dependent data to predict the stability of the samples at other temperatures, e.g., typical ambient storage temperatures at which the direct determination of oxidative stability would be too slow for most samples. The proposed method is instrumentally simple and is easily automated. The sample throughput rate is an order of magnitude faster relative to current alternatives; temperature-dependent stability characterization for a sample (three temperatures, triplicate measurement at each temperature) requires ≤ 2 h. The reproducibility of the results is excellent. For a cottonseed-oil sample studied over 3 days, the slope and intercept of the log(O(2) consumption) vs 1/T linear plot (for all the 45 measurements made) exhibited uncertainties of 2.1% and 2.0% for the slope and the intercept, respectively, with a linear r(2) value of 0.9929. In a high-temperature (160 °C) oxidation experiment with various oils, the oxygen consumption was well-correlated (linear r(2) 0.9692) with the concomitant decrease in iodine absorption number (IAN). In contrast, it was poorly and negatively correlated with an increase in the peroxide value.

Preconcentration/preelution ion chromatography for the determination of perchlorate in complex samples

The determination of perchlorate in complex matrices by ion chromatography (IC) with an online preconcentration and preelution technique is discussed. The method was applied to different sample types containing large concentrations of matrix anions that would otherwise interfere with analysis via conventional IC. The present approach was highly effective in removing most of the matrix anions and was thus resistant to the interferences commonly encountered in a high ionic strength background. Method performance was evaluated by analyzing for low-level perchlorate in synthetic high ionic strength solutions, tissue extracts, and hydroponic nitrate fertilizer samples. Not only is it easier to practice the present method compared to USEPA Method 314.0, but for most of these samples the present approach provided equal to or better recovery of perchlorate than Method 314.0. With a sample of specific conductance 12,650muScm(-1), for example, the present method provided a perchlorate recovery of 101% at the 25mugL(-1) level versus 89% by EPA Method 314.0. Method detection limits of perchlorate in hydroponic fertilizer samples with this method (130-190mugkg(-1)) are the lowest thus far reported.

Method optimization for quantitative analysis of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) by liquid chromatography-electrospray ionization mass spectrometry

A simple, sensitive LC-ESI-MS method was optimized for quantitative analysis of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) in environmental samples. Under negative ionization mode, HMX can form adduct ions with various organic acids and salts, including acetic acid, formic acid, propionic acid, ammonium nitrate, ammonium chloride, sodium nitrite, and sodium nitrate. Acetic acid was chosen as additive and the ion, [M+CH(3)COO](-) with m/z=355 was used for selective ion monitoring (SIM) in this study. Good sensitivity was achieved with low acetic acid concentration in the mobile phase and relatively low capillary temperature. The method detection limit was 0.78pg for HMX in standard solution. Linearity (R(2)>0.9998) was obtained at low concentrations (0.5-50mug/L). This method has been used to determine HMX concentrations in water samples and lizard egg samples from an animal exposure study.

Simultaneous flow-injection measurement of hydroxide, chloride, hypochlorite and chlorate in Chlor-alkali cell effluents

A flow injection method is reported for the simultaneous determination of hydroxide, chloride, hypochlorite, and chlorate ions that exist in Chlor-alkali cell effluents in concentrations ranging from sub-millimolar to several molar. The hydroxide concentration is determined by the heat of neutralization of the injected sample into an acidic carrier stream and the chloride concentration is calculated from the measured conductance data. For the measurement of hypochlorite and chlorate, colorimetric iodometry is used. Iodide is oxidized to iodine by OCl(-) and ClO(3)(-) in acid solutions. While room temperature is sufficient for the reaction between OCl(-) and I(-), the reaction between ClO(3)(-) and I(-) requires an elevated temperature. The different reaction requirements are utilized to differentiate between NaOCl and NaClO(3), respectively.

A permeable membrane capacitance sensor for ionogenic gases Application to the measurement of total organic carbon.

We introduce a unique sensor, a tubular membrane capacitance sensor (TMCS). Wire electrodes are wrapped on a porous hydrophobic membrane tube and capacitance is measured with an inexpensive universal serial bus powered capacitance to voltage digital converter. At low solution conductivities, the double layer capacitance of the ionic solution is approximately linearly related to the solution conductance. Porous polypropylene hollow fibers were coated repeatedly with silicone rubber solutions to render the pores highly hydrophobic and hence minimize gradual liquid intrusion into the pores that leads to baseline drift. We applied the sensor to the determination of TOC in water samples where the analyte carbon was oxidized to CO(2) by UV/persulfate. The evolved CO(2) is sensed by the TMCS as the water slowly passing through it absorbs the CO(2) resulting in a change in conductance and capacitance. The system was capable of detecting 800ngC as glucose and 100ngC as inorganic carbonate. The sensor combines the dual functions of gas collection and measurement, thus permitting a simple miniature economic device.

Development of an extraction method for perchlorate in soils

Perchlorate originates as a contaminant in the environment from its use in solid rocket fuels and munitions. The current US EPA methods for perchlorate determination via ion chromatography using conductivity detection do not include recommendations for the extraction of perchlorate from soil. This study evaluated and identified appropriate conditions for the extraction of perchlorate from clay loam, loamy sand, and sandy soils. Based on the results of this evaluation, soils should be extracted in a dry, ground (mortar and pestle) state with Milli-Q water in a 1 ratio 1 soil ratio water ratio and diluted no more than 5-fold before analysis. When sandy soils were extracted in this manner, the calculated method detection limit was 3.5 microg kg(-1). The findings of this study have aided in the establishment of a standardized extraction method for perchlorate in soil.