Biomedical subjects

Hian Kee Lee

Publications and source records attributed to Hian Kee Lee.

At least 19 recordsLinked to original sources

PubMed2005

Orthogonal array designs for the optimization of liquid-liquid-liquid microextraction of nonsteroidal anti-inflammatory drugs combined with high-performance liquid chromatography-ultraviolet detection.

Orthogonal array designs (OADs) were applied for the first time to optimize liquid-liquid-liquid microextraction (LLLME) conditions for the analysis of three nonsteroidal anti-inflammatory drug residues (2-(4-chlorophenoxy)-2-methylpropionic acid, ketoprofen, and naproxen) in wastewater samples. Six relevant factors were investigated: type of organic solvent, composition of donor phase and acceptor phase, stirring speed, extraction time and salt concentration. In the first stage, mixed-level orthogonal array design, an OA16 (4(1) x 2(12)) matrix was employed to study the effect of six factors, by which the effect of each factor was estimated using individual contributions as response functions. Based on the results of the first stage, 1-octanol was chosen as organic solvent for extraction. The other five factors were selected for further optimization using an OA16 (4(5)) matrix and a 4 x 4 table to locate more exact levels for each variable. The relative standard deviations for the reproducibility of optimized LLLME varied from 6.2 to 7.1%. The coefficients of determination for calibration curves were higher than 0.9950. The method detection limits for drugs spiked in ultrapure water were in the range of 0.03-0.3 ng/mL. The final optimized conditions were applied to the analysis of drug residues in three wastewater samples in Singapore.

Anti-Inflammatory Agents, Non-Steroidal↗

PubMed2005

Polymer-coated hollow-fiber microextraction of estrogens in water samples with analysis by gas chromatography-mass spectrometry.

A novel sorbent, dihydroxylated polymethylmethacrylate (DHPMM), coated on hollow-fiber membrane, is used for the polymer-coated hollow-fiber microextraction of trace amounts of natural and synthetic estrogens, such as diethylstilbestrol, estrone, 17beta-estradiol and 17alpha-ethynylestradiol, in aqueous samples. In this procedure, estrogens were extracted using the functionalized polar DHPMM polymer with derivatization using N-methyl-N-(trimethylsilyl)trifluoroacetamide followed by gas chromatography-mass spectrometric analysis. The detection limits for estrogens in aqueous sample were between 0.03 and 0.8 ng l(-1) and the calibration curves were linear over the concentration range 0.05-10 microgl(-1) and had correlation coefficients of >0.994. The relative standard deviations (RSDs) were <15% (n = 3). This simple, accurate, sensitive and selective analytical method is applicable to the determination of trace amounts of estrogens in reservoir and potable water samples.

Chemical Fractionation↗

PubMed2005

Sol-gel-coated oligomers as novel stationary phases for solid-phase microextraction.

Amphiphilic and hydrophilic oligomers were synthesized and coated on fused silica capillaries using a sol-gel technique. Sol-gel-coated capillaries were evaluated for the solid-phase microextraction and preconcentration of a wide variety of non-polar and polar analytes. Both types of coatings were stable under high temperature (up to 280 degrees C). The extraction efficiency of the sol-gel coatings was evaluated for the extraction of both non-polar and polar analytes, including organochlorine pesticides, triazine herbicides, estrogens and alkylphenols (APs) and bisphenol-A (BPA). Compared with commercially available solid-phase microextraction (SPME) adsorbents such as poly(dimethylsiloxane)divenylbenzene and polyacrylate, the new materials showed comparable selectivity and sensitivity towards both non-polar and polar analytes. The new coatings gave good linearity and detection limits. For example with triazines, a detection limit of <0.005 microl l(-1), precision from 5.0 to 11.0% (n = 6) and linearity of the calibration plots (0.5 to 50 microl l(-1)) were obtained. The sol-gel coated SPME capillaries were used for the determination of triazine herbicides in reservoir water samples collected in Singapore.

Estrogens↗

PubMed2005

Determination of alkylphenols and bisphenol-A. A comparative investigation of functional polymer-coated membrane microextraction and solid-phase microextraction techniques.

