MyMedR

Comparison of solvent extraction and solid-phase extraction for the determination of organochlorine pesticide residues in water

Tan GH

Analyst, 1992 Jul;117(7):1129-32.

PMID: 1524230

Solid-phase extraction (SPE) of organochlorine pesticide residues from environmental water samples was evaluated using octadecyl (C18)-bonded porous silica. The efficiency of SPE of these pesticide residues from reagent water samples at 1-5 micrograms dm-3 levels was compared with those obtained by solvent extraction with hexane and Freon TF (trichlorotrifluoroethane). Average recoveries exceeding 80% for these organochlorine pesticides were obtained via the SPE method using small cartridges containing 100 mg of 40 microns C18-bonded porous silica. The average recovery by solvent extraction with hexane and Freon TF exceeded 90% in both instances. It was concluded that the recoveries and precision for the SPE of organochlorine pesticides were poorer than those for the solvent extraction method. Organochlorine pesticide residue levels in environmental water samples from two major rivers flowing through predominantly rice-growing areas were monitored by gas chromatography using the solvent extraction method with hexane. Exceptionally high levels of organochlorine pesticide residues such as BHC, DDT, heptachlor, endosulfan and dieldrin were found in these water samples.

Residue levels of phthalate esters in water and sediment samples from the Klang River basin

Tan GH

Bull Environ Contam Toxicol, 1995 Feb;54(2):171-6.

PMID: 7742623

Fulltext The authors reply: Urethral strictures after bipolar transurethral resection of prostate may be linked to slow resection rate

Tan GH

Investig Clin Urol, 2018 01;59(1):68.

PMID: 29333519 DOI: 10.4111/icu.2018.59.1.68

Organochlorine pesticide residue levels in peninsular Malaysian rivers

Tan GH, Vijayaletchumy K

Bull Environ Contam Toxicol, 1994 Sep;53(3):351-6.

PMID: 7919710

Chemometric approach to the optimization of HS-SPME/GC-MS for the determination of multiclass pesticide residues in fruits and vegetables

Abdulra'uf LB, Tan GH

Food Chem, 2015 Jun 15;177:267-73.

PMID: 25660885 DOI: 10.1016/j.foodchem.2015.01.031

An HS-SPME method was developed using multivariate experimental designs, which was conducted in two stages. The significance of each factor was estimated using the Plackett-Burman (P-B) design, for the identification of significant factors, followed by the optimization of the significant factors using central composite design (CCD). The multivariate experiment involved the use of Minitab® statistical software for the generation of a 2(7-4) P-B design and CCD matrices. The method performance evaluated with internal standard calibration method produced good analytical figures of merit with linearity ranging from 1 to 500 μg/kg with correlation coefficient greater than 0.99, LOD and LOQ were found between 0.35 and 8.33 μg/kg and 1.15 and 27.76 μg/kg respectively. The average recovery was between 73% and 118% with relative standard deviation (RSD=1.5-14%) for all the investigated pesticides. The multivariate method helps to reduce optimization time and improve analytical throughput.

Multivariate study of parameters in the determination of pesticide residues in apple by headspace solid phase microextraction coupled to gas chromatography-mass spectrometry using experimental factorial design

Abdulra'uf LB, Tan GH

Food Chem, 2013 Dec 15;141(4):4344-8.

PMID: 23993624 DOI: 10.1016/j.foodchem.2013.07.022

Solid-phase microextraction (SPME) is a solvent-less sample preparation method which combines sample preparation, isolation, concentration and enrichment into one step. In this study, multivariate strategy was used to determine the significance of the factors affecting the solid phase microextraction of pesticide residues (fenobucarb, diazinon, chlorothalonil and chlorpyrifos) using a randomised factorial design. The interactions and effects of temperature, time and salt addition on the efficiency of the extraction of the pesticide residues were evaluated using 2(3) factorial designs. The analytes were extracted with 100 μm PDMS fibres according to the factorial design matrix and desorbed into a gas chromatography-mass spectrometry detector. The developed method was applied for the analysis of apple samples and the limits of detection were between 0.01 and 0.2 μg kg(-)(1), which were lower than the MRLs for apples. The relative standard deviations (RSD) were between 0.1% and 13.37% with average recovery of 80-105%. The linearity ranges from 0.5-50 μg kg(-)(1) with correlation coefficient greater than 0.99.

