J. Verdú-Andrés

Evaluation and elimination of the blank bias error using the H-point standard addition method : application to spectrophotometric determinations using absorbent blank

Abstract The basis of the H-point standard addition method in the cases where the analyte determination requires the use of an absorbent reagent is considered. The method evaluates and eliminates the blank bias error present in such procedures by using the absorbance increment at two selected wavelengths as the analytical signal of the calibration graphs. Three different determinations were tested: the determination of proteins with the biuret method, thorium with thoron and of magnesium with Titan Yellow. These procedures are better described by the proposed method than by the traditional approach using absorbance values against reagent blank.

New micromethod combining miniaturized matrix solid-phase dispersion and in-tube in-valve solid-phase microextraction for estimating polycyclic aromatic hydrocarbons in bivalves.

Miniaturized matrix solid-phase dispersion (MSPD) was developed for the extraction of common polycyclic aromatic hydrocarbons (PAHs) from bivalve samples (100mg, dry weight). Additional clean-up and analyte enrichment was accomplished by in-tube solid-phase microextraction (SPME). For this purpose the extracts collected after MSPD were diluted with water and injected into a capillary column coated with the extractive phase. This capillary column was connected to the analytical column by means of a switching valve. Separation and quantification of the PAHs were carried out using a monolithic LC column and fluorescence detection. Since the in-tube SPME device allowed the processing of large volumes of the extracts (2.0 mL) excellent sensitivity was achieved, thus making solvent evaporation operations unnecessary. The overall recoveries ranged from 10% to 28% for the studied compounds. The relative standard deviation (RSD) ranged from 2% to 10% for intra-day variation (n=3), and the limits of detection (LODs) were < or =0.6 ng/g (dry weight). The proposed procedure was very simple and rapid (total analysis time was approximately 20 min), and the consumption of organic solvents and extractive phases was drastically reduced. The reliability of the proposed MSPD/in-tube SPME method was tested by analysing several bivalves (mussels and tellins) as well as a standard reference material (SRM).

Generalized H-point standard additions method for analyte determinations in unknown samples

The generalized H-point standard additions method (GHPSAM) is proposed. The method locates the linear spectral behaviour for the unknown global interference, if it exists, in all the spectral regions measured. The linear spectral interval for the interference is located from second derivative data. After this, the method validates the linearity supposition and estimates the analyte concentration free from bias error. So, only the data measured from the method are enough to obtain the unbiased analyte concentration in the presence of unknown interferences. Mixtures of phenol and 4-chloro-2-nitrophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol have been tested in order to check the validity of the method. The method also has been applied to the determination of amphetamine in urine samples. It works well in a lot of spectral situations, except in the cases when the global interference and the analyte are linear in the same spectral region.

Study of the behaviour of the absorbent blanks in analytical procedures by using the H-Point standard additions method (HPSAM)

This paper studies the behaviour of reagent blank in different extractive-colorimetric procedures (determination of sympathomimetic amines with NQS reagent) by using the H-Point Standard Additions Method (HPSAM) in order to study and characterize the different possibilities that the blank can introduce in an analytical procedure. We define two kinds of blanks: the external blank (from reagent alone solutions data) and the internal blank (from extrapolation of reagent plus analyte solutions data). Comparison between both gives the information about the reproducibility of the behaviour of the reagent blank. A procedure to evaluate, and characterize, errors (if they exist) is described, and a guide for optimizing the measuring procedure is presented.

In-tube solid-phase microextraction coupled by in valve mode to capillary LC-DAD: Improving detectability to multiresidue organic pollutants analysis in several whole waters.

A simple and fast capillary chromatographic method has been developed to identify and quantify organic pollutants at sub-ppb levels in real water samples. The major groups of pesticides (organic halogens, organic phosphorous, and organic nitrogen compounds), some hydrocarbons (polycyclic aromatic hydrocarbons), phthalates and some phenols such as phenol and bisphenol A (endocrine disruptors) were included in this study. The procedure was based on coupling, in-tube solid-phase microextraction (IT-SPME) by using a conventional GC capillary column (95% methyl-5% phenyl substituted backbone, 80cmx0.32mm i.d., 3microm film thickness) in the injection valve to capillary liquid chromatography with diode array detection. A comparative study between the IT-SPME manifold and a column-switching device using a C(18) column (35mmx0.5mm i.d., 5microm particle size) has been performed. The IT-SPME procedure was optimal, it allows reaching limits of detection (LODs) between 0.008 and 0.2microg/L. No matrix effect was found and recoveries between 70 and 116% were obtained. The precision of the method was good, and the achieved intra- and inter-day variation coefficients were between 2 and 30%. This procedure has been applied to the screening analysis of 28 compounds in whole waters from several points of the Mediterranean coast (Valencia Community, Spain).

