No abstract

In this work, capillary electrophoresis chips with electrochemical detection have been assessed as creative and selective microfluidic platforms to integrate and simplify on a microscale the traditional methods for complex natural antioxidants determination. Depending on the acid-base properties of the analytes, two approaches (class-selective electrochemical index determination (CSEID) and individual antioxidant determination (IAD)) were investigated for the analysis of nine antioxidants ((+)-catechin, rutin, quercetin, chlorogenic, ferulic, caffeic, protocatechuic, vanillic, and gallic acids) in food samples. First, the novel concept of a class-selective electrochemical index is proposed allowing a fast and reliable determination of the main antioxidant classes (flavonoids and phenolic acids) in less than 100 s. In addition, an impressive separation of nine antioxidants is also offered in less than 260 s with the individual antioxidant determination approach. Qualitative and quantitative performances of both approaches were studied. The analytical figures of merit (i.e., electroosmotic flow (EOF) precision as relative standard deviation (RSD), resolution, signal precision as RSD, limit of detection, limit of quantification, and accuracy as recovery) of both approaches were <4%, approximately 1, < or = 5%, <8 microM, 30 microM, between 91% and 104%, and <4%, < or = 2%, <9%, < or = 6 microM, < or = 20 microM (with the exception of protocatechuic acid, which shows values of 40 and 130 microM, respectively), between 80% and 107% for the CSEID and IAD concepts, respectively, which are excellent for food samples analysis. A set of representative samples was analyzed including apple and pear skins and pulps, red and white wines, and green tea tablets. High agreement was observed between the results of the sample analyses from the two microchip-based approaches, and good correlation was observed with results obtained from traditional methods. Although the prominent phenolic antioxidant classes and compounds were successfully determined, some nonprominent peaks were not detected in the samples when applying the IAD approach. Different integration strategies on microchip platforms were further explored, looking for a simplification of the overall analytical process without losing the excellent analytical characteristics obtained in both approaches. The results are promising and indicative of the progress of analytical microfluidics toward the "plateau of productivity" and the routine laboratory application.

In this work, on-line calibration methods were applied for compensation for matrix effects in Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using three novel multiple sample introduction systems based on Flow Blurring Ò technology. The methods were compared with conventional calibration methods, using a Conikal nebulizer and a cyclonic spray chamber (i.e., Standard Sample Introduction (SSI) system). Experiments were carried out with synthetic samples containing different matrices. The total liquid flow through the multinebulizers was 400 mL min À1 whereas in the SSI system it was 1000 mL min À1 . One type of calibration method tested was external calibration. By using this calibration method, the mean of absolute values corresponding to the relative error values of different multiple sample introduction systems and all the matrices was 14% and uncertainty was 0.6%. When on-line internal standard calibration was used, the mean relative error value dropped to 3% and uncertainty was 0.6%. With on-line standard addition calibration, relative error values went down to 2%. However, uncertainty values increased to 2% in all cases. With all the calibration methodologies, the accuracy and uncertainty of the obtained results were very similar for both standard and multiple sample introduction systems. The main difference was a significant reduction in resource consumption (i.e., samples, reagents and time) when multinebulization systems were used. Sensitivity, precision and limits of detection were evaluated for the different Flow Blurring Ò based systems and SSI system. For most of the emission lines evaluated, all the Flow Blurring Ò based systems gave higher precision values and lower limits of detection than SSI system. A certified reference material (Estuarine Water, LGC6016), without prior sample treatment (i.e., dilution), was analyzed using external calibration with the SSI system and on-line standard addition calibration with Flow Blurring Ò based systems. The certified reference material analysis gave relative error values ranging between +20% and À30% for the SSI system, and between +4% and À2% for Flow Blurring Ò based systems.

A new multinebulizer, based on Flow BlurringÒ technology (FBMN), is evaluated for a simple, fast and direct analysis of organic samples in Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES). Organic samples are analyzed by on-line standard addition calibration using aqueous calibration standards. A Standard Sample Introduction (SSI) system (i.e., MicroMistÒ commercial nebulizer (MM) and a spray chamber) is used for comparison using a conventional standard addition with organic calibration standards. Both systems are coupled to the same cyclonic-type spray chamber and organic samples are nebulized at the same flow rate (100 mL min À1 ). Aerosol characterization revealed that when using the FBMN, practically all the organic primary aerosol volume is contained in droplets smaller than 33 mm, whereas when using the MM nebulizer, it is contained in droplets smaller than 114 mm. The on-line standard addition calibration was tested with diluted oil samples, providing results as accurate as those obtained with the reference system, with percent relative error values ranging from À5% to 4% for the reference system, and slightly lower, from À3% to 3% for the FBMN-based system. Figures of merit estimation shows that sensitivity, precision and limits of detection are better in the on-line calibration analysis than in the conventional one. In particular, long-term stability studies reveal that the addition of water in the on-line standard addition calibration significantly contributes to carbon compounds combustion, and therefore eliminates spectral interferences from carbon compounds and avoids carbon deposits in ICP components. After 2 hours of continuous organic sample introduction, the RSD (%) values ranged between 1.5% and 2% with the FBMN-based system and between 10% and 13% with the SSI system. Accuracy and uncertainty of the proposed on-line calibration was also evaluated in the analysis of various organic samples (i.e., biodiesel certified material and real diesel samples). In the analysis of certified reference material, the relative error values were found to be in the range from À4% to +4% for the SSI system and from À4% to +1.0% for the FBMN-based system. Recovery values of real samples of 5% biodiesel in diesel were, in all cases, close to 100%.