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G.H. Tan
A Department of Chemistry, Faculty of Science , University Malaya , Lembah Pantai , Kuala Lumpur , Malaysia

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Journal article
Published: 31 March 2019 in Toxins
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Mycotoxins are common food contaminants which cause poisoning and severe health risks to humans and animals. The present study applied chemometric approach in liquid chromatography-tandem mass spectrometry (LC-MS/MS) optimization for simultaneous determination of mycotoxins, i.e., aflatoxins B1, B2, G1, and G2, and ochratoxin A. The validated quick, easy, cheap, effective, rugged, and safe (QuEChERS)-LC-MS/MS method was used to study the occurrence of mycotoxins in 120 food matrices. The recovery ranges from 81.94% to 101.67% with relative standard deviation (RSD) lesser than 11%. Through the developed method, aflatoxins were detected in raisin, pistachio, peanut, wheat flour, spice, and chili samples with concentration ranges from 0.45 to 16.93 µg/kg. Trace concentration of ochratoxin A was found in wheat flour and peanut samples which ranged from 1.2 to 3.53 µg/kg. Some of the tested food samples contained mycotoxins of above the European legal maximum limit.

ACS Style

Ali Mohamed Ali Alsharif; Yeun-Mun Choo; Guan-Huat Tan. Detection of Five Mycotoxins in Different Food Matrices in the Malaysian Market by Using Validated Liquid Chromatography Electrospray Ionization Triple Quadrupole Mass Spectrometry. Toxins 2019, 11, 196 .

AMA Style

Ali Mohamed Ali Alsharif, Yeun-Mun Choo, Guan-Huat Tan. Detection of Five Mycotoxins in Different Food Matrices in the Malaysian Market by Using Validated Liquid Chromatography Electrospray Ionization Triple Quadrupole Mass Spectrometry. Toxins. 2019; 11 (4):196.

Chicago/Turabian Style

Ali Mohamed Ali Alsharif; Yeun-Mun Choo; Guan-Huat Tan. 2019. "Detection of Five Mycotoxins in Different Food Matrices in the Malaysian Market by Using Validated Liquid Chromatography Electrospray Ionization Triple Quadrupole Mass Spectrometry." Toxins 11, no. 4: 196.

Chemometrics
Published: 15 May 2018 in Analytical Letters
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Multivariate response surface methodology optimization using Placket–Burman and Box–Behnken designs were respectively used for the screening and optimization of significant factors for liquid chromatography–tandem mass spectrometry. Consequently, the optimized instrument successfully improved the sample preparation protocol and the method was validated. However, modified QuEChERS dispersive solid phase extraction coupled with ionic liquid-based dispersive liquid–liquid microextraction were used for the determination of multi-pesticide residues in fruit and vegetable samples. The analysed samples were jackfruit, strawberries, cucumber, pears, and carrots. The resulting linearity range (5–400 µg/kg) and regression coefficient (>0.99) results were satisfactory. The 94.2 and 95.8% accuracy (89–138%) and precision (0–25%) results were satisfactory and within the recommended ranges (≤20%) and (70–120%), respectively. The limits of detection (0.01–0.54 µg/kg) and quantitation (0.03–1.79 µg/kg) were excellent. The matrix effects (≤−87%) for all analysed samples were not significant. The estimated measurement uncertainties (≤27%) were within the acceptable range (≤50%). Justifiably, the response surface methodology optimized instrument and sample treatment techniques were reliable and convenient for multi-pesticide residue determination in various fruits and vegetables.

ACS Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf. Determination of Pesticide Residues in Fruit and Vegetables by High-Performance Liquid Chromatography–Tandem Mass Spectrometry with Multivariate Response Surface Methodology. Analytical Letters 2018, 52, 231 -248.

AMA Style

Abubakar Lawal, Richard Chee Seng Wong, Guan Huat Tan, Lukman Bola Abdulra’Uf. Determination of Pesticide Residues in Fruit and Vegetables by High-Performance Liquid Chromatography–Tandem Mass Spectrometry with Multivariate Response Surface Methodology. Analytical Letters. 2018; 52 (2):231-248.

Chicago/Turabian Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf. 2018. "Determination of Pesticide Residues in Fruit and Vegetables by High-Performance Liquid Chromatography–Tandem Mass Spectrometry with Multivariate Response Surface Methodology." Analytical Letters 52, no. 2: 231-248.