H-Point standard additions method for resolution of binary mixtures with simultaneous addition of both analytes

Abstract The basis of the H-point standard additions method, HPSAM, with simultaneous addition of both analytes is proposed for the resolution of binary mixtures. It is a modification of the previously described H-point standard additions method that permits the resolution of both species from a unique calibration set by making the simultaneous addition of the two analytes. The method uses as analytical signals the absorbances at pairs of wavelengths where each species shows the same absorbance. The required data to apply the method are the absorbance values at the previously selected wavelengths for the sample alone and spiked with both species at known concentrations. Linear relations between absorbance values and added concentration of analyte are found, and the intersection of the lines at the previously selected wavelengths permits to obtain the analyte concentration in the sample. Wavelength pairs can be selected to obtain the most precise results. The effect of the relation of the concentrations of the two species in the standard and in the sample has been studied. Some mixtures of phenol and o-cresol, species highly overlapped (absorption maxima: for phenol at 234.6 and 287.0 nm; for o-cresol at 236.6 and 288.4 nm) and with similar absorptivities have been tested. If the matrix effect is known to be absent, the method can be employed with absortivity coefficients of the pure compounds.

Development of the H-point standard additions method for the use of spectrofluorimetry and synchronous spectrofluorimetry

The basis of the H-point standard additions method (HPSAM) is developed for the use of fluorimetric and synchronous spectrofluorimetric measurements. The method permits the transformation of the indeterminate error resulting from the presence of an interference in the determination of an analyte into a systematic constant error that can be evaluated and permits the determination of the analyte free from bias error. Some examples are presented. Phenol fluorescence calibrations in water have a high interference from the Raman emission band of the solvent. The HPSAM permits the elimination of this interference, because the method can eliminate the blank bias error. Resolution of highly overlapped species, such as phenol-o–cresol mixtures, can be carried out with the method. In order to show this possibility, synchronous spectrofluorimetry was chosen to eliminate the blank signal.

Strategies for the enantiomeric determination of amphetamine and related compounds by liquid chromatography

This paper summarizes recent research on the stereospecific analysis of amphetamine, its analogs and metabolites, by liquid chromatography. The different methods proposed have been evaluated and compared in terms of resolution power, time of analysis, sensitivity, or potential for automation. Chiral derivatization, followed by separation of the diastereomers formed in achiral chromatographic systems, is still the method preferred for the analysis of amphetamines at trace levels, as derivatization also improves analyte detectability. This is the method of choice for the enantiomeric analysis of amphetamines at the low concentrations typically encountered in biological samples. In recent years, special attention has been devoted to the development of alternatives for the automation of the analytical process by integrating the derivatization step into the chromatographic scheme. A promising alternative is the employment of beta-cyclodextrins as chiral selectors, both immobilized on the stationary phase and added to the mobile phase. However, with a few exceptions, beta-cyclodextrins perform better for non-derivatized amphetamines. Therefore, the utility of these selectors in the analysis of biological samples is limited. The reliability of less-used chiral stationary phases (Pirkle type, cellulose based or protein based), as well as methods based on the mathematical treatment of the chromatographic signal, are also discussed.

Liquid chromatographic determination of aliphatic amines in water using solid support assisted derivatization with 9-fluorenylmethyl chloroformate

SummaryA simple and sensitive method has been developed for the liquid chromatographic determination of short-chain aliphatic amines in water. Analytes are retained in solid-phase extraction (SPE) cartridges, and then derivatized by drawing an aliquot of the fluorogeneic reagent 9-fluorenylmethyl chloroformate (FMOC) through the cartridges. After a certain reaction time the derivatives formed are desorbed with acetonitrile. The collected extracts are then chromatographed on a LiChrospher 100 RP18 125 mm×4 mm i.d., 5 μm, column using an acetonitrile-water gradient. The influence of experimental conditions (SPE material, volume of sample, concentration of FMOC, time of reaction and pH) has been investigated. Optimal results have been obtained with C18 SPE cartridges using a sample volume of 5.0 mL. For derivatization, 0.25 mL aliquots of 25 mM FMOC have been used, the reaction time being only 2 min. The method has been applied to the quantification of several aliphatic amines: methylamine, ethylamine, dimethylamine,n-butylamine,n-pentylamine andn-hexylamine. Under the proposed conditions the percentages of analytes retained plus derivatized were of about 54–107% compared to those obtained with direct solution derivatization. The method provided good reproducibility, linearity and accuracy within the 0.050–1.0 mg L−1 concentration range. The limits of detection were in the 0.25–5.0 μg L−1 range. The utility of the described approach has been tested by analysing tap water, river water and industrial waste water.

Application of the H-point standard additions method by using absorbance increment values as analytical signals

This paper demonstrates how the absorbance increment (DeltaA) between two wavelengths selected according to the fundamental criteria for application of the H-Point standard additions method (HPSAM) are only related to the analyte concentration. A procedure for calculation of the unknown analyte concentration with no bias error by applying HPSAM to DeltaA values in much the same way as the method of standard additions (MOSA), is reported. The method was also applied to a calibration with a single standard. The results obtained on 6 samples with maximal separations between 65 and 0 nm are reported. Finally, the proposed method was applied to the resolution of different phenol-o-cresol mixtures.