Speciation of lead in environmental waters by preconcentration on a new fullerene derivative

A novel fullerene derivative including a chelating group attached to the fullerene core has been prepared by photoreaction of C60 and sodium diethyldithiocarbamate (NaDDC) in toluene-methanol medium. The optimization of the synthesis procedure was monitored by UV-visible spectroscopy using the spectrum of C60 fullerene as reference; the new material, a C60-NaDDC monoadduct, showed a shoulder at 430 nm and an increase in the absorption band comprised between 425 and 500 nm. The solid was obtained by photolysis reaction in approximately 24 h, requiring further purification by preparative chromatography. Characterization of the brown product was accomplished by XRD and solid-state 13C MAS NMR; chemical modification was confirmed through the appearance of peaks close to the fullerene core peak located at 146.9 ppm, which can be assigned to carbon atoms of C60 that are covalently bonded to the diethyldithiocarbamate group to form a pyrrolidine ring-fused fullerene monoadduct. Using lead species as a model assessed the ability of the new material for the preconcentration of metallic and organometallic compounds, providing detection limits of 4-15 ng/L. The most interesting conclusions of the results were high adsorption efficiency, selectivity, and stability of the C60-NaDDC derivative (the packed material can be used for at least six months).     

Determination of condensed tannin monomers in environmental samples by capillary gas chromatography of acid depolymerization extracts

A method for molecular-level quantification of condensed tannin is described that uses acid depolymerization and carbocation capture by phloroglucinol. Resulting monomers and phloroglucinol adducts are trimethylsilyl derivatized, separated by capillary gas chromatography, detected by flame ionization, and quantified relative to standards. Optimal depolymerization conditions were determined for acid strength, phloroglucinol concentration, time, and temperature. The method gives reproducible results in leaf litter that are linear over 2 orders of magnitude with detection limits down to approximately 100 ng condensed tannin. In addition to tannin, triterpenoids were also identified and can be quantified with this method. Analyses of soils and sediments indicate that mineral interactions with condensed tannin are important both analytically and environmentally.     

On-line preconcentration and determination of mercury in biological and environmental samples by cold vapor-atomic absorption spectrometry

An on-line procedure for the determination of traces of total mercury in environmental and biological samples is described. The present methodology combines cold vapor generation associated to atomic absorption spectrometry (CV-AAS) with preconcentration of the analyte on a minicolumn packed with activated carbon. The retained analyte was quantitatively eluted from the minicolumn with nitric acid. After that, volatile specie of mercury was generated by merging the acidified sample and sodium tetrahydroborate(III) in a continuous flow system. The gaseous analyte was subsequently introduced via a stream of Ar carrier into the atomizer device. Optimizations of both, preconcentration and mercury volatile specie generation variables were carried out using two level full factorial design (2(3)) with 3 replicates of the central point. Considering a sample consumption of 25mL, an enrichment factor of 13-fold was obtained. The detection limit (3sigma) was 10ngL(-1) and the precision (relative standard deviation) was 3.1% (n=10) at the 5microgL(-1) level. The calibration curve using the preconcentration system for mercury was linear with a correlation coefficient of 0.9995 at levels near the detection limit up to at least 1000microgL(-1). Satisfactory results were obtained for the analysis of mercury in tap water and hair samples.     

Determination of trace perchlorate in high-salinity water samples by ion chromatography with on-line preconcentration and preelution

A simple, automated system for the determination of trace perchlorate by ion chromatography (IC) with an online preconcentration technique is reported. The sample is preconcentrated, and less strongly held ions preeluted before the analyte is transferred to the principal separation system. This approach provides low limits of detection (LOD) and is particularly robust toward the effect of high concentrations of common anions, such as those present in groundwater samples. It compares favorably with currently promulgated EPA method 314.0. The LOD (S/N = 3) is 0.77 microg/L for a 2-mL reagent water sample and decreases more-or-less proportionately with increasing sample volume, at least up to 20 mL. Even with a sample of conductivity 14.7 mS/cm (approximately that of 0.1 M Na2SO4), the recovery of added perchlorate at the 25.0 microg/L level was still 92%. The concentration of added perchlorate in the range of 1-400 microg/L was linearly correlated to the peak area, with an r2 value of 0.9997. The recovery of perchlorate from artificial samples with different conductivity by the present method compares favorably with those from the currently recommended EPA Method. The ability of this approach to remove matrix interferences suggests that it would be also promising for perchlorate analysis in other challenging samples.     

