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Analytical and Bioanalytical Chemistry (v.374, #5)
International comparability of chemical measurement results by R. Wielgosz (pp. 767-771).
The international system of units (SI) is an internationally recognized system based on standards of long-term stability; by the use traceable measurements it provides an international infrastructure for realizing comparable measurements. The work of the Consultative Committee for Amount of Substance (CCQM) and the implementation of the Mutual Recognition Arrangement (MRA) are facilitating an international programme for metrology in chemistry to extend this infrastructure to the field of chemical measurements. The major points of this programme, which include the execution of international comparisons and the construction of a key comparison and calibration database at the BIPM, are described.
Keywords: Metrology Key comparison SI Measurement
New IUPAC recommendations on the measurement of pH – background and essentials by Friedrich G. Baucke (pp. 772-777).
The IUPAC Recommendations on pH (1985) have serious metrological deficiencies (recommendation of two pH scales and of several pH definitions and procedures to measure pH). Background and essential features of new recommendations, which replace the 1985 document, are reported in this paper. The new document is strictly based on metrological principles. pH is defined (notionally) by the negative logarithm of the hydrogen ion activity according to Sørensen and Linderstrøm-Lang (1924), that is pH=–lga H. Because pH is a single ion quantity it is immeasurable and is therefore experimentally verified, with stated uncertainties, by pH(PS) values of primary standard buffer solutions. The assignment of pH(PS) is carried out in a Harned cell (without transference), which is defined as a primary method of measurement, and involves the Bates-Guggenheim convention. pH(PS) is thus a conventional quantity. Consideration of the uncertainty of the Bates-Guggenhein convention, however, permits its incorporation into the internationally accepted SI system of measurement. Comparison of the pH of secondary buffer solutions with pH(PS) values in recommended cells with transference yields secondary standards, whose pH(SS) can be traced back to pH(PS) and consequently to the definition of pH. The traceability chain is continued "downwards" by practical cells with transference containing glass electrodes for the measurement of pH(X) values of unknown solutions, for which three calibration procedures are recommended. The measurement of pH is thus represented by the traceability chain pH(X)→pH(SS)→pH(PS)→pH as defined, each step having stated uncertainties. This hierarchical system of measurement excludes any pH 'scale'. Tabulated pH(PS) values are given as examples, and it is recommended that actual pH(PS) and pH(SS) be taken from certificates, which are to accompany each lot of certified reference material (CRM). Target uncertainties and examples of their calculation, a sign convention for pH cells and conventions for presenting cell schemes are given in the new document.
Keywords: IUPAC recommendations pH definition pH measurement Traceability Standard buffer solutions
Traceability of pH measurements by glass electrode cells: performance characteristic of pH electrodes by multi-point calibration by R. Naumann; Ch. Alexander-Weber; R. Eberhardt; J. Giera; P. Spitzer (pp. 778-786).
Routine pH measurements are carried out with pH meter-glass electrode assemblies. In most cases the glass and reference electrodes are thereby fashioned into a single probe, the so-called 'combination electrode' or simply 'the pH electrode'. The use of these electrodes is subject to various effects, described below, producing uncertainties of unknown magnitude. Therefore, the measurement of pH of a sample requires a suitable calibration by certified standard buffer solutions (CRMs) traceable to primary pH standards. The procedures in use are based on calibrations at one point, at two points bracketing the sample pH and at a series of points, the so-called multi-point calibration. The multi-point calibration (MPC) is recommended if minimum uncertainty and maximum consistency are required over a wide range of unknown pH values. Details of uncertainty computations for the two-point and MPC procedure are given. Furthermore, the multi-point calibration is a useful tool to characterise the performance of pH electrodes. This is demonstrated with different commercial pH electrodes. Electronic supplementary material is available if you access this article at http://dx.doi.org/10.1007/s00216-002-1506-5. On that page (frame on the left side), a link takes you directly to the supplementary material.
Keywords: Metrology pH Measurement traceability Measurement uncertainty
Improved reliability of pH measurements by Petra Spitzer; Barbara Werner (pp. 787-795).
Measurements of pH are performed on a large scale at laboratory level, and in industry. To meet the quality-control requirements and other technical specifications there is a need for traceability in measurement results.The prerequisite for the international acceptance of analytical data is reliability. To measure means to compare. Comparability entails use of recognised references to which the standard buffer solutions used for calibration of pH meter–electrode assemblies can be traced.The new recommendation on the measurement of pH recently published as a provisional document by the International Union on Pure and Applied Chemistry (IUPAC) enables traceability for measured pH values to a conventional reference frame which is recognised world-wide. The primary method for pH will be described.If analytical data are to be accepted internationally it is necessary to demonstrate the equivalence of the national traceability structures, including national measurement standards. For the first time key comparisons for pH have been performed by the Consultative Committee for Amount of Substance (CCQM, set up by the International Bureau of Weights and Measures, BIPM) to assess the equivalence of the national measurement procedures used to determine the pH of primary standard buffer solutions. The results of the first key comparison on pH CCQM-K9, and other international initiatives to improve the consistency of the results of measurement for pH, are reported.
