Yiu-Tung Wong

Distribution of polycyclic aromatic hydrocarbons in the atmosphere of Hong Kong

This paper reports the monitoring results of eleven polycyclic aromatic hydrocarbons (PAHs), four to six-ring, at two urban sites-Central & Western (CW) and Tsuen Wan (TW) in Hong Kong from January to December 2000; and the findings of a study conducted in 2001 of the partitioning of the gaseous and particulate phases of PAHs. The sum of the eleven PAHs under study (sigmaPAHs) was found to range from 6.46 to 38.8 ng m(-3). The annual mean levels at 12.2 ng m(-3) and 15.8 ng m(-3) for CW and TW respectively are comparable to those recorded for the previous two years and are also within the reported ranges for other metropolitan cities in the Asia Pacific region. Amongst the selected eleven PAHs, fluoranthene and pyrene were the two most abundant found in the urban atmosphere of Hong Kong during the study period accounting for approximately 80%, of the total PAHs. The ratios of benzo(a)pyrene to benzo(g,h,i)perylene (BaP/BghiP) and indeno(1,2,3-cd)pyrene to benzo(g,h,i)perylene (IDP/BghiP) indicate that diesel and gasoline vehicular exhausts were the predominant local emission sources of PAHs. Seasonal variations with high winter to summer ratios for each of the individual PAHs (CW: 1.6-16.7 and TW: 0.82-8.2) and for sigmaPAHs (CW: 1.9 and TW: 1.8) and a spatial variation of BaP amongst the air monitoring stations are noted. Results of correlation studies illustrate that local meteorological conditions such as ambient temperature, solar radiation, wind speed and wind direction have significant impact on the concentrations of atmospheric PAHs accounting for the observed seasonal variations. A snapshot comparison of the concentrations of PAHs at four sites including a roadside site, a rural site and the two regular urban sites CW and TW was also performed using the profiles of PAHs recorded on two particulate episode days in March 2000.

Screening of closantel and rafoxanide in animal muscles by HPLC with fluorescence detection and confirmation using MS

A sensitive and selective LC-FLD method was described for determination of closantel and rafoxanide in bovine and ovine muscles. Bovine and ovine muscles were extracted with ACN and acetone mixture (80:20, v/v). After cleanup with Oasis MAX SPE cartridges, the sample was analyzed by LC-FLD using the control point approach. No false-negative result was observed at or below maximum residue limits and the false-positive rate was below 5%. Suspected positive sample was confirmed by LC-MS/MS. This method was suitable for screening of large batch of samples and hence considerably reduced the time and cost required for quantitation and confirmatory analyses.

Phenylboronic Acid Solid Phase Extraction Cleanup and Isotope Dilution Liquid Chromatography-Tandem Mass Spectrometry for the Determination of Florfenicol Amine in Fish Muscles.

Florfenicol (FFC) residues in foods are regulated as the sum of florfenicol and its metabolites measured as florfenicol amine (FFA). An isotope dilution LC-MS/MS method utilizing phenylboronic acid (PBA) SPE cleanup is established for the accurate determination of FFA in fish muscles (i.e., salmon and tilapia) after acid catalyzed hydrolysis. Comparisons of the PBA SPE cleanup procedure with other cleanup procedures such as mixed-mode cationic (MCX) SPE and solid supported liquid-liquid extraction were performed. Quantification of FFA in fish muscles was accomplished by using matrix-matched calibration with FFA-D3 as the internal standard. The method was validated with FFA fortified fish muscles at three different levels (50, 100, and 200 μg/kg). Conversion of FFC to FFA by acid catalyzed hydrolysis was evaluated and found to be ≥88%. The recoveries of FFA in fish muscles at the three fortification levels ranged from 89 to 106%, and RSDs were ≤9% in all cases. The LOD values in salmon and tilapia muscles were 0.13 and 1.64 μg/kg, respectively. The LOQ values in salmon and tilapia muscles were 0.29 and 4.13 μg/kg, respectively. This method is suitable for the application in routine control of FFC in fishes according to its residue definition.

Ultra-trace determination of sodium fluoroacetate (1080) as monofluoroacetate in milk and milk powder by GC-MS/MS and LC-MS/MS

A sensitive analytical method based on derivatization with 3-nitroaniline is established for the trace determination of sodium fluoroacetate (1080) in milk and milk powder. The method reported here utilizes protein precipitation with tungstic acid, solid phase extraction (SPE) clean-up in pass-through mode with a mixed-mode anionic exchange (MAX) cartridge and N,N′-dicyclohexylcarbodiimide (DCC) mediated derivatization with 3-nitroaniline (3-NA). The 2-fluoro-N-(3-nitrophenyl)acetamide (MFA–3NA) derivative can be determined with both gas chromatography tandem mass spectrometry (GC-MS/MS) and liquid chromatography tandem mass spectrometry (LC-MS/MS). In our study, negative chemical ionization (NCI) GC-MS/MS and electrospray ionization (ESI) LC-MS/MS in negative ionization mode were used. The quantification of MFA–3NA was accomplished by using matrix-matched calibration with stable isotope labeled sodium monofluoroacetate (13C2D2–MFA) as an internal standard (I.S.). The method was validated with MFA fortified milk samples at 0.1, 0.2 and 0.4 μg L−1 and milk powder samples at 0.5, 1 and 2 μg kg−1. In GC-MS/MS, the LOQs for milk and milk powder samples were 0.0042 μg L−1 and 0.0085 μg kg−1, respectively. The RSDs at all spike levels for milk and milk powder samples were less than 6% with recoveries ranging from 90 to 105%. In LC-MS/MS, the LOQs for milk and milk powder samples were 0.00033 μg L−1 and 0.0088 μg kg−1, respectively. The RSDs at all spike levels for milk and milk powder samples were less than 13% with recoveries ranging from 90 to 112%. The proposed method has been applied to detect 1080 as monofluoroacetate (MFA) in milk and milk powder samples purchased from the local market.