James A. Holcombe

Magnetic γ-Fe2O3 nanoparticles coated with poly-l-cysteine for chelation of As(III), Cu(II), Cd(II), Ni(II), Pb(II) and Zn(II).

Poly-l-cysteine (PLCys(n)) (n=20) was immobilized onto the surface of commercially available magnetic gamma-Fe(2)O(3) nanoparticles, and its use as a selective heavy metal chelator was demonstrated. Magnetic nanoparticles are an ideal support because they have a large surface area and can easily be retrieved from an aqueous solution. PLCys(n) functionalization was confirmed using FTIR and the quantitative Ellmans test. Metal binding capacities for As(III), Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II) were determined at pH 7.0 and compared to adsorption capacities for unfunctionalized gamma-Fe(2)O(3) nanoparticles. The effect of pH on the PLCys(n) functionalized nanoparticles was also investigated. For all of the metals examined, binding capacities (mumol metal/g support) were more than an order of magnitude higher than those obtained for PLCys(n) on traditional supports. For As(III), Cu(II), Ni(II) and Zn(II), the binding capacities were also higher than the metal adsorption capacities of the unfunctionalized particles. Metal uptake was determined to be rapid (< 2.5 min) and metal recoveries of >50% were obtained for all of the metals except As(III). PLCys(n), which has a general metal selectivity towards soft metals acids, was chosen to demonstrate the proof of concept. Greater metal selectivity may be achievable through the use of combinatorial peptide library screening or by using peptide fragments based on known metal binding proteins.

Fluorescent peptide sensor for the selective detection of Cu2

A new fluorescent peptidyl chemosensor for Cu(2+) ions with fluorescence resonance energy transfer (FRET) capabilities has been synthesized via Fmoc solid-phase peptide synthesis. The metal chelating unit, which is flanked by the fluorophores tryptophan (donor) and dansyl chloride (acceptor), consists of the amino acids glycine and aspartic acid (Gly-Gly-Asp-Gly-Gly-Asp-Gly-Gly-Asp-Gly-Gly-Asp-Gly-Gly). Coordination of the Cu(2+) ions to the metal chelating unit results in fluorescent quenching of both the donor and acceptor fluorophores. Although it was determined that Cu(2+) binding causes no change in FRET efficiency, emission and Cu(2+)-induced quenching of the acceptor dye can be used to monitor the concentration of the copper ions, with a detection limit of 32microgL(-1). The sensor also demonstrated sensitivity, reversibility and selectivity towards Cu(2+) in a transition metal matrix at pH 7.0.

Time and spatial absorbance profiles within a graphite furnace atomizer

Abstract It is often assumed that the free atom distribution within an electrothermally heated graphite atomizer is uniformly distributed within the furnace volume. Time and spatial absorbance traces taken within the furnace show that this is not the case until a majority of the analyte has been released from the surface, i.e. after the absorbance peak. The distributions within a 3 mm diameter CRA-90 furnace show absorbances which decrease in going from the bottom, where the sample was initially deposited, to the top of the furnace. Distribution profiles for Ag, Cu, Fe, Cr, Ni, Zn and Pb are presented. The more volatile elements show non-uniformities which may be attributed to mass transport limiting processes. A model based on desorption kinetics is used to explain the extreme nonuniformity observed for the less volatile elements studied. This approach not only accounts for the spatial nonuniformity but also explains the variations in the general absorbance-time profiles noted previously by various researchers. The alteration of the graphite surface by matrix decomposition products is also discussed for Pb and Zn. Changes in peak shapes are accounted for by alterations in the desorption process.

A Monte Carlo Simulation for Graphite Furnace Atomization of Copper

Monte Carlo simulation techniques are used to model the release of Cu from a graphite furnace atomizer during the heating pulse. The simulation allows for control of nonuniform axial temperature distribution of the furnace, surface readsorption of the analyte, and various furnace geometries and initial sample location. Spatial maps of the particles are obtained as a function of time during the simulations, and axial and radial spatial distributions of the analyte particles are presented for Cu. Spatially integrated and spatially resolved absorbance profiles from experimental measurements are also presented for comparison. Explanations of the Monte Carlo approach are given and the equations used to govern release and loss are presented. The approach relies heavily on physical/chemical constants rather than curve fitting approaches. The release of Cu is a first-order process proceeding from submonolayer coverages on the surface with an activation energy of 29 ± 3 kcal. With the use of a kinetic approach for the release and readsorption it was found that the rate of release and rate of readsorption were nearly identical. This suggests that equilibrium is approached at the gas/graphite interface for Cu under these conditions. At the peak approximately 10% of the original sample is within the furnace volume as a vapor. At 900 K/s heating rate, convective expulsion of the analyte is negligible in comparison to diffusive loss. However, in a mini-Massman-type furnace the presence of the sample introduction hole is responsible for 35% of the analyte lost. This is in contrast to the 11% predicted by area considerations alone.

