Ming Keong Wong

Optimization of microwave-assisted solvent extraction of polycyclic aromatic hydrocarbons in marine sediments using a microwave extraction system with high-performance liquid chromatography-fluorescence detection and gas chromatography-mass spectrometry

Abstract In the present study, a microwave-assisted solvent extraction (MASE) technique using a microwave extraction system (MES) has been developed for the extraction of polycyclic aromatic hydrocarbons (PAHs) from marine sediment. The optimum MASE conditions can be obtained by performing the mixed-level orthogonal array design (OAD) procedure. A comparison between the Soxhlet extraction method and the MASE technique showed that although both techniques gave comparable results on certified reference materials (CRM) HS-4 and HS-6, the MASE technique allows the use of less solvent and is also time-saving and cost-effective without affecting its extraction efficiency. The optimum MASE technique was coupled to two analytical techniques: high-performance liquid chromatography (HPLC) with both ultraviolet (UV) and fluorescence detectors and gas chromatography-mass spectrometry (GC-MS) for the qualitative and quantitative screening of PAHs in CRM and “real world” samples. Recoveries of PAHs from two CRMs were all above 73.3%. The concentration of PAHs in marine sediment collected from primary industrial areas was between 0.03 and 0.35 μg/g on a dry weight basis.

Heavy metals in some Chinese herbal plants

The concentrations of nine heavy metals, cadmium, cobalt, copper, iron, manganese, nickel, lead, zinc and mercury in 42 Chinese herbal medicinal plants were determined. Generally, all the samples studied had, relative to the other trace metals, higher concentrations of iron, manganese, and zinc. The concentration range of the metals determined was comparable to that in many of the East Asian vegetables and fruits. A few samples were found to contain relatively higher concentrations of the toxic metals such as cadmium, lead, and mercury. This was probably caused by contamination during air-drying and preservation.

Solid-phase microextraction using pencil lead as sorbent for analysis of organic pollutants in water

Most solid-phase microextraction methods are based on poly (dimethylsiloxane)-coated silica fibers. Is the present study, pencil lead was used as an alternative sorbent for solid-phase imcroextraction. Its application to three organic pollutants with different polarity in water was investigated. The detection limit for determination by gas chromatography with electron capture detector was 0.005 ng ml−1 for lindane, 0.05 ng ml−1 for methyl parathion and 1 ng ml−1 for 2-chlorophenol. The presence of dissolved humic substances (10 mg l−1) in water did not affect the extraction of the three analytes.

Orthogonal array design as a chemometric method for the optimization of analytical procedures. Part 4. Mixed-level design and its application to the high-performance liquid chromatographic determination of polycyclic aromatic hydrocarbons

The theory and methodology of a mixed-level orthogonal array design for the optimization of analytical procedures is described. Firstly, the method of construction of a mixed-level orthogonal array design is discussed in detail, followed by the assignment of experiments and the data analysis strategy in the design. The optimization of the parameters involved in the high-performance liquid chromatographic determination of polycyclic aromatic hydrocarbons is then employed to demonstrate the practical application of the proposed mixed-level orthogonal array design in the area of analytical chemistry.

Combination of orthogonal array design and overlapping resolution mapping for optimizing the separation of heterocyclic amines by capillary zone electrophoresis

Abstract This paper describes the application of combined orthogonal array design and overlapping resolution mapping to the optimization of capillary zone electrophoresis for the separation of heterocyclic amines. The factors affecting resolution, such as buffer pH, organic modifier, concentration of buffer solution, capillary tubing temperature, and electric field strength, were studied in two steps. In the first step, orthogonal array design was used to determine the most important factors and interactions. The experiments were carried out according to an OA16 (215) matrix through sixteen pre-designed trials. Based on the results of the first step, the second set of experiments was performed according to a three-dimensional overlapping resolution mapping scheme, in which eleven pre-planned trials were executed and global optimum conditions for the separation within a reasonable analysis time were obtained. The proposed conditions were successfully applied in the separation of thirteen heterocyclic amines.

