Raúl A. Gil

Cloud point extraction of mercury with PONPE 7.5 prior to its determination in biological samples by ETAAS

Cloud point extraction (CPE) has been used for the pre-concentration of mercury, after the formation of a complex with 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol (5-Br-PADAP), and later analysis by electrothermal atomic absorption spectrometry (ETAAS) using polyethyleneglycolmono-p-nonyphenylether (PONPE 7.5) as surfactant. The chemical variables affecting the separation step were optimized. Under the optimum conditions, i.e, pH 8.5, cloud point temperature 80 degrees C, 5-Br-PADAP=4x10(-5) mol L(-1), PONPE 7.5=0.2%, sample volume=1.0 mL, an enhancement factor of 22-fold was reached. The lower limit of detection (LOD) obtained under the optimal conditions was 0.01 microg L(-1). The precision for 10 replicate determinations at 2.0 microg L(-1) Hg was 4.0% relative standard deviation (R.S.D.). The calibration graph using the pre-concentration system for mercury was linear with a correlation coefficient of 0.9994 at levels near the detection limits up to at least 16 microg L(-1). The method was successfully applied to the determination of mercury in biological samples and in certified reference material (QC METAL LL3).

Application of multi-walled carbon nanotubes as substrate for the on-line preconcentration, speciation and determination of vanadium by ETAAS

This paper presents the development of a pre-concentration and speciation method for inorganic vanadium species by flow injection solid phase extraction-electrothermal atomic absorption spectrometry (FI-SPE-ETAAS). This was carried out by employing a conical mini-column filled with multi-walled carbon nanotubes mounted in the arm of the autosampler of the ETAAS. This system was applied to the on-line pre-concentration and speciation of vanadium in natural waters in conjunction with ETAAS determination. The time required for the pre-concentration of 1.0 ml of sample (1.0 ml min−1), elution/injection (0.2 ml min−1), reading/data acquiring and conditioning was about 2.9 min, resulting in a sample throughput of 20 samples per hour. A 20-fold total enrichment factor for a sample volume of 1.0 ml was obtained with respect to the vanadium determination by ETAAS without pre-concentration. The relative standard deviation for six replicates containing 200 ng l−1 was 3.3%. A limit of detection (3s) of 19 ng l−1 was achieved and the limit of quantification was estimated (10s), obtaining a characteristic concentration of 63 ng l−1. The calibration curve was linear from the quantification limit up to 1500 ng l−1, with a correlation coefficient of 0.9996. This method permitted us to determine the total inorganic vanadium in natural waters. For speciation purposes, V(IV) was masked with 1,2 cyclohexanediaminetetraacetic acid (CDTA) in order to determine the V(V) concentration selectively.

Biosorption: A new rise for elemental solid phase extraction methods

Biosorption is a term that usually describes the removal of heavy metals from an aqueous solution through their passive binding to a biomass. Bacteria, yeast, algae and fungi are microorganisms that have been immobilized and employed as sorbents in biosorption processes. The binding characteristics of microorganisms are attributed to functional groups on the surface providing some features to the biosorption process like selectivity, specificity and easy release. These characteristics turn the biosorption into an ideal process to be introduced in solid phase extraction systems for analytical approaches. This review encompasses the research carried out since 2000, focused on the employment of biosorption processes as an analytical tool to improve instrumental analysis. Since aminoacids and peptides as synthetic analogues of natural metallothioneins, proteins present in the cell wall of microorganisms, have been also immobilized on solid supports (controlled pore glass, carbon nanotubes, silica gel polyurethane foam, etc.) and introduced into solid phase extraction systems; a survey attending this issue will be developed as well in this review.

L-tyrosine immobilized on multiwalled carbon nanotubes: a new substrate for thallium separation and speciation using stabilized temperature platform furnace-electrothermal atomic absorption spectrometry.

