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Dive into the research topics where Margaret H. Back is active.

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Featured researches published by Margaret H. Back.


Environmental Science & Technology | 1994

Kinetic studies of metal speciation using chelex cation exchange resin : application to cadmium, copper, and lead speciation in river water and snow

Chuni L. Chakrabarti; Yanjia Lu; D.C. Grégoire; Margaret H. Back; William H. Schroeder

Kinetic studies of the rate of uptake of the metal ions Cd, Cu, and Pb by the Chelex batch technique have been made on model solutions containing the metal ions and the complexants EDTA, NTA, and fulvic acid. The metal ions were quantitated by inductively coupled plasma mass spectrometry, and the rates of uptake of the metal ions were analyzed by the iterative deconvolution method. In the presence of an excess of EDTA or NTA, all the metal ion was complexed with the chelating agent, forming a slowly dissociating complex. In the presence of fulvic acid, these metals formed strongly bound complexes with a range of slow dissociation rates. The results reveal the importance of [fulvic acid]/[metal ion] ratio and, hence, the extent of occupation of binding sites in fulvic acid (a polyfunctional complexes


Analytica Chimica Acta | 2003

Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Jeffrey W. Guthrie; Rupasri Mandal; Mohamed S. A. Salam; Nouri M. Hassan; J. Murimboh; Chuni L. Chakrabarti; Margaret H. Back; D.C. Grégoire

Abstract Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)–HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)–HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)–HA complexes decreased, and the proportion of free Ni2+ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)–HA complexes was slower than that of Ni(II)–FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.


Journal of Analytical Atomic Spectrometry | 1996

Speciation of some metals in river surface water, rain and snow, and the interactions of these metals with selected soil matrices

J. Y. Lu; Chuni L. Chakrabarti; Margaret H. Back; A. L. R. Sekaly; D. C. Gregoire; W. H. Schroeder

Some metal species in river surface water, rain and snow were characterized by size and molecular weight, using filtration and ultrafiltration, and by their chemical reactivity on Chelex cation-exchange resin using both column and batch techniques. Trace metal species were determined in samples of atmospheric precipitation, and wet-deposition of trace metal species through runoff into the Rideau river (Ottawa, Ontario, Canada) was estimated by determining the concentrations of metal species in the rain water before and after contact with selected soils in the laboratory. Two types of soils, Plainfield Sandy soil and Green Belt soil (Ottawa, Ontario, Canada), were used as models for studying the transformation of metal species from rain water to soil. Aluminium, Cd, Cu, Ni, Pb and Zn complexes in the samples of rain and snow were found to have relatively low molecular weight ( 3000). Kinetic studies of the metal speciation revealed that the metal species in the sample of snow were similar to those in the model solutions studied earlier by the present authors, and were probably complexes of inorganic ligands or small organic ligands, and that the metal species in the Rideau river surface water had low dissociation rates and were probably metals bound to strong binding sites of humic compounds and/or bound to or trapped in colloidal materials. Most metal species in the sample of rain water were transformed from simple, small and labile species originally present in the rain water to larger, non-labile species (the metals were probably bound to humic compounds, and/or colloidal materials) after the rain water was equilibrated with the soils investigated.


Analytica Chimica Acta | 1999

Effect of the competition of copper and cobalt on the lability of Ni(II)–organic ligand complexes. Part I. In model solutions containing Ni(II) and a well-characterized fulvic acid

Rupasri Mandal; Amina L.R. Sekaly; J. Murimboh; Nouri M. Hassan; Chuni L. Chakrabarti; Margaret H. Back; D.C. Grégoire; William H. Schroeder

Abstract The competitive binding of Cu(II), Co(II) and Ni(II) ions by a well-characterized fulvic acid (FA) in model aqueous solutions has been investigated by employing the competing ligand exchange method (CLEM) with Chelex-100 as the competing ligand. The reaction rate between a metal ion and a ligand is dependent on the rate of exchange of the coordinated water, which varies greatly for the above metals. Fulvic acid is a polyfunctional, polyelectrolytical, physically-heterogeneous, organic complexant having binding sites that can be roughly classified into two categories: minor (∼1–10%), strong sites, and major (∼99–90%), weak sites. The strong binding sites are first occupied, and after all the strong binding sites are occupied, the weak binding sites are occupied. Experiments have been done using model aqueous solutions containing various concentrations of Cu, Co and Ni, and the above well-characterized FA. Graphite furnace atomic absorption spectrometry and inductively-coupled plasma mass spectrometry were employed to monitor the rate of uptake of the metals by Chelex-100. The above metals, in the metals/FA mole ratios used in this study, have been found to compete with one another for the relatively small number of strong binding sites of the FA, and Cu(II) and Co(II) which have much greater rate constant for water exchange than that of Ni(II) win the competition over Ni(II). In the absence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Ni(II), forming strong complexes, which are inert, whereas, in the presence of Cu(II) and Co(II), the strong binding sites of the FA are occupied by Cu(II); Co(II), Ni(II), and the remaining Cu(II) occupy the weak binding sites of the FA, forming weak complexes, which are labile. The enhanced lability of the Ni–FA complexes in the presence of Cu(II) and Co(II) indicates that Cu(II) and Co(II) successfully compete with Ni(II) for the strong binding sites of the FA.


