Matrix effect in second-order data: determination of dyes in a tanning process using vegetable tanning agents.

The aim of this paper is to determine the concentration of three dyes throughout the tanning process of leather using vegetable tanning agents with a sequential injection analyser with second-order data treatment. As the vegetable tanning agents used are highly absorbent species, we focus on three aspects: (i) difficulties with the resolution (ii) the reduction in the working concentration range; and (iii) matrix effects. Ideally, second-order instruments provide "second-order advantage"; i.e. calibration is possible in the presence of uncalibrated interfering species. However, if the interfering species change the instrumental response of the analyte (in scale or shape), standard additions must be used to ensure the accuracy of the estimated analyte concentration. Here we study the presence of matrix effects for three dyes in several samples in order to significantly improve the accuracy of predictions in the presence of such effects. We found that there were matrix effects in at least 80% of the samples with an alpha risk of 5%. We used this method to study the exhaustion of dyes in the dyeing process.

Kinetic and adsorption study of acid dye removal using activated carbon.

The adsorption of three acid dyes, Acid Red 97, Acid Orange 61 and Acid Brown 425 onto activated carbon was studied for the removal of acid dyes from aqueous solutions at room temperature (25 degrees C). The adsorption of each dye with respect to contact time was then measured to provide information about the adsorption characteristics of activated carbon. The rates of adsorption were found to conform to the pseudo-second-order kinetics with a good correlation. The experimental isotherms obtained, except for Acid Orange 61 studied in mixture, were of the S-type in terms of the classification of Giles and co-workers. The best fit of the adsorption isotherm data was obtained using the Freundlich model. When a comparative study was made of the results obtained with single and mixed dyes, it can be seen that some of them affect others and modify their behavior in the adsorption process. The results indicate that activated carbon could be employed for the removal of dyes from wastewater.

Sequential injection analysis with second-order treatment for the determination of dyes in the exhaustion process of tanning effluents.

A sequential injection (SI)-DAD spectrophotometric method to control the exhaustion of dyes in a mixture of three dyes from a tanning industry process has been developed. It is based on an interdiffusion process of the sample and reagents which leads to a gradual fall in pH through the channel to the detector recording a data matrix. The aim of this paper is to develop a second-order calibration model that is unaffected by interferents by applying multivariate curve resolution with alternating least squares (MCR-ALS). We obtained a linear calibration in the 5-30mgl(-1) range with a correlation coefficient of 0.999 for each dye with detection limits of 2.6, 3.9 and 2.1mgl(-1) for Acid Red, Acid Brown and Acid Orange, respectively. The simultaneous determination of the three dyes from tanning samples showed a satisfactory precision for the three analytes. The method has been validated comparing the concentration of some spiked samples with the expected concentration using a t-paired test. When we used this method to study the exhaustion of dyes, we found that there were several stages in this process. These data may be the key to optimising the exhaustion process.

Resolution of phenol, and its di-hydroxyderivative mixtures by excitation-emission fluorescence using MCR-ALS Application to the quantitative monitoring of phenol photodegradation.

The photodegradation of phenol using TiO(2) as catalyst was studied and monitored by fluorescence excitation-emission matrix (EEM). Hydroquinone, catechol and resorcinol were the dihydroxyderivative intermediates although in lower concentrations than phenol. The data were analyzed using a three-way multivariate curve resolution alternating least squares method (MCR-ALS) and augmented matrices. The procedure was assessed using synthetic samples prepared with a {4,3} Simplex-lattice design that considered a representative range of analyte concentrations. The results were analyzed in terms of overall RMSEP for the overall data set. A detailed study was made of how the analytes behaved at each concentration level and how the concentration of the other species affected the process. The method was used to quantify phenol in photodegradation samples with an overall prediction error of 5.37%. The conversion values were fitted to pseudo first-order kinetics and the apparent rate constant was calculated to be -4.9x10(-4)+/-5.2x10(-5)min(-1).

Sequential injection analysis for the simultaneous determination of clavulanic acid and amoxicillin in pharmaceuticals using second-order calibration.

