Changes In Chemical Concentrations Research Articles

Time-Resolved Microdialysis for In Vivo Neurochemical Measurements and Other Applications

Monitoring changes in chemical concentrations over time in complex environments is typically performed using sensors and spectroscopic techniques. Another approach is to couple sampling methods, such as microdialysis, with chromatographic, electrophoretic, or enzymatic assays. Recent advances of such coupling have enabled improvements in temporal resolution, multianalyte capability, and automation. In a sampling and analysis method, the temporal resolution is set by the mass sensitivity of the analytical method, analysis time, and zone dispersion during sampling. Coupling methods with high speed and mass sensitivity to microdialysis sampling help to reduce some of these contributions to yield methods with temporal resolution of seconds. These advances have been primarily used in monitoring neurotransmitters in vivo. This review covers the problems associated with chemical monitoring in the brain, recent advances in using microdialysis for time-resolved in vivo measurements, sample applications, and other potential applications of the technology such as determining reaction kinetics and process monitoring.

Fricke xylenol gel characterization using a photoacustic technique

Fricke chemical dosimetry measurements of the absorbed dose of ionizing radiation depend on the quality and characteristics of the system that reads each dosimeter. The final accuracy is significantly dependent on the technique used for measuring the chemical concentration changes in the dosimeters. We have used a photoacoustic technique to detect the Fricke xylenol gel (FXG) optical absorbance. The FXG, a derivation of the aqueous Fricke dosimeter, is made more sensitive and stable with addition of gelatin (300 Bloom) and xylenol orange. The light intensity transmitted through an FXG sample before and after irradiation was measured with an acoustic detector. The incremental optical absorbance is directly proportional to the ionizing radiation absorbed dose. We present the optical absorbance measurements as a function of absorbed dose and of post-irradiation time. We apply our photoacoustic technique to determine absorbed dose profiles. The results show that the photoacoustic technique applied to FXG provides a new dosimetric system, as good as those already established using spectrophotometric techniques.

The 8.2 ka event from Greenland ice cores

We present a collection of high-resolution chemistry and stable isotope records from the plateau of the Greenland ice cap during the cold event 8200 yr ago. Using a composite of four records, the cold event is observed as a 160.5 yr period during which decadal-mean isotopic values were below average, within which there is a central event of 69 yr during which values were consistently more than one standard deviation below the average for the preceding period. Four cores in north, south, and central Greenland show differences at decadal and shorter timescales; it is not yet clear if this represents significant spatial differences in response. The results show clear evidence for colder temperatures and a decrease in snow-accumulation rate. However, the changes in chemical concentrations for the ions looked at here are small, suggesting only minor changes in atmospheric circulation for this event. Apart from the decrease in methane concentration, Greenland ice cores give only weak evidence for effects outside the North Atlantic region.

Processes that account for the ozone maximum at the mesopause

The presence of a maximum in ozone density and mixing ratio in the mesopause region has been known for several decades although the measurement database is still quite limited. This ozone layer is formed in the vicinity of the maximum in atomic oxygen number density. In this paper, we present simulations of the ozone maximum from a three‐dimensional dynamical chemical model. Ozone variability is dominated by the diurnal cycle in ozone, which fluctuates between low concentrations in sunlight and high concentrations in darkness. However, the diurnal variability also has strong contributions from atmospheric tides and from slow changes in chemical concentrations following sunrise and sunset. In this study, we find that the magnitude of the ozone secondary maximum is closely tied to the temperature. The very low temperatures at the mesopause accelerate the formation of ozone and inhibit the loss. This factor and the location of the atomic oxygen density maximum both contribute in approximately equal measure to determining the altitude of the ozone layer. The magnitude of the nighttime ozone maximum is sensitive to the eddy and molecular diffusion rates primarily through the influence of these processes on the concentration of hydrogen.

Physical limits to biochemical signaling.