Keywords: Metrology pH measurement Primary standard Traceability
Extended traceability of pH: an evaluation of the role of Pitzer's equations by Günther Meinrath (pp. 796-805).
The 2002 IUPAC recommendation on pH (provisional) has taken its own philosophy to provide a basis for comparable and traceable assignment of a value, from a measurement, to the quantity pH. Whereas the substituted 1983 IUPAC recommendation relied heavily on precisely prescribed experimental techniques and procedures, the current recommendation defines a hierarchical relationship between references for comparison (primary and secondary standards) and objective criteria on the comparison of measurements with these standards. The recommendation aims at a traceability chain from the national metrological institution (NMI) level down to field and laboratory measurements. Currently, however, the traceability chain is developed to the level of certified reference materials (CRM), namely the above mentioned primary and secondary standards. To complete the traceability chain, several theoretical and practical aspects have to be pondered. In part, the methods for comparative assessment of different options have yet to be developed. As an illustrating example of the complexity of issues to be considered in a further extension of the traceability chain is estimation of the doubt associated with Pitzer coefficients. The Pitzer equations for activity coefficient modelling are explicitly mentioned in the 2002 IUPAC recommendation on pH (provisional) as enabling possible improvement in the ionic strength extrapolations to zero ionic strength. An assessment of uncertainty of ternary Pitzer coefficients is given for the first time.
Keywords: Metrology pH measurement Pitzer coefficients Uncertainty analysis Latin hypercube sampling Bootstrap
Acidity function p(a H γ Cl) as a step to pH assessment by Maria Camões; Maria Guiomar Lito (pp. 806-812).
The conventional assignment of pH reference buffer standards, pH(S), is achieved by means of a series of procedures that follow from measurement of Harned cell potentials for an electrolyte solution which is the buffer solution of interest. An intermediate step is assessment of the acidity function p(a H γ Cl)0, the extrapolated value of a linear representation of the dependence of p(a H γ Cl) on m Cl for at least three different molalities, m Cl, of added alkali chloride (0.005; 0.010; 0.015 mol kg–1 KCl). This experimental value can be compared with a theoretically expected value calculated from the dissociation constants of the buffer species. Whereas these calculations always give negative slopes for diprotic and triprotic acids and zero slope for monoprotic acids, experimental values with negative or positive slopes can be obtained for well fitting straight lines obtained for buffer solutions with ionic strengths from 0.0025 to 0.144 mol kg–1. Such disagreement between theoretically and experimentally obtained values introduce an extra source of uncertainty in the establishment of pH(S) and on its traceability chain. In this work examples are presented and discussed for which the discrepancy between expected and experimental values leads to different intercept p(a H γ Cl)0.
Keywords: Acidity function Extrapolation Uncertainty Added chloride
pH measurements in non-aqueous and aqueous–organic solvents – definition of standard procedures by Sandra Rondinini (pp. 813-816).
pH standardisation procedures in non-aqueous and aqueous–organic solvents are discussed in the light of the newly prepared IUPAC recommendation on measurement of pH in dilute aqueous solutions. Both scientific and metrological aspects are considered, as required by the definitions of primary and secondary methods of measurements recently endorsed by BIPM (Bureau International de Poids et de Mesures, France).
Keywords: pH measurement pH standards Primary methods Secondary methods
Secondary pH standards and their uncertainty in the context of the problem of two pH scales by Rouvim Kadis (pp. 817-823).
Establishing a traceability route with all measurement and uncertainty relationships determined is an important aspect of traceability, and seems to be particularly striking in pH measurement. In this paper the issue of evaluation of secondary pH standards measured with reference to a primary standard in a differential cell with free diffusion type liquid junctions is considered. Relatively high uncertainty, U=0.015, has been assigned to such standards in the recent IUPAC Recommendations on pH (2001), because of a specific residual liquid-junction potential treated statistically as a contribution to the combined uncertainty. Close inspection of the problem leads to the conclusion that a correction for the residual liquid-junction potential should be applied to the measured value of a secondary pH standard. This can be considered as a correction for a known systematic effect on the traceability route. With available experimental data it is demonstrated that such a correction can reasonably be made for well-studied standard buffer systems. In this way the uncertainty associated with secondary pH standards is kept to a low level, and, what is more, the problem of two pH scales, a multi-standard scale and a single-standard scale, gains a proper solution. The need for different treatment of residual liquid-junction potentials at different levels in the measurement hierarchy is noted. Much attention is also given to rational categorization of pH standards in the hierarchy.
Keywords: pH measurement pH scales pH standards Measurement uncertainty Traceability chain
Uses of pH electrodes in nuclear chemistry by C. Ekberg; S. Andersson; M. Olsson (pp. 824-830).
