Designs of experiment for proficiency testing with a limited number of participants

Metrological designs of experiment for proficiency testing (PT) with a limited number of participants are discussed. The designs are based on development of in-house reference materials (IHRMs) with traceable assigned values and fit-for-purpose uncertainties, used in a PT scheme as the measurement standards. When adequate certified reference materials (CRMs) are available, a comparative approach for IHRM development, using simultaneous analysis of IHRM and CRM test portions in pairs, is proposed for PT objectives. In the case where adequate CRMs are not available – for example, in the field of analysis of unstable aqueous systems – the function of the measurement standard for PT can be fulfilled by a synthetic IHRM prepared gravimetrically using non-adequate, non-aqueous CRMs or pure substances. The CRM or the pure substance is used as a spike for fortification of a natural water sample, while the natural water sample is used as a working IHRM for the spike determination. In this case the traceability chain is longer, since two IHRMs are added for one quantity determination, but it remains unbroken.Presented at the Second International Conference on Metrology –Trends and Applications in Calibration and Testing Laboratories, 4–6 November 2003, Eilat, Israel     

In-house reference materials of mometasone furoate with traceable assay certified values

Characterization of in-house reference materials (IHRMs) with traceable property values for the mometasone furoate assay is discussed. The traceability of the value carried by the IHRM has been established to the value carried by a higher metrological status United States Pharmacopoeia Reference Standard (USP RS). A comparative approach is used to overcome systematic errors in measurement results, specific to the measurement method and/or to the laboratory developing the IHRM. The traceability chain was realized by the simultaneous analysis of the IHRM and the USP RS test portions under the same conditions.     

Arsenic speciation in marine biological materials by LC-UV-HG-ICP/OES

Arsenic compounds have been extracted with methanol/water (1:1) from three Certified Reference Materials (CRMs): CRM 278 (mussel tissue), CRM 422 (cod muscle) and DORM-1 (dogfish muscle). The extracts obtained were analyzed by an LC-UV-HG-ICP/OES coupled system, which permits the one-line determination of arsenocholine, arsenobetaine, dimethylarsinate, monomethylarsonate, As(III) and AS(V) at concentration in the range of gl^–1. The main species found in all CRMs extracts was arsenobetaine (97.3% of total As in CRM 422, 89.5% of total As in DORM-1 and 22.2% of total As in CRM 278).     

Implementation of traceability – needs and perspective of the in-vitro-diagnosticum industry

Manufacturers support the concept of traceability. However, only a small number of the medically relevant measurands can be traced to the highest metrological order. In many cases, the measured substances are heterogeneous mixtures where traceability can be established only to either an international conventional reference measurement procedure or to a manufacturers own in-house reference system. The traceability concept needs to be seen in the context that the results of medical laboratories are not an aim per se, but are meant to provide useful medical information to clinicians, and that pre- and post-analytical steps may also contribute significantly to errors. There is a need for the further development of suitable reference measurement systems, but in view of the multitude of tasks and limited resources, priorities need to be set.Presented at BERM-9 – 9^th International Symposium on Biological and Environmental Reference Materials, 15–19 June 2003, Berlin, Germany     

In-House pH Reference Materials

 In-house pH reference materials (IHRMs) traceable to the corresponding NIST pH standards have been developed. Characterization of the IHRMs was based on a comparative approach implying transmission of measurement information from the corresponding standard to the IHRM by simultaneous pH measurements in solutions of both the IHRM and the standard under the same conditions. While the measurements were performed, a statistically detectable temporal drift of the measurement system took place. This did not hinder extraction of the necessary measurement information by way of the comparative approach applied. Correspondence: The National Physical Laboratory of Israel (INPL), Givat Ram, Jerusalem 91904, Israel. e-mail: ilya_kus@netvision.net.il Received August 20, 2002; accepted November 8, 2002     

Flow injection chemiluminescence determination of l-cysteine in amino acid mixture and human urine with the BrO3 −–quinine system

In this paper, a novel flow injection chemiluminescence (CL) determination of l-cysteine is proposed. The method is based on the CL reaction of l-cysteine and KBrO₃ in acidic medium. The CL intensity was greatly enhanced in the presence of quinine. The CL intensity was linear with l-cysteine concentration in the range of 0.2–80 g L^–1, and the detection limit was 0.1 g L^–1 (3). A complete analysis, including sampling and injecting, could be performed in 1 min, giving a throughput of about 60 h^–1. The relative standard deviation was 1.6% for 0.8 g L^–1 l-cysteine (n=11). The proposed method was satisfactorily applied to the determination of cysteine in an amino acid mixture and human urine. The mechanism of the CL reaction is also discussed.     