Many crucial biological processes operate with surprisingly small numbers of molecules, and there is renewed interest in analyzing the impact of noise associated with these small numbers. Twenty-five years ago, Berg and Purcell showed that bacterial chemotaxis, where a single-celled organism must respond to small changes in concentration of chemicals outside the cell, is limited directly by molecule counting noise and that aspects of the bacteria's behavioral and computational strategies must be chosen to minimize the effects of this noise. Here, we revisit and generalize their arguments to estimate the physical limits to signaling processes within the cell and argue that recent experiments are consistent with performance approaching these limits.

Comparison of two headspace solid phase microextraction fibres for the detection of volatile chemical concentration changes due to industrial processing of orange juice

AbstractSolid phase microextraction (SPME) with two polymeric coatings, polydimethylsiloxane (PDMS) and polyacrylate (PA), was used to isolate and quantitate orange juice volatile compounds from the headspace of fresh orange juice after the finishing, deaeration and pasteurisation processes. The results from the two fibres were largely consistent. Statistically significant changes in concentration due to the deaeration process were detected for medium‐volatility alcohols, hexanal and seven terpenes as measured by SPME‐PDMS. However, when using the PA‐coated fibre, more statistically significant changes in concentration were detected for aldehydes and esters. Alcohols and terpenes presented similar results using both polymeric coatings. The pasteurisation process did not modify the aromatic profile of the deaerated orange juice, except for methyl butyrate. These results indicate that the PA coating seemed to be more suitable for the analysis of the evolution of the orange juice aromatic fraction during industrial processing. Copyright © 2005 Society of Chemical Industry

Mechanistic Study of the Role of One-Component Resins in Rubber-to-Brass Bonding in Tires

Abstract Adhesion between rubber and brass-coated steel cords is enhanced by using resins as adhesion promoters. Experiments were carried out using a squalene liquid rubber modeling approach to study the effect of resins on the chemistry of the vulcanization reaction. The formation of new intermediates during vulcanization and changes in chemical concentrations with reaction time was studied using Gel Permeation Chromatography (GPC) analysis of the reacted squalene mixtures. Also, the effect of presence of resins on the surface of sulfidized brass cords was studied by analyzing the adhesion layer's elemental composition using the Electron Dispersive X-ray Spectroscopy (EDX) and Secondary Ion Mass Spectrometry (SIMS) characterization techniques. The changes in surface morphology of the adhesion layer with reaction time was noted by taking micrographs using the Scanning Electron Microscopy (SEM) technique. In this paper, a new mechanism is proposed for the role of resins in the improvement of initial and aged adhesion performance between rubber and brass-coated steel tire cords.

Interannual variability and trend of CH4 lifetime as a measure for OH changes in the 1979–1993 time period

The interannual variability and trend in the CH4 lifetime, as a measure for global mean OH concentration, have been analyzed systematically with three‐dimensional (3‐D) chemistry‐transport model simulations. It is shown that the global mean OH concentration is highly variable from year to year due to changes in meteorology, changes in tropospheric UV radiation intensities, and changes in chemical concentrations owing to variable emissions of photochemical precursor gases (CH4‐CO‐NMVOC‐NOx). The meteorological variability is taken into account with the ECMWF‐ERA15 1979–1993 reanalysis. Satellite observations provide the observed changes in the stratospheric ozone concentrations. Emission inventories are used to account for trends in anthropogenic emissions and their patterns. For the period 1979–1993, our simulations indicate a decrease of the calculated global tropospheric methane lifetime from 9.2 to 8.9 years, corresponding to a positive OH trend of 0.24 ± 0.06% yr−1. The modeled trend is mostly determined by changes in the tropical tropospheric water vapor content, while the changes in photolysis rates and in surface emissions of reactive trace gases compensate in their effect on the calculated OH trend over the analyzed time period.

