M. K. Wong

Minimization of solvent consumption in pesticide residue analysis

The present paper reviews various pathways that will lead to reduction of organic solvent usage in pesticide residue analysis. Considerable reduction of solvent consumption can be achieved by miniaturizing the scale of sample extraction and cleanup and by simplifying the analytical procedures. Adoption of new analytical techniques, such as solid-phase microextraction and supercritical fluid extraction can also help to reduce the solvent consumption considerably. With the development of various new techniques in analytical chemistry, reduction of solvent consumption in pesticide residue analysis should not be a big technical problem anymore. The important point is to treat the issue as equally important as sensitivity and accuracy when developing a method.

Microwave-assisted solvent elution technique for the extraction of organic pollutants in water

Solid phase extraction (SPE) with appropriate solid sorbents has been commonly used in the routine extraction of organic pollutants in water. The elution of analytes from the solid sorbents normally takes place by organic solvents under an applied vacuum. In this study, a microwave-assisted solvent elution technique was developed for the elution of analytes from C18 membrane disks during microwave irradiation from a microwave extraction system (MES). Several parameters, namely, elution solvent, elution temperature, duration of elution and the volume of solvent which may affect the elution efficiency of microwave-assisted solvent elution (MASE) technique towards organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), phthalate esters (PAEs), organophosphorus pesticides (OPs), fungicides, herbicides and insecticides from the membrane disk were investigated. Good recoveries above 75% were obtained for most of the organic pollutants using the optimum SPE-MASE technique. The effect of sodium chloride and humic acid on the recoveries on the target analytes were also investigated.

Determination of organochlorine pesticides in water by membranous solid-phase extraction, and in sediment by microwave-assisted solvent extraction with gas chromatography and electron-capture and mass spectrometric detection

Abstract C 18 -impregnated disks for solid-phase extraction were evaluated for the extraction of organochlorine pesticides (OCPs) from water samples, and stability of the pesticides in storage. Good recoveries from different water samples were obtained. Microwave-assisted solvent extraction (MASE) was employed for the extraction of marine sediments spiked with OCPs. Factors affecting the efficiency of MASE were also investigated. Recoveries of 17 OCPs and 2 OCP surrogates were in the range 70.6–93.1% with mean deviation between 2.0 to 8.0%.

Microwave extraction of phthalate esters from marine sediment and soil

SummaryAs part of an on-going ASEAN+)-Canada Cooperative Programme on Marine Science, microwave-assisted solvent extraction has been employed for the extraction of six phthalate esters from marine sediment and soil samples. Five of the six esters studied are among the United States Environmental Protection Agencys list of top priority pollutants. The effects of extraction solvent, extraction temperature, duration of extraction and extraction volume on the mean recoveries of the six phthalate esters were quantitatively evaluated by means of an analysis of variance, followed by testing the differences among the level means for each condition with least significant difference method. Microwave-assisted solvent extraction allowed comparable or higher recoveries of the six phthalate esters (70.1–91.0%) in comparison with conventional soxhlet (65.5–89.5%) and sonication (64.6–88.6%). The precision of results by microwave-assisted solvent extraction was improved significantly compared to the conventional techniques. The microwave extraction system has many advantages over the soxhlet and sonication extraction, e.g., no laborious clean-up procedure, lower usage of hazardous organic solvent, and larger sample throughput. The technique has been employed for the analysis of native marine sediment and soil samples in Singapore.

Membrane solid-phase extraction with closed vessel microwave elution for the determination of phenolic compounds in aqueous matrices

Preconcentration of phenolic compounds was carried out with Empore C18 membrane disks, which were eluted with a closed-vessel microwave extraction system. The optimum microwave-elution conditions were obtained by optimizing the elution solvent, elution temperature, duration of elution and volume of elution solvent. The recoveries of eleven phenolic compounds spiked at 10 and 500 μg/L levels into water, using the optimum conditions, were all above 85% with relative standard deviations between 4.0 and 10.0%, except for phenol and 4-nitrophenol. Extraction of an industrial waste water sample suspected of containing phenolic compounds by SPE-CVME showed similar recoveries of phenol (the only phenolic compound detected) when compared with LLE and C18 SPE cartridge techniques. Analysis of phenolic compounds was carried out using liquid chromatography coupled to a UV detection system.

