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Hian Kee Lee

Publications and source records attributed to Hian Kee Lee.

At least 19 recordsLinked to original sources
PubMed2005

Headspace water-based liquid-phase microextraction.

A new organic-free mode of headspace liquid-phase microextraction termed headspace water-based liquid-phase microextraction has been developed. Five phenols were used as model compounds in the development and evaluation of the procedure. Parameters affecting the extraction efficiency were investigated. Under optimal extraction conditions, very high enrichment factors were achieved. Since only water (with sodium hydroxide to adjust the pH) and a conventional microsyringe were used, this new technique is affordable, efficient, and convenient for extraction of volatile and semivolatile ionizable compounds. In addition, in combination with capillary electrophoresis, the entire analytical process is totally organic solvent-free and is thus environmentally friendly.

Chemistry Techniques, Analytical↗
PubMed2005

Analysis of persistent organic pollutants in marine sediments using a novel microwave assisted solvent extraction and liquid-phase microextraction technique.

A simple and novel analytical method for quantifying persistent organic pollutants (POPs) in marine sediments has been developed using microwave assisted solvent extraction (MASE) and liquid-phase microextraction (LPME) using hollow fibre membrane (HFM). POPs studied included twelve organochlorine pesticides (OCP) and eight polychlorinated biphenyl (PCB) congeners. MASE was used for the extraction of POPs from 1 g of sediment using 10 ml of ultrapure water at 600 W for 20 min at 80 degrees C. The extract was subsequently subjected to a single step LPME-HFM cleanup and enrichment procedure. Recovery varied between 73 and 111% for OCPs; and 86-110% for PCBs, and exceeded levels achieved for conventional multi-step Soxhlet extraction coupled with solid-phase extraction. The method detection limit for each POP analyte ranged from 0.07 to 0.70 ng g(-1), and peak areas were proportional to analyte concentrations in the range of 5-500 ng g(-1). Relative standard deviations of less than 20% was obtained, based on triplicate sample analysis. The optimized technique was successfully applied to POP analysis of marine sediments collected from the northeastern and southwestern areas of Singapore's coastal environment.

Environmental Monitoring↗
PubMed2005

Dynamic liquid-liquid-liquid microextraction with automated movement of the acceptor phase.

A new dynamic liquid-liquid-liquid microextraction procedure, with the automated movement of acceptor phase (LLLME/AMAP) to facilitate mass transfer, was developed in this study. Four compounds, 3-nitrophenol, 4-nitrophenol, 3,4-dinitrophenol, and 2,4-dichlorophenol, were used as model compounds to be preconcentrated from water samples. The extraction involved filling a 2-cm length of hollow fiber with 4 muL of acceptor solution using a conventional microsyringe, followed by impregnation of the pores of the fiber wall with 1-octanol. The fiber was then immersed in 4 mL of aqueous sample solution. The analytes in the sample solution were extracted into the organic solvent and then back-extracted into the acceptor solution. During extraction, the acceptor phase was repeatedly moved in and out of the hollow fiber channel and the syringe controlled by a syringe pump. Separation and quantitative analyses were then performed by using high-performance liquid chromatography. The results indicated that up to 400-fold enrichment of the analytes could be obtained under the optimized conditions. The enrichment factors were two times those of static liquid-liquid-liquid microextraction. Good repeatabilities (RSD values below 9.30%) were obtained. The calibration linear range was from 10 to 1000 ng/mL with the square of the correlation coefficient (r2) >0.9916. Detection limits were in the range of 0.45-0.98 ng/mL. In addition, as compared with the previously reported dynamic three-phase microextraction in which there was no relative movement between the acceptor and the organic phase (which is not conducive to effective mass transfer), this new method shows much higher extraction efficiency. All these results suggest that this new dynamic LLLME/AMAP technique could be a better alternative to the previous LLLME for the extraction of analytes from aqueous samples.

Journal Article↗
PubMed2005

Effect of cooking on the loss of persistent organic pollutants from salmon.

