Biomedical subjects

Hian Kee Lee

Publications and source records attributed to Hian Kee Lee.

At least 19 recordsLinked to original sources

PubMed2006

On-site polymer-coated hollow fiber membrane microextraction and gas chromatography--mass spectrometry of polychlorinated biphenyls and polybrominated diphenyl ethers.

Porous polypropylene hollow fiber membrane coated with a conjugated polymer was used as an on-site sampling device for the extraction of polychlorinated biphenyls and polybrominated biphenyl ethers from coastal sea water samples. The coated hollow fiber membrane was placed in a vial containing the sample, and the target compounds extracted via manual shaking of the vials at the site of sample collection. For each extraction, two fibers were used. After extraction, the fibers with the adsorbed analytes were brought back to the laboratory for further processing. Care was taken to preserve the integrity of the analytes and to avoid contamination during transport; after extraction, the fibers were carefully removed and placed in air-tight crimper vials which were stored in an ice-box. The analytes were desorbed by solvent in the laboratory and analyses were carried out using gas chromatography/mass spectrometry. This method was highly reproducible with relative standard deviations in the range of 1-9%. Recoveries from spiked water samples ranged from 83% to 98%. Low limits of detections between 0.04 and 0.21ngl(-1) were achieved. The extraction efficiency was compared with solid-phase microextraction.

Gas Chromatography-Mass Spectrometry↗

PubMed2006

Developments in single-drop microextraction.

Single-drop microextraction (SDME) has become a very popular liquid-phase microextraction technique because it is inexpensive, easy to operate and nearly solvent-free. Essentially, SDME combines extraction (and conceivably, cleanup) and concentration in a minimum number of steps, and thereafter, direct extract introduction into an analytical system. In this review, in order to encourage further development of SDME, we focus on its recent developments in its various guises. Its applications when used in combination with different analytical techniques, such as gas chromatography, high-performance liquid chromatography, inductively-coupled plasma mass spectrometry, capillary electrophoresis, mass spectrometry and electrothermal atomic absorption spectrometry, are summarized. SDME does have some limitations, and these are also discussed as well. Finally, an outlook on the future of the technique is given.

Analytic Sample Preparation Methods↗

PubMed2006

Injection port derivatization following ion-pair hollow fiber-protected liquid-phase microextraction for determining acidic herbicides by gas chromatography/mass spectrometry.

Injection port derivatization following ion-pair hollow fiber-protected liquid-phase microextraction (LPME) for the trace determination of acidic herbicides (2,4-dichlorobenzoic acid, 2,4-dichlorophenoxyacetic acid, 2-(2,4-dichlorophenoxy)propionic acid, 3,5-dichlorobenzoic acid, 2-(2,4,5-trichlorophenoxy)propionic acid) in aqueous samples by gas chromatography/mass spectrometry (GC/MS) was developed. Prior to GC injection port derivatization, acidic herbicides were converted into their ion-pair complexes with tetrabutylammonium chloride in aqueous samples and then extracted by 1-octanol impregnated in the hollow fiber. Upon injection, ion pairs of acidic herbicides were quantitatively derivatized to their butyl esters in the GC injection port. Thus, several parameters related to the derivatization process (i.e., injection temperature, purge-off time) were evaluated, and main parameters affecting the hollow fiber-protected LPME procedure such as extraction organic solvent, ion-pair reagent type, pH of aqueous medium, concentration of ion-pair reagent, sodium chloride concentration added to the aqueous medium, stirring speed, and extraction time profile, optimized. At the selected extraction and derivatization conditions, no matrix effects were observed. This method proved good repeatability (RSDs <12.3%, n = 6) and good linearity (r2 > or = 0.9939) for spiked deionized water samples for five analytes. The limits of detection were in the range of 0.51-13.7 ng x L(-1) (S/N =3) under GC/MS selected ion monitoring mode. The results demonstrated that injection port derivatization following ion-pair hollow fiber-protected LPME was a simple, rapid, and accurate method for the determination of trace acidic herbicides from aqueous samples. In addition, this method proved to be environmentally friendly since it completely avoided open derivatization with potentially hazardous reagents.

Acids↗

PubMed2006

Ion-pair dynamic liquid-phase microextraction combined with injection-port derivatization for the determination of long-chain fatty acids in water samples.

