Biomedical subjects

Wennan Zhao

Publications and source records attributed to Wennan Zhao.

4 recordsLinked to original sources

PubMed2006

Automation and optimization of liquid-phase microextraction by gas chromatography.

Several fully automated liquid-phase microextraction (LPME) techniques, including static headspace LPME (HS-LPME) (a drop of solvent is suspended at the tip of a microsyringe needle and exposed to the headspace of the sample solution), exposed dynamic HS-LPME (the solvent is exposed in the headspace of sample vial for different time, and then withdrawn into the barrel of the syringe. This procedure is repeated a number of times), unexposed dynamic HS-LPME (the solvent is moved inside the needle and the barrel of a syringe, and the gaseous sample is withdrawn into the barrel and then ejected), static direct-immersed LPME (DI-LPME) (a drop of solvent is suspended at the tip of a microsyringe needle and directly immersed into the sample solution), dynamic DI-LPME (the solvent is moved inside the needle and the barrel of a syringe, and the sample solution is withdrawn and ejected), and two phase hollow fiber-protected LPME (HF-LPME) (a hollow fiber is used to stabilize and protect the solvent), auto-performed with a commercial CTC CombiPal autosampler, are described in this paper. Critical experimental factors, including temperature, choice of extraction solvent, solvent volume, plunger movement rate, and extraction time were investigated. Among the three HS-LPME techniques that were evaluated, the exposed dynamic HS-LPME technique provided the best performance, compared to the unexposed dynamic HS-LPME and static HS-LPME approaches. For DI-LPME, the dynamic process can enhance the extraction efficiency and the achieved method precision is comparable with the static DI-LPME technique. The precision of the fully automated HF-LPME is quite acceptable (RSD values below 6.8%), and the concentration enrichment factors are better than the DI-LPME approaches. The fully automated LPME techniques are more accurate and more convenient, and the reproducibility achieved eliminates the need for an internal standard to improve the method precision.

Chemistry Techniques, Analytical↗

PubMed2006

Time-weighted average water sampling with a diffusion-based solid-phase microextraction device.

A new diffusion-based solid-phase microextraction (SPME) time-weighted average (TWA) field water sampling device was developed and investigated by field trial. The sampler is constructed with copper tube and caps and a commercial SPME fiber assembly. The device possesses all advantages of SPME; it is solvent-free, reusable, combines sampling, isolation and enrichment into one step, and the fiber can be directly injected into a gas chromatograph for analysis with a commercial SPME fiber holder, without further treatment. Field trials in Laurel Creek (Waterloo, Ont., Canada) and Hamilton Harbour (Hamilton, Ont., Canada) illustrated that the device is durable, easy to deploy, and the mass uptake of the device is independent of the face velocity. The device provides good precision [relative standard deviations (RSDs) are less than 20%] and the data obtained with this device are quite comparable to those obtained with the spot sampling method, which demonstrates that the newly developed SPME water sampling device is suitable for long-term monitoring of organic pollutants in water.

Diffusion↗

PubMed2006

On-rod standardization technique for time-weighted average water sampling with a polydimethylsiloxane rod.

In this study, a polydimethylsiloxane (PDMS) rod was developed as a passive sampler, based on the solid-phase microextraction (SPME) technique. The on-rod standardization technique was applied to the PDMS rod passive sampler. Using the desorption of the pre-loaded standard on the PDMS rod to calibrate the absorption of the target analytes allows for the determination of the time-weighted average (TWA) concentrations of pollutants in the aqueous media. The PDMS rod passive sampler with the on-rod standardization technique was tested in the laboratory with a flow-through system and was subsequently applied to measure TWA concentrations of polycyclic aromatic hydrocarbons (PAHs) in the field (Hamilton Harbour, Hamilton, Ont., Canada). Both the laboratory and field experiments demonstrated that, with the on-rod standardization technique, the PDMS rod can be successfully used as a passive sampler for TWA water sampling in the field. The PDMS rod passive sampler benefits from the inherent advantages of the SPME approach: it incorporates sampling, isolation and enrichment into one step. The design of this system also addressed the additional needs for passive sampling techniques, providing an economical approach to field sampling that is also easy to deploy. Rather, with this approach, TWA concentrations of target analytes can be obtained by one sampler, and can be analyzed directly, with no further sample preparation treatment required.

Chromatography, Liquid↗

PubMed2005

Kinetic calibration for automated headspace liquid-phase microextraction.

The kinetics of the absorption and desorption of analytes for headspace liquid-phase microextraction (HS-LPME) were studied. It was found that the desorption of analytes from the extraction phase into the sample matrix is isotropic to the absorption of the analytes from the sample matrix into the extraction phase under the same conditions. This therefore allows for the calibration of absorption using desorption. Calibration was accomplished by exposing the extraction phase, which contained a standard, to the sample matrix. The information from the desorption of the standard, such as time constant a, could be directly used to estimate the concentration of the target analyte in the sample matrix. This new kinetic calibration method for headspace LPME was successfully used to correct the matrix effects in the BTEX analysis of an orange juice sample. In this study, the headspace LPME techniques were successfully fully automated, for both static and dynamic methods, with the CTC CombiPal autosampler. All operations of headspace LPME, including sample transfer and agitation, filling of extraction solvent, exposing the solvent in the headspace, withdrawing the solvent to syringe and introducing the extraction phase into injector, were autoperformed by the CTC autosampler. The fully automated headspace LPME technique is more convenient and improved the precision and sensitivity of the method. This automated dynamic headspace LPME technique can be also used to obtain the distribution coefficient between the sample matrix (aqueous or another solution) and the extraction phase (1-octanol or another solvent). The distribution coefficient between 1-octanol and orange juice, at 25 degrees C, was obtained with this technique.

Adsorption↗