Sandra Gil

Microvolume turbidimetry for rapid and sensitive determination of the acid labile sulfide fraction in waters after headspace single-drop microextraction with in situ generation of volatile hydrogen sulfide.

In this work, we demonstrate the feasibility of applying headspace single-drop microextraction with in-drop precipitation for the quantitative determination of the acid labile sulfide fraction (H2S, HS-, and S2- (free sulfide), amorphous FeS and some metal sulfide complexes-clusters as ZnS) in aqueous samples by microvolume turbidimetry. The methodology lies in the in situ hydrogen sulfide generation and subsequent sequestration into an alkaline microdrop containing ZnO(2)(2-) and exposed to the headspace above the stirred aqueous sample. The ZnS formed in the drop was then determined by microvolume turbidimetry. The optimum experimental conditions of the proposed method were: 2 microL of a microdrop containing 750 mg L(-1) Zn(II) in 1 mol L(-1) NaOH exposed to the headspace of a 20-mL aqueous sample stirred at 1600 rpm during 80 s after derivatization with 1 mL of 6 mol L(-1) HCl. An enrichment factor of 1710 was achieved in only 80 s. The calibration graph was linear in the range of 5-100 microg L(-1) with a detection limit of 0.5 microg L(-1). The repeatability, expressed as relative standard deviation, was 5.8% (N = 9). Finally, the proposed methodology was successfully applied to the determination of the acid labile sulfide fraction in different natural water samples.

Evaluation of ultrasound-assisted extraction as sample pre-treatment for quantitative determination of rare earth elements in marine biological tissues by inductively coupled plasma-mass spectrometry.

In this work, the determination of rare earth elements (REEs), i.e. Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu in marine biological tissues by inductively coupled-mass spectrometry (ICP-MS) after a sample preparation method based on ultrasound-assisted extraction (UAE) is described. The suitability of the extracts for ICP-MS measurements was evaluated. For that, studies were focused on the following issues: (i) use of clean up of extracts with a C18 cartridge for non-polar solid phase extraction; (ii) use of different internal standards; (iii) signal drift caused by changes in the nebulization efficiency and salt deposition on the cones during the analysis. The signal drift produced by direct introduction of biological extracts in the instrument was evaluated using a calibration verification standard for bracketing (standard-sample bracketing, SSB) and cumulative sum (CUSUM) control charts. Parameters influencing extraction such as extractant composition, mass-to-volume ratio, particle size, sonication time and sonication amplitude were optimized. Diluted single acids (HNO(3) and HCl) and mixtures (HNO(3)+HCl) were evaluated for improving the extraction efficiency. Quantitative recoveries for REEs were achieved using 5 mL of 3% (v/v) HNO(3)+2% (v/v) HCl, particle size <200 μm, 3 min of sonication time and 50% of sonication amplitude. Precision, expressed as relative standard deviation from three independent extractions, ranged from 0.1 to 8%. In general, LODs were improved by a factor of 5 in comparison with those obtained after microwave-assisted digestion (MAD). The accuracy of the method was evaluated using the CRM BCR-668 (mussel tissue). Different seafood samples of common consumption were analyzed by ICP-MS after UAE and MAD.

Greener analytical method for determination of thiomersal (sodium ethylmercurithiosalicylate) in ophthalmic solutions using sono-induced cold vapour generation-atomic absorption spectrometry after UV/H2O2 advanced oxidation

A new method is described for the determination of thiomersal (i.e., a stabilizing agent) in eye-drop solutions by sono-induced cold vapour generation-atomic absorption spectrometry following an UV/H2O2 advanced oxidation process. A green methodology has been developed that eliminates the need for both strong oxidising conditions for degradation of thiomersal and conventional chemical reducing agents, such as tin chloride or sodium tetrahydroborate(III), for cold vapour generation. Conversion of thiomersal into Hg(II) is performed under 10 min UV irradiation of the eye-drop solution in the presence of 100 μL of H2O2. Sono-reduction of Hg(II) to yield Hg(0) is carried out in a batch reactor coupled to a quartz furnace atomizer kept at room temperature for measurement of the Hg atomic absorption. This reduction/vaporization process was enhanced by addition of formic acid to the sample solution prior to ultrasound irradiation. The method has been applied to the determination of thiomersal in seven commercial eye-drop solutions. The limit of detection was 0.04 μg mL−1 of thiomersal and the between-batch precision expressed as relative standard deviation for n = 4 separate UV oxidations ranged from 3 to 7%.

Directly suspended droplet microextraction in combination with microvolume UV-vis spectrophotometry for determination of phosphate.

A miniaturized methodology for the determination of phosphate in waters has been developed by combining directly suspended droplet microextraction (DSDME) with microvolume spectrophotometry. The method is based on the extraction of the ion pair formed between 12-molybdophosphate and malachite green onto a microdrop of methyl isobutyl ketone and subsequent spectrophotometric determination with no dilution. An enrichment factor of 325 was obtained after 7.5 min of microextraction. The detection limit was 6.1 nM phosphate and the repeatability, expressed as relative standard deviation, was 2.7% (n=6). The method was successfully applied to the determination of dissolved reactive phosphorus in different freshwater samples.

Dispersive liquid-liquid microextraction combined with microvolume spectrophotometry to turn green the 5530 APHA standard method for determining phenols in water and wastewater.