Recent studies have raised concern over the presence of high levels of persistent organic pollutants (POPs) in farmed fish relative to wild specimens of the same species, particularly salmon. Although cooking is known to reduce the burden of POPs in fish, the mechanisms of loss/degradation are not clearly understood. This study investigated the loss of POPs, including polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), p,p'-DDT [2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane] and its related metabolites (sum noted as DDTs), and chlordane congeners, from salmon (Salmo salar) steaks when subjected to baking, boiling, frying, or microwave cooking. Ranges in the raw flesh were 25.1-62.9 ng/g wet weight (ww) for PCBs, 2.5-7.6 ng/g ww for PBDEs, 2.4-5.3 ng/g ww for chlordanes, and 17.5-43.8 ng/g ww for DDTs. Analysis of raw steaks from along the fish body revealed a significant variation of POP concentrations along the fish body, with higher concentrations at the head end than the tail, with a peak in the central section. After cooking, levels of POPs decreased in salmon steak with an average loss of 26 +/- 15% relative to the initial POP load in the raw steak. The removal of the skin from the cooked salmon steak resulted in a further average loss of 9 +/- 3%. The loss of POPs did not differ significantly between cooking methods. Losses of POPs were significantly and linearly correlated with the losses of lipid during cooking, suggesting removal of lipids is the critical factor for POPs reduction in cooked fish. Cooking of raw fish contaminated with POPs can therefore be expected to reduce the consumption exposure risk to human health.

Animals↗
PubMed2005

Persistent organic pollutants and heavy metals in typical seafoods consumed in Singapore.

In this study, the levels of several heavy metals and persistent organic pollutants (POPs) were measured in the edible portions of 20 different seafood types consumed in Singapore (2 < n < 12). The mean heavy metal concentrations among the seafood types ranged from below detection limits (BLD) to 14.2 microg/g wet weight (ww) for As (shark), to 0.50 microg/g ww for Cd (kunning), to 25.5 microg/g ww for Cu (gray prawn), to 0.58 microg/g ww for Hg (eel), and to 1.21 microg/g ww for Pb (salmon). Chlordane, polychlorinated biphenyls (PCBs), and p, p'-DDT [2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane] and its related metabolites (sum noted as DDTs) were the main POPs found among the seafood types, with highest concentrations in salmon fillets and green mussels. Polybrominated diphenyl ether (PBDE) concentrations in salmon fillets (2.75 ng/g ww) were one order of magnitude lower than the highest concentration of PCBs (28.5 ng/g ww). The mean daily intake of contaminants from seafood was calculated for the general population of Singapore. Daily intakes of heavy metals and POPs from seafood are below the oral reference dose set by the U.S. Food and Drug Administration (FDA), except for As, although our study did not characterize the species of As present. Daily intake of As, DDTs, heptachlor, and PCBs in seafood exceeded the conservative cancer benchmark concentrations set by the U.S. Environmental Protection Agency (EPA), suggesting that a significant number of people are potentially at risk in Singapore over a lifetime from seafood consumption.

Animals↗
PubMed2005

Bioaccumulation of DDT pesticide in cultured Asian seabass following dietary exposure.

Bioaccumulation and metabolism of p,p'-DDT was studied in the marine carnivorous fish Lates calcarifer, Asian seabass, in a controlled aquaculture experiment. Over a 42-d period, seabass were fed pellets dosed with p,p'-DDT at environmentally realistic levels. Virtually all p,p'-DDT in pellets bioaccumulated in the fish with an uptake efficiency of 98%. The levels of p,p'-DDT and the metabolites p,p'-DDD and p,p'-DDE were analyzed in muscle, liver, visceral fat, brain, and remaining tissues. Partitioning of p,p'-DDT and its metabolites among the control, low-dose, and high-dose exposed seabass were 14.8% in muscle, 3.5% in liver, 37.1% in visceral fat, 0.11% in brain, and 45.5% in remaining tissues, where partitioning between tissues was a function of tissue lipid content. p,p'-DDT bioaccumulation increased linearly with exposure in visceral fat and muscle tissue. The metabolism of p,p'-DDT, which occurs mainly in the liver, resulted in the degradation of 2.5% of p,p'-DDT into p,p'-DDD. These new findings show that bioaccumulation processes at environmentally realistic ingestion exposure levels (ng/g) differ from previous DDT ingestion studies conducted at unrealistically high DDT levels (microg/g), highlighting the need to revise models on the transfer of persistent organic pollutants in the marine environment and aquaculture systems.

Animal Feed↗
PubMed2004

Development and application of a simple capillary-microreactor for oxidation of glucose with a porous gold catalyst.

An efficient oxidation of glucose to gluconic acid was performed using a porous gold(0) catalyst in a low-cost microreactor designed from Pyrex glass capillary tubing; compared with the conventional synthesis procedure this novel approach of using a capillary-microreactor offers a convenient and highly efficient means to optimise reaction conditions and catalytic activities.