An approach for the separation and determination of inorganic thallium species is described. A new sorbent, L-tyrosine-carbon nanotubes (L-tyr-CNTs), was used and applied to the analysis of tap water samples. At pH 5.0, L-tyr was selective only towards Tl(III), while total thallium was determined directly by stabilized temperature platform furnace-electrothermal atomic absorption spectrometry (STPF-ETAAS). The Tl(III) specie, which was retained by L-tyrosine, was quantitatively eluted from the column with 10% of nitric acid. An on-line breakthrough curve was used to determine the column capacity, which resulted to be 9.00 micromol of Tl(III) g(-1) of L-tyr-CNTs with a molar ratio of 0.14 (moles of Tl bound to moles of L-tyr at pH 5). Transient peak areas revealed that Tl stripping from the column occurred instantaneously. Effects of sample flow rate, concentration and flow rate of the eluent, and interfering ions on the recovery of the analyte were systematically investigated. The detection limit for the determination of total thallium (3sigma) by STPF-ETAAS was 150 ng L(-1). The detection limit (3sigma) for Tl(III) employing the separation system was 3 ng L(-1), with an enrichment factor of 40. The precision of the method expressed as the relative standard deviation (RSD) resulted to be 3.4%. The proposed method was applied to the speciation and determination of inorganic thallium in tap water samples. The found concentrations were in the range of 0.88-0.91 microg L(-1) of Tl(III), and 3.69-3.91 microg L(-1) of total thallium.

Total and inorganic mercury determination in biodiesel by emulsion sample introduction and FI-CV-AFS after multivariate optimization

An automated procedure for total and inorganic mercury determination in biodiesel by CV-AFS was studied. The samples were introduced directly as oil-in-water emulsions in a flow injection manifold followed by cold vapor generation coupled to atomic fluorescence spectrometry (FI-CV-AFS). After irradiation with an UV source, organic mercury (e.g.MeHg+ and PhHg+ among others) was decomposed. Mercury vapors were generated using an acidic SnCl2 solution in a continuous flow system and were then determined. This strategy reduced sample handling, avoiding sample contamination and analyte losses. The limit of detection was calculated as 0.2 µg Kg−1 (0.03 µg L−1 for the emulsions) and the relative standard deviation was better than 8% at levels of 3.0 µg L−1 in the emulsion, calculated from the peaks obtained. The accuracy was verified by comparing the results with a total microwave-assisted digestion.

Slurry sampling in serum blood for mercury determination by CV-AFS

The heavy metal mercury (Hg) is a neurotoxin known to have a serious health impact even at relatively low concentrations. A slurry method was developed for the sensitive and precise determination of mercury in human serum blood samples by cold vapor generation coupled to atomic fluorescence spectrometry (CV-AFS). All variables related to the slurry formation were studied. The optimal hydrochloric concentration and tin(II) chloride concentration for CV generation were evaluated. Calibration within the range 0.1-10 microg L(-1) Hg was performed with the standard addition method, and compared with an external calibration. Additionally, the reliability of the results obtained was evaluated by analyzing mercury in the same samples, but submitted to microwave-assisted digestion method. The limit of detection was calculated as 25 ng L(-1) and the relative standard deviation was 3.9% at levels around of 0.4 microg L(-1)Hg.

Determination of chemical oxygen demand by a flow injection method based on microwave digestion and chromium speciation coupled to inductively coupled plasma optical emission spectrometry

The present paper evaluates the applicability of a new FIA system for COD determination. The new system, flow injection microwave solid phase extraction by means of activated carbon (FI-MW-SPE), consists of a digestion circuit, placed in a home MW oven, coupled to an ICP-OES spectrophotometer. Doehlert experimental design was used to speed up the optimization of different experimental variables studied for assisted digestion methods. The method provided a high throughput of about 18 samples h(-1). To assess the accuracy of analytical methods linear regression, elliptic joint confidence region (EJCR) was used. A large linear range of 2.78-850 mg O(2) L(-1) with an excellent detection limit of 0.94 mg O(2) L(-1) was obtained. The interference by high chloride concentration was studied, and values below 3000 mg Cl(-) ions L(-1), allowed the estimation of COD load without any masking agents. COD values for various types of wastewater samples were correlated with those obtained by standard manual methods. Moreover, interferences due to matrix nature are absent; since matrix is washed out of the column before Cr (III) is eluted. This method reduces the time, reagent volume, hazardous emission, external contamination, with a good reproducibility and accuracy.

The use of electrothermal vaporizer coupled to the inductively coupled plasma mass spectrometry for the determination of arsenic, selenium and transition metals in biological samples treated with formic acid.