Review
Published: 21 April 2018 in Journal of Chromatographic Science
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Fruits and vegetables constitute a major type of food consumed daily apart from whole grains. Unfortunately, the residual deposits of pesticides in these products are becoming a major health concern for human consumption. Consequently, the outcome of the long-term accumulation of pesticide residues has posed many health issues to both humans and animals in the environment. However, the residues have previously been determined using conventionally known techniques, which include liquid-liquid extraction, solid-phase extraction (SPE) and the recently used liquid-phase microextraction techniques. Despite the positive technological effects of these methods, their limitations include; time-consuming, operational difficulty, use of toxic organic solvents, low selective property and expensive extraction setups, with shorter lifespan of instrumental performances. Thus, the potential and maximum use of these methods for pesticides residue determination has resulted in the urgent need for better techniques that will overcome the highlighted drawbacks. Alternatively, attention has been drawn recently towards the use of quick, easy, cheap, effective, rugged and safe technique (QuEChERS) coupled with dispersive solid-phase extraction (dSPE) to overcome the setback challenges experienced by the previous technologies. Conclusively, the reviewed QuEChERS-dSPE techniques and the recent cleanup modifications justifiably prove to be reliable for routine determination and monitoring the concentration levels of pesticide residues using advanced instruments such as high-performance liquid chromatography, liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry.

ACS Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf; Ali Alsharif. Recent Modifications and Validation of QuEChERS-dSPE Coupled to LC–MS and GC–MS Instruments for Determination of Pesticide/Agrochemical Residues in Fruits and Vegetables: Review. Journal of Chromatographic Science 2018, 56, 656 -669.

AMA Style

Abubakar Lawal, Richard Chee Seng Wong, Guan Huat Tan, Lukman Bola Abdulra’Uf, Ali Alsharif. Recent Modifications and Validation of QuEChERS-dSPE Coupled to LC–MS and GC–MS Instruments for Determination of Pesticide/Agrochemical Residues in Fruits and Vegetables: Review. Journal of Chromatographic Science. 2018; 56 (7):656-669.

Chicago/Turabian Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf; Ali Alsharif. 2018. "Recent Modifications and Validation of QuEChERS-dSPE Coupled to LC–MS and GC–MS Instruments for Determination of Pesticide/Agrochemical Residues in Fruits and Vegetables: Review." Journal of Chromatographic Science 56, no. 7: 656-669.

Original
Published: 06 April 2018 in Chromatographia
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Pesticides play vital roles in agricultural practices, but the nature and mishandling of the chemicals have led to their accumulation in moist soil, vegetables and fruits. Thus, development of efficient methods for pesticides determination is one of the most important ways to address such challenges. Multivariate response surface methodology optimisation using Placket–Burman and Box–Behnken designs were respectively used for screening and optimisation of significant factors of quick, easy, cheap, effective, rugged and safe (QuEChERS) with dispersive solid-phase extraction (SPE). Consequently, the optimised QuEChERS-dSPE technique coupled with modified ionic liquid-based (IL-based) dispersive liquid–liquid microextraction was used for sample preparation before LC–MS/MS. The developed method was validated (SANTE 11831-2017) for multi-pesticide residues determination in samples of bananas, cabbages, tomatoes, oranges and onions. The precision results were satisfactory in terms of relative standard deviation (≤ 20%) as recommended. The results of accuracy for relative recoveries (82–137%) were satisfactory because 92.5% of results were within the recommended range (70–120%). The matrix effects in all the samples were very weak (less effective) (≤ − 80%). The linearity of the evaluated results was 5–400 µg kg−1 and regression coefficients (R2) were > 0.99. The resulting limits of detection and quantitation were 0.02–0.32 and 0.07–1.06 µg kg−1, respectively, and therefore satisfactory. Certifiably, the estimated measurement uncertainties range (1–16%) was acceptable (≤ 50%). Thus, the developed method could be reliable and suitable for routine determination of multiple pesticide residues in various vegetable and fruit samples.

ACS Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf; Ali Alsharif. Multi-pesticide Residues Determination in Samples of Fruits and Vegetables Using Chemometrics Approach to QuEChERS-dSPE Coupled with Ionic Liquid-Based DLLME and LC–MS/MS. Chromatographia 2018, 81, 759 -768.