Journal Article↗
PubMed2004

Analysis of endocrine disrupting alkylphenols, chlorophenols and bisphenol-A using hollow fiber-protected liquid-phase microextraction coupled with injection port-derivatization gas chromatography-mass spectrometry.

Liquid-phase microextraction (LPME) coupled with gas chromatography-mass spectrometry were used to determine alkylphenols (APs), chlorophenols (CPs) and bisphenol-A (BPA) in aqueous samples. APs, CPs and BPA are highly polar compounds and need to be derivatized before analysis by GC-MS. In this work, they were derivatized in the GC injection port with bis(trimethylsilyl)trifluoroacetamide (BSTFA). The analytes were extracted directly from 5 ml of sample solution using 5 microl of organic solvent though a porous polypropylene hollow fiber. The hollow fiber, filled with an immiscible organic solvent (ca. 5 microl), was immersed in the sample solution which was stirred during the 30-min extraction. An aliquot (2 microl) of the extract and 2 microl of BSTFA were then consecutively injected into the GC injection port. Extraction parameters such as extraction time, pH of sample, concentration of salt added, and stirring rate were optimised. The proposed LPME provided a good average enrichment factor of up to 162-fold, reproducibility ranging from 5.9 to 13.9% (n = 4), and good linearity (r2 = 0.995) for spiked water samples. The limits of detection (LODs) ranged between 0.005 and 0.015 microgl(-1) (S/N = 3) using GC-MS with selective ion monitoring and limits of quantification were in the range of 0.012-0.026 microg l(-1). A comparative study was performed between LPME, headspace solid-phase microextraction (HS-SPME) and liquid-liquid extraction (LLE). The results obtained suggested that hollow fiber LPME was a rapid, simple and efficient technique for APs, CPs and BPA, and provided a good alternative to SPME and LLE. Finally, the proposed method was applied to monitor Singapore coastal water samples.

Endocrine Glands↗
PubMed2004

Solvent bar microextraction.

In this work, a new and simple microextraction method termed solvent bar microextraction (SBME) was developed. In this method, the organic extractant solvent (1-octanol) was confined within a short length of a hollow fiber membrane (sealed at both ends) that was placed in a stirred aqueous sample solution. Tumbling of the extraction device within the sample solution facilitated extraction. Pentachlorobenzene (PCB) and hexachlorobenzene (HCB) were used as model compounds to investigate the extraction performance. Analysis was carried out by gas chromatography/electron capture detection. This new method provided very high enrichment (approximately 110-fold for PCB and approximately 70-fold for HCB) in 10 min and good reproducibility (<4%, n = 6). Since the hollow fiber membrane was sealed, it could be used for extraction from "dirty" samples, such soil slurries. This novel microextraction method was compared with single-drop microextraction and static hollow fiber membrane microextraction in which the extractant solvent was also held within a hollow fiber but with the latter fixed to a syringe needle (i.e., there was no tumbling effect). Comparison between SBME and conventional solid-phase microextraction in a soil slurry sample was also investigated.

Journal Article↗
PubMed2004

Hollow fiber membrane-protected solid-phase microextraction of triazine herbicides in bovine milk and sewage sludge samples.

A porous polypropylene hollow fiber membrane (HFM)-protected solid-phase microextraction (HFM-SPME) procedure in conjunction with gas chromatography/mass spectrometric analysis for use in the determination of triazine herbicides in bovine milk samples is described. A 65-microm polydimethylsiloxane-divinylbenzne (PDMS-DVB) SPME fiber was protected by an HFM. HFM-SPME experimental parameters such as fiber type, extraction time, extraction temperature and salt concentration were investigated and optimized. The relative standard deviations for the reproducibility of the optimized HFM-SPME method varied from 4.30 to 12.37%. The correlation coefficients of the calibration curves were between 0.9799 and 0.9965 across a concentration range of 0-200 microg l(-1). The method detection limits for triazines in bovine milk were in the range of 0.003-0.013 microg l(-1) and limits of quantification were in the range of 0.006-0.021 microg l(-1). The suitability of HFM-SPME was extended to the analysis of the herbicides in sewage sludge samples. The results demonstrate that HFM-SPME was an efficient pretreatment and enrichment procedure for complex matrices.

Animals↗
PubMed2004

Determination of pesticides in soil by liquid-phase microextraction and gas chromatography-mass spectrometry.

