Ricardo Q. Aucélio

Determination of Cu, Mn, Ni and Sn in gasoline by electrothermal vaporization inductively coupled plasma mass spectrometry, and emulsion sample introduction

Trace metals in fuels, except in the case of additives, are usually undesirable and normally they occur in very low concentrations in gasoline, requiring sensitive techniques for their determination. Coupling of electrothermal vaporization with inductively coupled plasma mass spectrometry minimizes the problems related to the introduction of organic solvents into the plasma. Furthermore, sample preparation as oil-in-water emulsions reduces problems related to gasoline analysis. In this work, a method for determination of Cu, Mn, Ni and Sn in gasoline is proposed. Samples were prepared by forming a 10-fold diluted emulsion with a surfactant (Triton X-100), after treatment with concentrated HNO3. The sample emulsion was pre-concentrated in the graphite tube by repeated pipetting and drying. External calibration was used with aqueous standards in a purified gasoline emulsion. Six samples from different gas stations were analyzed, and the analyte concentrations were found to be in the μg l−1 range or below. The limits of detection were 0.22, 0.02, 0.38 and 0.03 μg l−1 for Cu, Mn, Ni and Sn, respectively. The accuracy of the method was estimated using a recovery test.

Evaluation of electrothermal atomic absorption spectrometry for trace determination of Sb, As and Se in gasoline and kerosene using microemulsion sample introduction and two approaches for chemical modification

Studies aimed at the development of analytical methods based on electrothermal atomic absorption spectrometry for the determination of antimony, selenium and arsenic in gasoline and kerosene were performed. The goal was to develop sensitive methods that allow a simple calibration procedure to be used through direct correlation between analyte signals measured in the sample and signals from aqueous standards. This was possible by optimization of both the sample preparation procedure and furnace conditions. The platform atomization technique was used together with chemical modification (ruthenium permanent modifier or Pd/Mg nitrate chemical modifier). Better results were obtained when samples were introduced as microemulsions prepared with a surfactant (Triton X-100) rather than sample introduction as detergentless microemulsions (three component solution). Good sensitivity was achieved, which allowed the determination of analytes in gasoline and kerosene samples in the µg L−1 range. Linear ranges up to three decades were observed for the analytical curves. Laboratory-prepared samples were analyzed with good precision and recovery.

Analysis of soil and sediment samples by laser ablation inductively coupled plasma mass spectrometry

The analysis of soil and sediment samples using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was studied. Solution-based calibration was used for the quantification of trace elements in these samples. In most cases, the measured concentrations were within ±20% of the certified values using 60 Ni or 107 Ag as the internal standard. Measurements with Ag were carried out to investigate whether an internal standard could be spiked into soils for quantification purposes. The influence of particle size on the applicability of sample spiking was briefly studied, and it was demonstrated that particle size could significantly influence measurements if only the surface constituents of the particle were ablated. Use of 43 Ca or 44 Ca as an internal standard produced poorer results owing to interferences at these masses. In some cases, such as with Sr, Ba, Y and Rb, the measured concentrations were low by a factor of 2-3. This could be remedied by using one of these elements as an internal standard for the others. The effects of elemental speciation, organic content and particle size were investigated. Elemental speciation and organic content of the soils did not appear to significantly affect the LA-ICP-MS measurements. Particle size, however, was found to influence the precision and sensitivity of the measurements. Samples with smaller particle sizes yielded higher signal levels and better precision.

Trace elemental determination in alcohol automotive fuel by electrothermal atomic absorption spectrometry

Abstract Suitable analytical methodologies were developed allowing direct determination of As, Cu, Fe, Pb, Sb and Sn in alcohol fuel samples by electrothermal atomic absorption spectrometry. Different chemical modification approaches were tested and compared in terms of analytical performance and in practical terms. Experimental conditions were optimized allowing little sample dilution and use of calibration curves prepared with aqueous inorganic analyte standards. Methodologies were tested with analyte spiked alcohol samples. Good analyte recoveries from spiked alcohol samples, precision better than 10% and limits of detection in the sub μg l−1 range were achieved.

Determination of trace elements in crude oil by ICP-OES using ultrasound-assisted acid extraction

The determination of Cr, Mo, Ti, V, Cd, Fe, Mn, Ni and Zn in crude oil by ICP-OES using ultrasound-assisted acid extraction is proposed. The crude oil was treated with concentrated nitric acid and heated at 85 °C for 30 min. Then, the mixture was placed in an ultrasonic bath for 15 min in order to obtain total extraction of trace elements from the oil sample into the acid solution, which was directly aspirated into the ICP to be quantified using analytical curves constructed with inorganic standards. Principal component analysis (PCA) and factor analysis (FA) were used to evaluate the effect and importance of the experimental variables in this sample preparation procedure. The proposed method was tested using NIST 1634c (Trace Elements in Residual Fuel Oil) and two petroleum samples. Recoveries between 90 and 105% were achieved. Limits of quantification in the sample ranged from 0.008 to 0.62 μg g−1, depending on the analyte.