AMA Style

Abubakar Lawal, Richard Chee Seng Wong, Guan Huat Tan, Lukman Bola Abdulra’Uf, Ali Alsharif. Multi-pesticide Residues Determination in Samples of Fruits and Vegetables Using Chemometrics Approach to QuEChERS-dSPE Coupled with Ionic Liquid-Based DLLME and LC–MS/MS. Chromatographia. 2018; 81 (5):759-768.

Chicago/Turabian Style

Abubakar Lawal; Richard Chee Seng Wong; Guan Huat Tan; Lukman Bola Abdulra’Uf; Ali Alsharif. 2018. "Multi-pesticide Residues Determination in Samples of Fruits and Vegetables Using Chemometrics Approach to QuEChERS-dSPE Coupled with Ionic Liquid-Based DLLME and LC–MS/MS." Chromatographia 81, no. 5: 759-768.

Journal article
Published: 18 July 2017 in Journal of Applied Sciences and Environmental Management
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A headspace solid-phase microextraction (HS-SPME) method was developed as a preliminary investigation using univariate approach for the analysis of 14 multiclass pesticide residues in fruits and vegetable samples. The gas chromatography mass spectrometry parameters (desorption temperature and time, column flow rate, interface temperature) and solid phase microextraction parameters (fiber coating type, extraction temperature and time, pH, salt addition, stirring rate, dilution factor, organic solvent type and amount) were all investigated and optimized. The optimum values for the optimized parameters are as follows: Injection Temperature, 270 °C; Interface temperature, 300 °C; Column flow rate, 1.3 mL/min; Fiber coating, PDMS/DVB; Extraction time, 30 mins; Extraction temperature, 60 °C; Stirring rate, 300 rpm; Salt addition, 10% (v/w) NaCl; pH, 7; Desorption time, 7 min; Desorption temperature, 270°C; Organic solvent 3 % (methanol/acetone, 21:79%). The optimization of the mixture of organic solvents was optimized using design of experiment (DOE) with simplex lattice, designed using Minitab Statistical Software®. The developed method was then applied to the analysis of samples of apple, tomato, broccoli, lettuce, grape, cucumber, cabbage and pear. The investigated pesticides were found to be below the maximum residue levels, while some were not found. This shows that the fruit and vegetable samples are safe for consumption and do not pose any health risk for consumers.Keywords: Univariate design, Simple lattice design, design of experiment, GC-MS, HS-SPME

ACS Style

Lukman Bola Abdulra’Uf; Amudalat Ranti Lawal; Guan Huat Tan. Development of headspace solid-phase microextraction method for the analysis of pesticide residues in fruit and vegetable samples using OFAT design. Journal of Applied Sciences and Environmental Management 2017, 21, 455 .

AMA Style

Lukman Bola Abdulra’Uf, Amudalat Ranti Lawal, Guan Huat Tan. Development of headspace solid-phase microextraction method for the analysis of pesticide residues in fruit and vegetable samples using OFAT design. Journal of Applied Sciences and Environmental Management. 2017; 21 (3):455.

Chicago/Turabian Style

Lukman Bola Abdulra’Uf; Amudalat Ranti Lawal; Guan Huat Tan. 2017. "Development of headspace solid-phase microextraction method for the analysis of pesticide residues in fruit and vegetable samples using OFAT design." Journal of Applied Sciences and Environmental Management 21, no. 3: 455.

Review
Published: 01 November 2016 in Journal of AOAC International
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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.

ACS Style

Abubakar Lawal; Guan Huat Tan; Ali Alsharif. Recent Advances in Analysis of Pesticides in Food and Drink Samples Using LPME Techniques Coupled to GC-MS and LC-MS: a Review. Journal of AOAC International 2016, 99, 1383 -1394.

AMA Style

Abubakar Lawal, Guan Huat Tan, Ali Alsharif. Recent Advances in Analysis of Pesticides in Food and Drink Samples Using LPME Techniques Coupled to GC-MS and LC-MS: a Review. Journal of AOAC International. 2016; 99 (6):1383-1394.