Remediation of Petroleum Refinery Waste Pond Contents: A Case Study

The objectives of this field study were to (1) remediate the contents of a pond containing petroleum refinery waste products, and (2) characterize the performance of the remediation effort in terms of changes in chemical concentration, toxicity of the waste material, and mobility of the remediated waste constituents. The study was conducted in a field land treatment unit (LTU) in two phases over 140 weeks. Site remediation and closure decisions should be based on criteria that indicate that protection of human health and the environment has been achieved. Thus, criteria used to determine performance should include measures of chemical loss as well as measures of mobility (leaching) and toxicity. In this field study, changes in both general (oil and grease, total petroleum hydrocarbons) and specific (benzene, toluene, ethylbenzene and xylene compounds and polycyclic aromatic hydrocarbons) were determined. Several measures of chemical mobility and LTU soil toxicity also were used. The data indicated that the land treatment of this waste material reduced the concentrations of general and specific hydrocarbons, reduced the toxicity of the material being treated, and contained the waste constituents, that is, little to no vertical migration of the chemicals measured occurred. At the end of both field phases of the study, considerable amounts of general hydrocarbons remained in the LTU soil, as indicated by total petroleum hydrocarbons, oil and grease, and total polycyclic aromatic hydrocarbon compound measurements. However, there were little chemical mobility and relative toxicity of the soil at the end of these phases.

Fate of Treated and Weathered Hydrocarbons in Soil—Long-Term Changes

Information about the loss and mobility of anthropogenic chemicals in soils over long periods of time—years to decades—was evaluated, with the purpose of providing a perspective on the extent to which such chemicals remain sequestered over time. Such information is needed to help ensure that decisions made to leave concentrations of chemicals in soil—based on short-term chemical availability data—are environmentally sound and protective. The information evaluated was obtained from field studies that considered changes in chemical concentrations over extended periods of time. The chemicals evaluated were petroleum hydrocarbons, polynuclear aromatic hydrocarbons (PAH), and pentachlorophenol (PCP). The sites considered were those where decisions had been made to let such organic chemicals remain in the soil after some degree of treatment or weathering. The evaluation indicated that only a few long-term studies provide adequate data to identify what happens to weathered or treated hydrocarbons, PCP, and PAH that are allowed to remain at a site. However, for the examples evaluated, the initial chemical mobility was low and there were no indications that the residual hydrocarbons, PCP, or PAH became more mobile over time. In addition, the data indicated that there was continued slow degradation over time. The results suggest that the current measures of chemical release and mobility, even though they may measure only short-term effects, may be reasonable indicators of the long-term effects.

Chemical mapping of anxiety in the brain of healthy humans: an in vivo 1H-MRS study on the effects of sex, age, and brain region.

We recently presented results in an in vivo study of human brain chemistry in 'physiologic' anxiety, i.e., the anxiety of normal everyday life. Normal subjects with high anxiety demonstrated increased concentration of chemicals in orbital frontal cortex (OFC) as compared to lower anxiety. In a separate study of aging we demonstrated a decrease of total chemical concentration in OFC of middle-aged subjects, as compared with younger age. This brain region also showed gender dependence; men demonstrating decreased chemical concentration compared to women. We hypothesized that these sex- and age-dependent differences in OFC chemistry changes are a result of anxiety effects on this brain region. In the present study we examined these sex- and age-differential regional brain chemistry changes (as identified by localized in vivo proton magnetic resonance spectroscopy [1H-MRS]) in relation to the state-trait-anxiety (as measured by the State-Trait Anxiety Inventory) in 35 healthy subjects. The concentrations for all nine chemicals of 1H-MRS spectra were measured relative to creatine across multiple brain regions, including OFC in the left hemisphere. Analysis of variance showed anxiety-specific effects on chemical concentration changes in OFC, which were different for both sexes and age groups. Male subjects showed larger effect of anxiety on OFC chemistry as compared to females when the same sex high-anxiety subjects were compared to lower anxiety. Similarly, middle-aged subjects showed larger effect of anxiety on OFC chemistry as compared to younger age when the same age subjects with high anxiety were compared to lower anxiety. Largest effect of anxiety on OFC chemistry was due to changes of N-Acetyl aspartate. The results indicate that the state-trait anxiety has sex- and age-differential patterns on OFC chemistry in healthy humans, providing new information about the neurobiological roots of anxiety.