Trace monitoring of water-borne phenolics in the Klang River Basin

Tan GH, Chong CL

Environ Monit Assess, 1993 Feb;24(3):267-77.

PMID: 24227384 DOI: 10.1007/BF00545983

The Klang River Basin is located in the most densely populated region in Malaysia, with its heavy concentration of industries and population. A systematic study of the pollution to this river system caused by phenolic compounds have been carried out under this project. Analyses of water samples from the Klang River by high performance liquid chromatography (HPLC) with an ultraviolet (UV) detector at 280 nm have shown the presence of some priority phenolic pollutants.

Use of Carbon Nanotubes for the Analysis of Pesticide Residues in Fruits and Vegetables

Abdulrauf LB, Tan GH

J AOAC Int, 2016 Nov 01;99(6):1415-1425.

PMID: 28206878 DOI: 10.5740/jaoacint.16-0275

This review presents the application of carbon nanotubes as sorbent materials in the analysis of pesticide residues in fruits and vegetables. The advantages, limitations, and challenges of carbon nanotubes, with respect to their use in analytical chemistry, are presented. The efficiency of their application as extraction sorbent materials (in terms of LOD, LOQ, linearity, relative recovery, and RSD) in SPE, solid-phase microextraction, multi-plug filtration clean-up, matrix solid-phase dispersion, and the quick, easy, cheap, effective, rugged and safe method is reported. The synthesis, functionalization, purification, and characterization methods of carbon nanotubes are also discussed.

Applications of solid-phase microextraction for the analysis of pesticide residues in fruits and vegetables: a review

Abdulra'uf LB, Chai MK, Tan GH

J AOAC Int, 2012 11 28;95(5):1272-90.

PMID: 23175958

This paper reviews the application of various modes of solid-phase microextraction (SPME) for the analysis of pesticide residues in fruits and vegetables. SPME is a simple extraction technique that eliminates the use of solvent, and it is applied for the analysis of both volatile and nonvolatile pesticides. SPME has been successfully coupled to both GC and LC. The coupling with GC has been straightforward and requires little modification of existing equipment, but interfacing with LC has proved challenging. The external standard calibration technique is widely used for quantification, while standard addition and internal or surrogate standards are mainly used to account for matrix effects. All parameters that affect the extraction of pesticide residues from fruits and vegetables, and therefore need to be optimized, are also reviewed. Details of the characteristics of analytical procedures and new trends in fiber production using sol-gel technology and molecularly imprinted polymers are discussed.

A direct high-performance liquid chromatographic method for the determination of therminol 66 contamination in commercial glycerin and fatty acids

Moh MH, Tang TS, Tan GH

J Chromatogr Sci, 2001 Dec;39(12):508-12.

PMID: 11767238

A simple and sensitive high-performance liquid chromatographic method for the determination of Therminol 66 thermal heating fluid in glycerin and fatty acids is developed. Sample solutions dissolved in methanol-tetrahydrofuran (50:50, v/v) are injected directly into a reversed-phase C18 column and eluted with a methanol and water mixture (88:12, v/v). The concentration of the thermal heating fluid is monitored by fluorescence detection at 257 nm (excitation) and 320 nm (emission). The calibration graph obtained from various concentrations of the thermal heating fluid in the methanol and tetrahydrofuran mixture is linear (correlation coefficient = 0.999), and the limit of detection is 0.01 microg/mL. Spiked glycerin containing 0.1 to 1.0 microg/g of the thermal heating fluid also gives good linearity with a mean recovery of 95.3%. The mean intra- and interassay precision are 1.80-6.51% and 5.71-9.03%, respectively, at the 0.1-microg/g level. The method is simple and does not require any pretreatment step, thus it is ideal for quality assurance purposes.

Recent Advances in Analysis of Pesticides in Food and Drink Samples Using LPME Techniques Coupled to GC-MS and LC-MS: a Review

Lawal A, Tan GH, Alsharif AM

J AOAC Int, 2016 Nov 01;99(6):1383-1394.

