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Biomedical subjects

D E Armstrong

Publications and source records attributed to D E Armstrong.

10 recordsLinked to original sources
PubMed2001

Partitioning of total mercury and methylmercury to the colloidal phase in freshwaters.

Using tangential flow ultrafiltration, total mercury (HgT) and methylmercury (MeHg) concentrations in the colloidal phase (0.4 microm-10 kDa) were determined for 15 freshwaters located in the upper Midwest (Minnesota, Michigan, and Wisconsin) and the Southern United States (Georgia and Florida). Unfiltered concentrations were typical of those reported for freshwater and ranged from 0.9 to 27.1 ng L(-1) HgT and from 0.08 to 0.86 ng L(-1) MeHg. For some rivers, HgT and MeHg in the colloidal phase comprised up to 72% of the respective unfiltered concentration. On average, however, HgT and MeHg concentrations were evenly distributed between the particulate (>0.4 microm), colloidal, and dissolved (<10 kDa) phases. The pool of Hg in the colloidal phase decreased with increasing specific conductance. Results from experiments on freshwaters with artificially elevated specific conductance suggest that HgT and MeHg may partition to different subfractions of colloidal material. The colloidal-phase HgT correlation with filtered organic carbon (OC(F)) was generally poor (r2 < 0.14; p > 0.07), but the regression of MeHg with OC(F) was strong, especially in the upper Midwest (r2 = 0.78; p < 0.01). On a mass basis, colloidal-phase Hg concentrations were similar to those of unimpacted sediments in the Midwest. Mercury to carbon ratios averaged 352 pg of HgT/mg of C and 25 pg of MeHg/mg of C and were not correlated to ionic strength. The log of the partition coefficient (log K(D)) for HgT and MeHg ranged from 3.7 to 6.4 and was typical of freshwater values determined using a 0.4 microm cutoff between the particulate phase and the dissolved phase. Log K(D) calculated using the <10 kDa fraction as "dissolved" ranged from 4.3 to 6.6 and had a smaller standard deviation about the mean. In addition, our data support the "particle concentration effect" (PCE) hypothesis that the association of Hg with colloids in the filter-passing fraction can lower the observed log K(D). The similarity between colloidal and particulate-phase partition coefficients suggests that colloidal mass and not preferential colloidal partitioning drives the PCE.

Carbon↗
PubMed2001

Determination of silver speciation in natural waters. 1. Laboratory tests of Chelex-100 chelating resin as a competing ligand.

Batch and column experiments were performed to investigate the suitability and chemical characteristics of Chelex-100 for use as a competing ligand in ionic silver (Ag(I)) speciation determinations in natural waters. A conditional stability constant (Kcond) for Ag+ chelation by iminodiacetate groups on the surface of Chelex resin was determined by fitting results of batch and column experiments with an equilibrium speciation model. Results of experiments in which Chelex competed with cyanide ion and thiosulfate ion for aqueous Ag+ were fitted well by a model in which log Kcond(Ag-Chelex) was set to 7.2. This value is similar to literature equilibrium constants for a 1:1 Ag(+)-EDTA chelate. In batch experiments with Chelex, equilibration times of 24 h were found to be sufficient to bring samples close to equilibrium. Effects of resin counterion and total Ag(I) concentration on extent of Ag(I) chelation were found to be minor. Effect of pH on Ag(I) chelation was minor over a range of 6-10. Column experiments (detention time = 6 s, empty-column basis) in which thiosulfate competed with Chelex for Ag(I) gave similar results to batch experiments with thiosulfate. This implies that batch and column experiments could be compared to explore ligands in natural water systems with different rates of dissociation.

Chelating Agents↗
PubMed2001

Determination of silver speciation in natural waters. 2. Binding strength of silver ligands in surface freshwaters.