Chicago/Turabian Style

Abubakar Lawal; Guan Huat Tan; Ali Alsharif. 2016. "Recent Advances in Analysis of Pesticides in Food and Drink Samples Using LPME Techniques Coupled to GC-MS and LC-MS: a Review." Journal of AOAC International 99, no. 6: 1383-1394.

Review
Published: 01 September 2015 in Journal of AOAC INTERNATIONAL
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Sample preparation has been identified as the most important step in analytical chemistry and has been tagged as the bottleneck of analytical methodology. The current trend is aimed at developing cost-effective, miniaturized, simplified, and environmentally friendly sample preparation techniques. The fundamentals and applications of multivariate statistical techniques for the optimization of microextraction sample preparation and chromatographic analysis of pesticide residues are described in this review. The use of Placket-Burman, Doehlert matrix, and Box-Behnken designs are discussed. As observed in this review, a number of analytical chemists have combined chemometrics and microextraction techniques, which has helped to streamline sample preparation and improve sample throughput.

ACS Style

Lukman Bola Abdulra'Uf; Ala Yahya Sirhan; Guan Huat Tan. Applications of Experimental Design to the Optimization of Microextraction Sample Preparation Parameters for the Analysis of Pesticide Residues in Fruits and Vegetables. Journal of AOAC INTERNATIONAL 2015, 98, 1171 -1185.

AMA Style

Lukman Bola Abdulra'Uf, Ala Yahya Sirhan, Guan Huat Tan. Applications of Experimental Design to the Optimization of Microextraction Sample Preparation Parameters for the Analysis of Pesticide Residues in Fruits and Vegetables. Journal of AOAC INTERNATIONAL. 2015; 98 (5):1171-1185.

Chicago/Turabian Style

Lukman Bola Abdulra'Uf; Ala Yahya Sirhan; Guan Huat Tan. 2015. "Applications of Experimental Design to the Optimization of Microextraction Sample Preparation Parameters for the Analysis of Pesticide Residues in Fruits and Vegetables." Journal of AOAC INTERNATIONAL 98, no. 5: 1171-1185.

Journal article
Published: 01 June 2015 in Food Chemistry
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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.

ACS Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. Chemometric approach to the optimization of HS-SPME/GC–MS for the determination of multiclass pesticide residues in fruits and vegetables. Food Chemistry 2015, 177, 267 -273.

AMA Style

Lukman Bola Abdulra’Uf, Guan Huat Tan. Chemometric approach to the optimization of HS-SPME/GC–MS for the determination of multiclass pesticide residues in fruits and vegetables. Food Chemistry. 2015; 177 ():267-273.

Chicago/Turabian Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. 2015. "Chemometric approach to the optimization of HS-SPME/GC–MS for the determination of multiclass pesticide residues in fruits and vegetables." Food Chemistry 177, no. : 267-273.

Journal article
Published: 08 January 2015 in Sample Preparation
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The various microextraction techniques have been developed in order to reduce sample preparation time, improve sensitivity and selectivity as well as to corroborate the recent advances in the development of highly sensitive and efficient analytical instrumentation. The current trend is aimed at the simplification and miniaturization of extraction steps, which has led to the combination of multi-step extraction and analytical analysis into single uninterrupted step. The method development involves the screening and subsequent optimization of both gas chromatography-mass spectrometry (GC-MS) and solid phase microextraction (SPME) parameters using multivariate experimental design, which has been shown to be efficient and effective with little experimental runs. The use of microextraction has been very effective in the analysis of contaminants in food, water and the environments to ensure they are safe and does not pose any health risk to human.

ACS Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. Design of experiment in the development of spme method for the determination of pesticide residues in fruits and vegetables. Sample Preparation 2015, 2, 1 .

AMA Style

Lukman Bola Abdulra’Uf, Guan Huat Tan. Design of experiment in the development of spme method for the determination of pesticide residues in fruits and vegetables. Sample Preparation. 2015; 2 (1):1.

Chicago/Turabian Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. 2015. "Design of experiment in the development of spme method for the determination of pesticide residues in fruits and vegetables." Sample Preparation 2, no. 1: 1.

