Diogo L. Rocha

A fast and environmental friendly analytical procedure for determination of melamine in milk exploiting fluorescence quenching.

An environmental friendly procedure was developed for fast melamine determination as an adulterant of protein content in milk. Triton X-114 was used for sample clean-up and as a fluorophore, whose fluorescence was quenched by the analyte. A linear response was observed from 1.0 to 6.0mgL(-1) melamine, described by the Stern-Volmer equation I°/I=(0.999±0.002)+(0.0165±0.004) CMEL (r=0.999). The detection limit was estimated at 0.8mgL(-1) (95% confidence level), which allows detecting as low as 320μg melamine in 100g of milk. Coefficients of variation (n=8) were estimated at 0.4% and 1.4% with and without melamine, respectively. Recoveries to melamine spiked to milk samples from 95% to 101% and similar slopes of calibration graphs obtained with and without milk indicated the absence of matrix effects. Results for different milk samples agreed with those obtained by high performance liquid chromatography at the 95% confidence level.

A critical review on photochemical conversions in flow analysis.

Photochemical conversions are cost-effective and environmental friendly processes that require mild experimental conditions and avoid generation of highly acidic wastes. Treated samples are then compatible with most of the analytical techniques. These characteristics become more relevant when the photoconversions are accomplished to flow analysis, thus allowing exploitation of incomplete reactions, the effective use of the photogenerated unstable radicals and in-line sample treatment. Decreasing of reagent consumption and waste generation, sample processing in a closed environment, and improvement of efficiency of the photochemical processes are other inherent advantages. These aspects are critically reviewed in this article, which emphasizes applications to fractionation and speciation analysis, photo-induced luminescence, miniaturization, and in-line waste treatment. Design of flow-through photochemical cells, use of auxiliary reagents in homogeneous and heterogeneous media, and configurations of flow manifolds are also discussed.

A multi-pumping flow system with on-line photochemical conversion and improved sensitivity for phosphorus fractionation in freshwaters

A flow-based procedure with solenoid micro-pumps was developed for phosphorus fractionation (dissolved organic and inorganic phosphorus) in freshwaters. The spectrophotometric detection was based on the formation of molybdenum blue and the organic species were on-line photo-converted to orthophosphate. The analytical response was linear within 10 and 75 µg L−1 with a detection limit of 2.0 µg L−1 (99.7% confidence level). Coefficient of variation of 1.8% (50 µg L−1 P, n = 20) and sampling rate of 40 determinations per hour were achieved. Per determination, 160 µg (NH4)6Mo7O24, 10 µg SnCl2, 640 µg K2S2O8 and 10 mg NaOH were consumed, generating 2.0 mL of waste. Slopes of analytical curves obtained for four different organic phosphorus species agreed with those obtained for orthophosphate, indicating quantitative conversion. The results for freshwater samples agreed with those obtained by the AOAC reference procedure at 95% confidence level. The organic matter did not interfere in the photo-oxidative process. The proposed procedure is a fast and environmentally friendly alternative for the phosphorus fractionation in freshwaters.

A sensitive flow-based procedure for spectrophotometric speciation analysis of inorganic bromine in waters

A flow-based system with solenoid micro-pumps and long path-length spectrophotometry for bromate and bromide determination in drinking water is proposed. The method is based on the formation of an unstable dye from the reaction between bromate, 2-(5-dibromo-2-pyridylazo)-5-(diethylamino)phenol (5-Br-PADAP) and thiocyanate ions. A multivariate optimization was carried out. A linear response was observed between 5.0 and 100 µg L(-1) BrO3(-) and the detection limit was estimated as 2.0 µg L(-1) (99.7% confidence level). The coefficient of variation (n=20) and sampling rate were estimated as 1.0% and 40 determinations per hour, respectively. Reagent consumption was estimated as 0.17 µg of 5-Br-PADAP and 230 μg of NaSCN per measurement, generating 6.0 mL of waste. Bromide determination was carried out after UV-assisted conversion with K2S2O8 using 300 µL of sample within the range 20-400 µg L(-1) Br(-). The generated bromate was then determined by the proposed flow system. The results for tap and commercial mineral water samples agreed with those obtained with the reference procedure at the 95% confidence level. The proposed procedure is therefore a sensitive, environmentally friendly and reliable alternative for inorganic bromine speciation.

Flow-based food analysis: an overview of recent contributions

Analysis of food and beverages (e.g. determination of nutrients, additives, and contaminants) is benefited by the advantages provided by flow systems such as high precision and sampling rate, as well as low reagent consumption and waste generation. Flow manifolds have been based on several detection techniques (e.g. luminescence, spectrophotometry, electroanalysis, and mass spectrometry), and have been employed for sample handling and chemical derivatization also coupled to separation techniques. Time-consuming steps such as sample preparation and calibration, including exploitation of the standard addition method have been performed on-line in a faster and cleaner way. In addition, flow systems are advantageous for the use of modified electrodes, biosensors, and optosensors. This review is focused on the critical evaluation of the potential of selected flow-based analytical systems for food analysis, highlighting the applications after 2012.

Tracer-monitored flow titrations

The feasibility of implementing tracer-monitored titrations in a flow system is demonstrated. A dye tracer is used to estimate the instant sample and titrant volumetric fractions without the need for volume, mass or peak width measurements. The approach was applied to spectrophotometric flow titrations involving variations of sample and titrant flow-rates (i.e. triangle programmed technique) or concentration gradients established along the sample zone (i.e. flow injection system). Both strategies required simultaneous monitoring of two absorbing species, namely the titration indicator and the dye tracer. Mixing conditions were improved by placing a chamber with mechanical stirring in the analytical path aiming at to minimize diffusional effects. Unlike most of flow-based titrations, the innovation is considered as a true titration, as it does not require a calibration curve thus complying with IUPAC definition. As an application, acidity evaluation in vinegars involving titration with sodium hydroxide was selected. Phenolphthalein and brilliant blue FCF were used as indicator and dye tracer, respectively. Effects of sample volume, titrand/titrant concentrations and flow rates were investigated aiming at improved accuracy and precision. Results were reliable and in agreement with those obtained by a reference titration procedure.

In-Line Incomplete Solid-Liquid Extraction for Spectrophotometric Determination of Soluble Inorganic Phosphorus in Plants and Foods

Phosphorus determination in plant materials and foods is important. For accessible inorganic P assessment, solid-liquid extraction (SLE) is required. In this work, a flow-based analytical procedure employing zones penetration approach was developed aiming at the soluble inorganic phosphorus (PI) determination in plants and foods. The analyte was quantified by the spectrophotometric molybdenum blue method. Linear response was observed between 2.0 and 30 mg L PI, and detection limit was estimated at 0.50 mg L PI (99.7% confidence level, n = 20). Determination rate was estimated at 20 h using in-line extraction with 20 mg of sample. Precise timing of the flow system allowed the use of incomplete SLE for the first time prior to PI determination. The online extraction was carried out for 60 s, leading to an extraction efficiency of 65 ± 3% in comparison to batch quantitative extraction. The results obtained by the proposed procedure were in agreement with those obtained with batch extractions (95% confidence level).

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.