Ion-pair dynamic liquid-phase microextraction coupled to injection-port derivatization has been developed for the determination of long-chain fatty acids in water samples by gas chromatography-mass spectrometry (GC-MS). In this procedure, long-chain fatty acids (C(14), C(16) and C(18)) were converted into their ion-pair complexes with tetrabutylammonium hydrogen sulfate and then extracted by organic solvent (1-octanol) impregnated in the hollow fiber. The dynamic nature of the extraction was represented by the repeated movement of the acceptor phase (organic solvent) in the hollow fiber that was controlled by a syringe pump. Ion pairs of fatty acids quantitatively formed butyl esters in the injection-port of the gas chromatography. Several parameters such as injection temperature, purge-off time, organic solvent, ion-pair reagent, pH, agitation speed, extraction time and the syringe pump parameters (plunger speed and dwell time) have been optimized. The limits of detection were in the range 0.0093-0.015 ng mL(-1) (at a signal-to-noise ratio of 3) under GC-MS-selected ion monitoring mode and the relative standard deviations were between 7.7% and 11.5%. The method was successfully applied to measure long-chain fatty acids in real water samples.

Chemical Fractionation↗

PubMed2006

Exposure and response of aquacultured oysters, Crassostrea gigas, to marine contaminants.

Comparative growth rates were monitored in the Pacific oyster, Crassostrea gigas, at two sites in Singapore, one uncontaminated and the other contaminated with respect to ambient seawater quality. Growth rates differed significantly at the two sites, revealing that marine water quality can have potentially adverse effects for the oyster aquaculture industry in Singapore. Shell abnormalities (chambering) were observed for juvenile and mature oysters at the contaminated site. Water quality parameters including temperature, salinity, dissolved oxygen, total organic carbon and chlorophyll-a were essentially similar at both sites. Differences in the levels of tributyl tin detected in soft tissues were not observed, but significant differences in the burden of persistent organic pollutants (POPs) existed between the two sites. On a positive note, the effects of pollution on oysters were found to be reversible, where transplantation of individuals to the uncontaminated site resulted in the ability of C. gigas to recover in terms of growth rate and the burden of bioaccumulated POPs.

Animals↗

PubMed2006

Polymer-coated hollow fiber microextraction combined with on-column stacking in capillary electrophoresis.

In this work, a novel microextraction method termed polymer-coated hollow fiber microextraction (PC-HFME) was developed in combination with capillary electrophoresis (CE). Polar dihydroxylated polymethylmethacrylate polymer was coated onto a porous propylene hollow fiber membrane and used as an adsorbent and that was placed in a stirred aqueous sample solution. Tumbling of the extraction device within the sample solution facilitated extraction. The amino alcohols (2-amino-1-phenylethanol, norephedrine, alprenolol and atenolol which are beta-blocker drugs), were used as model compounds to investigate the extraction performance. No organic solvent was used in this procedure. The extract was then further concentrated through on-column stacking (normal stacking mode) during CE analysis. The detection limits ranged from 0.9 to 7 ng ml(-1). Relative standard deviations (n=6) ranged from 4 to 6%. The extraction of the amino alcohols in spiked wastewater effluent (representing a complex matrix) was evaluated using the developed procedure.

Amino Alcohols↗

PubMed2006

Continuous flow microextraction combined with high-performance liquid chromatography for the analysis of pesticides in natural waters.

Continuous flow microextraction (CFME) combined with high-performance liquid chromatography-ultraviolet (HPLC-UV) detection has been applied to the analysis of five widely used pesticides, simazine, fensulfothion, etridiazole, mepronil and bensulide, present at trace levels in water samples. CFME employs a single organic solvent drop positioned at the tip of a polyether ether ketone (PEEK) tubing, which is immersed in a continuous flowing aqueous sample solution in a 0.5-ml glass chamber. The PEEK tubing acts as the organic drop holder and fluid delivery duct. Analytes are partitioned between the organic drop and the bulk sample solution. Important extraction factors including type of solvent, its volume, sample solution flow rate, extraction time, its pH and addition of salt were investigated. All pesticides exhibit good linearity in the investigated concentration range of 25-250 ng ml(-1) with coefficients of determination (R2) ranging from 0.9879 to 0.9999 under the optimized conditions. Detection limits lower than 4 ng ml(-1) were obtained for all analytes. The method was evaluated by analyzing natural water sample collected from a reservoir in Singapore. This study for the first time demonstrated the compatibility of CFME procedure and HPLC separation.