A fast method for the determination of As, Co, Cu, Fe, Mn, Ni, Se and V in biological samples by ETV-ICP-MS, after a simple sample treatment with formic acid, is proposed. Approximately 75 mg of each sample is mixed with 5 mL of formic acid, kept at 90°C for 1 h and then diluted with nitric acid aqueous solution to a 5% (v/v) formic acid and 1% (v/v) nitric acid final concentrations. A palladium solution was used as a chemical modifier. The instrumental conditions, such as carrier gas flow rate, RF power, pyrolysis and vaporization temperatures and argon internal flow rate during vaporization were optimized. The formic acid causes a slight decrease of the analytes signal intensities, but does not increase the signal of the mainly polyatomic ions ((14)N(35)Cl(+), (14)N(12)C(+), (40)Ar(12)C(+), (13)C(37)Cl(+), (40)Ar(36)Ar(+), (40)Ar(35)Cl(+), (35)Cl(16)O(+), (40)Ar(18)O(+)) that affect the analytes signals. The effect of charge transfer reactions, that could increase the ionization efficiency of some elements with high ionization potentials was not observed due to the elimination of most of the organic compounds during the pyrolysis step. External calibration with aqueous standard solutions containing 5% (v/v) formic acid allows the simultaneous determination of all analytes with high accuracy. The detection limits in the samples were between 0.01 (Co) and 850 μg kg(-1) (Fe and Se) and the precision expressed by the relative standard deviations (RSD) were between 0.1% (Mn) and 10% (Ni). Accuracy was validated by the analysis of four certified reference biological materials of animal tissues (lobster hepatopancreas, dogfish muscle, oyster tissue and bovine liver). The recommended procedure avoids plasma instability, carbon deposit on the cones and does not require sample digestion.

Multielemental analysis in vegetable edible oils by inductively coupled plasma mass spectrometry after solubilisation with tetramethylammonium hydroxide.

Trace metals have negative effects on the oxidative stability of edible oils and they are important because of possibility for oils characterisation. A single-step procedure for trace elemental analysis of edible oils is presented. To this aim, a solubilisation with tetramethylammonium hydroxide (TMAH) was assayed prior to inductively coupled plasma mass spectrometry detection. Small amounts of TMAH were used, resulting in high elemental concentrations. This method was applied to edible oils commercially available in Argentine. Elements present in small amounts (Cu, Ge, Mn, Mo, Ni, Sb, Sr, Ti, and V) were determined in olive, corn, almond and sunflower oils. The limits of detection were between 0.004 μg g(-1) for Mn and Sr, and 0.32 μg g(-1) for Sb. Principal components analysis was used to correlate the content of trace metals with the type of oils. The two first principal components retained 91.6% of the variability of the system. This is a relatively simple and safe procedure, and could be an attractive alternative for quality control, traceability and routine analysis of edible oils.

On-line preconcentration and speciation analysis of Se(IV) and Se(VI) using L-methionine immobilised on controlled pore glass

A study was undertaken to evaluate the analytical capabilities of the aminoacid L-methionine as a possible substrate for the selective sorption of selenium inorganic species for the determination of Se(IV) and Se(VI) in aqueous solution. Methionine was covalently immobilised on controlled pore glass (CPG), packed in a minicolumn and incorporated in an on-line flow injection system. Se was determined by flow injection hydride generation-inductively coupled plasma optical emission spectrometry (FI-HG-ICP OES). Chemical and physical variables affecting the sorption process were tested in order to select the optimal analytical conditions for the Se retention by methionine. Se(VI) was selectively retained by methionine at pH 3.0, while Se(IV) was not retained. Se(VI) was subsequently removed from the conical minicolumn with 10% HCl and on-line reduced with 12 mol l−1 HCl in a water bath (100 °C). A preconcentration factor of 90 was achieved when 30 mL of sample were processed reaching a detection limit of 11 ng l−1, with a sample throughput of 17 samples h−1 for total selenium determination and 8 samples h−1 for speciation [Se(IV) and Se(VI)]. The speciation analysis of inorganic Se in drinking water was performed following the proposed method and the recoveries were in all cases between 98 and 100%.