Comparison of silica-immobilized poly(L-cysteine) and 8-hydroxyquinoline for trace metal extraction and recovery

Poly(l-cysteine) (PLC) and 8-hydroxyquinoline (8HQ) were immobilized on controlled-pore glass and used in a flow injection system for the separation of Cd, Pb and Cu from synthetic sea-water, Co and Ni matrices as well as CRM sea-water. Both resins allowed for the quantitative recovery of 50 µg L –1 Cd and Pb in synthetic sea-water. However, low recoveries of 2-4% and 40-50% were observed using 8HQ for the separation of 50 µg L –1 Cd and Pb, respectively, from a 10000-fold excess of Co and Ni, while PLC maintained quantitative recoveries. Neither 8HQ nor PLC showed reproducible or complete recoveries of Cu 2+ from the columns using the typical means for stripping (1 M HNO 3 ). On-line breakthrough experiments showed that 8HQ had a significant strong binding site capacity for Cd, Pb, Cu, Co and Ni. PLC also had strong sites for Cd, Pb and Cu but showed only weak binding of Co and Ni. The selectivity of PLC against these harder acid metals allowed for quantitative recovery of Cd, Pb and Cu in Co and Ni matrices. Extracting low level spikes of Cd and Pb from CRM sea-water (CASS-1 and NASS-2) tested the application to ‘real’ samples. Recovery efficiencies of Cd were high for both CRM matrices studied. Pb recovery was good for CASS-1; however, recovery from NASS-2 was unexpectedly low. Mass transfer limitations were observed for both resins in the flow system, resulting in apparent decreased capacities at faster flow rates. Stability constants governing Cd adsorption by PLC and 8HQ were obtained by a non-linear least-squares regression analysis of the Cd binding data and revealed that at least four classes of binding site were present on both resins. Stability constants for the most stable sites were estimated using EDTA or ethylenediamine (en) as competing ligands. 8HQ had no sites that were competitive with EDTA, whereas PLC had an EDTA-competitive site with a stability constant of 1×10 13 and a capacity of 1 µmol g –1 . Both PLC and 8HQ had sites that were stronger than Cd(en) 2 with estimated stability constants ranging from 10 9 to 10 11 . Weaker sites on the resins had stability constants that ranged from 10 4 to 10 6 . Cd was used to demonstrate the viability of this method for stability constant determination as it is well characterized for both 8HQ and PLC.

Mass spectral and atomic absorption studies of selenium vaporization from a graphite surface

Abstract Selenium desorbs from a graphite surface primarily as SeO 2 which dissociates in the gas phase to form atomic Se with an appearance temperature of 1175 K in atmospheric GFAAS. Mass spectrometry studies in vacuum show that the SeO 2 desorption occurs at 425 K without the addition of any modifiers and that elemental Se does not desorb from the heated graphite flat. Vacuum MS studies show that the addition of Ni delays the appearance temperature of a significant amount of SeO 2 to 850 K. The SeO 2 lost at this temperature does not contribute to the Se absorbance signal which has a delayed appearance in atmospheric GFAA of 1575 K with the addition of Ni. Above 850 K, the remaining SeO 2 is attached to NiO which has formed on the surface. Then, NiO is reduced by the carbon forming CO and CO 2 which desorb at 1200 K while elemental Ni remains in the carbon. At the same time, SeO 2 is released from NiO and desorbs from the graphite in vacuum as SeO 2 , SeO, Se and Se 2 . Some reduction of SeO 2 on the surface has occurred at this high temperature. These Se species dissociate in the high temperature gas phase atmosphere of GFAA forming elemental Se which is then detected as a delayed Se AA signal. Much of the SeO 2 desorbed at low temperatures can be lost through diffusion from the furnace even when Ni is added, if the gas phase is not preheated. XPS and SEM studies confirm the existence of NiO on the surface at 1100 K with the remaining Se dispersed in the Ni so that it is not observable. Elemental Ni desorbs from the graphite at 1675 K. in vacuum MS, corresponding to the Ni signal observed in atmospheric GFAA. Following high temperature cleaning steps, elemental Ni is detected by XPS and MS to remain in the graphite. This amount of Ni is not significant enough to contribute to the Ni AA signal, nor does the elemental Ni affect the Se AA signal. An oxygenated surface, like that produced by the addition of Ni, also causes the appearance of the Se AA signal in atmosphere to be delayed to a higher temperature. The signal is enhanced, although not to the extent of that with the addition of Ni. SEM data show SeO 2(s,1) congregating at active sites on the oxygenated surface following a dry step. Under vacuum conditions, the desorption of SeO 2 from an oxygenated surface was not delayed. This indicates that atmospheric pressure is necessary to allow mobile SeO 2 species to move to oxygenated active sites which then delay the release. The mobile SeO 2 species are desorbed from the surface under vacuum conditions before reaching the active sites.

