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Yong S.,Applied science Group | Yong S.,National University of Singapore | Chen Y.,Applied science Group | Lee T.K.,Applied science Group | Lee H.K.,National University of Singapore
Talanta | Year: 2014

Determination of total thyroxine in human serum using hollow fiber liquid-phase microextraction (HF-LPME) has been accomplished for the first time. HF-LPME serves as an inexpensive sample pretreatment and the cleanup method that is nearly solvent-free. Thyroxine was extracted through a water immiscible organic solvent immobilized in the wall pores of a polypropylene hollow fiber into 20 μl of an aqueous acceptor phase inside the lumen of the hollow fiber. This technique produced extracts that had comparable cleanness with those obtained using solid-phase extraction (SPE). Serum samples with endogenous thyroxine were spiked with isotopically-labeled thyroxine and analyzed by liquid chromatography-tandem mass spectrometry after HF-LPME extraction. Extraction parameters including the organic phase, acid/base concentration of acceptor phase, stirring speed and extraction time were optimized. The calibration range was found to be linear over 1-1000 ng/g with the limit of detection (LOD) of 0.3 -ng/g. For quantification of total thyroxine in human serum, 6 subsamples were prepared and the results indicated very good precision with a relative standard deviation of <1.3%. The difference from the SPE method was less than 1.2%, with independent t-test showing insignificant bias. Two reference materials of human serum were analyzed, and our obtained values were compared with the reference values. The results showed very good precision with RSD around 0.2% and the deviation from the reference values were -3.1% and -2.1%. The newly developed method is precise, accurate, inexpensive, and environmentally friendly. © 2014 Elsevier B.V.


Bay L.J.,National University of Singapore | Chan S.H.,Applied science Group | Walczyk T.,National University of Singapore
Journal of Analytical Atomic Spectrometry | Year: 2015

A new approach to normalize measured isotope ratios (carbon and nitrogen) by elemental analyser continuous flow isotope ratio mass spectrometry (EA-CF-IRMS) was evaluated. Isotope ratios of samples are altered during the IRMS measurement and must be corrected for both instrumental drifts as well as instrumental isotope fractionation for comparing data within and between laboratories. Traditionally, the isotope ratio of a reference gas is measured intermittently to correct for time dependent changes in isotope fractionation over the course of the measurement. However, this step appears to be redundant as bracketing standards are usually included in a measurement run for comparisons between measurements and laboratories and they can serve, in principle, the same purpose. Here we show that measurements without normalization to the reference gas are on a par in terms of accuracy and precision with those where the reference gas was used when employing an optimized strategy for bracketing samples with reference standards. Abolishment of intermittent reference gas measurements in EA-CF-IRMS analysis has the potential to cut short the analysis time significantly, can help to save costs in commercial IRMS laboratories and may open a new door for instrument developers to design high through-put IRMS instruments. This journal is © The Royal Society of Chemistry.


Cheah N.P.,Applied science Group | Chong N.W.L.,Health Products Regulation Group | Tan J.,Applied science Group | Morsed F.A.,Applied science Group | Yee S.K.,Health Products Regulation Group
Tobacco Control | Year: 2014

Objective: Many electronic nicotine delivery systems (ENDS) are marketed as safer tobacco alternative products or effective cessation therapies. ENDS samples were evaluated for design features, including nicotine and glycols content. This could be useful in developing a legal framework to handle ENDS. Methods: Identification of the nicotine, glycerol and propylene glycol (PPG) contents was conducted using gas chromatography mass spectrometry with quantification performed using flame ionisation techniques. Results: Varying nicotine amounts were found in ENDS cartridges which were labelled with the same concentration. Chemicals such as PPG and glycerol were found to be present in the nicotine-containing liquid of the cartridges. ENDS varied in their contents and packaging information. Limited information was available on the contents of nicotine and other chemicals present in a variety of ENDS sampled. Conclusions: Based on samples tested in this study, many contain misleading information on product ingredients. The results show poor consistency between actual nicotine content analysed on ENDS cartridges and the amount labelled. These findings raise safety and efficacy concerns for current and would-be recreational users or those trying to quit smoking.