The determination and definition of pH is a controversial subject in many areas in chemistry. For these reasons the International Union of Pure and Applied Chemistry (IUPAC) has developed recommendations for pH measurement. These recommendations are currently (winter 2001) under revision – there will be increased emphasis on traceability of uncertainties in pH measurement. Here we describe how glass electrodes designed for measurement of pH are used in nuclear chemistry. The use of pH electrodes is then related to the IUPAC recommendations. In applied chemistry, e.g. nuclear chemistry, a pH is not sought as often as a hydrogen ion concentration or a simple equilibrium point during a titration. Ionic strengths are, moreover, often above the range in which the IUPAC recommendations apply. In these instances uncertainties must be assessed individually.
Keywords: pH measurements Potentiometric titrations Surface charge Solvent extraction
Measurement uncertainty in the pH measurement procedure by Ralf Degner (pp. 831-834).
A measured value without even an approximate knowledge of the uncertainty is worthless. The uncertainty is part of every measured value and specification of the uncertainty is part of every analytical procedure. The uncertainty makes the value independent of its origin. The basis for estimation of the uncertainty is the "Guide to the Expression of Uncertainty in Measurement ". For some procedures, however, for example pH measurement, several problems arise in practice. This article describes a practical and inexpensive way of calculating the uncertainty of pH values.
Keywords: Uncertainty pH Process optimization Quality control
Separation and identification of organic acid-coenzyme A thioesters using liquid chromatography/electrospray ionization-mass spectrometry by Joseph J. Dalluge; Steven Gort; Russell Hobson; Olga Selifonova; Frank Amore; Ravi Gokarn (pp. 835-840).
A method has been developed for the direct determination of coenzyme A (CoA) and organic acid-CoA thioesters in mixtures using directly combined liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS). Mixtures of CoA and organic acid-CoA thioesters were analyzed by LC/ESI-MS with detection of protonated molecular ions and characteristic fragment ions for each compound. The identities of the CoA-thioesters were established based on LC retention times and simultaneously recorded mass spectra. Monitoring of the CoA specific fragment ion at m/z 428 throughout the chromatogram provides a unique fingerprint for CoA content in the samples that corroborates the identification of organic acid-CoA thioesters in the mixtures. Furthermore, fragment ions arising from the ester linkage portion of the molecule allow unambiguous identification of the CoA esters in the samples. A second LC elution system was developed that allows the simultaneous separation and identification of 2-hydroxypropionyl-CoA (lactyl-CoA) and 3-hydroxypropionyl CoA (3HP-CoA), which have the same mass and identical MS fragmentation behavior. The utility of LC/ESI-MS employing this elution system is demonstrated by the determination of 3HP-CoA and lactyl-CoA (converted to CoA-thioesters from their corresponding free acids using CoA-transferase) in fermentation broths from Escherichia coli strains engineered for the production of 3-hydroxypropionic acid (3HP). External calibration employing a purified 3HP-CoA standard allowed indirect quantification of 3HP content in the broth with a precision of 1% (RSD). The feasibility of extending the method described above to perform LC/selected reaction monitoring-tandem mass spectrometry for direct determination of organic acid-CoA thioesters in cells was also demonstrated.
Keywords: Liquid chromatography/mass spectrometry Liquid chromatography/tandem mass spectrometry Metabolite Metabolomics Metabolic engineering
Whole-cell biosensing of 3-chlorocatechol in liquids and soils by X. Guan; Elisa d'Angelo; W. Luo; S. Daunert (pp. 841-847).
A rapid and sensitive technique is needed to analyze water and soils for chlorocatechols, common environmental pollutants produced from wood pulp chlorination and other processes. The soil bacteria Pseudomonas putida, harboring plasmid pSMM50R-B′, selectively express β-galactosidase in response to 3-chlorocatechol in pure water samples. The objective of the study was to determine whether background matrices in fresh water, sea water, soils, and organic solvents interfered with 3-chlorocatechol analysis by use of a bacteria-sensing system and by high-performance liquid chromatography (HPLC). Although 3-chlorocatechol detection by HPLC was not substantially affected by the background composition of aqueous or organic solvents, HPLC was ineffective in the analysis of contaminated soils due to irreversible contaminant sorption. Whereas detection by the bacteria-sensing system was reduced in the presence of aqueous and organic solvents, interferences could be reduced by sample dilution. 3-Chlorocatechol was detected when the bacteria were added directly to contaminated soils, suggesting that the organism enhanced desorption or had access to the sorbed compounds. Results indicate that the bacteria-sensing system has wide application for detection of 3-chlorocatechols in environmental samples, especially in soils where extraction and HPLC analysis are not efficient due to extensive contaminant sorption.