Real-time amperometric measurements of zeptomole quantities of dopamine released from neurons.

Amperometry with carbon-fiber microelectrodes provides a unique way to measure very small chemical concentration changes at the surface of biological cells. In this work, an investigation of dopamine release from individual neurons isolated from the mouse retina is described. The mice were genetically modified so that, in cells that expressed the protein responsible for catecholamine synthesis, tyrosine hydroxylase, the marker protein, placental alkaline phosphatase, was also expressed. This modification allowed for identification of the dopamine-containing cells among the many present in the freshly dissociated retina. Release of dopamine was evoked by chemical secretagogues delivered from micropipets that were calibrated with respect to response time and concentration delivered. Amperometric measurements were recorded with low-noise patch clamp amplifiers, and the primary noise source was found to be the electrode capacitance. Dopamine release occurred in the form of transient concentration spikes, consistent with release from small intracellular vesicles. With optimized filtering of the data, the quantity secreted during each release event could be determined. The average quantity determined at one cell was 52 zmol. However, the spikes were quite variable in size and the amount released per event ranged from 8 to 170 zmol. These measurements allow an estimation of the concentration of released transmitter in a synapse.

The international campaign on intercomparison between electrodes for geoelectrical measurements

In many applications of electrical geophysics, the electrodes used to detect signal seem to be an important source of noise. To identify which electrodes were most stable for electric field measurements; an international campaign to evaluate electrodes for long-term monitoring of the telluric field was held at Garchy, France from April 1995 to April 1996. More than fifty different electrode pairs from researches around the world were tested in the campaign. Most of electrodes were metal-electrolyte designs, but with different construction details. The inter-comparison was carried out through laboratory and field experiments. The results of the experiment showed a surprising variation between the performances of different electrodes. Stability as determined in a brief laboratory experiment proved to be an excellent predictor of long-term field stability. Instabilities of differing qualitative character were observed during the year-long experiment. Linear drift may be caused by gradual changes in ground contact, leaking of electrolyte, or changes in chemical concentration of electrolyte within the electrodes. Sudden onset transient noise, which could be as large as 20 mV, is likely to be an electrochemical process such as reaction charge release and charge accumulation within the electrodes. Other physical causes of drift could be due to mechanical contact problems. Of metal electrodes, stainless steel is by far the best.The conclusions from the Garchy experiment are relevant to exploration geophysics: self-potential surveys are strongly affected by the long-term stability of electrodes and some electrodes may also have internal noise exceeding 1 mV per Hz in the frequency band around 1✓Hz, thus potentially affecting measurements of Induced Polarisation data.

A FEASIBILITY STUDY OF DIRECT INJECTION FOR VARIABLE-RATE HERBICIDE APPLICATION

Variable-rate application of herbicides based on soil properties and crop yield potential may be a viableapproach to reducing the quantities of herbicides used in crop production. Application accuracy of direct injection equipment is an important issue for variable-rate application. A simulation model was developed using SLAM II to assess the performance of a direct injection sprayer used for site-specific application of preemergence herbicides in corn. Field data from a university research farm were used as input to the model (i.e., soil texture, organic matter content, and yield potential). Several factors such as in-line mixing location, hose diameter, nozzle spacing and size, and ground speed were investigated for their effects on system performance. A complement of system parameters was identified to reduce application errors. Multiple system errors were combined using the mean square error approach. Results of the simulation were input to a GIS software to generate herbicide application rate maps and the corresponding error rate maps. Application errors for direct injection systems were as high as 40% for mistreated areas of the field, with changes in chemical concentration at the remote nozzles occurring after as much as 80 m of travel past the point of a step change of the input command to the controller. Recommendations for reducing delay times at the nozzle include moving the in-line mixing location close to the boom and using smaller line sizes to reduce line volume, without appreciably increasing pressure drop.