Speciation of Cr(III) and Cr(VI) in environmental samples determined by selective separation and preconcentration on silica gel chemically modified with niobium(V) oxide

In this study a new method for chromium speciation in water using solid phase extraction coupled to a flow injection system and flame atomic absorption spectrometry was developed. The adsorption behavior of Cr(III) and Cr(VI) on Nb2O5-SiO2 allowed the selective separation of Cr(III) from Cr(VI) in the pH range of 6-9. Thus, a method for Cr(III) preconcentration and extraction using Nb2O5-SiO2 was developed. Total chromium was determined after the reduction of Cr(VI) to Cr(III) using sodium sulfite in acidic medium. The operational variables of the preconcentration and reduction procedures were optimized through full factorial and Doehlert designs. The limit of detection for Cr(III) was 0.34microgL(-1) and the precision was below 4.6%. Results for recovery tests using different environmental samples were between 90 and 105%. Certified reference materials (NIST 1640 and NIST 1643e) were analyzed in order to check the accuracy of the proposed method, and the results were in agreement with the certified values.     

Speciation of chromium in water samples with cloud point extraction separation and preconcentration and determination by graphite furnace atomic absorption spectrometry

A novel method has been developed for the speciation of chromium in natural water samples based on cloud point extraction (CPE) separation and preconcentration, and determination by graphite furnace atomic absorption spectrometry (GFAAS). In this method, Cr(III) reacts with 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (PMBP) yielding a hydrophobic complex, which then is entrapped in the surfactant-rich phase, whereas Cr(VI) remained in aqueous phase. Thus, separation of Cr(III) and Cr(VI) could be realized. Total chromium was determined after the reduction of Cr(VI) to Cr(III) by using ascorbic acid as reducing reagent. PMBP was used not only as chelating reagent in CPE procedure, but also as chemical modifier in GFAAS determination of chromium. The detection limit for Cr(III) was 21 ng L(-1) with an enrichment factor of 42, and the relative standard deviation was 3.5% (n=7, c=10 ng mL(-1)). The proposed method has been applied to the speciation of chromium in natural water samples with satisfactory results.     

Multiwalled carbon nanotubes for speciation of chromium in environmental samples

A solid phase extraction procedure has been established for chromium speciation in natural water samples. The procedure is based on the solid phase extraction of the Cr(VI)-ammonium pyrrolidine dithiocarbamate (APDC) chelate on multiwalled carbon nanotubes (MWNTs). After oxidation of Cr(III) to Cr(VI) by using hydrogen peroxide, the presented method was applied to the determination of the total chromium. The level of Cr(III) is calculated by difference of total chromium and Cr(VI) levels. The procedure was optimized for some analytical parameters including pH, eluent type, flow rates of sample and eluent, matrix effects, etc. The detection limit based on 3 sigma criterion was 0.90 microg L(-1) for Cr(VI). The adsorption capacity of multiwalled carbon nanotubes was 9.50 mg g(-1) Cr (VI). The presented method was applied for the speciation of chromium in natural water sample with satisfactory results (recoveries>95%, R.S.D.'s<9%).     

Hole-doping of fullerenes and nanotubes by way of intercalation chemistry

Succeeding to the electron-doping processes of carbon nanostructures, chemical methods devoted to the hole-doping of the latter have significantly developed over the past ten years. Intercalation chemistry remains a top-rated technique in this purpose, among the variety of available chemical doping schemes. A review of the p-type doping of fullerenes and nanotubes by this method is exposed, which also includes a wide range of derived potential applications and prospects regarding the materials thus obtained.     