Journal article
Published: 01 July 2014 in Journal of AOAC INTERNATIONAL
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Solid-phase microextraction (SPME) is a solventless sample preparation method that combines sample preparation, isolation, concentration, and enrichment into one step. A simple and effective method coupling headspace-SPME to GC/MS was developed for the analysis of chlorpyrifos, fenitrothion, endosulfan I, and endosulfan II pesticide residues in cocoa powder. In this study, multivariate strategy was used to determine the significance of the factors affecting the SPME of the pesticides using a Plackett-Burman design, and the significant factors were optimized using central composite design. The analytes were extracted with 100 μm polydimethylsiloxane fibers according to the factorial design matrix and desorbed into a GC/MS instrument. The developed method was applied for the analysis of a cocoa powder sample, and it exhibited good figures of merit for the analytical methodology. Using the optimized conditions, the linearity ranged from 2.5 to 500 μg/kg (R2 > 0.99) using an internal standard calibration method, and the average recoveries were between 75 and 95%, with RSD values between 3.8 and 9.7%.

ACS Style

Lukman Bola Abdulra'Uf; Guan Huat Tan. Chemometric Study and Optimization of Headspace Solid-Phase Microextraction Parameters for the Determination of Multiclass Pesticide Residues in Processed Cocoa from Nigeria Using Gas Chromatography/Mass Spectrometry. Journal of AOAC INTERNATIONAL 2014, 97, 1007 -1011.

AMA Style

Lukman Bola Abdulra'Uf, Guan Huat Tan. Chemometric Study and Optimization of Headspace Solid-Phase Microextraction Parameters for the Determination of Multiclass Pesticide Residues in Processed Cocoa from Nigeria Using Gas Chromatography/Mass Spectrometry. Journal of AOAC INTERNATIONAL. 2014; 97 (4):1007-1011.

Chicago/Turabian Style

Lukman Bola Abdulra'Uf; Guan Huat Tan. 2014. "Chemometric Study and Optimization of Headspace Solid-Phase Microextraction Parameters for the Determination of Multiclass Pesticide Residues in Processed Cocoa from Nigeria Using Gas Chromatography/Mass Spectrometry." Journal of AOAC INTERNATIONAL 97, no. 4: 1007-1011.

Evaluation study
Published: 15 December 2013 in Food Chemistry
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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 23 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.

ACS Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. 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. Food Chemistry 2013, 141, 4344 -4348.

AMA Style

Lukman Bola Abdulra’Uf, Guan Huat Tan. 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. Food Chemistry. 2013; 141 (4):4344-4348.

Chicago/Turabian Style

Lukman Bola Abdulra’Uf; Guan Huat Tan. 2013. "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." Food Chemistry 141, no. 4: 4344-4348.

Original articles
Published: 13 November 2013 in Journal of Liquid Chromatography & Related Technologies
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A method for the analysis of aflatoxins in food samples was developed. The preliminary work focused on optimizing sample pretreatment conditions such as the extraction solvent; the type and amount of drying agents; the extraction time; and the solvent-sample ratio. A 60:40 methanol/acetonitrile (%, v/v) extraction mixture provided recoveries in the range of 71.8–107.7% at spike levels of 1.5–50.0 µg/kg with relative standard deviations (RSDs) lower than 15%. The separation of the four aflatoxin compounds with high selectivity and sensitivity, and within reasonable retention times, was performed by a C18 250 mm × 4.6 mm × 5 µm column (Nucleodur 250-5) at 45°C. The developed QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) procedure, high performance liquid chromatography (HPLC) method was then compared with the fluorometric determination method. It was found that the fluorometric determination method had a poor precision and positive bias as compared to QuEChERS-HPLC method. A total of 669 domestic and imported food samples in Jordan were analyzed for their aflatoxins content. Peanut and peanut butter samples showed the highest incidence of contamination (10 contaminations) followed by pistachio nut samples (6 contaminations) and sesame seed samples (2 contaminations).

ACS Style

Ala’ Yahya Sirhan; Guan Huat Tan; Ahmad Al-Shunnaq; Lukman Abdulra'uf; Richard C. S. Wong. QuEChERS-HPLC METHOD FOR AFLATOXIN DETECTION OF DOMESTIC AND IMPORTED FOOD IN JORDAN. Journal of Liquid Chromatography & Related Technologies 2013, 37, 321 -342.