Keywords: Bioavailable Matrix interference High-performance liquid chromatography Genetically engineered microorganism Galactosidase Pseudomonas putida
Assessing the transfer of pesticides to the atmosphere during and after application. Development of a multiresidue method using adsorption on Tenax and thermal desorption-GC/MS by Olivier Briand; Maurice Millet; Florence Bertrand; Michel Clément; René Seux (pp. 848-857).
An air sampling and analytical method based on adsorption on porous polymer (Tenax TA) followed by automatic thermal desorption (ATD) and GC/MS analysis was developed for ten pesticides commonly used on major crops in Britanny and some of their metabolites in air (from spray drift and volatilisation transfer processes): alachlor, atrazine (and two major degradation products: deethylatrazine and deisopropylatrazine), carbofuran, cyprodinil, epoxyconazole, iprodione (and 3,5-dichloroaniline), lindane (and -HCH, its isomer), metolachlor, terbuconazole and trifluralin. This method was established with special consideration for optimal thermal desorption conditions, linear ranges, limits of detection and quantification. Moreover, collection efficiencies of Tenax TA at room temperature were examined. This method was then applied to the determination of ambient pesticide levels during the spraying season at a rural area. The method was also applied to determine the vertical gradient of alachlor concentrations on a treated maize parcel to evaluate volatilisation fluxes.
Keywords: Pesticides Atmosphere Thermal desorption GC-MS
Quantification of the refrigerants R22 and R134a in mixtures by means of different polymers and reflectometric interference spectroscopy by F. Dieterle; G. Belge; C. Betsch; G. Gauglitz (pp. 858-867).
The aim of this study was the quantification of vapors of the ozone-depleting refrigerant R22 in the presence of its most important substitute R134a, by the use of the reflectometric interference spectroscopy and polymers as sensitive layers. First, the sorption characteristic of different types of polymers exposed to the vapors of the two analytes was investigated. Then, binary mixtures of the two refrigerants were measured with an array set-up on the basis of six polymer sensors. The measurements were evaluated by the use of neural networks, whereby low limits of detection of 0.45 percentage volume (vol. %)for R22 and 1.45 vol. % for R134a could be established. Additionally, one polar polymer and one microporous polymer were selected for the measurements with a low-cost set-up. The quantification of R22 in the presence of R134a with this low-cost set-up was possible with a limit of detection of 0.44 vol. %, which would enable a fast and economical monitoring at recycling stations.
Keywords: Refrigerants Reflectometric interference spectroscopy Artificial neural networks R22 R134a
Determination of cationic surfactants in water samples by their enhanced resonance light scattering with azoviolet by Chuan Yang; Yuan Li; Cheng Huang (pp. 868-872).
A simple assay of cationic surfactants in water samples was developed based on the measurements of enhanced resonance light scattering (RLS). At pH 6.09 and ionic strength 0.03 M, the interactions of azoviolet (AV) with cationic surfactants, including zephiramine (Zeph) and cetyl trimethyl ammonium bromide (CTMAB), result in enhanced RLS signals characterized by the peaks of 470.0, 485.0 and 495.0 nm. The enhanced RLS intensity is proportional to the concentration of cationic surfactant of Zeph in the range of 0.2~6.0×10–6 M, and to that of CTMAB in the range of 0.4~4.8×10–6 M. The limit of determination (3σ) is 2.1×10–8 M and 3.8×10–8 M for the two surfactants, respectively. Determinations of cationic surfactants in synthetic and tap water samples were successfully made with a recovery of 90.5~108.6%.
Keywords: Resonance light scattering (RLS) Cationic surfactants Azoviolet (AV) Zephiramine (Zeph) Cetyl trimethyl ammonium bromide (CTMAB)
Determination of trace levels of dissolved vanadium in seawater by use of synthetic complexing agents and inductively coupled plasma–atomic emission spectroscopy (ICP–AES) by Ghiasse Abbasse; Baghdad Ouddane; Jean Fischer (pp. 873-878).
In the determination of traces of dissolved vanadium in complex matrices such as seawater, separation and enrichment from the matrix is of special importance. A wide variety of methods has been proposed for preconcentration, depending to the nature of samples and the methods to be used for measurement. Among these methods separation techniques based on sorption on to chelating resins seem convenient, rapid, and capable of achieving a high concentration factor. The methods proposed in this paper are based on the transformation of all dissolved vanadium species in seawater into organic complexes by use of synthetic complexing agents such as dithizone, luminol, or 8-hydroxyquinoline; the resulting vanadium–organic complexes were sorbed on to a C18 column at a flow rate of 5 mL min–1. The vanadium sorbed on the C18 columns was then stripped by use of nitric acid (2 mol L–1) and analysed by inductively coupled plasma–atomic emission spectroscopy, ICP–AES. This method was optimised and use of other chelating resins, such as chelamine, chelex-100, and immobilised 8-hydroxyquinoline and was compared by passing seawater samples directly over the resins. The experimental conditions (pH, acid used for elution, and contact time between the liquid sample and the resin) were optimised. The results were compared for all the resins used and were indicative of excellent and coherent reproducibility.