Characterizing the surface properties of carbon nanotubes by inverse gas chromatography

Inverse gas chromatography (IGC) was used to characterize the surface properties of pristine multi-walled carbon nanotubes (MWNTs), as well as the poly(acrylic acid) sidewall covalently functionalized MWNTs (PAA-g-MWNTs) and hydroxyl group directly grafted MWNTs (MWNTols). The dispersive component of the surface energy ( ) and the acid/base character of these samples’ surfaces were estimated by the retention time with different non-polar and polar probes at infinite dilution region. The specific free energy (ΔG ^AB) and the enthalpy (ΔH ^AB) of adsorption corresponding to acid–base surface interactions were determined. By correlating ΔH ^AB with the donor and acceptor numbers of the probes, the acidic (K _A) and the basic K _D parameters of the samples’ were calculated. The results show that chemical modification successfully reduces the dispersive component of the surface energy of MWNTs. Furthermore, MWNTs grafted with hydroxyl groups exhibit a more basic character, while MWNTs grafted with poly(acrylic acid) show a more acidic character. Overall, IGC provides useful complementary information on the changes resulted from the chemical modifications of the surface.     

Solid phase extraction of heavy metal ions in environmental samples on multiwalled carbon nanotubes

Multiwalled carbon nanotubes (MWNTs) were used as solid phase extractor for Cu(II), Cd(II), Pb(II), Zn(II), Ni(II) and Co(II) ions as ammonium pyrrolidine dithiocarbamate (APDC) chelates, in the present study. The influences of the experimental parameters including pH of the solutions, amounts of MWNTs, amounts of APDC, eluent type and volume, sample volume etc. on the quantitative recoveries of analyte ions were investigated. The effects of matrix ions of natural waters and some transition metals on the recoveries of the analyte ions were also examined in the model solutions. Tests of addition/recovery for analyte ions in real samples were performed with satisfactorily results. The detection limits (3s) for the analyte ions were in the range of 0.30-0.60 microg l(-1). The concentrations of analytes in standard reference materials (NIST RM 8418 Wheat gluten, LGC 6010 Hard drinking water and NIST SRM 1515 Apple leaves) pretreated by the presented method were measured with FAAS and the analytical values were well agreed with the certified values and the reference values without the interference of major components. The presented method has been applied to the determination of analytes in food and environmental samples with satisfactory results.     

Individualization of gasoline samples by covariance mapping and gas chromatography/mass spectrometry

A set of 10 fresh (unevaporated) gasoline samples from a single metropolitan area were differentiated based on a covariance mapping method combined with a t-test statistic. The covariance matrix for each sample was calculated from the retention time-ion abundance data set obtained by gas chromatography/mass spectrometry analysis. Distance metrics were calculated between the covariance matrices from replicate analyses of the same sample and between the replicate analyses of different samples. The distance metric for the same-sample comparisons were shown to constitute a population significantly different from the distance metric for the different-sample comparisons. A power analysis was performed to estimate the number of analyses needed to discriminate between two samples while maintaining a probability of type II error, beta, below 1%, e.g., a test power greater than 99%. Triplicate analyses of two gasoline samples was shown to be sufficient to discriminate between the two using a t-test, while keeping beta<0.01 at a significance level, alpha, of 0.05. Analysis of the 45 possible pairwise comparisons between samples found that 100% of the samples were statistically distinguishable, and no type II errors occurred. Blind tests were conducted wherein 2 of the 10 gasoline samples where presented as unknowns. One of the unknowns was found to be indistinguishable from the original source, and one unknown was determined to be statistically different from the original source, constituting a type I error. The effects of evaporation on sample comparison are not addressed in this paper. The results from this study demonstrate a statistically acceptable method of physical evidence comparison in forensic casework.