Sidnei G. Silva

A flow injection procedure based on solenoid micro-pumps for spectrophotometric determination of free glycerol in biodiesel

A flow system designed with solenoid micro-pumps is proposed for fast and greener spectrophotometric determination of free glycerol in biodiesel. Glycerol was extracted from samples without using organic solvents. The determination involves glycerol oxidation by periodate, yielding formaldehyde followed by formation of the colored (3,5-diacetil-1,4-dihidrolutidine) product upon reaction with acetylacetone. The coefficient of variation, sampling rate and detection limit were estimated as 1.5% (20.0 mg L(-1) glycerol, n=10), 34 h(-1), and 1.0 mg L(-1) (99.7% confidence level), respectively. A linear response was observed from 5 to 50 mg L(-1), with reagent consumption estimated as 345 μg of KIO(4) and 15 mg of acetylacetone per determination. The procedure was successfully applied to the analysis of biodiesel samples and the results agreed with the batch reference method at the 95% confidence level.

Direct determination of sodium, potassium, chromium and vanadium in biodiesel fuel by tungsten coil atomic emission spectrometry

High levels of sodium and potassium can be present in biodiesel fuel and contribute to corrosion, reduced performance and shorter engine lifetime. On the other hand, trace amounts of chromium and vanadium can increase the emission of pollutants during biodiesel combustion. Sample viscosity, immiscibility with aqueous solutions and high carbon content can compromise biodiesel analyzes. In this work, tungsten filaments extracted from microscope light bulbs are used to successively decompose biodiesels organic matrix, and atomize and excite the analytes to determine sodium, potassium, chromium and vanadium by tungsten coil atomic emission spectrometry (WCAES). No sample preparation other than simple dilution in methanol or ethanol is required. Direct analysis of 10-μL sample aliquots using heating cycles with less than 150 s results in limits of detection (LOD) as low as 20, 70, 70 and 90 μg kg(-1) for Na, K, Cr and V, respectively. The procedures accuracy is checked by determining Na and K in a biodiesel reference sample and carrying out spike experiments for Cr and V. No statistically significant differences were observed between reference and determined values for all analytes at a 95% confidence level. The procedure was applied to three different biodiesel samples and concentrations between 6.08 and 95.6 mg kg(-1) for Na and K, and between 0.22 and 0.43 mg kg(-1) for V were obtained. The procedure is simple, fast and environmentally friendly. Small volumes of reagents, samples and gases are used and no residues are generated. Powers of detection are comparable to other traditional methods.

Tungsten coil atomic emission spectrometry combined with dispersive liquid–liquid microextraction: A synergistic association for chromium determination in water samples

A novel and environment friendly analytical method is reported for total chromium determination and chromium speciation in water samples, whereby tungsten coil atomic emission spectrometry (WCAES) is combined with in situ ionic liquid formation dispersive liquid-liquid microextraction (in situ IL-DLLME). A two stage multivariate optimization approach has been developed employing a Plackett-Burman design for screening and selection of the significant factor involved in the in situ IL-DLLME procedure, which was later optimized by means of a circumscribed central composite design. The optimum conditions were complexant concentration: 0.5% (or 0.1%); complexant type: DDTC; IL anion: PF6(-); [Hmim][Cl] IL amount: 60 mg; ionic strength: 0% NaCl; pH: 5 (or 2); centrifugation time: 10 min; and centrifugation speed: 1000 rpm. Under the optimized experimental conditions the method was evaluated and proper linearity was obtained with a correlation coefficient of 0.991 (5 calibration standards). Limits of detection and quantification for both chromium species were 3 and 10 µg L(-1), respectively. This is a 233-fold improvement when compared with chromium determination by WCAES without using preconcentration. The repeatability of the proposed method was evaluated at two different spiking levels (10 and 50 µg L(-1)) obtaining coefficients of variation of 11.4% and 3.6% (n=3), respectively. A certified reference material (SRM-1643e NIST) was analyzed in order to determine the accuracy of the method for total chromium determination and 112.3% and 2.5 µg L(-1) were the recovery (trueness) and standard deviation values, respectively. Tap, bottled mineral and natural mineral water samples were analyzed at 60 µg L(-1) spiking level of total Cr content at two Cr(VI)/Cr(III) ratios, and relative recovery values ranged between 88% and 112% showing that the matrix has a negligible effect. To our knowledge, this is the first time that combines in situ IL-DLLME and WCAES.

