Simple multi-signal calibrations exploiting flow analysis systems.

Multi-signal calibrations have been recently exploited in molecular spectrochemical analysis alternatively to traditional calibration methods, improving analytical frequency and accuracy. The application of these strategies is simple and minimizes efficiently matrix effects by analyzing two calibration solutions comprising sample plus standard (S1), and sample plus blank (S2). The plot of the signals obtained with S1 and S2 at multiple settings (e.g. different wavelengths) yield a slope that can be related to the analyte concentration in the sample. Similarly, transient signals could also be related to the analyte concentration exploiting a similar strategy. Thus, in this work, two multi-signal approaches developed in flow-based systems are proposed, based on the responses at multiple wavelengths (online multi-energy calibration, OMEC), and on the dispersion profile of the samples, herein denominated multi-dispersion calibration (MDC). The calibrations were carried out with sample solutions after 2-fold dilution with a standard solution and with water. The feasibility of OMEC and MDC were demonstrated using KMnO4 solutions (without chemical reactions) under continuous and pulsed flow regimes. The applicability of this strategy was also demonstrated by the spectrophotometric determination of urea in milk and pet potty spray in a multi-pumping flow system, based on the color change of bromothymol blue after catalyzed hydrolysis by urease from jack beans (Canavalia ensiformis). MDC and OMEC were compared with external calibrations (EC) and classical standard addition. The limits of detection for urea were estimated at 13 mg L-1, 16 mg L-1, and 10 mg L-1 using MDC, MEC and EC, respectively. Recoveries from 93 to 101%, and the agreement of sample analyzes with the reference procedure demonstrated the good accuracy achieved by the proposed methods. Therefore, it was demonstrated the feasibility of MDC and OMEC for analytical purposes in a simple and efficient way with the advantages of flow-based manifolds.

An automated flow system for in-line extraction of proteins and separation of jacalin using a low-cost stationary phase.

Lectins are carbohydrate binding proteins with many physiological and biotechnological applications. Isolation of proteins is normally time-consuming and encompasses multiple and, sometimes, complicated steps that hinder reproducibility and yield. Affinity chromatography is an efficient way to simplify and improve protein purification, however often requiring an expensive and fragile stationary phase. In this regard, automated flow-based systems minimize the time for extraction of species from solid samples without hindering the features of batch procedures. In this work, a new inexpensive affinity-based stationary phase was developed for in-line separation of jacalin, a galactose-binding lectin from jackfruit seeds. In the flow manifold, in-line extraction of proteins was also carried out with continuous monitoring using the spectrophotometric Biuret assay. For protein determination, linear response was observed from 3.0 to 15 g L-1. The results of the analysis of protein extracts from jackfruit seeds obtained with the herein described procedure and batch procedure agreed with 95% confidence level. Quantitative extraction of proteins from jackfruit seed powder required recirculation of extraction buffer for 15 min through a lab-made polymethylmethacrylate (PMMA) column containing 200 mg of the crude seed powder. In the chromatographic step, jacalin was isolated after 300 s. Therefore, three essential steps for jacalin isolation were performed in one manifold in a fast way, minimizing sample consumption and solution handling. Additionally, the versatile and multi-task developed flow manifold can be useful for routine analysis and preparative procedures, being adaptable for the extraction and separation of other species from solid matrixes with continuous monitoring of the processes.

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.

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 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 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.