Chromatography, High Pressure Liquid↗

PubMed2006

Development and application of microporous hollow fiber protected liquid-phase microextraction via gaseous diffusion to the determination of phenols in water.

A new organic solvent-free microextraction technique termed liquid-gas-liquid microextraction (LGLME) was developed. In this technique, a small amount (6 microl) of aqueous acceptor solution (0.5M NaOH) is introduced into the channel of a 2.65 cm polypropylene hollow fiber. The hollow fiber is then immersed in an aqueous sample donor solution. The aqueous acceptor phase in the channel of the hollow fiber is separated from the sample solution by the hydrophobic microporous hollow fiber wall with air inside its pores. The analytes (phenols) passed through the microporous hollow fiber membrane by gas diffusion and were then trapped by the basic acceptor solution. After extraction, the acceptor solution was withdrawn into a microsyringe and injected into a capillary electrophoresis sample vial for subsequent analysis. Limits of detection of between 0.5 and 10 microg/l for eight phenols could be achieved. The relative standard deviations (n=6) of this technique between 2.7 and 7.6%. The technique also provides good enrichment factors for all the eight analytes.

Chromatography, Gas↗

PubMed2006

Application of liquid-phase microextraction and on-column derivatization combined with gas chromatography-mass spectrometry to the determination of carbamate pesticides.

A method has been established for the determination of five carbamate pesticides in water samples using liquid-phase microextraction (LPME) followed by on-column derivatization and gas chromatography-mass spectrometric (GC-MS) determination. Trimethylphenylammonium hydroxide (TMPAH) and trimethylsulfonium hydroxide (TMSH) were used as derivatization reagent for extracts prior to GC-MS analysis as carbamate pesticides are thermally labile compounds. Parameters that affect the extraction efficiency (selection of organic solvent and extraction time) and derivatization efficiency (choice of derivatization reagent and concentration of derivatization reagent) were investigated. The proposed method provided good enrichment factors up to 224, with reproducibility ranging from 4.86 to 7.81%, and good linearity from 1 to 400 microg/L. The limits of detection (LODs) ranged between 0.2 and 0.8 microg/L (S/N = 3) using GC-MS with selective ion monitoring. This method was applied to the determination of carbamate pesticides in tap water and waste water.

Carbamates↗

PubMed2006

Development and application of porous membrane-protected carbon nanotube micro-solid-phase extraction combined with gas chromatography/mass spectrometry.

A novel, multiwalled carbon nanotube (MWCNT)-supported micro-solid-phase extraction (mu-SPE) procedure has been developed. A 6-mg sample of MWCNTs was packed inside a (2 cm x 1.5 cm) sheet of porous polypropylene membrane whose edges were heat-sealed to secure the contents. The mu-SPE device, which was wetted with dichloromethane, was then placed in a stirred sewage sludge sample solution to extract organophosporous pesticides, used here as model compounds. Tumbling of the extraction device within the sample solution facilitated extraction, and the porous membrane acted as a filter to exclude the extraction of extraneous materials. After extraction, analytes were desorbed in hexane and analyzed using gas chromatography/mass spectrometry. Since the porous membrane afforded protection of the MWCNTs, no further cleanup of the extract was required. The pi-pi electrostatic interaction with the analytes and the large surface area of MWCNTs facilitated the adsorption of analytes, with good selectivity and reproducibility. Under the optimized extraction conditions, the method showed good linearity in the range of 0.1-50 mug/L, repeatability of the extractions (RSD 2-8%, n = 4), and low limits of detection (1-7 pg/g). No analyte carryover effect was observed, and each mu-SPE device could be used for up to 30 extractions. Comparison was made with hollow fiber protected solid-phase microextraction and headspace solid-phase microextraction; mu-SPE was demonstrated to be a fast, accurate, and cost-effective pretreatment method for sewage sludge samples.

Journal Article↗

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↗