Keywords: Vanadium 8-Hydroxyquinoline Chelex-100 C18 column Seawater
New procedures for determination of acids in coffee extracts, and observations on the development of acidity upon ageing by Giancarlo Verardo; Federico Cecconi; Paola Geatti; Angelo G. Giumanini (pp. 879-885).
Analysis of the acid content of coffee extracts has been performed by adaptation of a procedure, recently introduced by our laboratory, to the task of evaluating both the "volatile" and "non volatile" fractions. Besides useful changes in the preparation of the acid pool, diazobutane was used to obtain esters of volatile acids suitable for GC–MS analysis, to minimize losses of material by evaporation. The new procedures have been used to evaluate changes in the acid content of some Italian-style roasted coffee extracts after accelerated ageing (65 °C, 72 h) and in coffee beans after different amounts of roasting. The origin of formation of volatile acidity was also investigated. Electronic supplementary material is available if you access this article at http://dx.doi.org/10.1007/s00216-002-1561-y On that page (frame on the left side) a link takes you directly to the supplementary material.
Keywords: Organic acids Diazomethane Diazobutane Methyl esters Butyl esters Coffee extract
Origin assessment of green coffee (Coffea arabica) by multi-element stable isotope analysis of caffeine by Bernhard Weckerle; Elke Richling; Sandra Heinrich; Peter Schreier (pp. 886-890).
The δ13CVPDB, δ2HVSMOW and δ18OVSMOW values of caffeine isolated from Arabica green coffee beans of different geographical origin have been determined by isotope ratio mass spectrometry (IRMS) using elemental analysis (EA) in the "combustion" (C) and "pyrolysis" (P) modes (EA-C/P-IRMS). In total, 45 coffee samples (20 from Central and South America, 16 from Africa, six from Indonesia, and three from Jamaica and Hawaii) were analysed, as well as three reference samples of synthetic caffeine. Validation was performed by excluding isotope discrimination in the course of sample preparation and determining linear dynamic ranges for EA-P-IRMS measurements. The values for caffeine from green coffee ranged from –25.1 to – 29.9‰, –109 to –198‰, and +2.0 to –12.0‰ for δ13CVPDB, δ2HVSMOW, and δ18OVSMOW, respectively. Data evaluation by linear discrimination analysis (LDA) and by classification and regression tree (CART) analysis revealed the δ18OVSMOW values to be highly significant. Use of LDA on the δ2HVSMOW and δ18OVSMOW data from coffee of African and Central/South American provenance led to error rates of 5.7% and 7.7% for adaption and cross validation, respectively.
Keywords: Multi-element stable isotope analysis Coffee Coffea arabica Caffeine Origin assessment
Electrocatalytic activity of cobalt phthalocyanine stabilized by different matrixes by B. Ballarin; M. Gazzano; J. Hidalgo-Hidalgo de Cisneros; D. Tonelli; R. Seeber (pp. 891-897).
The behavior of cobalt phthalocyanine complexes incorporated inside an hydrotalcite-like clay (HT) or a sonogel–carbon composite has been investigated in order to develop chemically modified electrodes suitable for use as amperometric detectors. The electrocatalytic oxidation process of cysteine at this new electrode has been studied by cyclic voltammetry. For comparison, the oxidation of cysteine catalyzed by the cobalt phthalocyanine complex as a redox mediator, either dissolved in solution or entrapped inside the HT structure, has been followed by polarography. The sonogel–carbon composite electrode is stable and its response is repeatable. Cysteine oxidation is actually induced by the electrogenerated Co(III) complex, and the relevant anodic peak current varies linearly with cysteine concentration within the range 9.0×10–4 to 1.0×10–2 mol L–1.
Keywords: Sonogel–carbon electrode Hydrotalcite Cobalt phthalocyanine complexes Electrocatalysis Cysteine
Environmetric modeling and interpretation of river water monitoring data by V. Simeonov; J. Einax; I. Stanimirova; J. Kraft (pp. 898-905).
This environmetric study deals with modeling and interpretation of river water monitoring data from the basin of the Saale river and its tributaries the Ilm and the Unstrut. For a period of one year of observation between September 1993 and August 1994 a data set from twelve campaigns at twenty-nine sampling sites from the Saale river and six campaigns from the river Ilm at seven sampling sites and from river Unstrut at ten sampling sites was collected. Twenty-seven chemical and physicochemical properties were measured to estimate the water quality. The application of cluster analysis, principal components analysis, and apportioning modeling on absolute principal components scores revealed important information about the ecological status of the region of interest:identification of two separate patterns of pollution (upper and lower stream of the rivers);identification of six latent factors responsible for the data structure with different content for the two identified pollution patterns; anddetermination of the contribution of each latent factor (source of emission) to the formation of the total concentration of the chemical burden of the river water.As a result more objective ecological policy and decision making is possible.