Orthogonal array design as a chemometric method for the optimization of analytical procedures. Part 3. Five-level design and its application in a polarographic reaction system for selenium determination

The theory and methodology of a five-level orthogonal array design for the optimization of analytical procedures were developed. In the theoretical section, the construction and characteristics of the OA25(56) matrix are described in detail while orthogonality is proved by means of a fourth-order polynomial model. Next, the assignment of experiments in the OA25(56) matrix is illustrated, followed by the data analysis strategy, in which significant/non-significant influence for each factor is quantitatively evaluated by the analysis of variance (ANOVA) technique including the percentage contribution, and the differences among five levels for each factor that has a significant influence are determined by Duncans multiple F-test. Finally, the response surface methodology for the OA25(56) matrix is developed by using the equations obtained from the proof on orthogonality. In the application section, the reaction system for selenium determination by differential-pulse polarography, as a practical example, is employed to demonstrate the application of the OA25(56) matrix in the area of analytical chemistry.

Orthogonal array design for the optimization of closed-vessel microwave digestion parameters for the determination of trace metals in sediments

Abstract Orthogonal array design was applied to the optimization of the closed-vessel microwave digestion of sediment samples. Five variables related to the digestion efficiency were studied: the addition of HF in digestion acid, the ratio between HCl and HNO 3 , the maximum pressure setting, the output power and the digestion time. In the experimental assignment, the digestion time at two levels and other variables at four levels were tested using an OA 16 (4 4 × 2 3 ) matrix. The interactions between the parameters were neglected. From the results, the optimum conditions for the closed vessel microwave digestion for sediment samples were established.

Microwave digestion of biological samples with tetramethylammonium hydroxide and ethylenediaminetetraacetic acid for element determination

A microwave-digestion system with a closed PTFE vessel was used to improve the leaching of inorganic constituents from biological samples with tetramethylammonium hydroxide (TMAH) and ethylenediaminetetraacetic acid (EDTA). The effects of microwave parameter settings and the quantities of TMAH and EDTA used on leaching efficiency were evaluated. This new digestion method has been applied to the standard reference materials NIST SRM 1577 B Bovine Liver 1515 Apple Leaves and NIES CRM No. 1 Pepperbush, No. 3 Chorella, No. 6 Mussel and No. 7 Tea Leaves. The major and minor elements in the digests were analyzed by flame atomic absorption spectrometry or graphite furnace atomic absorption spectrometry. Good agreement of the analytical results with the certified values was obtained.

Determination of serum aluminium using an ion-pair reversed-phase high-performance liquid chromatographic-fluorimetric system with lumogallion

An ion-pair reversed-phase high-performance liquid chromatographic method with fluorimetric detection, using lumogallion [4-chloro-3-(2,4-dihydroxyphenylazo)-2-hydroxybenzene-1-sulphonic acid] as a ligand, has been successfully applied to the determination of aluminium in human serum. The highly fluorescent aluminium-lumogallion complex (lambda ex 505 nm, lambda em 574 nm) was separated on a LiChrosorb RP-18 column with an eluent consisting of 30% acetonitrile, 70% 0.02 M potassium hydrogen phthalate and 10 microM lumogallion. The proposed system offers a simple, rapid, selective and sensitive method for the determination of aluminium in serum. The detection limit for aluminium was 0.05 microgram/l in aqueous solution and the limit of determination was 2.2 micrograms/l in serum. The recovery of the method is generally over 90%.

The behaviour of leached aluminium in tea infusions

Abstract The behavior of leached aluminium (Al) was studied using GF-AAS with different sample pretreatments and RP-HPLC with lumogallion as the precolumn reagent. The results showed that Al species in tea infusion could be categorized into three groups: large organic compounds, small stable organic compounds and free form Al. The compositions of Al species in a tea infusion would vary with the method of the tea production. For non-fermented teas such as Long-Jin and Moli-Hua, most of the leached Al in the first tea infusion was in large organic compounds and small stable organic compounds, and free form Al consisting of less than 10%. In the case of semi-fermented and fully fermented teas such as Shuihsien and Pu-Erh, the main compositions of Al species in the first infusion were free form Al and small stable organic compounds, while Al in large organic compounds made up about 14% and 4%, respectively. When 1-g tea leaves were infused with 100 ml boiling water, about 21% Al in the tea leaves was leached into the water in the first infusion and a total of about 27% of the Al in the tea leaves was leached into the water in four infusions.