In this work, a new method based on the combination of dispersive liquid-liquid microextraction (DLLME) with microvolume spectrophotometry has been developed as a greener and miniaturized alternative to the 5530 APHA standard method for determining phenols in water and wastewater. The method relies on the oxidative coupling of phenols with 4-aminoantipyrine (4-AAP). In order to preconcentrate the dye formed, the classical liquid-liquid extraction used in the 5530 APHA method (involving 500 mL of sample and 50 mL of trichloromethane) has been replaced by DLLME (with 5 mL of sample, 50 μL of trichloromethane and 200 μL of acetonitrile). After optimization, the method yielded limits of detection and quantification (0.8 and 2.5 μg L(-1), respectively) that were comparable with those obtained by the 5530 APHA standard method. Repeatability, expressed as relative standard deviation, was 5.2% (N=6), and the enrichment factor (EF) was 700. The proposed method was applied to the determination of phenols in different water samples and a wastewater with recoveries in the range 90-99%. The greenness profile was established in accordance with the suggestions made by the NEMI (National Environmental Methods Index). The absence of PBT (persistent bioaccumulative and toxic chemicals) and corrosive reagents and a drastic reduction of generated wastes can be emphasized.

On-line UV photoreduction in a flow-injection/stopped-flow manifold for determination of mercury by cold vapour-atomic absorption spectrometry.

Photo-chemical vapour generation has been applied in a flow-injection system under stopped-flow conditions for determination of Hg by atomic absorption spectrometry. The system allows mercury vapour generation without the need for conventional reduction reactions based on sodium/potassium tetrahydroborate (III) or tin chloride in acid medium. The photo-induced reaction is accomplished by applying ultraviolet irradiation (UV) to the sample solution containing Hg(II) in the presence of an organic acid (i.e., acetic, citric, oxalic, ethylendiaminetetraacetic) as precursor of reducing species. A remarkable improvement in sensitivity is observed with acetic acid when stopped-flow is employed as compared to continuous operation, meaning that kinetics play an important role in the photo-induced reaction. A detection limit of 0.3 μg L−1 can be obtained, which represents a 6-fold improvement in respect to that obtained without stopped-flow. The repeatability expressed as relative standard deviation was about 2.7% (n = 15) for a 50 μg L−1 Hg standard. The effect of potential interferences on the photo-generation of Hg vapour was investigated. In the UV-photo-induced CVG, both inorganic Hg and organomercury species can be reduced to elemental mercury with the same efficiency. The method was applied to determination of Hg in several enriched natural water samples and recoveries of MeHg+, Thiomersal, EtHg+ and PhHg+ were in the range 97% to 102%.

Simultaneous ultrasound-assisted emulsification–derivatization as a simple and miniaturized sample preparation method for determination of nitrite in cosmetic samples by microvolume UV–vis spectrophotomety

A simple and miniaturized approach based on ultrasound-assisted emulsification-derivatization is proposed for the determination of nitrite in cosmetic samples by UV-vis micro-spectrophotometry. Oil/water emulsions were formed using 15 mg of cosmetic sample and 1 mL of an aqueous medium containing 0.5% w/v SDS and 1% v/v acetic acid. When powerful sonication systems were used to make emulsions, i.e. probe or cup-horn sonoreactor, stable and transparent emulsions were obtained in one or half minute per sample, respectively. The Griess reaction in these special conditions (i.e. sonication and the presence of an organized medium) was investigated. The absence of matrix effects allows external calibration with aqueous standards for nitrite quantification. Analytical features were compared to those of the European official method 82/434/EEC. Detection limit, sample throughput and reagent consumption were significantly improved.

Simplified and miniaturized procedure based on ultrasound-assisted cytosol preparation for the determination of Cd and Cu bound to metallothioneins in mussel tissue by ICP-MS

A simplified and miniaturized procedure for the determination of Cd and Cu bound to metallothioneins (MTs) by ICP-MS in mussel tissue has been developed. Cytosol preparation was based on the indirect sonication of slurries containing the lyophilized sample dispersed in 1 mL of extractant by means of a sonoreactor Cup-Horn. Rabbit liver MTs (Apo-MT-I, Apo-MT-II and Cd(7)-MT-II) and a conventional cytosol preparation procedure were used for validation purposes. The usual heating step and additional centrifugations of the conventional procedure for cytosol preparation can be omitted when using ultrasound treatment. The possible effect of denaturation on MTs and its effect on the metal bound to MTs were evaluated. Variables influencing the ultrasound-assisted cytosol preparation procedure were carefully optimized for simultaneous determination of both metals. Chromatographic conditions to separate the MT fraction from other proteins present in cytosols were also studied. Six samples can be processed within 3 min of sonication. An acid ultrasound-assisted extraction procedure with diluted acid was also proposed for determining total Cd and Cu. Finally, Cd and Cu bound to MTs as well as total Cd and Cu were determined in mussels from Pontevedra and Ares-Betanzos coastal inlets (Galicia, Spain).

Determination of total lead and lead species according to their lability in coastal seawater by Chelex-100 titration and electrothermal-atomic absorption spectrometry

Abstract A speciation method based on Chelex-100 titration in combination with electrothermal-atomic absorption spectrometry has been developed to study the distribution of Pb among different fractions in coastal seawater. The method allows an accurate determination of total Pb, thereby avoiding the effect of the saline matrix. Additionally, the distribution patterns of Pb among species with different lability can be established in seawater as a function of pH. Four Pb fractions have been obtained in a model seawater and two seawater samples from Ria de Vigo (NE Atlantic ocean), i.e. free Pb ion, inorganic Pb fraction, organic Pb fraction and inert Pb fraction. The inert and labile organic Pb fractions dominate at natural pH (≈8). Complexation mechanisms of Pb were investigated from the sorption curves of lead on Chelex-100 according to the Gibbs–Donnan model. Three complexes were formed, PbL, PbL2 and Pb(HL)2 with the following intrinsic complexation constants logβ(1np) = −3.25, −10.10 and −1.98, respectively. The formation of the complex PbL is negligible in all range of pH studied, while the complex Pb(HL)2 predominates up to pH 4 and PbL2 from pH 4.