Marcin Wieczorek

An automatic, vigorous-injection assisted dispersive liquid–liquid microextraction technique for stopped-flow spectrophotometric detection of boron

A novel automatic vigorous-injection assisted dispersive liquid-liquid microextraction procedure based on the use of a modified single-valve sequential injection manifold (SV-SIA) was developed and applied for determination of boron in water samples. The major novelties in the procedure are the achieving of efficient dispersive liquid-liquid microextraction by means of single vigorous-injection (250 µL, 900 µL s(-1)) of the extraction solvent (n-amylacetate) into aqueous phase resulting in the effective dispersive mixing without using dispersive solvent and after self-separation of the phases, as well as forwarding of the extraction phase directly to a Z-flow cell (10 mm) without the use of a holding coil for stopped-flow spectrophotometric detection. The calibration working range was linear up to 2.43 mg L(-1) of boron at 426nm wavelength. The limit of detection, calculated as 3s of a blank test (n=10), was found to be 0.003 mg L(-1), and the relative standard deviation, measured as ten replicable concentrations at 0.41 mg L(-1) of boron was determined to be 5.6%. The validation of the method was tested using certified reference material.

Generalized Calibration Strategy in Analytical Chemistry

A novel calibration methodology named ‘the generalized calibration strategy’ (GCS) is presented that is aimed at verifying and improving the accuracy of analytical determinations. It is based on integration of calibration methods together with such operations as standard addition and sample dilution. At each stage of the sample dilution, the analytical result is estimated on the basis of six values, which are differently resistant to errors caused by interference and non-linearity effects. The process of stepwise elimination of both effects is controlled by comparison of these values. A dedicated flow system is presented. The GCS can be adapted to calibration by all well-known methods: conventional, internal standard, and indirect. This novel calibration strategy has been tested on indirect determination of chlorites in water samples by UV/Vis spectrophotometry.

Univariate analytical calibration methods and procedures. A review

An original focus on univariate calibration as an experimental process of quantitative analysis is presented. A novel classification system is introduced against the background of the present situation concerning nomenclature of calibration methods. Namely, it has been revealed that four methods well-known in analytical chemistry: the conventional method, the internal standard method, the indirect method and the dilution method, can be split into those carried out in both the interpolative and the extrapolative mode. It is then shown that the basic procedures of all these methods can be modified including different approaches, such as matrix-matched technique, spiking the sample with a reactant, bracketing calibration, and others. For the first time (as compared to monographies dealing with univariate calibration) it is reviewed how the methods are mixed and integrated with one another thereby creating new calibration strategies of extended capabilities in terms of enhanced resistance to the interference and non-linear effects - as the main sources of systematic calibration errors. As additional novelty, rationally possible combinations of the calibration methods - not met hitherto in the literature - have been predicted. Finally, some general rules relating to calibration are formulated and the main calibration problems that still need to be solved are displayed.

Development of microextraction by packed sorbent for toxicological analysis of tricyclic antidepressant drugs in human oral fluid.

The aim of this study was to apply microextraction by packed sorbent (MEPS) to the isolation of six tricyclic antidepressants (TCADs): nordoxepin, doxepin, desipramine, nortriptyline, imipramine, and amitriptyline from human oral fluid. Samples were collected from healthy volunteers via free spillage from the oral cavity to disposable test tubes. A method of oral fluid sample pretreatment was developed and optimized in terms of suitability for MEPS extraction and removing of interfering agents (protein, food debris, or air bubbles). Moreover, it was short and simple to perform with limited sample consumption (150μL). Extracts were analysed by UHPLC-MS. The MEPS/UHPLC-MS method was validated at three concentration levels (2.00, 4.00 and 8.00ng/mL) of all analytes in the range 1.25-10.0ng/mL. The following parameters were determined: limit of detection, limit of quantification, precision, and accuracy. For all tested concentration levels, the intra- and inter-day repeatability did not exceeded 8.1% and 12.2%, respectively. Gained LOQ value, 0.50ng/mL, made the MEPS/UHPLC-MS method to be a useful tool in clinical and forensic laboratories, which was demonstrated on the basis of analysis of real samples.

Complementary dilution method: A new version of calibration by the integrated strategy

Abstract A novel version of the integrated approach to calibration is presented as “the complementary dilution method” (CDM). In the series of six solutions a standard and a sample are suggested to be complementarily diluted with each other and with the blank solution, hence four linear calibration graphs may be constructed on the basis of the measurement data obtained. The analyte concentration in a sample is estimated by four values calculated in both the interpolative and extrapolative way. When comparing statistically these values with one to another, the calibration errors caused by both the non‐linear and interference effect can be detected. The flow injection system designed for realization of the CDM method is presented. The manners how the system can be adapted to examination and elimination of the calibration errors are described. The usefulness and performance of the CDM method have been confirmed experimentally on the examples of the iron determination by UV/VIS spectrophotometry and the calcium and magnesium determination by flame atomic absorption spectrometry.

Simultaneous spectrophotometric flow injection determination of phosphate and silicate.