A Study of the Heavy Metal Concentrations of the Singapore River

The concentrations of lead, cadmium, copper, zinc, iron, and mercury in the water, sediment and biota of the Singapore River were determined. The concentration of the various metals in water showed significant variation at different sampling times and there was enrichment of lead content. The spatial patterns of metal distribution in the sediment could be due to the different pollution sources such as exhaust emission from boats and other environmental variables such as clay content of sediment or tide level. In the biota, higher amounts of metals were found in species inhabiting the river bed while organisms of higher trophic level such as free-swimming fishes and crabs accumulated lower amounts. In general, the flesh of fishes and crabs had the least content of metals compared to other tissues. The Concentration of various metals in bivalve, Mytilopsis sallei, which were found extensively in the river did not reflect the pollution distribution.

Optimization of liquid chromatographic parameters for the separation of priority phenols by using mixed-level orthogonal array design

Abstract Mixed-level orthogonal array design (OAD) as a chemometric method has been employed to optimize liquid Chromatographic (LC) conditions for the separation of environmental pollutants. Six parameters were examined by OAD, namely reversed-phase (C18) LC columns from different manufacturers, content of methanol and acetonitrile in the mobile phase, initial time-duration of a fixed mobile phase composition, time-duration of a gradient elution programme, and mobile phase flow rate. The optimization of these parameters for the determination of eleven priority substituted phenols was carried out to demonstrate the applicability of mixed-level OAD in environmental analytical chemistry in which the use of trial-and-error procedures is often unsatisfactory due to sample complexity. The advantages and the disadvantages of mixed-level OAD were discussed.

Optimization by orthogonal array design of solid phase extraction of organochlorine pesticides from water

SummaryA two-level orthogonal array design (OAD) to optimize the solid phase extraction of organochlorine pesticides is described. Parameters including eluting solvents, SPE cartridges packed with C18-bonded silica from different manufacturers, duration of air drying, pH, salinity and humic acid have been examined using OAD. This systematic approach was then used to optimize the relevant parameters required for off-line solid phase extraction of organochlorine pesticides from water. The optimized parameters were employed to perform extraction of the pesticides from natural waters.

Soil lead and other metal levels in industrial, residential and nature reserve areas in Singapore

The use of pressure feedback microwave digestion technique has permitted rapid and efficient digestion of soil and sediment samples. The evaluation of different acid mixtures to digest soil samples were studied by mixed-level orthogonal array design. The selected acid mixture of HCl-HNO3-HF was employed in the survey of Pb, Zn, Cu, Cr and Mn in soil samples. Surface soil samples were collected from industrial, residential and nature reserve areas in Singapore. The five metal concentrations were determined by flame atomic absorption spectrometry and graphite furnace atomic absorption spectrometry. The lead and other metal contents in NBS SRM 1645, NIES CRM No. 2 and NRCC BCSS-1 sediment standard references were determined concurrently with the survey samples. Five measured metal loading on the surface soils was in the order: industrial area > residential area > nature reserve area. The trace metal concentrations in surface soil from areas of heavy traffic are higher than those from residential areas. The main sources of trace metal pollution are vehicular exhaust and industrial activities.

Determination of Polycyclic Aromatic Hydrocarbons in Rougan, a Traditional Chinese Barbecued Food, by Capillary Gas Chromatography

Several polycyclic aromatic hydrocarbons (PAHs), including those which are carcinogenic, have been detected in rougan, a traditional Chinese barbecued pork dish. The food samples were cleaned up by caustic digestion, solvent extraction, liquid-liquid partition, and column chromatography. The determination was carried out using capillary gas-chromatography with a flame ionization detector. A 25-m HP-1 capillary column was used. Fluorene, phenanthrene, anthracene, benzo[a]anthracene, chrysene and benzo[k]fluoranthene were detected in three of the five samples within the range of 3-17 ng/g. Fluoranthene and pyrene were detected in only two of the samples at similar levels. Two potent carcinogenic compounds, benzo[a]pyrene and benzo[b,j,k]fluoranthene, were not detected in any of the samples. No detectable PAHs have been found in meat cooked without the direct contact of food with flame and smoke. In barbecuing over an open charcoal fire, animal fat and juices dripping onto the open fire probably enhanced the formation of PAHs.