Richardson M.F.,Deakin University | Sherman C.D.H.,Deakin University | Lee R.S.,Applied science Group | Bott N.J.,RMIT University | Hirst A.J.,Deakin University
Molecular Ecology | Year: 2016

The establishment and subsequent spread of invasive species is widely recognized as one of the most threatening processes contributing to global biodiversity loss. This is especially true for marine and estuarine ecosystems, which have experienced significant increases in the number of invasive species with the increase in global maritime trade. Understanding the rate and mechanisms of range expansion is therefore of significant interest to ecologists and conservation managers alike. Using a combination of population genetic surveys, environmental DNA (eDNA) plankton sampling and hydrodynamic modelling, we examined the patterns of introduction of the predatory Northern Pacific seastar (Asterias amurensis) and pathways of secondary spread within southeast Australia. Genetic surveys across the invasive range reveal some genetic divergence between the two main invasive regions and no evidence of ongoing gene flow, a pattern that is consistent with the establishment of the second invasive region via a human-mediated translocation event. In contrast, hydrodynamic modelling combined with eDNA plankton sampling demonstrated that the establishment of range expansion populations within a region is consistent with natural larval dispersal and recruitment. Our results suggest that both anthropogenic and natural dispersal vectors have played an important role in the range expansion of this species in Australia. The multiple modes of spread combined with high levels of fecundity and a long larval duration in A. amurensis suggests it is likely to continue its range expansion and significantly impact Australian marine ecosystems. © 2016 John Wiley & Sons Ltd


Kee C.-L.,Applied science Group | Ge X.,Applied science Group | Low M.-Y.,Applied science Group
Food Additives and Contaminants - Part A Chemistry, Analysis, Control, Exposure and Risk Assessment | Year: 2015

Two groups of isomeric phosphodiestrase-type 5 inhibitors (PDE-5), consisting of four sildenafil- and three thiosildenafil-like analogues, have been successfully differentiated using high-resolution MS/MS. The optimised MS/MS data obtained from each compound were used to build a database with the aid of mass processing software. Isomeric compounds with very close chromatographic separation like dimethylsildenafil and homosildenafil could be distinguished by their unique fingerprint fragment ions in the MS/MS database. All fragment ions were within the mass tolerance of 5 ppm. One case study using an adulterated dietary supplement is included to provide more insights into this application. © 2015 Health Sciences Authority, Singapore. Published by Taylor & Francis.


Venhuis B.J.,RIVM National Institute for Public Health and the Environment | Tan J.,Applied science Group | Vredenbregt M.J.,RIVM National Institute for Public Health and the Environment | Ge X.,Applied science Group | And 2 more authors.
Forensic Science International | Year: 2012

Following a health complaint a food supplement was brought in for analysis on the suspicion of being adulterated with a synthetic drug substance. When the capsule content did not show evidence of adulteration, the capsule shell was investigated. Using HPLC-DAD and HPLC-MS the capsule shell was found to contain 2.85. mg of the erectile dysfunction drug tadalafil. Using microscopy and RAMAN spectroscopy the presence of tadalafil was shown throughout the gelatine matrix as particles and dissolved into the matrix. The adulteration is probably carried out by adding tadalafil powder to a gelatine jelly in the manufacturing of the capsules shells. Because this technique may also be used for other drug substances, capsules shells should be considered a vehicle for hiding drug substances in general. © 2011 Elsevier Ireland Ltd.


Chen Y.,Applied science Group | Liu Q.,Applied science Group | Yong S.,Applied science Group | Lee T.K.,Applied science Group
Clinica Chimica Acta | Year: 2012

Background: An isotope dilution mass spectrometry (IDMS) technique has been developed for high accuracy analysis of glucose in human serum. Currently, all the IDMS methods for glucose analysis are based on gas chromatography-mass spectrometry (GC-MS). In this study, isotope dilution liquid chromatography-tandem mass spectrometry (ID LC-MS/MS) was investigated. Methods: NIST SRM 965b glucose in frozen human serum was analyzed by linear regression IDMS based on both LC-MS/MS and GC-MS. Serum samples were spiked with isotope labeled glucose and deproteinized by acetonitrile. For LC-MS/MS measurement, the supernate was injected directly after filtration and dilution. For GC-MS measurement, the supernate was evaporated to dryness and went through a two-step derivatization before injection. Results: All measurements had good precision with CVs of < 1%. Results from GC-MS agreed very well with results from LC-MS/MS, with a difference of < 0.7%. The final reporting values in this study, based solely on LC-MS/MS, were within the certified ranges. The relative expanded uncertainties were within the range of 1.37% to 1.69% for the 4 levels of glucose, which were comparable with uncertainties from the certificate. Conclusions: The IDMS method based on LC-MS/MS is precise and accurate. It does not require lengthy derivatization steps and thus, greatly simplifies the sample preparation procedure. © 2012 Elsevier B.V.