PMID: 27667201 DOI: 10.5740/jaoacint.16-0272

Food quality and food safety are major challenges affecting agricultural and industrial aspects of production. Many contaminants from different sources contaminate foods and drinks, leading to disastrous health problems like gene mutations and cancer. Previously, many different methods have been used for the analysis of these contaminants. Liquid-liquid extraction (LLE) has been the most well-known conventional technique used, but its limitations are its tediousness, time required, and the use of large quantities of toxic organic solvents. These limitations have led to the search for other, efficient techniques that are more environmentally friendly. Hence, this review highlights recent advances in liquid-phase (single-drop, hollow fiber, and dispersive liquid-liquid) microextraction procedures for food and drink analyses. Such modifications can be justified for solving limitations associated with the traditional LLE method. The objective of this review is to serve as a reference platform for providing effective management tools for solving problems of pollution, clean-up, and control of food quality and safety globally.

Graphite nanocomposites sensor for multiplex detection of antioxidants in food

Ng KL, Tan GH, Khor SM

Food Chem, 2017 Dec 15;237:912-920.

PMID: 28764086 DOI: 10.1016/j.foodchem.2017.06.029

Butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tert-butylhydroquinone (TBHQ) are synthetic antioxidants used in the food industry. Herein, we describe the development of a novel graphite nanocomposite-based electrochemical sensor for the multiplex detection and measurement of BHA, BHT, and TBHQ levels in complex food samples using a linear sweep voltammetry technique. Moreover, our newly established analytical method exhibited good sensitivity, limit of detection, limit of quantitation, and selectivity. The accuracy and reliability of analytical results were challenged by method validation and comparison with the results of the liquid chromatography method, where a linear correlation of more than 0.99 was achieved. The addition of sodium dodecyl sulfate as supporting additive further enhanced the LSV response (anodic peak current, Ipa) of BHA and BHT by 2- and 20-times, respectively.

Fulltext Low power upconversion mixer for medical remote sensing

Lioe de X, Shafie S, Ramiah H, Tan GH

ScientificWorldJournal, 2014;2014:923893.

PMID: 25133266 DOI: 10.1155/2014/923893

This work presents the design of a low power upconversion mixer adapted in medical remote sensing such as wireless endoscopy application. The proposed upconversion mixer operates in ISM band of 433 MHz. With the carrier power of -5 dBm, the proposed mixer has an output inferred 1 dB compression point of -0.5 dBm with a corresponding output third-order intercept point (OIP3) of 7.1 dBm. The design of the upconversion mixer is realized on CMOS 0.13 μm platform, with a current consumption of 594 μA at supply voltage headroom of 1.2 V.

Statistical validation for the profiling of heroin by associating simulated postcut samples with the corresponding precut sample

Chan KW, Tan GH, Wong RC

J Forensic Sci, 2013 Jan;58 Suppl 1:S199-207.

PMID: 23013257 DOI: 10.1111/j.1556-4029.2012.02285.x

Statistical validation is crucial for the clustering of unknown samples. This study aims to demonstrate how statistical techniques can be optimized using simulated heroin samples containing a range of analyte concentrations that are similar to those of the case samples. Eight simulated heroin distribution links consisting of 64 postcut samples were prepared by mixing one of two mixtures of paracetamol-caffeine-dextromethorphan at different proportions with eight precut samples. Analyte contents and compositional variation of the prepared samples were investigated. A number of data pretreatments were evaluated by associating the postcut samples with the corresponding precut samples using principal component analysis and discriminant analysis. Subsequently, combinations of seven linkage methods and five distance measures were explored using hierarchical cluster analysis. In this study, Ward-Manhattan showed better distinctions between unrelated links and was able to cluster all related samples in very close distance under the known links on a dendogram. A similar discriminative outcome was also achieved by 90 unknown case samples when clustered via Ward-Manhattan.

Internal hernia through the mesosalpinx: a lesson to learn

Tan GH, Harunarashid H, Das S, Goh YH, Ramzisham AR

Clin Ter, 2010;161(6):533-4.

PMID: 21181082

An internal hernia through the mesosalpinx is a rare condition which is often overlooked. We report the case of a 65-year-old lady who presented with features of small bowel obstruction. At laparotomy, a gangrenous ileum was found to have herniated through a defect in the right mesosalpinx. We discuss this rare cause of a small bowel obstruction and its diagnostic dilemma.

Factors associated with HER2 overexpression in breast cancer: Experience in an Asian developing country

Tan GH, Choo WY, Taib NA, Yip CH

Asian Pac J Cancer Prev, 2009;10(5):837-40.

PMID: 20104975

INTRODUCTION: The HER2 gene is amplified in up to 30% of human breast cancers, leading to overexpression of the HER2 protein on the cell surface. Overexpression of HER2 is associated with a more aggressive cancer and hence a poorer overall survival.

