L.A Tortajada-Genaro

A new flow cell design for chemiluminiscence analysis

The present study proposes a new flow cell called a bundle cell for chemiluminescence analysis. The results obtained were compared with those achieved by manual and automated batch procedures and flow manifolds with different cells: an common quartz flow cell, a helix cell and the most used spiral cell. Figures of merit such as limit of detection, sensitivity, accuracy and precision for the Cr(III) determination were established with light emission produced by catalysed Cr(III) luminol oxidation by hydrogen peroxide in a basic aqueous solution. An improvement in sensitivity about 50% as compared with the spiral cell and even larger with respect to the other flow cells tested was observed. The limit of detection provided was lower than those obtained with the other flow cells. In reference to the batch mode, similar results were obtained with the bundle flow cell. Good results were obtained for several real water samples containing chromium at different concentrations.

Influence of water sample storage protocols in chemiluminescence detection of trace elements

This paper shows the influence of different sample storage protocols, on the chemiluminescence signal of some metal ions. The storage protocols studied were: acid addition (HCl or HNO(3)) and no reagent addition to filtered and refrigerated (T=4 degrees C) samples. Light emission was produced for the chemiluminescence reaction between luminol and hydrogen peroxide in buffer carbonate conditions (pH 10.8) catalysed by Cr(III), Co(II) and Cu(II). Batch and/or flow modes in different conditions were tested. Fe(II), Fe(III), Ni(II) and Mn(II) did not give chemiluminescence in the studied conditions. A parallel study of sensitivity and selectivity was performed. Then the presence or absence of the masking agent EDTA, added to samples or used in the carrier stream, is assayed. If the samples are acidified with HNO(3), a previous neutralisation is needed using batch mode. The determination of Cr(III) is independent of storage protocol by flow injection (FI) method; however, the determination of Co(II) or Cu(II) or total determination of three metals requires the conditioning of standards. Detection limits achieved are ranged between 0.5 and 2 mug l(-1). For batch mode, detection limits are better for unacidified samples and worse for carbonate-neutralised samples. The influence of storage protocols was validated using standard metal mixtures and calibration solutions. The use of standard reference material (SRM(c) 1640) (Trace elements in natural water) corroborates the previous statements and validates the accuracy of the different approaches underlined. This paper demonstrates that it is possible to determine Cr(III) selectively in natural waters.

The generalized H-point standard-additions method to determine analytes present in two different chemical forms in unknown matrix samples. Part I. General considerations

This paper shows how the generalized H-point standard-additions method (GHPSAM) can be used to obtain the total concentration or concentrations of different chemical forms of an analyte when the matrix of the sample is completely unknown. The spectral regions where the interferent behaviour can be considered as linear are found and the analyte concentration free from bias error is estimated. This paper includes the already published features of the GHPSAM and a new modification of this method which allows the simultaneous determination of two chemical forms of an analyte in a sample.

A Guide to Avoid Method Bias of Chromium (III, VI) Chemiluminescence Determination by Luminol-Hydrogen Peroxide Reaction - Application to Water Samples

Cr(III) and/or Cr(VI) determinations based on light emission produced by luminol oxidation by hydrogen peroxide in basic aqueous solution catalyzed by Cr(III) were studied in order to diagnose and/or avoid method bias. The calibration step was optimized, and the usefulness of the method for speciating chromium was tested. The use of the standard addition method in the linear interval concentration range made it possible to diagnose the accuracy of the method for real samples. Good results were obtained for several real water samples containing chromium at different concentrations. The proposed protocol made the method traceable with an appropriate certified reference material and with the reference method.