Keywords: Data analysis River water data Cluster analysis Principal components analysis Source apportioning model
A mathematical model and mechanism of sublation of dye-surfactant ion complexes by Yujuan Lu; Xihai Zhu (pp. 906-914).
A study of dye-surfactant ion complexes, bromophenol blue (BB), an anionic dye, with hexadecyl pyridium chloride (HPC) complex, and methane violet (MV), a cationic dye, with sodium docedylbenzensulfonate complex (DBS), was carried out. On the base of the complete transport mechanisms, the Langmuir adsorption and the ion complex equilibrium in aqueous phase, a mathematic model for the ion complex system is obtained with the aid of the 4th Runge-Kuta method and the Mathematic 4.0 and Matlab programs. The effects of many parameters are investigated. A substantial difference is posed between the solvent sublation and solvent extraction. Furthermore, the simulation shows that the model is substantiated with experiments on the solvent sublation of the two kinds of complexes. The results are very different from the models proposed by Wilson et al., which predict very different experimental results.
Keywords: Solvent sublation Mathematical model Mechanism Dye Surfactant
The acetylsalicylic acid–bromine system for multicomponent kinetic determinations by Guillermo López-Cueto; Miren Ostra; Carlos Ubide (pp. 915-922).
When acetylsalicylic acid (ASA) is added to a bromine solution, slow decay of the bromine concentration occurs. Hydrolysis of ASA yields salicylic acid (SA) slowly, and bromine reacts rapidly with SA but not with ASA. Simulated reaction profiles based on a two-step reaction scheme agree closely with the experimental profiles. This behaviour can be used to develop kinetic methods for resolution of mixtures of ASA and a second component that reacts rapidly with bromine. A remarkable practical feature of these methods is that the faster the reaction between bromine and the second component, the simpler and easier the analytical method. The reaction between hydroquinone (HQ) and bromine is rapid and a very simple analytical method is proposed. Mean validation errors of 2.8% for HQ and 7.2% for ASA have been found with concentration ratios [ASA]:[HQ] ranging between 0.32 and 19.4. The reaction between paracetamol (AAP) and bromine is not so fast and more complicated calibration methods are required. After use of a calibration plane mean validation errors of 2.7% for AAP and 8.1% for ASA have been found with concentration ratios ranging between 1.28 and 77.5. Similar kinetic approaches should be possible with many other mixtures of ASA and a second fast-reacting component, because bromine reacts with many inorganic and organic species by oxidation–reduction, substitution, and addition reactions.
Keywords: Kinetic determination Bromination Acetylsalicylic acid
One-step SFE-plus-C18 selective extraction of low-polarity compounds, with lipid removal, from smoked fish and bovine milk by Md. Ali; Richard B. Cole (pp. 923-931).
Co-extraction of lipid materials is the major source of interference in determinations of low-polarity compounds in many biological matrixes. "SFE-plus-C18", a recently developed supercritical fluid extraction method employing C18 adsorbent in the extraction chamber, can enable selective extraction of low-polarity compounds in lipid-rich biological matrixes without a cleanup step. This study reports the application of the SFE-plus-C18 method to the quantification of: 1. polycyclic aromatic hydrocarbons (PAH) in commercially purchased smoked fish; and 2. anti-cancer agents cyclophosphamide (CP) and suberoylanilide hydroxamic acid (SAHA) spiked into homogenized whole bovine milk.Over the course of SFE-plus-C18 extraction, indigenous lipids are preferentially retained on the C18 adsorbent. Compared with the conventional method, only 8–15% of the lipids in the smoked fish sample, and only 6–18% of the lipids in the milk sample, were co-extracted by SFE-plus-C18. This reduction in the quantity of background lipids significantly improved chromatographic separations, retarded deterioration of the column, and dramatically improved the ability to quantify PAH present at trace levels in smoked fish by GC–MS. Using the SFE-plus-C18 method, ten targeted PAH were detected in the range 9.5–13.5 ng g–1 in the smoked fish sample. Compared with these levels, PAH extractions by use of conventional SFE gave values that were lower by 38–86%. Recoveries of CP and SAHA spiked into milk were close to 100% in both SFE-plus-C18 and conventional SFE, where the lipid background during the chromatographic elution of CP and SAHA was not so severe.
Keywords: SFE lipid removal Polycyclic aromatic hydrocarbons GC–MS Smoked fish Bovine milk
Validation of an SPME method, using PDMS, PA, PDMS–DVB, and CW–DVB SPME fiber coatings, for analysis of organophosphorus insecticides in natural waters by D. Lambropoulou; V. Sakkas; T. Albanis (pp. 932-941).