Changes in Toxicity and Mobility Resulting from Bioremediation Processes

Bioremediation is a commonly used process for the remediation of soils and sludges containing hydrocarbon compounds. The extent of chemical concentration reduction that occurs in bioremediation processes and the concentration of residual chemicals varies widely for different soils and sludges and for different processes. Along with changes in chemical concentration, measures of toxicity and chemical mobility are important information as site remediation decisions are increasingly being made within a risk-based corrective action framework. This review article presents illustrative data from studies that evaluated the effectiveness of bioremediation processes and that contained information about changes in chemical mobility and soil or sludge toxicity. The weight-of-evidence data presented indicated that, as part of the bioremediation process, there is a reduction of the apparent toxicity of the soils and sludges that were treated. In addition, remaining chemical constituents generally were less mobile. The patterns were consistent for both laboratory and field-scale bioremediation studies.

AN INTERPRETATION OF SOME OBSERVATIONS ON KARST SPRING REGIMES AND THEIR IMPLICATION FOR THE UNDERGROUND DRAINAGE SYSTEMS IN THE LHASA AREA, TIBET

Tibet is an arid and cold plateau and the springs which occur there have rarely been studied. The chemical concentration, hydrograph and other physical behavior of the karst springs around the Lhasa area have been analysed in order to understand the underground karst development in this area. The springs on Mt. West located to the West of Lhasa have a unified water table and their discharge, lag-time and chemical concentration changes depend on the hydrological zones in which they are located and their regimes mainly respond to rainfall. The aquifer of the Jiala warm spring has the same hydrogeological structure, but its water source comes from the meltwater and its discharge changes are controlled by surface temperature variations, with a stagnant period ranging from 3 months to 10 days. The Quesang hot spring is also supplied by meltwater but with a half year lag-time because of long distance from the source. A cold spring located at the same point as the hot spring reflects the fact that they individually have their independent drainage systems in the same limestone mass, just like some other karst springs in the world. The occurrence of conduit flows is not corespondent with the present-day karst development. Thus such conduits in limestone terrain might be the inherited development of old karst.

Evaluation of Potential Contamination of Groundwater Following Liquid Manure Application on Grassland in Newfoundland, Canada

Abstract An experimental site representative of Newfoundland soil and climate conditions has been instrumented and investigated for possible groundwater contamination due to the disposal of liquid waste from farming operations. The data obtained from field observations and laboratory analysis for the 1992-93 period have been interpreted using non-parametric statistical methods. Background concentration levels and individual well signatures have been defined and used for subsequent detection of changes in chemical concentrations of selected contaminants. Present farming disposal practices as applied on this site, typical of climate and soil conditions in Newfoundland, Canada, seem to have no significant impact on groundwater quality.

