Biomedical subjects

Thanh H Nguyen

Publications and source records attributed to Thanh H Nguyen.

7 recordsLinked to original sources

PubMed2026

A quantitative microbial risk assessment of activated carbon point-of-use filters and the risk of Legionella pneumophila.

Activated carbon point-of-use (POU) filters have become increasingly popular. Often certified to remove lead and improve water taste, these filters are not certified to remove bacteria. In this study, we examined the effects of using POU filters on the risk posed by Legionella pneumophila (L. pneumophila) in a quantitative microbial risk assessment model. Release rates of L. pneumophila were measured for new and artificially aged (ripened) POU filters. Additionally, we measured aerosol concentrations produced by water fixtures that might have a POU filter (e.g., drinking fountain bubbler, faucet). We modeled scenarios without filters (with and without residual chlorine) and with filters (new and ripe filters), four water usage activities (drinking from a drinking fountain, handwashing, dishwashing, and showering), and three age groups (children, adults, and elderly). Systems with the highest to lowest risk were those without filters and without chlorine, those with ripe filters, those without filters and with chlorine, and finally, those with new filters. The risk of illness decreased for activities with shorter exposure times, such as drinking and handwashing. The L. pneumophila concentrations, aerosol concentrations, and exposure time had the largest impacts on the risk of illness. The maximum allowable concentration of L. pneumophila needed to achieve a 10-4 target annual risk of illness was 0.02 copies/mL. This study showed that POU filters may reduce L. pneumophila risk but only if the filter is new (≤ 1 week old before the filter ripens significantly), and other strategies for reducing L. pneumophila concentrations or decreasing exposure time may be needed to further decrease the risk of illness.

Legionella pneumophila↗

PubMed2007

Plasmid DNA adsorption on silica: kinetics and conformational changes in monovalent and divalent salts.

A quartz crystal microbalance with dissipation (QCM-D) is used to determine the adsorption rate of a supercoiled plasmid DNA onto a quartz surface and the structure of the resulting adsorbed DNA layer. To better understand the DNA adsorption mechanisms and the adsorbed layer physicochemical properties, the QCM-D data are complemented by dynamic light scattering measurements of diffusion coefficients of the DNA molecules as a function of solution ionic composition. The data from simultaneous monitoring of variations in frequency and dissipation energy with the QCM-D suggest that the adsorbed DNA layer is more rigid in the presence of divalent (calcium) cations compared to monovalent (sodium) cations. Adsorption rates are significantly higher in the presence of calcium, attaining a transport-limited rate at about 1 mM Ca2+. Results further suggest that in low ionic strength solutions containing 1 mM Ca2+ and in moderately high ionic strength solutions containing 300 mM NaCl, plasmid DNA adsorption to negatively charged mineral surfaces is irreversible.

Adsorption↗

PubMed2006

Absorption and adsorption of hydrophobic organic contaminants to diesel and hexane soot.

Soot particles vary in pore structure, surface properties, and content of authigenic (native) extractable organic chemicals. To better understand the effects of these properties on sorption, aqueous sorption isotherms for 14C-labeled phenanthrene and 1,2,4-trichlorobenzene were obtained for four soots of varying properties: two diesel reference soots, a hexane soot, and an ozonated hexane soot. Substantial isotherm nonlinearity was observed. In comparison to diesel soot SRM 2975, diesel soot SRM 1650b had a much higher content of extractable authigenic organic chemicals, showed less sorption of 14C-labeled sorbate at low relative concentrations (Ce/Sw), and showed higher sorption at high Ce/Sw. In comparison to normal hexane soot, the ozonated hexane soot had a higher surface O/C ratio and showed substantially less sorption at all concentrations studied. The sorption differences were attributed to the noted differences in properties, and results were interpreted through a dual-mode sorption model that included the possibility of both surface adsorption (modeled using a Polanyi-based approach) and simple phase partitioning (linear absorption). Generally, such modeling indicated that overall uptake at low concentrations in all four soots was dominated by surface adsorption but that sorption at higher sorbate concentrations in SRM 1650b was heavily influenced by linear absorption within the natively bound organic phase.

Absorption↗

PubMed2005

Immobilization of soot particles in a silica matrix: A sorbent-carrier system for studying organic chemical sorption.

