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M W Cheung

Publications and source records attributed to M W Cheung.

12 recordsLinked to original sources
PubMed2000

Analytical method for the determination of atrazine and its dealkylated chlorotriazine metabolites in water using gas chromatography/mass selective detection.

A multiresidue method is reported for the determination of atrazine and its dealkylated chlorotriazine metabolites in water. Water samples are buffered to pH 10 and partitioned in ethyl acetate. Final analysis is accomplished using gas chromatography/mass selective detection (GC/MSD) in the selected ion monitoring (SIM) mode. The limit of detection (LOD) is 0.050 ng and the limit of quantification (LOQ) is 0.10 ppb for 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 2-amino-4-chloro-6-(isopropylamino)-s-triazine (G-30033), 2-amino-4-chloro-6-(ethylamino)-s-triazine (G-28279), and 2, 4-diamino-6-chloro-s-triazine (G-28273). The mean procedural recoveries were 90, 92, 98, and 85% and the standard deviations were 12, 13, 16, and 20% for atrazine, G-30033, G-28279, and G-28273, respectively (n = 30). The study was conducted under U.S. EPA FIFRA Good Laboratory Practice Guidelines 40 CFR 160 for method validation. The reported procedure accounts for residues of G-28273 in water that are not included in EPA Method 507.

Atrazine↗
PubMed2000

Analytical method for the determination of cyromazine and melamine residues in soil using LC-UV and GC-MSD.

A method is reported for the determination of cyromazine and melamine residues in soil. Soil samples are extracted twice via mechanical shaking, each time with 70% acetonitrile/30% 0.050 M ammomium carbonate for 30 min. An aliquot portion of the pooled extracts is subjected to strong cation exchange (SCX) purification on AG 50W-X4 resin. Final analysis is accomplished using liquid chromatography-ultraviolet (LC-UV) detection at a wavelength of 214 nm. Confirmatory analyses can be performed using gas chromatography-mass selective detection (GC-MSD) in the selected ion monitoring (SIM) mode. The limit of detection (LOD) is 2.5 ng injected and the limit of quantification (LOQ) is 10 ppb when using LC-UV for the analysis of N-cyclopropyl-1,3,5-triazine-2,4, 6-triamine (cyromazine) and 1,3,5-triazine-2,4,6-triamine (melamine). The LOD is 0.050 ng injected and the LOQ is 10 ppb when using GC-MSD for confirmatory analyses. The mean procedural recoveries were 97 and 95% and the standard deviations were 16 and 11% for cyromazine and melamine, respectively (n = 24), when using LC-UV. The mean procedural recoveries were 107 and 92% and the standard deviations were 9.9 and 16% for cyromazine and melamine, respectively (n = 29), when using GC-MSD. The method validation study was conducted under U.S. EPA FIFRA Good Laboratory Practice Guidelines 40 CFR 160. The method also passed an Independent Laboratory Validation (ILV) as per U.S. EPA FIFRA Subdivision N.

Chromatography, Liquid↗
PubMed1999

Sub-lethal effects of heavy metals on activated sludge microorganisms.

The effects of heavy metals, at sub-lethal concentrations, on activated sludge microbial ecosystem were investigated. Adsorption capacity and rate of copper, chromium, lead and zinc on microbial flocs were much faster than that of organic matters. Metals affected not only the adsorption rate of organic matters but also the COD adsorption capacity of the activated sludge. Effects of heavy metals, on wastewater treatment performance of a sequencing batch reactor were also studied. Metal-laden wastewater at sub-lethal levels affected the performance to different extents, depending on the hydraulic retention time. Metal ions acted as a strong competitor against the organic compounds for active sites on the bioflocs instead of acting as a toxic microbial inhibitor, thus hampering organic adsorption and affecting the COD removal efficiency under shorter HRTs.

Adsorption↗
PubMed1997

Anesthetic management of a parturient implanted with a permanent pacemaker--a case report.