A green analytical procedure for determination of copper and iron in plant materials after cloud point extraction

A green analytical procedure exploiting cloud point extraction for determination of copper and iron in plant materials is presented. After sample digestion with a diluted oxidant mixture (HNO3 + H2O2) in a closed vessel microwave oven, metal ions were simultaneously extracted as hydrophobic complexes formed with 1,2-tiazolylazo-2-naphthol (TAN) using Triton X-114 as extracting agent. Copper and iron were determined by flame atomic absorption spectrometry (FAAS) and iron also by UV-Vis spectrophotometry. For FAAS, the detection limits (99.7% confidence level) were estimated as 1 and 10 μg L-1 for copper and iron, respectively. Linear responses for both analytes were observed in the 25-200 μg L-1 range. The enrichment factors were estimated as 30 for both analytes and the extraction was quantitative, as evaluated by measurements in the supernatant solution remaining after extraction. For iron determination by UV-Vis spectrophotometry, the detection limit was estimated as 1 μg L-1, with linear response within 6 and 60 μg L-1. The accuracy of the procedure was evaluated by determination of the metals in certified reference materials and the results agreed with the certified values at the 95% confidence level. In the whole procedure, the reagent consumption was 1 mL HNO3, 1.1 mL H2O2, 50 mg Triton X-114 and 150 μg TAN per determination.

An environmentally friendly analytical procedure for nickel determination by atomic and molecular spectrometry after cloud point extraction in different samples

Cloud point extraction (CPE) was employed for separation and preconcentration prior to the determination of nickel by graphite furnace atomic absorption spectrometry (GFAAS), flame atomic absorption spectrometry (FAAS) or UV-Vis spectrophotometry. Di-2-pyridyl ketone salicyloylhydrazone (DPKSH) was used for the first time as a complexing agent in CPE. The nickel complex was extracted from the aqueous phase using the Triton X-114 surfactant. Under optimized conditions, limits of detection obtained with GFAAS, FAAS and UV-Vis spectrophotometry were 0.14, 0.76 and 1.5 μg L−1, respectively. The extraction was quantitative and the enrichment factor was estimated to be 27. The method was applied to natural waters, hemodialysis concentrates, urine and honey samples. Accuracy was evaluated by analysis of the NIST 1643e Water standard reference material.

Cobalt as chemical modifier to improve chromium sensitivity and minimize matrix effects in tungsten coil atomic emission spectrometry

Cobalt is used as chemical modifier to improve sensitivity and minimize matrix effects in Cr determinations by tungsten coil atomic emission spectrometry (WCAES). The atomizer is a tungsten filament extracted from microscope light bulbs. A solid-state power supply and a handheld CCD-based spectrometer are also used in the instrumental setup. In the presence of 1000 mg L(-1) Co, WCAES limit of detection for Cr (λ=425.4 nm) is calculated as 0.070 mg L(-1); a 10-fold improvement compared to determinations without Co modifier. The mechanism involved in such signal enhancement is similar to the one observed in ICP OES and ICP-MS determinations of As and Se in the presence of C. Cobalt increases the population of Cr(+) by charge transfer reactions. In a second step, Cr(+)/e(-) recombination takes place, which results in a larger population of excited-state Cr atoms. This alternative excitation route is energetically more efficient than heat transfer from atomizer and gas phase to analyte atoms. A linear dynamic range of 0.25-10 mg L(-1) and repeatability of 3.8% (RSD, n=10) for a 2.0 mg L(-1) Cr solution are obtained with this strategy. The modifier high concentration also contributes to improving accuracy due to a matrix-matching effect. The method was applied to a certified reference material of Dogfish Muscle (DORM-2) and no statistically significant difference was observed between determined and certified Cr values at a 95% confidence level. Spike experiments with bottled water samples resulted in recoveries between 93% and 112%.