Solid-phase microextraction (SPME) has been optimized and applied to the determination of the organophosphorus insecticides diazinon, dichlofenthion, parathion methyl, malathion, fenitrothion, fenthion, parathion ethyl, bromophos methyl, bromophos ethyl, and ethion in natural waters. Four types of SPME fiber coated with different stationary phases (PDMS, PA, PDMS–DVB, and CW–DVB) were used to examine their extraction efficiencies for the compounds tested. Conditions that might affect the SPME procedure, such as extraction time and salt content, were investigated to determine the analytical performance of these fiber coatings for organophosphorus insecticides. The optimized procedure was applied to natural waters – tap, sea, river, and lake water – spiked in the concentration range 0.5 to 50 µg L–1 to obtain the analytical characteristics. Recoveries were relatively high – >80% for all types of aqueous sample matrix – and the calibration plots were reproducible and linear (R2>0.982) for all analytes with all the fibers tested. The limits of detection ranged from 2 to 90 ng L–1, depending on the detector and the compound investigated, with relative standard deviations in the range 3–15% at all the concentration levels tested. The SPME partition coefficients (Kf) of the organophosphorus insecticides were calculated experimentally for all the polymer coatings. The effect of organic matter such as humic acids on extraction efficiency was also studied. The analytical performance of the SPME procedure using all the fibers in the tested natural waters proved effective for the compounds.
Keywords: Organophosphorus insecticides SPME Environmental water analysis Gas chromatography Partition coefficients.
Microwave extraction of heavy metals from wet rhizosphere soils and its application to evaluation of bioavailability by Shuzhen Zhang; Anxiang Lu; Xiao-quan Shan; Zhongwen Wang; Songxue Wang (pp. 942-947).
Single microwave extraction of heavy metals Cr, Ni, Zn, Cu, Cd, and Pb by the extractants CaCl2, EDTA, CH3COOH and HCl has been established. The experimental conditions were optimized. A microwave power of 60% and an extraction time of 25 min were adopted. Comparison of microwave extraction and conventional extraction methods revealed that results obtained by microwave techniques were generally in a good agreement with those obtained by use of extraction methods; use of the former resulted in substantial time saving and a better precision, however. Microwave extraction was used to evaluate the bioavailability of heavy metals in soils. Single correlation analysis was performed to establish the relationship between the concentrations of the extractable heavy metals in wet rhizosphere soils and their concentrations in wheat (Troticum aestivum L.) grown on the soils under greenhouse conditions. The correlation coefficients between the extractable metals in soils and their concentrations in wheat roots were found to be 0.5398–0.7819, 0.6425–0.8073, 0.5568–0.8276, 0.5851–0.6087 for the extractants CaCl2, EDTA, CH3COOH, and HCl, respectively.
Keywords: Microwave extraction Rhizosphere soil Heavy metals Bioavailibility.
Improved immobilization of 8-hydroxyquinoline on polyacrylonitrile fiber and application of the material to the determination of trace metals in seawater by inductively coupled plasma mass spectrometry by Bei Wen; Xiao-quan Shan (pp. 948-954).
A modified synthetic method has been developed for immobilization of 8-hydroxyquinoline on polyacrylonitrile fiber. The synthetic conditions, e.g. reagent concentration, reaction temperature and time, were optimized. The features of the newly-modified fiber are higher exchange capacity compared with most other materials containing immobilized 8-hydroxyquinoline, better mechanic characteristics, high stability at both high and low pH, and ease of synthesis. This modified fiber can concentrate and separate trace metals from matrices with higher concentrations of alkali and alkaline earth elements. To validate the characteristics of the modified fiber, trace metals Ag, Be, Cd, Co, Cu, Mn, Ni, Pb, and Zn in the certified reference materials river water (SLRS-4) and seawater (CASS-4, NASS-5, SLEW-3) were preconcentrated before determination by inductively coupled plasma mass spectrometry. Good agreement was obtained between the data obtained by this method and the certified values.
Keywords: 8-Hydroxyquinoline immobilized polyacrylonitrile fiber Trace metals Sea water Inductively coupled plasma mass spectrometry
Application of a heated electrospray interface for on-line connection of the AAS detector with HPLC for detection of organotin and organolead compounds by P. Rychlovský; P. Černoch; M. Skleničková (pp. 955-962).
A heated electrospray interface that affords high sensitivity and long-term signal stability for AAS detection of metal-containing analytes in organic or organic–water solvents after HPLC separation is described. The vitreous body of the electrospray interface is externally heated above the boiling point of the solvent and quartz furnace AAS is used for detection. Interface working conditions were optimized with a full experimental design for the detection of tin- (tetramethyl-, tetraethyl-, tetrabutyl-, and tetrapentyltin, tributyltin chloride, dibutyltin dichloride, and butyltin trichloride) and lead- (tetraethyl- and tetraphenyllead) containing compounds in the column eluate. The heated electrospray interface enables use of a wide range of flow rates – from 50 to 1000 µL min–1. The measurement sensitivity and detection limit achieved were compared with those obtained by use of the thermospray interface and post-column conversion of the organotin compounds to gaseous hydrides. The detection limits for the low-molecular weight species of the homologous series (2.8±0.1 ng (140±5 ng mL–1) for tetramethyltin and 3.1±0.2 ng (155±10 ng mL–1) for tetraethyltin) were obtained approximately one order of magnitude lower than those obtained by use of the thermospray interface. With this HPLC–ES–QFAAS system the tributyltin content of BCR reference material 477, mussel tissue, was analyzed. This system was also applied to analysis of tetraethyllead in gasoline samples.