Trace amounts of pesticides in soil were determined by liquid-phase microextraction (LPME) coupled to gas chromatography-mass spectrometry (GC-MS). The technique involved the use of a small amount (3 microl) of organic solvent impregnated in a hollow fiber membrane, which was attached to the needle of a conventional GC syringe. The organic solvent was repeatedly discharged into and withdrawn from the porous polypropylene hollow fiber by a syringe pump, with the pesticides being extracted from a 4 ml aqueous soil sample into the organic solvent within the hollow fiber. Aspects of the developed procedure such as organic solvent selection, extraction time, movement pattern of plunger, concentrations of humic acid and salt, and the proportion of organic solvent in the soil sample, were optimized. Limits of detection (LOD) were between 0.05 and 0.1 microg/g with GC-MS analysis under selected-ion monitoring (SIM). Also, this method provided good precision ranging from 6 to 13%; the relative standard deviations were lower than 10% for most target pesticides (at spiked levels of 0.5 microg/g in aqueous soil sample). Finally, the results were compared to those achieved using solid-phase microextraction (SPME). The results demonstrated that LPME was a fast (within 4 min) and accurate method to determine trace amounts of pesticides in soil.

Gas Chromatography-Mass Spectrometry↗
PubMed2004

Determination of polybrominated diphenyl ethers in marine biological tissues using microwave-assisted extraction.

Growing concern on the environmental impact of polybrominated diphenyl ethers (PBDEs) has created the need for rapid and quality assured analytical methods to quantify PBDEs in a spectrum of matrix types. This study presents the first validated method for the quantification of major PBDE congeners (47, 99 and 100) in marine biological tissues using microwave-assisted extraction (MAE). The recovery of polychlorinated biphenyls and various organochlorine pesticides has also been ascertained. Analytical accuracy, precision, limits of detection and cleanup efficiency were evaluated for PBDE congeners, and empirical data justifies the use of MAE for the extraction and analysis of PBDEs in biological matrices. MAE was also compared to Soxhlet extraction efficiency for PBDEs in the standard reference materials SRM2978 and SRM1588a and gave comparable results (<15% variation).

Ethers↗
PubMed2004

Development and application of polymer-coated hollow fiber membrane microextraction to the determination of organochlorine pesticides in water.

A novel extraction procedure coupled with gas chromatography-mass spectrometric detection for quantification of organochlorine pesticides (OCPs) in water is described. Amphiphilic polyhydroxylated polyparaphenylene (PH-PPP) was synthesized and coated on the surfaces of a porous polypropylene hollow fiber membrane (HFM). Due to the high porosity of the HFM, maximum active surface area to achieve high extraction efficiency is expected. The polymer-coated HFM was used for the extraction of 15 OCPs from water. The extraction efficiency was compared with emerging and established methods such as liquid-phase microextraction (LPME), solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE) techniques. We term the current procedure as polymer-coated hollow fiber microextraction (PC-HFME). PC-HFME showed good selectivity and sensitivity. Detection limits for OCPs were in the range of 0.001-0.008 microg l(-1). The sensitivity and selectivity of the coated HFM could be adjusted by changing the characteristics of the coated PH-PPP film.

Hydrocarbons, Chlorinated↗
PubMed2004

Application of liquid-phase microextraction and gas chromatography-mass spectrometry for the determination of polychlorinated biphenyls in blood plasma.

This study investigated the feasibility of applying liquid-phase microextraction combined with gas chromatography-mass spectrometry (GC-MS) to determine polychlorinated biphenyls (PCBs) in blood plasma. An efficient and simple extraction technique has been developed for the enrichment of PCBs from human blood plasma samples using single-step liquid-phase microextraction (LPME) in conjunction with a hollow fibre membrane (HFM). An eight PCB congener mixture was spiked into 2.5 ml of blood plasma, and the solution was then adjusted to pH 10.5 with a salinity of 20% (w/v) prior to making the total volume to 5 ml with ultrapure water. The porous HFM, filled with 3 microl of organic solvent, was then immersed into the solution, which was continuously agitated at 700 rpm for 30 min. Extract (1 microl) containing the pre-concentrated analytes was then injected into a GC-MS without further pre-treatment. Using an optimised extraction procedure, a large enrichment factor of the analytes, i.e. up to 241-fold was achieved in 30 min. The procedure resulted in a relative standard deviation of < 11% (n = 6), and a linear calibration range from 2.5 to 150 microg/l (r > 0.999), and detection limits between 0.07 and 0.94 microg/l, respectively. To demonstrate the feasibility of the procedure, PCB concentrations were determined in actual blood samples collected from the local population in Singapore using the optimised LPME technique.