AMA Style

Ala’ Yahya Sirhan, Guan Huat Tan, Ahmad Al-Shunnaq, Lukman Abdulra'uf, Richard C. S. Wong. QuEChERS-HPLC METHOD FOR AFLATOXIN DETECTION OF DOMESTIC AND IMPORTED FOOD IN JORDAN. Journal of Liquid Chromatography & Related Technologies. 2013; 37 (3):321-342.

Chicago/Turabian Style

Ala’ Yahya Sirhan; Guan Huat Tan; Ahmad Al-Shunnaq; Lukman Abdulra'uf; Richard C. S. Wong. 2013. "QuEChERS-HPLC METHOD FOR AFLATOXIN DETECTION OF DOMESTIC AND IMPORTED FOOD IN JORDAN." Journal of Liquid Chromatography & Related Technologies 37, no. 3: 321-342.

Review
Published: 06 October 2013 in Chromatographia
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Stir bar sorptive extraction (SBSE) is a microextraction technique, introduced to overcome the problem of limited extraction capacity and fragile fiber coatings inherent in the solid phase microextraction technique. The major limitations of the SBSE technique are that only polydimethylsiloxane has been commercially available, this reduces its use to non-polar analytes, and its tedious reconstitution step which can lead to loss of analytes and introduction of impurities. The current trend has been aimed at the use of other materials, some of which are commercially available, such as restricted access materials, carbon adsorbents, molecularly imprinted polymers, ionic liquids, microporous monoliths, sol–gel prepared coatings and dual phase material. This has greatly helped in widening the applications of SBSE for pesticide analysis in fruits and vegetables and other matrices. The introduction of a thermal desorption unit which eliminates the reconstitution step of the stir bar in organic solvents before instrumental analysis has helped to automate the extraction method online with gas chromatography. This paper reviews the use of SBSE in pesticide residues analysis in fruits and vegetables, with a view on sample preparation steps, method optimization and validation of analytical figures of merit.

ACS Style

Lukman Bola Abdulra'Uf; Guan H. Tan. Review of SBSE Technique for the Analysis of Pesticide Residues in Fruits and Vegetables. Chromatographia 2013, 77, 15 -24.

AMA Style

Lukman Bola Abdulra'Uf, Guan H. Tan. Review of SBSE Technique for the Analysis of Pesticide Residues in Fruits and Vegetables. Chromatographia. 2013; 77 (1-2):15-24.

Chicago/Turabian Style

Lukman Bola Abdulra'Uf; Guan H. Tan. 2013. "Review of SBSE Technique for the Analysis of Pesticide Residues in Fruits and Vegetables." Chromatographia 77, no. 1-2: 15-24.

Original articles
Published: 01 June 2013 in Australian Journal of Forensic Sciences
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This paper describes the steps involved in harmonizing a statistical method for the classification of illicit heroin using manufacturing impurity data. Sixteen linked samples were prepared from each of the five unrelated heroin seizures. Five links, totaling 80 samples were subjected to two sample weight approaches for liquid–liquid extraction. A wide range of intra-batch variations were observed in the dataset and this was probably due to the poor extraction efficiency and weight differences. Notwithstanding this, this paper aims to find the best clustering technique (pretreatment and clustering tool) that is able to minimize the intra-batch variation without undermining the inter-batch variation for sample classification. Three pretreatment methods were first evaluated by principle component analysis (PCA) and normalization coupled with standardization was found to be able to show promising results. Subsequently, hierarchical cluster analysis (HCA) was chosen to evaluate the pretreated data as this technique can clearly depict the relationships between samples on a dendogram. In the HCA, Ward-Squared Euclidean and Ward-Squared Pearson were found to be the successful linkage-distance combinations for clustering all the linked samples under five distinct groups.

ACS Style

Kar Weng Chan; Guan Huat Tan; Richard C.S. Wong. Harmonizing a chemometric procedure for the classification of illicit heroin using manufacturing impurities: preliminary investigation of heroin simulated links. Australian Journal of Forensic Sciences 2013, 45, 153 -163.

AMA Style

Kar Weng Chan, Guan Huat Tan, Richard C.S. Wong. Harmonizing a chemometric procedure for the classification of illicit heroin using manufacturing impurities: preliminary investigation of heroin simulated links. Australian Journal of Forensic Sciences. 2013; 45 (2):153-163.