OBJECTIVE: To evaluate the association between clinico-pathological features and HER2 overexpression in breast cancer.

METHODS: This is a retrospective study conducted in the Department of Surgery, University Malaya Medical Centre. The association between HER2 overexpression, determined by immunohistochemistry, and other clinicopathological factors was evaluated in 996 patients with newly diagnosed breast cancer treated from 2005 to 2007 using univariate and multivariate logistic regression.

RESULTS: HER2 overexpression occurred in 30.3% of patients. On bivariate analysis, HER2 overexpression was inversely related to ER expression (p<0.01) and PR expression (p<0.01). This overexpression was associated with a higher tumour grade, lymphovascular positivity and infiltrating ductal carcinoma subtype. On multivariate analysis, HER2 overexpression was significantly associated with higher tumour grade (p= 0.018, CI 1.25-11.04), PR negativity (p= 0.002, CI 0.30-0.77) and lymphovascular positivity (p= 0.042, CI 1.01-2.12).

CONCLUSIONS: HER2 overexpression was observed in 30.3% of Malaysian female breast cancer patients. This group of patients represents a more aggressive subtype of breast cancer with higher tumour grade, PR negativity and lymphovascular positivity. No significant relationship was established between HER2 overexpression and age, race, lymph node, ER, pathology subtype and stage of disease from this study.

Four tetracyclic oxindole alkaloids and a taberpsychine derivative from a Malayan Tabernaemontana

Lim KH, Sim KM, Tan GH, Kam TS

Phytochemistry, 2009 Jun;70(9):1182-1186.

PMID: 19643450 DOI: 10.1016/j.phytochem.2009.06.010

Four tetracyclic oxindole alkaloids, 7(R)- and 7(S)-geissoschizol oxindole (1 and 2), 7(R),16(R)- and 7(S),16(R)-19(E)-isositsirikine oxindole (3 and 4), in addition to a taberpsychine derivative, N(4)-demethyltaberpsychine (5), were isolated from the Malayan Tabernaemontana corymbosa and the structures were established using NMR and MS analysis.

A phage-displayed single chain variable fragment that interacts with hepatitis B core antigen: library construction, selection and diagnosis

Tan GH, Yusoff K, Seow HF, Tan WS

J Clin Virol, 2007 Jan;38(1):49-56.

PMID: 17074533

Phage display is an alternative method for constructing and selecting antibodies with desired specificity towards an antigen.

Antigenicity and immunogenicity of the immunodominant region of hepatitis B surface antigen displayed on bacteriophage T7

Tan GH, Yusoff K, Seow HF, Tan WS

J Med Virol, 2005 Dec;77(4):475-80.

PMID: 16254965

The immunodominant region of hepatitis B virus (HBV) located in the viral small surface antigen (S-HBsAg) elicits virus-neutralizing and protective antibodies. In order to develop an easy and inexpensive method to produce this region without the need for extensive purification, amino acid residues 111-156 of S-HBsAg were fused to the C-terminal end of the 10B capsid protein of T7 phage. Western blotting and ELISA confirmed the expression of the recombinant protein on the surface of the phage particles. The recombinant phage exhibited the antigenic and immunogenic characteristics of HBsAg, illustrating its potential as an immunological reagent and vaccine.

A phage-displayed cyclic peptide that interacts tightly with the immunodominant region of hepatitis B surface antigen

Tan WS, Tan GH, Yusoff K, Seow HF

J Clin Virol, 2005 Sep;34(1):35-41.

PMID: 16087122

The surface antigen (HBsAg) of hepatitis B virus (HBV) is highly conformational and generally evokes protective humoral immune response in human. A disulfide constrained random heptapeptide library displayed on the coat protein III of filamentous bacteriophage M13 was employed to select specific ligands that interact with HBsAg subtype ad. Fusion phages carrying the amino acid sequence ETGAKPH and other related sequences were isolated. The binding site of peptide ETGAKPH was located on the immunodominant region of HBsAg. An equilibrium binding assay in solution showed that the phage binds tightly to HBsAg with a relative dissociation constant (KDrel) of 2.9+/-0.9 nM. The phage bearing this peptide has the potential to be used as a diagnostic reagent and two assays for detecting HBsAg in blood samples are described.

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