A new method for studying sorption with diesel and hexane sootwas developed, tested, and applied. A commercial silica-based chromatography medium was used as an inert inorganic carrier for immobilization (entrapment) of soot particles and their aggregates, thus creating a combined sorbent for sorption of hydrophobic organic chemicals (HOCs). After precombustion to remove potential organic carbon contaminants, the silica particles and soot samples were mixed under dry conditions that allowed the soot to be incorporated within the pore structure of the much larger (> 180 microm) carrier particles. Unincorporated soot was removed by multiple rinses with Milli-Q water. Sorption rate and equilibrium experiments were conducted, using phenanthrene as a probe HOC. Strong nonlinear sorption of phenanthrene was observed, in agreement with results previously obtained using air-bridge and flocculation-based methods. Batch kinetic studies suggested that 60 d of prewetting is required to obtain full water saturation, as perhaps needed for proper assessment of phenanthrene uptake rate by soot in aqueous systems. Forthe determination of equilibrium phenanthrene sorption, however, 1-d prewetting is sufficient so long as final equilibration is for at least 60 d. The new method is a practical approach to sorption measurement that may prove especially useful for study of strongly sorbing chemicals.

Adsorption↗

PubMed2005

Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments.

Values of the organic-carbon-based partition coefficient (Koc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log Koc and log Kow, where Kow is the octanol-water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however,these methods are notsufficientlyaccurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log Koc for 75 chemicals, including apolar, monopolar, and bipolar compounds. The paradigm of representing SOM by a single pp-LFER is qualitatively supported by our results, and the set of coefficients for the regression log Koc= (1.10+/-0.10)E - (0.72+/-0.14)S + (0.15+/-0.15)A - (1.98+/-0.14)B + (2.28+/-0.14)V + (0.14+/-0.10) represents a proposed set of water-SOM-specific properties for estimating log Koc. The developed correlation outperformed other currently recommended approaches with the given Koc data set and also compared favorably against the use of new multiple class-specific op-LFER regressions. Overall, the pp-LFER approach is recommended over other current methods for the purpose of Koc estimation and especially for polar chemicals.

Environmental Pollutants↗

PubMed2004

Sorption nonlinearity for organic contaminants with diesel soot: method development and isotherm interpretation.

An experimentally practical and precise flocculation-based method was developed, tested, and applied to determine phenanthrene and 1,2,4-trichlorobenzene sorption with NIST SRM 2975 diesel particulate matter. Following an initial equilibration period, polyaluminum chloride (PACI) solution was added to the sorption tubes in order to facilitate the formation of flocculated aggregates of soot particles. After separation of the solids through centrifugation, supernatant concentrations were determined as with conventional batch methods. The flocculation-based method was tested on three kinds of soot and then used to evaluate sorption kinetics and equilibrium with SRM 2975. Kinetic results showed that wetting of the soot required more than 20 days, but that 60 days was sufficient to achieve equilibration with both water and phenanthrene. Sixty-day isotherms for both phenanthrene and 1,2,4-trichlorobenzene were strongly nonlinear. At approximate 10(-3) of solubility, carbon-normalized distribution coefficients (Koc) were 10-20 times higher than those for absorption to sediment organic matter. Measurements at closer to solubility indicated much lower Koc, suggesting a total sorption capacity at aqueous solubility that is of similar magnitude to that in sediment organic matter. Independent analysis of extractable hydrocarbons suggests that absorption into a native hydrocarbon phase was not a major component of sorption.

Adsorption↗

PubMed2003

Optimizing the patient positioning for PICC line tip determination.

Central venous catheters (CVCs) are used for both emergent and long-term vascular access for the infusion of numerous therapeutic agents such as chemotherapy, parenteral nutrition, antibiotics, and analgesics, as well as for temporary hemodialysis or hemoperfusion. Current standard of care dictates that CVC insertion should be followed by an immediate chest radiograph to confirm appropriate position. Radiographic confirmation of central venous line placement is important because it is not possible to determine CVC tip position clinically. Although many catheter tips can be localized on the standard frontal radiograph, there are occasions when a second radiograph is necessary to localize the position of the CVC tip accurately. We hypothesized that a right posterior obligue chest radiograph would more consistently enable the catheter tip to be seen as it reduces the superimposition of mediastinal structures. One hundred chest radiographs taken in an anteroposterior (AP) projection and 100 chest radiographs taken in a right posterior oblique (RPO) projection after a peripherally inserted central catheter (PICC) line placement at UCI Medical Center from June 2000 to November 2002 were read by two radiologists. Forty-one percent of AP readings were discrepant and 4% had the annotation "difficult to identify the position of the tip" although the identification of tip position was similar. Fifty-five percent of AP readings were in agreement with no note of any difficulty. Eighteen percent of RPO readings were discrepant and 2% had the annotation "difficult to identify the position of the tip" although the identification of tip position was similar. Eighty percent of RPO readings were in agreement with no note of any difficulty.

Catheterization, Central Venous↗