Chicago/Turabian Style

Kar Weng Chan; Guan Huat Tan; Richard C.S. Wong. 2013. "Harmonizing a chemometric procedure for the classification of illicit heroin using manufacturing impurities: preliminary investigation of heroin simulated links." Australian Journal of Forensic Sciences 45, no. 2: 153-163.

Journal article
Published: 01 March 2013 in Science & Justice
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Sixteen trace elements found in 309 street heroin samples, piped water and contaminated water were determined using inductively coupled plasma-mass spectrometry. All the street heroin samples were found to contain high levels of sodium, a reflection of the use of sodium bicarbonate during heroin synthesis. Additionally, this element was also found to be one of the potential contaminants acquired from the piped water. Calcium could be derived from lime while iron, aluminum and zinc could have come from the metallic container used in the processing/cutting stage. The levels of these elements remained low in the heroin and it could be due to the dilution effects from the addition of adulterants. Statistical validation was performed with six links of related heroin samples using principal component analysis to find the best pretreatment for sample classification. It was obtained that normalization followed by fourth root showed promising results with 8% errors in the sample clustering. The technique was then applied to the case samples. Finally, the result suggested that the case samples could have originated from at least two major groups respectively showing unique elemental profiles at the street level.

ACS Style

Kar-Weng Chan; Guan-Huat Tan; Richard C.S. Wong. Investigation of trace inorganic elements in street doses of heroin. Science & Justice 2013, 53, 73 -80.

AMA Style

Kar-Weng Chan, Guan-Huat Tan, Richard C.S. Wong. Investigation of trace inorganic elements in street doses of heroin. Science & Justice. 2013; 53 (1):73-80.

Chicago/Turabian Style

Kar-Weng Chan; Guan-Huat Tan; Richard C.S. Wong. 2013. "Investigation of trace inorganic elements in street doses of heroin." Science & Justice 53, no. 1: 73-80.

Journal article
Published: 01 January 2013 in Asian Journal of Chemistry
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ACS Style

Reza Hajian; Guan H. Tan. Spectrophotometric and Voltammetric Studies on the Interaction of 7-Ethyl-10-hydroxycamptothecin (SN-38) as the Metabolized Compound of CPT-11 with ds-DNA. Asian Journal of Chemistry 2013, 25, 436 -440.

AMA Style

Reza Hajian, Guan H. Tan. Spectrophotometric and Voltammetric Studies on the Interaction of 7-Ethyl-10-hydroxycamptothecin (SN-38) as the Metabolized Compound of CPT-11 with ds-DNA. Asian Journal of Chemistry. 2013; 25 (1):436-440.

Chicago/Turabian Style

Reza Hajian; Guan H. Tan. 2013. "Spectrophotometric and Voltammetric Studies on the Interaction of 7-Ethyl-10-hydroxycamptothecin (SN-38) as the Metabolized Compound of CPT-11 with ds-DNA." Asian Journal of Chemistry 25, no. 1: 436-440.

Journal article
Published: 07 December 2012 in Journal of Separation Science
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The sample preparation step has been identified as the bottleneck of analytical methodology in chemical analysis. Therefore, there is need for the development of cost-effective, easy to operate, and environmentally friendly miniaturized sample preparation technique. The microextraction techniques combine extraction, isolation, concentration, and introduction of analytes into analytical instrument, to a single and uninterrupted step, and improve sample throughput. The use of liquid-phase microextraction techniques for the analysis of pesticide residues in fruits and vegetables are discussed with the focus on the methodologies employed by different researchers and their analytical performances. Analytes are extracted using water-immiscible solvents and are desorbed into gas chromatography, liquid chromatography, or capillary electrophoresis for identification and quantitation.

ACS Style

Lukman Bola Abdulra'uf; Ala’ Yahya Sirhan; Guan Huat Tan. Recent developments and applications of liquid phase microextraction in fruits and vegetables analysis. Journal of Separation Science 2012, 35, 3540 -3553.

AMA Style

Lukman Bola Abdulra'uf, Ala’ Yahya Sirhan, Guan Huat Tan. Recent developments and applications of liquid phase microextraction in fruits and vegetables analysis. Journal of Separation Science. 2012; 35 (24):3540-3553.

Chicago/Turabian Style

Lukman Bola Abdulra'uf; Ala’ Yahya Sirhan; Guan Huat Tan. 2012. "Recent developments and applications of liquid phase microextraction in fruits and vegetables analysis." Journal of Separation Science 35, no. 24: 3540-3553.