A method for the simultaneous determination of phosphate and silicate based on spectrophotometric measurement at 385 nm of a single peak using a flow injection system with two component calibration is described. In the system, a stream of sample containing both analytes is merged with a stream of ammonium molybdate to form (at 1<pH<2) molybdophosphoric and molybdosilicic acids. Total absorbance of the compounds is registered in a form of a constant signal. Simultaneously, oxalic acid solution is injected into a carrier stream (H2SO4) and then merged with the stream of sample containing the mixture of heteropolyacids. A characteristic peak is registered as a result of selective decomposition of molybdophosphoric acid by oxalic acid. The area (or the absorbance measured at the constant signal) and the absorbance measured at the minimum of the peak can be used as measures corresponding to the phosphate and silicate concentrations, respectively. The time of the peak registration is about 3 min. Two-component calibration with the use of four standard solutions of the phosphate/silicate concentrations established in accordance with 2(2) factorial design was applied. Phosphate and silicate can be determined within the concentration ranges of 0.20-15.00 and 0.20-20.00 mg L(-1), with precision less than 2.7 and 0.9% (RSD), respectively and accuracy better than 6.2% (RE). The detection limit is 0.054 and 0.092 mg L(-1) for phosphate and silicate, respectively. The method was applied to determination of the analytes in certified reference materials of groundwater, wastewater, and river water giving results consistent with the certified values.

Calibration by the standard addition and indicative dilution method in flame atomic absorption spectrometry

A simple novel calibration approach called ‘the standard addition and indicative dilution method’ (SAIDM) is presented. It consists in addition of a standard solution to a sample and in successive dilution of this solution until the signal measured is equal to the signal produced by the undiluted sample. Due to the presence of the analyte in the sample environment, SAIDM can be effectively used when interference effects occur. It has been theoretically and experimentally proved that the method proposed is capable of overcoming strong interferences in flame atomic absorption spectrometry even in such cases where the standard addition method (SAM)—conventionally used for this purpose—fails. In addition, due to the possibility of avoiding the extrapolation process, it offers better precision than SAM. However, it has also been revealed that SAIDM cannot be considered as a versatile calibration approach because it leads to worse analytical results in terms of accuracy than SAM when the interference effect is of a multiplicative character.

Novel electroanalytical method based on the electrostriction phenomenon and its application to determination of Cr(VI) by the flow injection technique

The electrostriction phenomenon is observed in membranes of thickness in the nanometer range, e.g. bilayer lipid membranes or self-assembled monolayers. Strong electrical field of 105-106V/cm in intensity appears when applying the potential lower than 1V to these membranes. Electrostatic forces change the dimensions of a dielectric, hence the membrane compression and decrease in thickness are observed. As a result, increase in the membrane capacitance is recorded. The presented work covers development and application of a new analytical method based on the innovative capacitance-to-frequency conversion method and flow technique. Construction of the novel flow manifold designed for realization of the whole analytical procedure of chromium(VI) determination with the use of capacitance measurements and its operating rules are shown. The main element of the system is a specially designed measurement cell for realization of chromium determination using a three-electrode system. Au electrode with a self-assembled monolayer of thiols grafted with functional groups selectively interacting with the analyte is employed as a dielectric layer. Ag/AgCl electrode and Pt one are used as a reference and auxiliary electrode, respectively. Accuracy, repeatability and reproducibility of the proposed analytical procedure were tested in determination of Cr(VI) in synthetic solutions and environmental water matrices spiked with the analyte. Benefits and drawbacks of the developed manifold were critically discussed.

Single peak parameters technique for simultaneous measurements: Spectrophotometric sequential injection determination of Fe(II) and Fe(III)

Spectrophotometric sequential injection system (SI) is proposed to automate the method of simultaneous determination of Fe(II) and Fe(III) on the basis of parameters of a single peak. In the developed SI system, sample and mixture of reagents (1,10-phenanthroline and sulfosalicylic acid) are introduced into a vessel, where in an acid environment (pH≅3) appropriate compounds of Fe(II) and Fe(III) with 1,10-phenanthroline and sulfosalicylic acid are formed, respectively. Then, in turn, air, sample, EDTA and sample again, are introduced into a holding coil. After the flow reversal, a segment of air is removed from the system by an additional valve and as EDTA replaces sulfosalicylic acid forming a more stable colorless compound with Fe(III), a complex signal is registered. Measurements are performed at wavelength 530 nm. The absorbance measured at minimum of the negative peak and the area or the absorbance measured at maximum of the signal can be used as measures corresponding to Fe(II) and Fe(III) concentrations, respectively. The time of the peak registration is about 2 min. Two-component calibration has been applied to analysis. Fe(II) and Fe(III) can be determined within the concentration ranges of 0.04-4.00 and 0.1-5.00 mg L(-1), with precision less than 2.8% and 1.7% (RSD), respectively and accuracy better than 7% (RE). The detection limit is 0.04 and 0.09 mg L(-1) for Fe(II) and Fe(III), respectively. The method was applied to analysis of artesian water samples.

EXTRAPOLATIVE VERSION OF THE INDIRECT CALIBRATION METHOD

In this work, an idea is theoretically and experimentally developed the adaptation of the indirect method to calibration in the extrapolative mode. The proposed approach enables estimation of the analytical results through an extrapolative method in the case of both positive and negative slopes of calibration dependence. The principle of the method has been presented and its analytical usefulness has been experimentally tested, giving special attention to examination of the possibility of overcoming interferences. The experimental examples encompass the determination of phosphate and chlorite by flame atomic absorption spectrometry (FAAS) and UV/VIS spectrophotometry, respectively, in the presence of different strong interferents. It has been proven that the method can be an effective calibration tool provided the specific instrumental and chemical conditions are fulfilled.