Selective Determination of Rutin by Fluorescence Attenuation of the CdS-2-mercaptopropionic Acid Nanocrystal Probe

In this work, an analytical method for the determination of rutin was developed based on the CdS-2-mercatopropioinic acid (Cd-2MPA) luminescence attenuation. The optimized experimental parameters were: (1) the Tris-HCl buffer concentration (0.05 mol L−1); (2) the pH of the system (7.4); (3) the amount of organic solvents (30% of methanol and acetonitrile) and the time required to stabilize the signal (30 min). The luminescence (387/498 nm) signal presented a linear range up to 4 × 10−5 mol L−1 of rutin (Y = 3 × 104X + 0.99; r = 0.997). The limit of detection (based on xb − 3 sb of the signal attenuation curve) was 1.2 × 10−6 mol L−1. It was observed that the attenuation of signal was caused by two effects: the contribution from the inner filter effect and a static luminescence quenching component. Luminescence lifetimes of CdS-2MPA in the presence and in the absence of rutin were the same, indicating the static nature of the quenching caused by rutin. No interferences were observed from the flavonoids hesperidin and hesperetin when these were present in concentrations as high as 10 times the one of rutin. In samples containing the flavonoid quercetin, thin layer chromatography (TLC) was used in order to previously separate the analyte since an inner filter effect from quercetin was expected. The TLC/CdS-2MPA attenuation approach enabled recoveries for rutin of approximately 99%. In pharmaceutical formulations and in simulated saliva samples recoveries were between 100 and 104%.

High-performance liquid chromatography with fast-scanning fluorescence detection and multivariate curve resolution for the efficient determination of galantamine and its main metabolites in serum.

Based on green analytical chemistry principles, an efficient approach was applied for the simultaneous determination of galantamine, a widely used cholinesterase inhibitor for the treatment of Alzheimers disease, and its major metabolites in serum samples. After a simple serum deproteinization step, second-order data were rapidly obtained (less than 6 min) with a chromatographic system operating in the isocratic regime using ammonium acetate/acetonitrile (94:6) as mobile phase. Detection was made with a fast-scanning spectrofluorimeter, which allowed the efficient collection of data to obtain matrices of fluorescence intensity as a function of retention time and emission wavelength. Successful resolution was achieved in the presence of matrix interferences in serum samples using multivariate curve resolution-alternating least-squares (MCR-ALS). The developed approach allows the quantification of the analytes at levels found in treated patients, without the need of applying either preconcentration or extraction steps. Limits of detection in the range between 8 and 11 ng mL(-1), relative prediction errors from 7 to 12% and coefficients of variation from 4 to 7% were achieved.

Spectrofluorimetry in organized media coupled to second-order multivariate calibration for the determination of galantamine in the presence of uncalibrated interferences.

The present article describes the spectrofluorimetric determination of galantamine, a widely used acetylcholinesterase inhibitor, through excitation-emission fluorescence matrices and second-order calibration. With the purpose of enhancing the fluorescence intensity of this substance, the effect of different organized assemblies was evaluated. Although the interaction of galantamine with different cyclodextrins is weak, it was corroborated that the fluorescence intensity of this pharmaceutical in the presence of alpha-cyclodextrin is increased by a twofold factor. Among the studied micellar media, the anionic surfactant sodium dodecyl sulfate produced the largest signals for the compound of interest (sixfold enhancement), and was selected as auxiliary reagent for the subsequent determinations. The developed approach enabled the determination of galantamine at the ng mL(-1) level without the necessity of applying separation steps, and in the presence of uncalibrated interferences. The applied second-order chemometric tools were parallel factor analysis (PARAFAC), unfolded partial least-squares coupled to residual bilinearization (U-PLS/RBL), and multidimensional partial least-squares coupled to residual bilinearization (N-PLS/RBL). The ability of U-PLS/RBL to successfully overcome spectral interference problems is demonstrated. The quality of the proposed method was established with the determination of galantamine in both artificial and natural water samples.

Use of hydrogen peroxide to achieve interference-free stripping voltammetric determination of copper at the bismuth-film electrode

In this work, a new approach is presented to allow interference-free determination of Cu (II) by stripping voltammetry using the bismuth-film electrode. The addition of hydrogen peroxide to the electroanalytical cell has promoted complete resolution between re-dissolution peaks of Bi (III) and Cu (II). The absence of interference could be evaluated by the correlation coefficient (r>0.99) between Cu (II) concentration and its shifted current peak (at +212 mV) while achieving a slightly fluctuation of the bismuth current peak at -180 mV. Studies were performed aiming towards the optimum conditions for trace determination of Cu (II) using hydrogen peroxide. The methodology was applied to a real sample (sugarcane spirits) and the results were compared to those from graphite furnace atomic absorption spectrometry. The analytical parameters of merit and the results of the analysis indicated that the analytical methodology could be readily used for trace determination of Cu (II).

Determination of the antiretroviral drug nevirapine in diluted alkaline electrolyte by adsorptive stripping voltammetry at the mercury film electrode.

: This paper describes a stripping method for the determination of nevirapine at the submicromolar concentration levels. The method is based on controlled adsorptive accumulation of nevirapine at thin-film mercury electrode, followed by a linear cyclic scan voltammetry measurement of the surface species. Optimal experimental conditions include a 2.0 x 10(-3) mol L(-1) NaOH solution (supporting electrolyte), an accumulation potential of -0.20 V, and a scan rate of 100 mV s(-1). The response of nevirapine is linear over the concentration range 0.01-0.14 ppm. For an accumulation time of 6 minutes, the detection limit was found to be 0.87 ppb (3.0 x 10(-9) mol L(-1)). More convenient methods to measure the nevirapine in presence of the efavirenz, acyclovir, didanosine, indinavir, nelfinavir, saquinavir, lamivudine, zidovudine and metals ions were also investigated. The utility of this method is demonstrated by the presence of nevirapine together with ATP or DNA.