A functional polymer (hydroxylated polymethacrylate) coated on porous polysulfone hollow fiber membrane (PS-HFM) was used as an adsorbent for the extraction of alkylphenols and bisphenol-A from seawater samples. Analyses of the extracts were performed using gas chromatography-mass spectrometry (GC-MS) after injection-port derivatization using bis(trimethylsilyl)trifluoroacetamide (BSTFA). We term the procedure as polymer-coated hollow fiber microextraction (PC-HFME). Owing to high porosity PS-HFM coated with hydroxylated polymer showed high extraction efficiency. Compared with solid-phase microextraction (SPME), PC-HFME showed good selectivity and sensitivity. Detection limits of alkylphenols and bisphenol-A ranged between 0.07 and 2.34 ng l(-1). The linearity range was from 0.01 to 15 microg l(-1) and the correlation coefficient (r) up to 0.997. The sensitivity and selectivity of the coated HFM could be potentially tuned by changing the characteristics of the coated hydroxylated polymer. The PC-HFME procedure was applied to the detection of alkylphenols and bisphenol-A in the coastal waters of Singapore.

Benzhydryl Compounds↗

PubMed2005

Dynamic hollow fiber-supported headspace liquid-phase microextraction.

With the increasing concern over deteriorating environmental quality, the analysis of organic pollutants in air, water, and soil has become critically important. The development of simple, efficient, and inexpensive analytical sample pretreatment is crucial for monitoring and evaluating the environment. In this work, a dynamic hollow-fiber supported headspace liquid-phase microextraction (DHF-HS-LPME) approach was developed. In dynamic LPME, the extracting solvent is held within a hollow fiber, affixed to a syringe needle and immersed in the sample solution, and is moved to-and-fro by using a programmable syringe pump. The movement facilitates mass transfer from the sample to the solvent. Here, a similar approach was adopted, except that extraction was from the headspace rather than by direct immersion. Analysis of the extract was carried out by gas chromatography-mass spectrometry. The effect of sampling temperature, water, salt, dwelling time were investigated. Results indicated that this novel headspace microextraction method gave good analyte-enrichment factors, linear range, limits of detection and repeatability, all of which were evaluated by extracting PAHs from soil samples. This technique represents an inexpensive, convenient, fast and simple sample preparation of this class of semi-volatile organic compounds.

Chromatography, Liquid↗

PubMed2005

Automated dynamic liquid-liquid-liquid microextraction followed by high-performance liquid chromatography-ultraviolet detection for the determination of phenoxy acid herbicides in environmental waters.

Automated dynamic liquid-liquid-liquid microextraction (D-LLLME) controlled by a programmable syringe pump and combined with HPLC-UV was investigated for the extraction and determination of 5 phenoxy acid herbicides in aqueous samples. In the extraction procedure, the acceptor phase was repeatedly withdrawn into and discharged from the hollow fiber by the syringe pump. The repetitive movement of acceptor phase into and out of the hollow fiber channel facilitated the transfer of analytes into donor phase, from the organic phase held in the pore of the fiber. Parameters such as the organic solvent, concentrations of the donor and acceptor phases, plunger movement pattern, speed of agitation and ionic strength of donor phase were evaluated. Good linearity of analytes was achieved in the range of 0.5-500 ng/ml with coefficients of determination, r2 > 0.9994. Good repeatabilities of extraction performance were obtained with relative standard deviations lower than 7.5%. The method provided up-to 490-fold enrichment within 13 min. In addition, the limits of detection (LODs) ranged from 0.1 to 0.4 ng/mL (S/N = 3). D-LLLME was successfully applied for the analysis of phenoxy acid herbicides from real environmental water samples.

2,4,5-Trichlorophenoxyacetic Acid↗

PubMed2005

Persistent organic pollutants in mangrove food webs in Singapore.

In this study, persistent organic pollutants were quantified in sediments, subsurface seawater, sea-surface microlayer and twenty-four biota species collected at two separate mangrove habitats in Singapore. Data confirmed the ubiquity of POPs, including polybrominated diphenyl ethers (PBDEs), in the marine environment of Singapore. A biomagnification phenomenon was observed amongst the species collected and analysed from both mangrove sites studied. Thunder crabs and fish displayed the highest POP levels. Congener profiles of PBDEs varied amongst mangrove biota species and suggested different metabolic pathways exist for flame retardants. Similarly, crab species showed an ability to metabolize chlordane insecticide.

Animals↗

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↗