Multivariate versus univariate calibration for nonlinear chemiluminescence data. Application to chromium determination by luminol-hydrogen peroxide reaction

Abstract Multivariate calibration is tested as an alternative to model chromium(III) concentration versus chemiluminescence registers obtained from luminol-hydrogen peroxide reaction. The multivariate calibration approaches included have been: conventional linear methods (principal component regression (PCR) and partial least squares (PLS)), nonlinear methods (nonlinear variants and variants of locally weighted regression) and linear methods combined with variable selection performed in the original or in the transformed data (stepwise multiple linear regression procedure). Both the direct and inverse univariate approaches have been also tested. The use of a double logarithmic transformation previous to the linear regression has been also evaluated. A new double logarithmic transformation previous to the linear regression is proposed in order to avoid the effect of the noise in the calibration model. Pre-processing, optimization and prediction ability of the multivariate calibration models has been studied at nine different experimental conditions including batch and FIA measurements. Box-plots, PCA and cluster analysis have been employed to test the prediction ability of the different models tested. Nonlinear PCR and nonlinear PLS provide the best results. Real samples have been analyzed and compared with the reference method. The results confirm the successful use of the proposed methodology.

Calibration transfer in chemiluminescence analysis: Application to chromium determination by luminol–hydrogen peroxide reaction

Abstract Direct standardization (DS) and piecewise direct standardization (PDS) methods are applied to multivariate standardization of chemiluminescence signals using PLS model as the calibration model. The linear concentration interval of the chemiluminescence determination was determined. This interval was located using univariate robust calibration by least median of squares (LMS) method. The linear calibration model was corroborated by conventional least-squares method. The standardization subset and window size were optimized by means of the prediction residual error sum of squares. Several instrumental sources of variability have been studied: the detection cell, instrument, batch versus flow injection (FI) modes, time and their possible combinations. This study shows the calibration transfer can be employed successfully. The proposed strategy was used in the chemiluminescence determination of Cr(III), Cr(VI) and total Cr in water samples by means of a calibration transfer. The obtained results are in agreement with reference method values and known spiked amounts, and are independent of the standardization factors.

The generalized H-point standard-additions method to determine analytes present in two different chemical forms in unknown matrix samples. Part II. Cr(VI) determination in water samples by absorption spectrophotometry

The generalized H-point standard-additions method (GHPSAM) is used in order to obtain the total Cr(VI) and chromate concentration in water samples whose matrices are completely unknown. Moreover, a new methodology, which is a modification of the GHPSAM, is proposed for the simultaneous determination of the two major chemical forms of Cr(VI) present in the sample. The method is based on the location of spectral intervals where the behaviour of the interferent absorbance can be considered as linear. From these intervals, the analyte concentration free from bias error can be estimated. Spiked samples of dig and harbour water measured in the UV–visible spectral region have been tested to check the background signal cancellation.

Multivariate standardisation for non-linear calibration range in the chemiluminescence determination of chromium.

Multivariate standardisation is proposed for the successful chemiluminescence determination of chromium based on luminol-hydrogen peroxide reaction. In an extended concentration range, non-linear calibration model is needed. The studied instrumental situations were different detection cells, instruments, assemblies, time and their possible combinations. Chemiluminescence kinetic registers have been transferred using piecewise direct standardisation (PDS) method. The optimisation of transfer parameters has been carried out based on the prediction residual error criteria. Non-linear principal component regression (NL-PCR) and non-linear partial least square regression (NL-PLS) were chosen for modelling the relationship signal-concentration of transferred registers. Good accuracy and precision were obtained for water samples. The concentrations of chromium were statistically in agreement with reference method values and with recovery studies. Therefore, it is possible to transfer chemiluminescence curves without loosing ability of prediction, even the presence of a non-linear behaviour.

Spectrophotometric Determination of Phenols in Water Samples by the GHPSAM Method

Abstract The generalized H-point standard-additions method (GHPSAM) is proposed in order to obtain the phenol concentration in water samples when the matrix is completely unknown. The procedure involves solid-phase extraction in BondElut PPL cartridges and data handling of the UV-visible spectrophotometry measurements. The spectral regions where the unknown interferent behaviour can be considered as linear are found and the analyte concentration free from bias error is estimated. The percentages of recovery of phenols in spiked samples were similar to those obtained by HPLC. Cresols or chlorophenols can be also determined in real samples by this method. The concentration range tested was 0.075 – 12.5 mg L−1 and the limits of detection found were in the 2.4 – 5.4 μg L−1 range. The method has been applied to real harbour water samples. The results obtained are compared to those provided by HPLC.