At the present time, a patient with a pacemaker who undergoes an anesthesia for a surgical procedure is very common, but a parturient with a permanent pacemaker scheduled for elective cesarean section (c/s) is very rare. Complete heart block in pregnancy is not a common event and it may be congenital or acquired, particularly secondary to cardiac surgery. In normal women, the heart rate is physiologically increased commensurate with need, whereas in a parturient with installment of an implanted fixed rate pacemaker of VVI mode her heart rate cannot accelerate to cope with increased physiological demand because of the fixed pacing rate. For cesarean section, because of destabilized cardiovascular adaptation, an anesthesiologist must focus on detecting the early signs of compromised cardiac output in order to avoid maternal as well as fetal distress. He may run considerable risk to administer general or regional anesthesia to this kind of parturient. We present a case report in managing the parturient installed a with non-rate response type pacemaker undergoing C/S under epidural anesthesia.

Adult↗
PubMed1996

Filamentous growth in activated sludge.

Bulking and foaming in activated sludge have been associated with filamentous overgrowth. Filamentous Nocardia amarae and nonfilamentous Pseudomonas auruginosa were cultured using fatty acids (C2-C24) as the sole carbon. N. amarae could utilize all acids tested for growth, whereas P. auruginosa hardly grew on acids with 12 or more carbons. Maximum specific growth rate and saturation constant of N. amarae on C24, at 0.048 h-1 and 1.520 g COD/L, respectively, were much lower than that of P. auruginosa, showing that N. amarae had a relatively stronger affinity for long-chain fatty acids. N. amarae was competitive in activated sludge processes that receive sewage containing a high proportion of long-chain fatty acids, oils, and fats.

Carbon↗
PubMed1987

Diagnostic value of the Widal test in childhood typhoid fever.

The usefulness of the Widal test in diagnosing childhood typhoid fever in endemic areas was investigated. The test was done on 150 children with other febrile illnesses and 98 bacteriologically proved cases of typhoid fever. Of the 150 children with nontyphoidal fever, only one had an H agglutinin titer of 1:50. Using an H or O agglutinin titer of 1:50 or more as a criterion for diagnosis, a positive Widal test was found in 88% of typhoid fever cases on the first occasion on which the test was done. If the test was repeated at least 94% of the typhoid cases had a significant result. The Widal test is a useful diagnostic test in children in endemic areas, provided interpretation of the test is made against background information relating to agglutinin levels in normal children in the region.

Adolescent↗
PubMedDate not supplied

Determination of two oxy-pyrimidine metabolites of diazinon in urine by gas chromatography/mass selective detection and liquid chromatography/electrospray ionization/mass spectrometry/mass spectrometry.

An analytical method was developed for the determination in urine of 2 metabolites of diazinon: 6-methyl-2-(1-methylethyl)-4(1H)-pyrimidinone (G-27550) and 2-(1-hydroxy-1-methylethyl)-6-methyl-4(1H)-pyrimidinone (GS-31144). Two of the urine sample preparation procedures presented rely on gas chromatography/mass selective detection (GC/MSD) in the selected ion monitoring mode for determination of G-27550. For fast sample preparation and a limit of quantitation (LOQ) of 1.0 ppb, urine samples were purified by using ENV+ solid-phase extraction (SPE) columns. For analyte confirmation at an LOQ of 0.50 ppb, classical liquid/liquid partitioning was used before further purification in a silica SPE column. An SPE sample preparation procedure and liquid chromatography/electrospray ionization/mass spectrometry/mass spectrometry (LC/ESI/MS/MS) were used for both G-27550 and GS-31144. The limit of detection was 0.01 ng for G-27550 with GC/MSD, and 0.016 ng when LC/ESI/MS/MS was used for both G-27550 and GS-31144. The LOQ was 0.50 ppb for G-27550 when GC/MSD and the partitioning/SPE sample preparation procedure were used, and 1.0 ppb for the SPE only sample preparation procedure. The LOQ was 1.0 ppb for both analytes when LC/ESI/MS/MS was used.

Cholinesterase Inhibitors↗