Spectrochimica Acta Part B: Atomic Spectroscopy | 1998

Measurements and analysis of dissociation rate constants of metal–fulvic acid complexes in aqueous solutions: Part II: measurement of decay rates by inductively-coupled plasma mass spectrometry and determination of rate constants for dissociation

Amina L.R. Sekaly; Margaret H. Back; Chuni L. Chakrabarti; D.C. Grégoire; Julia Y. Lu; William H. Schroeder

Abstract Rates of removal of metal ions from metal–fulvic acid complexes in aqueous solutions by Chelex-100 cation exchange resin have been measured using inductively-coupled plasma mass spectrometry (ICP–MS). When metal ions are complexed by the complexing agents, fulvic acid and ethylenediaminetetraacetic acid (each complexing agent is taken separately), the rate of uptake of metal ions by Chelex-100 has been related to the rate of dissociation of the metal complexes. The data were analyzed using the statistical methods described in Part 1. Complexes of lead with fulvic acid have a broad distribution of rate constants for dissociation, whereas complexes of aluminum with fulvic acid have a narrow distribution of dissociation rate constants. The rate constants for dissociation of both of these complexes decrease as the concentration of fulvic acid is increased at constant concentration of the metal. The results show that neither Zn nor Cd form strong complexes with fulvic acid.


Journal of Analytical Atomic Spectrometry | 1988

Transient atomisation by cathodic sputtering in a glow discharge for atomic absorption spectrometry

Chuni L. Chakrabarti; Kurt L. Headrick; Peter C. Bertels; Margaret H. Back

Analytical applications of cathodic sputtering in a glow discharge have almost wholly involved producing a steady-state atomic vapour, which suffers from relatively poor sensitivity and limit of detection common to the steady-state mode of atomisation. The formation of atoms in and loss of atoms from an analysis volume (defined by the geometry of the incident radiation from the hollow-cathode lamp) can be described by consecutive, first-order reactions, [graphic omitted], where D is the pre-atomisation species, B is the analyte atom that absorbs, C is the analyte atom that is lost and k1 and k2 are first-order rate constants. For k2≫k1, d[B]/dt= 0 (t is time in seconds) and the system is said to have reached a steady state. At the steady state, [B]=[D]0k1/k2, which makes [B] much less than [D]0, the initial concentration of the analyte atoms. Hence, the steady-state mode of atomisation gives low sensitivity.The objective of transient atomisation is to maximise [B] by making k1≫k2. For k1≫k2, [B]≈[D]0, and [B] then reaches its maximum possible value, giving the maximum absorbance. This paper presents the results of a preliminary study on transient atomisation with an Atomsource, used in conjunction with a Perkin-Elmer Model 5000 atomic absorption spectrometer and a Perkin-Elmer Model 7500 professional computer. Aqueous solutions of salts of Al, Cd, Co, Cu, Fe, Mn, Mo, Ni and V, one element in one solution, were used. The rate constants, k1 and k2, were evaluated from kinetic profiles and yielded k1/k2≫ 1, which satisfied the condition required for making [B]≈[D]0, yielding high peak-height sensitivity and an excellent detection limit for the above elements. Kinetic analysis revealed that the rate constant for atom formation in the early stages of the reaction (after a transition period), attained a much larger value (to be called k3) at later times. This rate constant for atom formation, k3, obtained from later times of atomisation, rather than k1 obtained from earlier times of atomisation, was used (i.e., k3≫k2) as the criterion for the peak-height sensitivity.


Spectrochimica Acta Part B: Atomic Spectroscopy | 1998

Measurements and analysis of dissociation rate constants of metal-fulvic acid complexes in aqueous solutions. Part I : Simulation of decay curves obtained by inductively coupled plasma-mass spectrometry to evaluate a method to measure the distribution of first-order rate constants

Amina L.R. Sekaly; Margaret H. Back; Chuni L. Chakrabarti; D.C. Grégoire; Julia Y. Lu; William H. Schroeder

Abstract A method for the analysis of exponential decay curves involving several kinetic components has been evaluated using simulated data. The method was originally developed for determination of fluorescence lifetimes and is applied here to data similar to those obtained in the measurement of rates of dissociation of metal–fulvic acid complexes in aqueous solutions. The method allows for a distribution of rate constants for dissociation of complexes instead of the single value for each complex obtained from the analysis used in previous studies from this laboratory. The signal-to-noise ratio of the data was a crucial factor in the distribution of the rate constants recovered from the analysis. Complexes with rate constants for dissociation differing by a factor of five were easily recovered, but when the difference was only a factor of two the resolution was not achieved even with a low level of noise. These tests were made with equal concentrations of each kinetic component. When the concentration of one kinetic component was reduced by a factor of 10, this species was not recovered in the analysis. These results clearly show the limits of resolution for individual kinetic components and the uncertainty expected in the values of the rate constants obtained by statistical analysis.


Analytica Chimica Acta | 1999

Effect of metal/fulvic acid mole ratios on the binding of Ni(II), Pb(II), Cu(II), Cd(II), and Al(III) by two well-characterized fulvic acids in aqueous model solutions

Amina L.R. Sekaly; Rupasri Mandal; Nouri M. Hassan; J. Murimboh; Chuni L. Chakrabarti; Margaret H. Back; D.C. Grégoire; W.H. Schroeder


Environmental Science & Technology | 2002

Chemical speciation and toxicity of nickel species in natural waters from the Sudbury Area (Canada)

Rupasri Mandal; Nouri M. Hassan; John Murimboh; Chuni L. Chakrabarti; Margaret H. Back; Ucu Rahayu; David R. S. Lean


Environmental Science & Technology | 2000

Competition of Ca(II) and Mg(II) with Ni(II) for binding by a well-characterized fulvic acid in model solutions

Rupasri Mandal; Mohamed S. A. Salam; John Murimboh; Nouri M. Hassan; Chuni L. Chakrabarti; Margaret H. Back; D.C. Grégoire

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D.C. Grégoire

Geological Survey of Canada

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William H. Schroeder

Meteorological Service of Canada

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