Johnny V Nguyen

Centers for Disease Control and Prevention, Druid Hills, GA, United States

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Publications (8)31.17 Total impact

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    ABSTRACT: Melamine and cyanuric acid have been implicated as adulterants in baby formula in China and pet foods in North America. In China, the effect of melamine or melamine-cyanuric acid adulteration lead to kidney stone development and acute renal failure in thousands of Chinese infants. A selective and sensitive analytical method was developed to measure melamine in human urine in order to evaluate the extent of potential health implications resulting from the consumption of these types of adulterated products in the general US population. This method involves extracting melamine from human urine using cation-exchange solid-phase extraction, chromatographically separating it from its urinary matrix co-extractants on a silica-based, strong-cation exchange analytical column using high performance liquid chromatography, and analysis using positive mode electrospray ionization tandem mass spectrometry. Quantification was performed using modified, matrix-based isotope dilution calibration covering the concentration range of 0.50-100 ng/mL. The limit of detection, calculated using replicates of blank and low level spiked samples, was 0.66 ng/mL and the relative standard deviations were between 6.89 and 14.9%. The relative recovery of melamine was 101-106%. This method was tested for viability by analyzing samples collected from the general US population. Melamine was detected in 76% of the samples tested, with a geometric mean of 2.37 ng/mL, indicating that this method is suitable for reliably detecting background exposures to melamine or other chemicals from which it can be derived.
    Journal of chromatography. B, Analytical technologies in the biomedical and life sciences 03/2012; 887-888:48-54. · 2.78 Impact Factor
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    ABSTRACT: Concern has increased about the resulting health effects of exposure to melamine and its metabolic contaminant, cyanuric acid, after infants in China were fed baby formula milk products contaminated with these compounds. We have developed a selective and sensitive analytical method to quantify the amount of cyanuric acid in human urine. The sample preparation involved extracting free-form cyanuric acid in human urine using anion exchange solid phase extraction. Cyanuric acid was separated from its urinary matrix components on the polymeric strong anion exchange analytical column; the analysis was performed by high performance liquid chromatography-tandem mass spectrometry using negative mode electrospray ionization interface. Quantification was performed using isotope dilution calibration covering the concentration range of 1.00-200ng/mL. The limit of detection was 0.60ng/mL and the relative standard deviations were 2.8-10.5% across the calibration range. The relative recovery of cyanuric acid was 100-104%. Our method is suitable to detect urinary concentrations of cyanuric acid caused by either environmental exposures or emerging poisoning events.
    Journal of chromatography. B, Analytical technologies in the biomedical and life sciences 10/2010; 878(28):2916-22. · 2.78 Impact Factor
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    ABSTRACT: We have developed a method using on-line solid-phase extraction-high-performance liquid chromatography-tandem mass spectrometry (SPE-HPLC-MS/MS) and isotope dilution quantification to measure atrazine and seven atrazine metabolites in urine. The metabolites measured were hydroxyatrazine, diaminochloroatrazine, desisopropylatrazine, desethylatrazine, desethylatrazine mercapturate, atrazine mercaturate and atrazine itself. Our method has good precision (relative standard deviations ranging from 4 to 20% at 5, 10 and 50 ng/mL), extraction efficiencies of 67 to 102% at 5 and 25 ng/mL, relative recoveries of 87 to 112% at 5, 25, 50 and 100 ng/mL limits of detection (LOD) ranging from 0.03 to 2.80 ng/mL. The linear range of our method spans from the analyte LOD to 100 ng/mL (40 ng/mL for atrazine and atrazine mercapturate) with R (2) values of greater than 0.999 and errors about the slope of less than 3%. Our method is rapid, cost-effective and suitable for large-scale sample analyses and is easily adaptable to other biological matrices. More importantly, this method will allow us to better assess human exposure to atrazine-related chemicals.
    Analytical and Bioanalytical Chemistry 08/2008; 391(5):1931-9. · 3.66 Impact Factor
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    ABSTRACT: Atrazine (ATZ) is the second most abundantly applied pesticide in the United States. When we assessed exposure to ATZ by measuring its urinary mercapturic acid metabolite, general population data indicated that < 5% of the population was exposed to ATZ-related chemicals (limit of detection < 0.8 ng/mL). The aim of our study was to determine if we were underestimating ATZ exposure by measuring its urinary mercapturic acid metabolite and if the urinary metabole profile changed with the exposure scenario. We conducted a small-scale study involving 24 persons classified as high- (n = 8), low(n = 5), and environmental- (n = 11) exposed to ATZ. Using online solid phase extraction high performance liquid chromatography-tandem mass spectrometry, we measured nine ATZ-related metabolites in urine that included dealkylated, hydroxylated, and mercapturic acid metabolites. We found that the urinary metabolite profiles varied greatly among exposure scenarios and among persons within each exposure scenario. Although diaminochlorotriazine (DACT) appeared to be the predominant urinary metabolite detected in each exposure category, the variation in proportion of total ATZ metabolites among persons was consistently large, suggesting that one metabolite alone could not be measured as a surrogate for ATZ exposure. We have likely been underestimating population-based exposures by measuring only one urinary ATZ metabolite. Multiple urinary metabolites must be measured to accurately classify exposure to ATZ and its environmental degradates. Regardless, DACT and desethylatrazine appear to be the most important metabolites to measure to evaluate exposures to ATZ-related chemicals.
