Quantitative detection of trichloroacetic acid in human urine using isotope dilution high-performance liquid chromatography-electrospray ionization tandem mass spectrometry.
The chemical disinfection of drinking water to control microbial contaminants results in the formation of disinfection byproducts (DBPs). The volatile trihalomethanes and the nonvolatile haloacetic acids (HAAs) are the most prevalent DBPs. It is important to monitor human exposure to HAAs because of their potential adversehealth effects, such as cancer. Among the HAAs, urinary trichloroacetic acid (TCAA) is a potential valid biomarker for assessing chronic ingestion exposure to HAAs from drinking water. We have developed a rugged, high-throughput, sensitive, accurate, and precise assay for the measurement of trace levels of TCAA in human urine using a simple solid-phase extraction (SPE) cleanup followed by isotope dilution high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). TCAA is extracted from the urine using SPE, separated from other extract components by reversed-phase HPLC, and analyzed by negative ion electrospray ionization-isotope dilution-MS/MS using a multiple reaction monitoring experiment. The method is simple and fast and is not labor intensive (sample preparation and analysis can be performed in approximately 15 min) with a limit of detection of 0.5 ng/mL in 1 mL of urine.
Ionization enhancement in atmospheric pressure chemical ionization and suppression in electrospray ionization between target drugs and stable-isotope-labeled internal standards in quantitative liquid chromatography/tandem mass spectrometry.
The phenomena of ionization suppression in electrospray ionization (ESI) and enhancement in atmospheric pressure chemical ionization (APCI) were investigated in selected-ion monitoring and selected-reaction monitoring modes for nine drugs and their corresponding stable-isotope-labeled internal standards (IS). The results showed that all investigated target drugs and their co-eluting isotope-labeled IS suppress each other's ionization responses in ESI. The factors affecting the extent of suppression in ESI were investigated, including structures and concentrations of drugs, matrix effects, and flow rate. In contrast to the ESI results, APCI caused seven of the nine investigated target drugs and their co-eluting isotope-labeled IS to enhance each other's ionization responses. The mutual ionization suppression or enhancement between drugs and their isotope-labeled IS could possibly influence assay sensitivity, reproducibility, accuracy and linearity in quantitative liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS). However, calibration curves were linear if an appropriate IS concentration was selected for a desired calibration range to keep the response factors constant.
Thermal Conductivity of Toluene+Cyclopentane Mixtures: Measurements and Prediction
New measurements of the thermal conductivity of toluene, cyclopentane, and a binary mixture of 60 wt% toluene are presented. The measurements cover the temperature range from 235 to 345 K and from the saturation line up to 20 MPa pressure. The measurements were performed with a transient hot-wire instrument. The uncertainty of the measurements is estimated to be ±0.5%. The present results are employed to examine the predictive power of a theoretically based scheme for the calculation of the transport properties of mixtures.