Journal article
Published: 26 September 2012 in Journal of Forensic Sciences
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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.

ACS Style

Kar-Weng Chan; Guan-Huat Tan; Richard C. S. Wong. Statistical Validation for the Profiling of Heroin by Associating Simulated Postcut Samples with the Corresponding Precut Sample. Journal of Forensic Sciences 2012, 58, 1 .

AMA Style

Kar-Weng Chan, Guan-Huat Tan, Richard C. S. Wong. Statistical Validation for the Profiling of Heroin by Associating Simulated Postcut Samples with the Corresponding Precut Sample. Journal of Forensic Sciences. 2012; 58 ():1.

Chicago/Turabian Style

Kar-Weng Chan; Guan-Huat Tan; Richard C. S. Wong. 2012. "Statistical Validation for the Profiling of Heroin by Associating Simulated Postcut Samples with the Corresponding Precut Sample." Journal of Forensic Sciences 58, no. : 1.

Journal article
Published: 21 September 2012 in Food Control
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A rapid, reliable and confirmatory method has been developed to determine the levels of aflatoxins B1, B2, G1, and G2 in barley, wheat, soybean, corn, peanut and peanut butter. The method is based on a single extraction step followed by liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry (LC-ESI-QTOF-MS/MS). The extraction, dilution, chromatographic and MS conditions were optimized in order to increase the sample throughput and sensitivity. The quantification of the analytes was measured by performing low-energy collision induced tandem mass spectrometry (CID-MS/MS) using the product ion scan mode. The calibration curves were linear over the range from 0.195 to 50.000 μg/kg for aflatoxin B1, G1 and from 0.029 to 15.000 μg/kg for aflatoxin B2, G2. The correlation coefficients (r2) were higher than 0.998. The overall recoveries for aflatoxins were in the range of 57.1–108.7% at spike levels of 0.6–15.0 μg/kg with relative standard deviations lower than 16%. The limit of quantification (LOQ) for aflatoxins is less than 0.8 μg/kg.

ACS Style

Ala' Yahya Sirhan; Guan Huat Tan; Richard C.S. Wong. Determination of aflatoxins in food using liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry (LC-ESI-QTOF-MS/MS). Food Control 2012, 31, 35 -44.

AMA Style

Ala' Yahya Sirhan, Guan Huat Tan, Richard C.S. Wong. Determination of aflatoxins in food using liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry (LC-ESI-QTOF-MS/MS). Food Control. 2012; 31 (1):35-44.

Chicago/Turabian Style

Ala' Yahya Sirhan; Guan Huat Tan; Richard C.S. Wong. 2012. "Determination of aflatoxins in food using liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry (LC-ESI-QTOF-MS/MS)." Food Control 31, no. 1: 35-44.

Journal article
Published: 01 September 2012 in Science & Justice
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Statistical classification remains the most useful statistical tool for forensic chemists to assess the relationships between samples. Many clustering techniques such as principal component analysis and hierarchical cluster analysis have been employed to analyze chemical data for pattern recognition. Due to the feeble foundation of this statistics knowledge among novice drug chemists, a tetrahedron method was designed to simulate how advanced chemometrics operates. In this paper, the development of the graphical tetrahedron and computational matrices derived from the possible tetrahedrons are discussed. The tetrahedron method was applied to four selected parameters obtained from nine illicit heroin samples. Pattern analysis and mathematical computation of the differences in areas for assessing the dissimilarity between the nine tetrahedrons were found to be user-convenient and straightforward for novice cluster analysts.

ACS Style

Kar-Weng Chan; Guan-Huat Tan; Richard C.S. Wong. A simplified clustering method for novice narcotic chemists. Science & Justice 2012, 52, 136 -141.

AMA Style

Kar-Weng Chan, Guan-Huat Tan, Richard C.S. Wong. A simplified clustering method for novice narcotic chemists. Science & Justice. 2012; 52 (3):136-141.

Chicago/Turabian Style

Kar-Weng Chan; Guan-Huat Tan; Richard C.S. Wong. 2012. "A simplified clustering method for novice narcotic chemists." Science & Justice 52, no. 3: 136-141.