Keywords: Electrospray interface High-performance liquid chromatography Quartz furnace atomic-absorption spectrometry Organotin compounds Organolead compounds
A sequential-injection system for spectrophotometric determination of p-aminobenzoic acid in sunscreens. by Alberto Chisvert; Jose-Vicente Izquierdo; Amparo Salvador (pp. 963-967).
A sequential injection method is proposed for spectrophotometric determination of p-aminobenzoic acid (PABA) in cosmetic formulations. The method is based on diazotization of the analyte, coupling with 8-hydroxyquinoline, and the subsequent formation of a colored product. The experimental conditions used (coupling reagent, sandwich arrangement, volumes aspirated, propulsion flow rate, reaction coil length) were studied. Response of the sequential injection method were linearly dependent on concentrations up to 25 µg mL–1 and the detection limit was 2 µg mL–1. Throughput was 51 measurements per hour and a complete cycle, including three measurement per sample and a washing step, resulted in a sample throughput of 13.8 h–1. Sunscreen formulations containing PABA were analyzed by the proposed method and validated by use of an HPLC procedure. The two methods gave comparable results. Yields ranged between 97–103% and good precision (RSD between 2 and 6%) was obtained. The accuracy, speed, and selectivity of the method make it suitable for safe application for quality-control of final products.
Keywords: p-Aminobenzoic acid (PABA) Sunscreen Sequential-injection analysis (SIA) UV–visible spectrometry
Estimation of the homogeneity of reference materials by SS–ETAAS and use of the "tape-sandwich" sample-introduction technique by J. Štupar; F. Dolinšek; U. Koren (pp. 968-976).
The so called "tape sandwich", a novel sample-introduction technique developed for SS–ETAAS, has been employed in a study of the microheterogeneity of some biological CRM. Pb, Cd, and Cr were measured by use of a laboratory-assembled instrument with graphite-cup atomizer. Aqueous standards were found satisfactory for calibration. The average mass of sub-samples atomised was in the range 0.250–0.660 mg±30%. The relative sampling error found for the CRM investigated varied from 15–34%. Calculated relative homogeneity constants of the order of 4–21 mg1/2 were obtained, and were reduced considerably by reduction of the particle size. The minimum sample sizes representative of the CRM were, with one exception, much lower (11–75 mg) than those recommended in the documentation accompanying the standards (100–200 mg). In a Chinese hair reference sample Cr data were normally distributed but the presence of nuggets was observed for Cd and Pb; this was associated with severe radial and longitudinal gradients of these elements in hair.
Keywords: SS–ETAAS "Tape sandwich" sample introduction Homogeneity of CRM
Preconcentration and separation of copper(II), cadmium(II) and chromium(III) in a syringe filled with 3-aminopropyltriethoxysilane supported on silica gel by Nilgun Tokman; Suleyman Akman; Mustafa Ozcan; Unel Koklu (pp. 977-980).
In this study, a syringe was filled with silica gel loaded with 3-aminopropyltriethoxysilane, for the separation and preconcentration of copper, cadmium and chromium prior to their determination by graphite furnace atomic absorption spectrometry (GFAAS) in seawater. For this purpose, a syringe was filled with 0.5 g of modified silica gel and the sample solution was drawn into the syringe and ejected back again. The analyte elements were quantitatively retained at pH 5. Then, the elements sorbed by the silica gel were eluted with 2.0 M of HCl and determined by GFAAS. At optimum conditions, the recovery of Cu, Cd and Cr were 96–98%. Detection limits (3δ) were 6.6, 7.5 and 6.0 µg L–1 for Cu, Cd and Cr, respectively. The elements could be concentrated by drawing and discharging several portions of sample successively but eluting only once. Cu, Cd and Cr added to a seawater sample were quantitatively recovered (>95%) in the range of the 95% confidence level. The method proposed in this paper was compared with a column technique. Optimum experimental conditions, reproducibility, precision and recoveries of both techniques are the same, but the syringe technique is much faster, easier and more practical than the column technique. It is a portable system and allows one to make the sorption process in the source of sample. In addition, the risk of contamination is less than in the column technique.
Keywords: Syringe technique Preconcentration and separation Copper Cadmium Chromium Modified silica Graphite furnace atomic absorption spectrometry