Chemical compositions of ultrafine lamellae in the water-quenched Ti-48Al alloy

As Ti rich Ti-Al alloy shows ductile nature with the α2 + γ lamellar structure [1], the mechanism of microstructural evolution in Ti-Al alloys has attracted much interest. Since the lamellar structure is formed by the continuous cooling in industrial processes, cooling rate dependence of α → γ and α → α2 + γ transformations has been extensively studied by many researchers [2-5]. Wang et al. [2,3] reported that α transformed to γ massively, when Ti-48Al alloy was water-quenched from the single phase α. As a nature of the massive transformation, they reported that there was no detectable difference in composition between massively transformed γ (hereafter, denoted as γm) and α2 phase, where α2 is a result of ordering of untransformed α phase. Jones and Kaufman [4] constructed continuous cooling transformation (CCT) diagrams for Ti-Al with various concentrations and suggested that lamellar microstructure occurred when cooling rate was slow and massive γm is formed when cooling rate was fast. Since massive transformation is nucleated at grain boundaries, α can be fully transformed to γm massively when grain size is small. However, Kumagai et al. [6] reported that massive reaction does not complete when Ti-48Al was solution treated for an extended period of time. This is attributed to larger size of α grains which has grown during the solution heat treatment. In these untransformed regions, they found extremely fine lamellar of α2 + γ with an orientation relationship {111}γ//{0001}α2 and γ / / α2 . This lamellar microstructure is very unusual in view of the following two points: (1) the lamellar distance is extremely fine with an order of 5 to 10 nm, and (2) the α2 thin lamellae are completely reverted to γ lamellae upon annealing at 1273 K. Based on these observations, Kumagai et al. concluded that the fine lamellar structure observed in the untransformed region in the water-quenched Ti-48Al alloy is a precursor phase to the γm. Based on the ultrafine scale of the lamellar structure, Abe et al. [7] suggested that the ultrafine lamellae of α2 + γ are formed by a displacive (martensitic) transformation without change in chemical compositions. However, in this study, they did not determine the chemical compositions in the lamellae, and no evidence to prove the displacive transformation mechanism was provided. Hence, this study aims to measure the local chemical concentration changes across the fine lamellae directly and to discuss the mechanism of microstructural evolution of the ultrafine lamellae in water-quenched Ti-48at.%Al alloy. For local concentration determination of the ultrafine lamellae, a unique microanalytical technique with an atomic spatial resolution, atom probe field ion microscopy (APFIM), was employed.

SIGNAL TRANSDUCTION PATHWAYS INVOLVING PROTEIN PHOSPHORYLATION IN PROKARYOTES

Bacteria are capable of sensing a wide variety of environmental signals, including changes in chemical concentrations, the presence of a host organism, or variation in physical parameters such as temperature, osmolarity, viscosity, or light. One response to changing conditions is to move to a more favorable locale; changes in locomotive behavior can be observed less than one second after a change in chemical composition of the medium. Another possible course of action is to adapt the cell to the new environment, either by changing enzyme activity or by altering expression of specific genes or groups of genes. The bacterium may use the modified enzymes or new gene products to adjust to its surroundings temporarily, or to establish a new long-term state (e.g. the sporulation response to starvation).

Measurement of bacterial random motility and chemotaxis coefficients: I. Stopped‐flow diffusion chamber assay

Bacterial chemotaxis, the directed movement of a cell population in response to a chemical gradient, plays a critical role in the distribution and dynamic interaction of bacterial populations in nonmixed systems. Therefore, in order to make reliable predictions about the migratory behavior of bacteria within the environment, a quantitative characterization of the chemotactic response in terms of intrinsic cell properties is needed.The design of the stopped-flow diffusion chamber (SFDC) provides a well-characterized chemical gradient and reliable method for measuring bacterial migration behavior. During flow through the chamber, a step change in chemical concentration is imposed on a uniform suspension of bacteria. Once flow is stopped, diffusion causes a transient chemical gradient to develop, and bacteria respond by forming a band of high cell density which travels toward higher concentrations of the attractant. Changes in bacterial spatial distributions observed through light scattering are recorded on photomicrographs during a 10-min period. Computer-aided image analysis converts absorbance of the photographic negatives to a digital representation of bacterial density profiles. A mathematical model (part II) is used to quantitatively characterize these observations in terms of intrinsic cell parameters: a chemotactic sensitivity coefficient, chi(0), from the aggregate cell density accumulated in the band and a random motility coefficient, mu, from population dispersion in the absence of a chemical gradient.Using the SFDC assay and an individual-cell-based mathematical model, we successfully determined values for both of these population parameters for Escherichia coli K12 responding to fucose. The values obtained were mu = 1.1 +/- 0. 4 x 10(-5) cm(2)/s and chi(o) = 8 +/- 3 +/- 10(-5) cm(2)/s. We have demonstrated a method capable of determining these parameter values from the now validated mathematical model which will be useful for predicting bacterial migration in application systems.