Immobilized peptides/amino acids on solid supports for metal remediation*

Recently,a significant amount of work has focused on metal binding by natural systems for various applications. This review will focus on the utility of amino acids, short peptides, and proteins that have been immobilized onto solid supports for use in metal binding. These systems include single amino acids, poly-amino acids, and peptides immobilized onto supports such as silica, polymer resins, and membranes. Also included are the studies involving the use of immobilized amino acids in ion-exchange chromatography.

Particle size distribution of sample transported from an electrothermal vaporizer to an inductively coupled plasma mass spectrometer

Abstract The particles exiting the graphite furnace electrothermal vaporizer (ETV) cell of a commercial ETV inductively coupled plasma mass spectrometer (ICP-MS) were size characterized. It was found that there were a large number of particles leaving a furnace heated with no sample. Assuming all the particles were graphitic, approximately 0.1 μg of material exited the ETV. A common analyte carrier, NaCl, was also characterized and exhibited a bimodal size distribution with a consistent absence of particles in the 0.2–0.3 μm region, which may be explained by the presence of two mechanisms responsible for NaCl particle formation. The mass spectral signals of Na + and C + indicated the NaCl and carbon particles were temporally separated. Several parameters were varied (i.e. heating rates, gas flows, transport tube geometries) and their impact on the size distribution was noted. ETV heating rates for 440 and 2000 K s −1 and Ar flows of 1.0 and 1.8 l min −1 produced a statistically insignificant change in the number of particles or their size distribution, except for the 0.3–0.5 μm regime with the varying heating rates. Varying transport tube lengths from 0.2–2.0 m showed subtle effects that were notable for large particles and long tubes.

Monte Carlo optimization of graphite furnace geometry and sample distribution for copper

Abstract Optimization of graphite furnace in GFAA has been a difficult problem because of the absence of an analytical expression describing the absorbance signal in terms of all of the parameters affecting it. This work continues the development of a Monte Carlo approach that can accurately account for any furnace geometry and physical/chemical parameter, including vapor loss as well as vaporization energetics and the extent of interaction of the analyte with the surface upon release and during vapor/surface collisions. p]In the present work, the impact of furnace geometry and initial sample distribution on the absorbance profiles for Cu are studied and the resulting data are analyzed looking for optimum analytical conditions. The results imply that increasing furnace length produces a linear (and nearly proportional) increase in peak area with the slope and the intercept dependent on dosing hole size and heating program. A significant fraction of the sample vapor is lost through the hole and is greater than predicted by comparing this hole area with that of the ends. Increasing the furnace diameter increases the fraction of the sample found in the gas phase, but decreases the peak absorbance signal. An increase in the length and a decrease in the diameter of the furnace would significantly increase the signal but would also magnify the impact of the dosing hole. An improvement in precision and/or accuracy may be realized if the precision in initial sample positioning can be improved. This may prove to be particularly noticeable with solvents which wet the graphite. All improvements noted can be realized if other factors are held constant. For example, if an increase in the furnace length is accompanied by a reduction in heating rate or increasing nonuniformity, then a net loss in signal may actually be observed.

Tin atom formation in a graphite furnace atomizer

Absorbance measurements resolved in time and space are presented for tin vaporized under various conditions in a graphite furnace. Mass spectroscopie studies coupled with temporal and spatial absorbance measurements indicate that oxygen entrained in the inert sheath gas significantly attenuates the free atomic tin population through the rapid formation of SnO. Addition of 10% hydrogen to the sheath gas and enclosure of the atomizer, substantially reduce the effect of entrained oxygen. Similar release mechanisms are postulated with and without hydrogen, but it is suggested that the concentration of elemental tin on the surface at the time of vaporization is greater when hydrogen is included. The effects of added sulfate salts were also studied and it was concluded that the formation and prevolatilization of SnS is responsible for the extreme depressions reported in the literature.