Three competing ligand methods were compared to determine characteristics of Ag(I) complexation by dissolved and colloidal ligands present in three rivers and one sewage treatment plant effluent. Iminodiacetate groups on Chelex-100 resin (used in batch and column experiments) and diethyldithiocarbamate (DDC) were used as competing ligands. Results of batch Chelex and DDC competition experiments show good agreement with regard to relative extent of Ag binding by natural ligands among the three river systems. Results of both methods also show a possible correlation between extent of Ag(I) complexation and organic matter concentration and/or Fe concentration. Fraction of Ag(I) associated with Chelex in both batch and column Chelex experiments was similar in each of the four systems tested, indicating that lability of Ag complexes does not change significantly on time scales ranging from seconds to 24 h. Results of Chelex and DDC competition were compared using a model based on a hypothetical single natural ligand. Under the experimental conditions used, this model quantified Ag(I) complexes with log Kcond values from approximately 12 to 14. For the three rivers studied, ligands with silver-association characteristics similar to those of reduced sulfur groups (log K = 14-16) present at subnanomolar concentrations likely dominate Ag(I) speciation in these systems. A weaker ligand (e.g., log Kcond < 12) at concentrations > 0.7 nM dominated Ag(I) speciation in the treatment plant effluent. This may result from elevated concentration of metals that compete for reduced sulfur groups rather than from a lower total concentration of these groups.

Chelating Agents↗
PubMed1986

The effects of diazepam on "fear" reactions in rats are modulated by environmental constraints on the rat's defensive repertoire.

The simultaneous effects of diazepam on three shock-induced reactions in rats were studied in order to determine the reliability of these behaviors as indices of anxiolytic drug action. Rats were injected with 1 mg/kg of diazepam or vehicle, placed in a 2-compartment chamber containing bedding material, and shocked with 1, 2, or 6 mA when they first touched a wire-wrapped prod attached to one end of the chamber. Diazepam-treated animals displayed significantly less burying behavior, but paradoxically, they also displayed more passive avoidance behavior and fewer exploratory side-transitions than vehicle-injected controls. Defensive burying behavior tended to be negatively correlated with passive avoidance behavior and positively correlated with exploratory side transitions. When the "competitive" relationship between defensive burying and passive avoidance was eliminated by testing rats in a 2-compartment chamber not containing bedding material, diazepam produced a significant suppression of passive avoidance and a significant increase in exploratory side-transitions, compared to control. Taken together, these results suggested that the validity of any single behavioral model of anxiolytic drug action might vary as a function of environmental constraints on the subjects' defensive repertoire.

Animals↗
PubMed1975

A simplified phosphorus analysis technique.

A simplified method for the analysis of total P(TP), total dissolved P(TDP) and dissolved reactive P (DRP) in multiple water samples has been developed. The reported modification utilizes a single digestion reagent and a single "mixed reagent" to eliminate neutralization, transfer and dilution steps normally required in total P analyses. The method results in a 70 percent decrease in analysis time for multiple samples, a reduction in the glassware required, and about a 30 percent increase in sensitivity. The method can be directly applied to water samples with P levels of 2 to 1100 ug P/l. Reproducibility and precision measurements compare to or exceed commonly used P techniques.

Indicators and Reagents↗
PubMed1971

Adenosine Triphosphate Content of Selenastrum capricornutum.

The adenosine triphosphate content of Selenastrum capricornutum was maintained within 1.4 to 3.4 mug of adenosine triphosphate/mg (dry weight) of living biomass at different growth stages in media of different phosphorus concentrations.

Journal Article↗
PubMedDate not supplied

Trace metal concentrations in shallow ground water.

Trace metal clean sampling and analysis techniques were used to examine the temporal patterns of Hg, Cu, and Zn concentrations in shallow ground water, and the relationships between metal concentrations in ground water and in a hydrologically connected river. Hg, Cu, and Zn concentrations in ground water ranged from 0.07 to 4.6 ng L-1, 0.07 to 3.10 micrograms L-1, and 0.17 to 2.18 micrograms L-1, respectively. There was no apparent seasonal pattern in any of the metal concentrations. Filtrable Hg, Cu, and Zn concentrations in the North Branch of the Milwaukee River ranged from below the detection limit to 2.65 ng Hg L-1, 0.51 to 4.30 micrograms Cu L-1, and 0.34 to 2.33 micrograms Zn L-1. Thus, metal concentrations in ground water were sufficiently high to account for a substantial fraction of the filtrable trace metal concentration in the river. Metal concentrations in the soil ranged from 8 to 86 ng Hg g-1, 10 to 39 micrograms Cu g-1, and 15 to 84 micrograms Zn g-1. Distribution coefficients, KD, in the aquifer were 7900, 22,000, and 23,000 L kg-1 for Hg, Cu, and Zn, respectively. These values were three to 40 times smaller than KD values observed in the Milwaukee River for suspended particulate matter.

Environmental Monitoring↗