    Environmental Health Perspectives 11/2007; 115(10):1474-8. · 7.26 Impact Factor
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    ABSTRACT: We developed a high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS-MS) method to measure metabolites of atrazine, phenylurea, and sulfonylurea herbicides in human urine. The metabolites measured in the method include atrazine mercapturate, desethyl atrazine, and desisopropyl atrazine as markers of atrazine exposure; dichlorophenyl urea, dichlorophenylmethyl urea, diuron, and linuron as markers of phenylurea herbicide exposure; and dimethoxypyrimidine, dimethylpyrimidine, and methoxymethyl triazine as markers for sulfonylurea herbicide exposure. The metabolites were extracted from urine by simple solid-phase extraction using a mixed-bed cartridge and were analyzed by HPLC-MS-MS. Quantification of the atrazine metabolites was achieved using isotope-dilution calibration. The remaining metabolites were quantified using similarly structured chemicals as internal standards. Extraction recoveries ranged from 88% to 104% (n = 5). Limits of detection for the entire method ranged from 0.125 to 1 ng/mL, and the average relative standard deviation of repeat measurements was about 13% (n = 30).
    Journal of analytical toxicology 06/2007; 31(4):181-6. · 2.11 Impact Factor
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    ABSTRACT: We report population-based concentrations (stratified by age, sex, and composite race/ethnicity variables) of selective metabolites of chlorpyrifos (3,5,6-trichloro-2-pyridinol; TCPY), chlorpyrifos methyl (TCPY), malathion (malathion dicarboxylic acid; MDA), diazinon (2-isopropyl-4-methyl-6-hydroxypyrimidine; IMPY), methyl parathion (para-nitrophenol; PNP), and parathion (PNP). We measured the concentrations of TCPY, MDA, IMPY, and PNP in 1997 urine samples from participants, aged 6-59 years, of the National Health and Nutrition Examination Survey, 1999-2000. We detected TCPY in more than 96% of the samples tested. Other organophosphorus pesticide metabolites were detected less frequently: MDA, 52%; IMPY, 29%; and PNP, 22%. The geometric means for TCPY were 1.77 microg/L and 1.58 microg/g creatinine. The 95th percentiles for TCPY were 9.9 microg/L and 8.42 microg/g creatinine. The 95th percentiles for MDA were 1.6 microg/L and 1.8 microg/g creatinine. The 95th percentiles for IMPY and PNP were 3.7 microg/L (3.4 microg/g creatinine) and 5.0 microg/L (4.2 microg/g creatinine), respectively. Multivariate analyses showed that children aged 6-11 years had significantly higher concentrations of TCPY than adults and adolescents. Similarly, adolescents had significantly higher TCPY concentrations than adults. Although the concentrations between sexes and among composite racial/ethnic groups varied, no significant differences were observed.
    Environmental Research 12/2005; 99(3):314-26. · 3.24 Impact Factor
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    ABSTRACT: The ability to estimate low-dose human exposure to commonly used pesticides often is requested in epidemiologic studies. Therefore, fast and robust methods are necessary that can measure many analytes in the same sample. We have developed a method for high-throughput analysis of 19 markers of commonly used pesticides in human urine. The analytes were seven specific metabolites of organophosphorus pesticides, five metabolites of synthetic pyrethroids, six herbicides or their metabolites, and one insect repellant. Human urine (2 mL) was spiked with stable isotopically labeled analogues of the analytes, enzymatically hydrolyzed, extracted using solid-phase extraction, concentrated, and analyzed using high-performance liquid chromatography-tandem mass spectrometry. The sample was divided into two portions and analyzed on two different mass spectrometers, one using atmospheric pressure chemical ionization (APCI) and the other using turbo ion spray atmospheric pressure ionization (TIS). All analytes except the pyrethroid metabolites were analyzed using APCI. The detection limits for all analytes ranged from 0.1 to 1.5 ng/mL of urine, with the majority (17) below 0.5 ng/mL. The analytical precision for the different analytes, estimated as both the within-day and between-day variation, was 3-14 and 4-19%, respectively. The extraction recoveries of the analytes ranged from 68 to 114%. The throughput, including calibration standards and quality control samples, is approximately 50 samples a day. However, the analysis time with the TIS application is much shorter, and if only pyrethroid metabolite data are of interest, the throughput can be increased to 100-150 samples/day.
    Analytical Chemistry 06/2004; 76(9):2453-61. · 5.70 Impact Factor
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    ABSTRACT: Organophosphorus pesticides are commonly used in both agricultural and residential settings. The widespread use of these chemicals makes it almost impossible for humans to avoid exposure. In order to determine background human exposure, there is a need for fast, reliable, and sensitive analytical methods. We have developed a sensitive method to quantify specific biomarkers of the organophosphorus pesticides acephate, azinphos, chlorpyrifos, coumaphos, diazinon, isazofos, malathion, methamidophos, parathion and pirimiphos or their O,O-dimethyl analogues in human urine, as their selective metabolites or as the intact pesticide. Isotopically labeled internal standards were used for eight of the analytes. The use of labeled internal standards in combination with high-performance liquid chromatography electrospray ionization-tandem mass spectrometry provided a high degree of specificity. Repeated analysis of urine samples fortified with high and low concentrations of the analytes gave relative standard deviations (RSD) of less than 10% for the analytes with an isotopically labeled standard. Analytes without isotopically labeled standards had higher RSD. For all compounds except methamidophos and acephate, the recoveries were greater than 70%. The limits of quantification for most of the analytes were in the range of 0.1 to 1 ng/mL. We detected concentrations of most of these pesticides and/or their metabolites in urine samples from non-occupationally exposed persons using our method. Our frequencies of detection for the analytes measured ranged from 1% to 98%.
    Analytical and Bioanalytical Chemistry 08/2003; 376(6):808-15. · 3.66 Impact Factor