Yihdego Y.,La Trobe University | Yihdego Y.,Snowy Mountains Engineering Corporation SMEC | Webb J.,La Trobe University | Leahy P.,Applied science Group
Hydrological Sciences Journal | Year: 2016

Lake Purrumbete is a deep volcanic freshwater lake, cylindrical in shape, located within an extensive basalt plain in southeastern Australia. A modified difference water budget method for lakes, that estimates net groundwater flux through the difference between the level of the lake and the water table, along with the specific yield and area of the aquifer, successfully modelled the lake level fluctuations. The major influences are evaporation and direct precipitation; however groundwater fluxes are significant (~17%). The salt balance modelling shows that the salinity in Lake Purrumbete is largely controlled by groundwater and surface water fluxes. Lake Purrumbete has a greater interaction with the surrounding groundwater system than other nearby volcanic lakes due to its significant depth and the presence of highly permeable basalts along the upgradient shoreline. Its low salinity reflects the substantial salt export in the groundwater and surface outflow, as well as the lake’s relatively small evaporation due to the low surface area/volume ratio. The fluctuations in lake level and salinity for Lake Purrumbete are much less than for other lakes in the region, even during the 1997–2010 drought, due partly to the large, relatively constant groundwater flux. Low salinity, deep water and estimated depth of stratification from this study, which leads to a distinct limnology, implies that the creatures need to adapt to a different habitat to survive in the vicinity of Lake Purrumbete. Thus, Lake Purrumbete will be an important aquatic ecosystem refugium if climate change causes frequent seasonal drying of other lakes in the plain, and it is important to develop management strategies to maintain its water resources and quality. Editor D. Koutsoyiannis Associate editor Not assigned © 2015 IAHS.


Chen Y.,Applied science Group | Liu Q.,Applied science Group | Yong S.,Applied science Group | Teo H.L.,Applied science Group | Lee T.K.,Applied science Group
Clinica Chimica Acta | Year: 2014

Background: Triglycerides are widely tested in clinical laboratories using enzymatic methods for lipid profiling. As enzymatic methods can be affected by interferences from biological samples, this together with the non-specific nature of triglycerides measurement makes it necessary to verify the accuracy of the test results with a reference measurement procedure. Several such measurement procedures had been published. These procedures generally involved lengthy and laborious sample preparation steps. In this paper, an improved reference measurement procedure for triglycerides and total glycerides was reported which simplifies the sample preparation steps and greatly shortens the time taken. Methods: The procedure was based on isotope dilution gas chromatography-mass spectrometry (ID GC-MS) with tripalmitin as the calibration standard. Serum samples were first spiked with isotope-labeled tripalmitin. For the measurement of triglycerides, the serum samples were subjected to lipid extraction followed by separation of triglycerides from diglycerides and monoglycerides. Triglycerides were then hydrolyzed to glycerol, derivatized and injected into the GC-MS for quantification. For the measurement of total glycerides, the serum samples were hydrolyzed directly and derivatized before injection into the GC-MS for quantification. Results: All measurement results showed good precision with CV <. 1%. A certified reference material (CRM) of lipids in frozen human serum was used to verify the accuracy of the measurement. The obtained values for both triglycerides and total glycerides were well within the certified ranges of the CRM, with deviation <. 0.4% from the certified values. The relative expanded uncertainties were also comparable with the uncertainties associated with the certified values of the CRM. The validated procedure was used in an External Quality Assessment (EQA) Program organized by our laboratory to establish the assigned values for triglycerides and total glycerides. © 2013 Elsevier B.V.


Chen Y.,Applied science Group | Teo H.L.,Applied science Group | Liu Q.,Applied science Group | Lee T.K.,Applied science Group
Clinical Biochemistry | Year: 2015

Background: Free glycerol in human serum is measured in clinical laboratories using enzymatic methods, which can be affected by interferences from biological samples. These methods are not applicable when stable isotopic tracers are used to determine lipid kinetics. Hence, a reference measurement procedure for free glycerol in human serum is needed. Methods: A reference measurement procedure based on two-step gas chromatography-isotope dilution mass spectrometry (GC-IDMS) was developed for the measurement of free glycerol in human serum. This procedure involved spiking with 13C3-glycerol, protein precipitation and cation exchange SPE, followed by two-step derivatization with 1-butylboronic acid and N-methyl-N-trimethylsilyltrifluoroacetamide. Tripalmitin certified reference material (CRM) was used as the calibration standard to ensure metrological traceability. Results: Good precision and accuracy were obtained as demonstrated by relative standard deviation (RSD) of 1.51%-3.33%, with average recoveries over 98%. The relative measurement uncertainty was below 3% with major contributions from the concentration of glycerol calibration solution, choice of ion pair, linear regression, and measurement precision. Conclusions: With good accuracy and precision, as well as clear metrological traceability, the developed GC-IDMS procedure is useful in producing traceable and accurate measurement of free glycerol in human serum. © 2015 The Canadian Society of Clinical Chemists.

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