In this paper, we report on a method for quantifying clavulanic acid and amoxicillin simultaneously in pharmaceuticals using sequential injection analysis (SIA) with a diode-array spectrophotometric detector and multivariate curve resolution with alternating least squares (MCR-ALS). We optimized the experimental parameters so that the analytical sequence could distinguish the concentrations and spectrum profiles of the species of interest with optimum resolution quality. After establishing the optimum conditions, we quantified clavulanic acid and amoxicillin in four pharmaceuticals. In most cases our results were slightly higher than those in the prospectus of the pharmaceutical. The relative standard deviations were below 5% for amoxicillin and below 7% for clavulanic acid. These results are acceptable because, to prevent degradation due to bacteriostatic activity, the concentration of the main reactant is usually higher.

Chromium speciation using sequential injection analysis and multivariate curve resolution.

In this paper we develop a suitable method for the speciation of chromium in tanning and environmental water samples. We use sequential injection analysis (SIA) with a diode array detector linked to chemometric tools such as multivariate curve resolution-alternating least squares (MCR-ALS) to determine Cr(III) and Cr(VI) species. Although Cr(III) is an absorbent species, its sensitivity is much lower than that of Cr(VI). To increase its sensitivity, therefore, it was complexed with EDTA. This method involves generating a pH gradient in the system reactor that converts dichromate into chromate in such a way that, when the sample reaches the detector, selective areas are observed and a data matrix is obtained. Applying MCR enables Cr(III) and Cr(VI) to be successfully determined simultaneously in tanning and environmental wastewater samples.

Simultaneous analysis of the photocatalytic degradation of polycyclic aromatic hydrocarbons using three-dimensional excitation-emission matrix fluorescence and parallel factor analysis.

Polycyclic aromatic hydrocarbons (PAHs) may be photochemically degraded. Monitoring of degradation process of PAHs is carried out by traditional methods, which normally imply time-consuming procedures that do not allow the chemical process to be analyzed in real time. In the present study, photodegradation kinetics of dibenz[a,h]anthracene, benz[a]anthracene, benz[a]pyrene and benz[k]fluorantene were investigated in aqueous solutions under different conditions. A 2(3) factorial design was used for optimizing the degradation process. Fluorescence spectroscopy is a fast, cheap and sensitive analytical method, attractive for use in conjunction with chemometric methods; in this case three-way analytical methodology based on fluorescence excitation-emission matrix (EEM) and parallel factor analysis (PARAFAC) was employed. A four-factor PARAFAC model made it possible to resolve the species presents in the degradation mixture and quantify the relative concentration of the analytes throughout the degradation. Several different parameters, such as core consistency, percentage of fit and correlation coefficients between recovered and reference spectra were employed to determine the suitable number of factors for the PARAFAC model. This new methodology allows us to determine satisfactorily the PAHs concentration during the photodegradation in mixtures of arbitrary composition, representing an interesting alternative to the conventional techniques normally used for the monitoring of degradation reactions.

Use of multivariate curve resolution for determination of chromium in tanning samples using sequential injection analysis.

We report a method for determining total chromium in tanning samples using sequential injection analysis (SIA) with a diode-array spectrophotometric detector. With a suitable analytical sequence CrO4(2-) is converted to Cr2O7(2-) inside the tubes of the SIA system, after total oxidation of chromium(III). A data matrix is obtained and analysed by several chemometric techniques based on multivariate analysis: principal components analysis, simple-to-use interactive self-modelling mixture analysis, and multivariate curve resolution-alternating least-squares. We studied several samples from different stages of a tanning process. Two of these samples were easily oxidized but the others needed more extreme conditions. The analytical sequence prepared, which was based on obtaining a pH gradient and used H2SO4 as reagent, is valid and independent of the level of oxidation needed for the sample. We established a calibration model and evaluated the figures of merit. In some samples we found interferents. With this method the amounts of chromium in each sample were quantified and the results were statistically similar to those obtained by use of the reference method, atomic absorption spectrometry.

Standardization of a multivariate calibration model applied to the determination of chromium in tanning sewage.

A multivariate standardization procedure was used to extend the lifetime of a multivariate partial least squares (PLS) calibration model for determining chromium in tanning sewage. The Kennard/Stone algorithm was used to select the transfer samples and the F-test was used to decide whether slope/bias correction (SBC) or piecewise direct standardization (PDS) had to be applied. Special attention was paid to the transfer samples since the process can be invalidated if samples are selected which behave anomalously. The results of the F-test were extremely sensitive to heterogeneity in the transfer set. In these cases, it should be taken as an interpretation tool.