Magnesium nitrate as a chemical modifier to improve sensitivity in manganese determination in plant materials by tungsten coil atomic emission spectrometry

In this work, magnesium nitrate is used for the first time as a chemical modifier in atomic emission spectrometry to improve sensitivity and minimize matrix effects in Mn determination. Magnesium–Mn interactions in the condensed and gas phases contribute to increasing the population of excited-state Mn atoms, which result in improved sensitivity, precision and accuracy in plant analysis by tungsten coil atomic emission spectrometry (WCAES). The limit of detection calculated for Mn determination in the presence of 750 mg L−1 Mg was 0.05 mg L−1, which is a 17-fold improvement when compared to determination without using the chemical modifier. Repeatabilities also improved from 6.4 to 3.6% (RSD, n = 10, Mn 2.5 mg L−1) when Mg was used, and linear responses were observed in the 0.1–7.5 mg L−1 Mn range. Manganese concentrations in standard reference materials of plants were 2-fold higher than the certified values for determinations without the modifier. For determinations using Mg(NO3)2, no statistically significant difference was observed between determined and certified Mn values at the 95% confidence level.

A flow injection procedure using Layered Double Hydroxide for on line pre-concentration of fluoride

This work showed a flow system designed with solenoid valves for preconcentration of fluoride using SPADNS method in water samples. The analyte was preconcentrated in a mini-column coated with Layered Double Hydroxides (LDH) used as adsorbent. Then, the fluoride ions were eluted with 0.5molL-1 sodium hydroxide and determined by spectrophotometry. The variables that affect the system such adsorbent mass, type of eluent, solutions flow rate, reagent concentration and pH effect were critically evaluated. Under optimized conditions, the detection limit, coefficient of variation, linear range and preconcentration factor were estimated at 15µgL-1 (99.7% confidence level), 0.8% (500µgL-1, n = 10), 50-500µgL-1 and 10, respectively. The accuracy of the method was evaluated by analysis of ALPHA APS 1076 (Simulated Rain Water) certified material, the values were not significantly different at a 95% level of confidence. The method was applied for fluoride determination in water samples and the levels found were below the maximum values established by Brazilian environmental and health legislations.

A Multicommuted Flow System for Spectrophotometric Determination of Formaldehyde in Mushroom

Formaldehyde is a carcinogenic compound that was employed as fungicide in the past. Despite its disuse, some foods such as mushrooms naturally produce formaldehyde, showing the importance of its determination. In this work, a mechanized flow system based on solenoid valves is proposed for spectrophotometric formaldehyde determination in mushrooms. The analyte was extracted from fresh and dried mushrooms samples by steam distillation with deionized water in the presence of phosphoric acid. The determination was based on the reaction between formaldehyde and acetylacetone in presence of ammonium acetate (Hantzsch reaction), leading to the formation of 3,5-diacetyl-1,4-dihydrolutidine (DDL, λmax = 412 nm). Linear response from 0.2 to 7.0 mg L was achieved, with detection limit estimated at 0.02 mg L (99.7% confidence level). Coefficient of variation and sample throughput were 1.1% (n = 15) and 17 h, respectively. Per determination, 11 mg of acetylacetone and 30 mg of ammonium acetate were consumed, generating 4.2 mL of waste. The procedure was applied to formaldehyde determination in fresh and dried shiitake, and fresh shimeji, and the results agreed with those attained by reference method at the 95% confidence level.