Gas Chromatography-Mass Spectrometry↗
PubMed2004

Two-step liquid-liquid-liquid microextraction of nonsteroidal antiinflammatory drugs in wastewater.

A simple and novel two-step liquid-liquid-liquid microextraction technique combined with reversed-phase HPLC has been developed for the determination of the nonsteroidal antiinflammatory drugs ibuprofen and 2-(4-chlorophenoxy)-2-methylpropionic acid in wastewater samples. In the first step, the analytes were extracted from an acidified sample (donor solution) into 1-octanol immobilized in the pores of 10 pieces of polypropylene hollow fiber and further into a basic acceptor phase inside the hollow fiber channels. This first extraction step, using 0.01 M NaOH as the acceptor phase and 0.1 M HCl within the donor phase, had a 100% relative recovery with an enrichment factor of 100-fold. The extract in the first step was then adjusted to acidic condition with HCl. It now represented the donor phase for the second step of the extraction, using a single piece of hollow fiber, with 2 microL of 0.01 M NaOH solution as the acceptor phase. This analyte-enriched acceptor phase was subsequently withdrawn into a microsyringe and directly injected into an HPLC system for analysis. With this two-step microextraction, sensitivity enhancement of >15,000-fold could be obtained. Detection limits of < or =100 ng/L could be achieved for both compounds. The method was applied to the analysis of wastewater.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed2004

Androgenic and estrogenic response of green mussel extracts from Singapore's coastal environment using a human cell-based bioassay.

In the last decade, evidence of endocrine disruption in biota exposed to environmental pollutants has raised serious concern. Human cell-based bioassays have been developed to evaluate induced androgenic and estrogenic activities of chemical compounds. However, bioassays have been sparsely applied to environmental samples. In this study we present data on sex hormone activities in the green mussel, Perna viridis, in Singapore's coastal waters. P.viridis is a common bioindicator of marine contamination, and this study is a follow-up to an earlier investigation that reported the presence of sex hormone activities in seawater samples from Singapore's coastal environment. Specimens were collected from eight locations around the Singapore coastline and analyzed for persistent organic pollutants (POPs) and heavy metals. Tissue extracts were then screened for activities on androgen receptors (ARs) and estrogen receptors (ER-alpha and ER-beta) using a reporter gene bioassay based on a HeLa human cell line. Mussel extracts alone did not exhibit AR activity, but in the presence of the reference androgenic hormone dihydrotestosterone (DHT), activities were up to 340% higher than those observed for DHT alone. Peak activities were observed in locations adjacent to industrial and shipping activities. Estrogenic activities of the mussel extract both alone and in the presence of reference hormone were positive. Correlations were statistically investigated between sex hormone activities, levels of pollutants in the mussel tissues, and various biological parameters (specimen size, sex ratio, lipid and moisture content). Significant correlations exist between AR activities, in the presence of DHT, and total concentration of POPs (r= 0.725, p < 0.05).

Animals↗
PubMed2003

Determination of organic micropollutants in rainwater using hollow fiber membrane/liquid-phase microextraction combined with gas chromatography-mass spectrometry.

A simple and rapid liquid-phase microextraction (LPME) method using a hollow fiber membrane (HFM) in conjunction with gas chromatography-mass spectrometry (GC-MS) is presented for the quantitative determination of 16 polycyclic aromatic hydrocarbons (PAHs) and 12 organochlorine pesticides (OCPs) in rainwater samples. The LPME conditions were optimized for achieving high enrichment of the analytes from aqueous samples, in terms of hollow fiber exposure time, stirring rate, sample pH, and composition. Enrichment factors of more than 100 could be achieved within 35 min of extraction with relative standard deviations (R.S.D.s) 1.3-13.6% for PAHs and 1.7-13.8% for OCPs, respectively, over a wide range of analyte concentrations. Detection limits ranged from 0.002 to 0.047 microg l(-1) for PAHs, and from 0.013 to 0.059 microg l(-1) for OCPs, respectively. The newly developed LPME-GC-MS method has been validated for the analysis of PAHs and OCPs in rainwater samples. Extraction recoveries from spiked synthetic rainwater samples varied from 73 to 115% for PAHs and from 75 to 113% for OCPs, respectively. Real rainwater samples were analyzed using the optimized method. The concentrations of PAHs and OCPs in real rainwater samples were between 0.005-0.162, and 0.063 microg l(-1), respectively.

Gas Chromatography-Mass Spectrometry↗