Photometric detection of cyclodextrins in liquid chromatography by using iodine generated electrochemically in-situ

A method has been developed for photometric detection of cyclodextrins (CD) in liquid chromatography using iodine (I₂) generated electrochemically in-situ. Iodide ion in the mobile phase was electrochemically oxidized to I₂ which was subsequently reacted with I^–, in an electrochemical flow cell, forming I₃^–. The absorbance of I₃^– was found to be greatly enhanced when CD were present in the mobile phase. The absorbance enhancement was caused by the change in the mole fraction of I₃^–, because of the inclusion reaction of I₃^– with CD. On the basis of this phenomenon, CD were detected by means of a photodiode-array UV–visible detector positioned downstream of the electrochemical flow cell. The signals were found to be linearly dependent on CD concentration. Because the formation constants of I₃^– with CD decrease in the order -CD>-CD>-CD, -CD was most detectable by the method. Detection limits were 1.0 mol L^–1 for -CD, 65 mol L^–1 for monoG1--CD, 100 mol L^–1 for -CD, and 200 mol L^–1 for -CD.     

Using enzymes isolated from diverse sources to determine metal ion cofactors

Oxidoreductases and hydrolases isolated from different sources (horseradish and peanut peroxidases, alcohol dehydrogenases from bakers yeast and horse liver, and alkaline phosphatases from Escherichia coli, chicken and seal intestine) were used to determine their metal ion cofactors: Fe(III), Zn(II) and Mg(II), respectively. Studying the effects of the metal ion cofactors on the catalytic activity of the enzymes of different origin showed that the extent of their inhibition, activation, or reactivation of their apoenzymes depended on the structure and accessibility of the enzyme active site, which varies among the biocatalysts isolated from different sources. The developed procedures are based on the inhibiting (Zn(II)) or activating (Mg(II)) effects of the metal ions on the catalytic activity of the enzymes, or on reactivating effects (Fe(III) and Zn(II)) on the apoenzymes. The procedures are characterized by high sensitivity and selectivity; the detection limits of Fe(III) using horseradish peroxidase, Zn(II) using alcohol dehydrogenase from bakers yeast, alkaline phosphatase from seal intestine and its apoenzyme, and Mg(II) using alkaline phosphatase from chicken intestine equal 10 ng L^–1, 20 ng L^–1, 3 g L^–1, 8 g L^–1 and 0.2 g L^–1, respectively.Presented at the 8th Symposium Kinetics in Analytical Chemistry, Rome, Italy, 5–8 July 2004.     

Simultaneous determination of pentachlorophenol and carbaryl in water

Summary A rapid method for the simultaneous determination of pentachlorophenol (PCP) and Carbaryl in water is described. A 1 liter water sample is extracted with hexane after acidification with concentrated H₂SO₄. After evaporation to a small volume, the extract is analyzed by gas chromatography-mass spectrometry, operating in the selected ion monitoring mode. Estimated detection limits are 0.08 g L^–1 for PCP and 0.11 g L^–1 for Carbaryl, with recoveries of 96% for PCP and 97% for Carbaryl.     

Development of a stir-bar-sorptive extraction–liquid desorption–large-volume injection capillary gas chromatographic–mass spectrometric method for pyrethroid pesticides in water samples

Stir-bar-sorptive extraction followed by liquid desorption and large-volume injection capillary gas chromatography with mass spectrometric detection (SBSE–LD–LVI-GC–MS), had been applied for the determination of ultra-traces of eight pyrethroid pesticides (acrinathrin, cypermethrin, deltamethrin, esfenvalerate, fenpropathrin, fenvalerate, and permethrin cis and trans isomers) in water samples. Instrumental calibration for selected-ion monitoring acquisition and conditions that could affect the SBSE–LD efficiency are fully discussed. By performing systematic assays on 30-mL water samples spiked at the 0.10 g L^–1 level it was established that stir-bars coated with 47 L polydimethylsiloxane, an equilibrium time of 60 min (750 rpm), 5% methanol as organic modifier, and acetonitrile as back-extraction solvent, provided the best analytical performance to monitor pyrethroid pesticides in water matrices. Good accuracy (81.8–105.0%) and remarkable reproducibility (<11.7%) were obtained, and the experimental recovery data were in good agreement with the theoretical equilibrium described by octanol–water partition coefficients (log K_O/W), with the exception of acrinathrin for which lower yields were measured. Excellent linear dynamic ranges between 25 and 400 ng L^–1 (r²>0.994), low quantification (3.0–7.5 ng L^–1) and detection (1.0–2.5 ng L^–1) limits were also achieved for the eight pyrethroid pesticides studied. The method was successfully used for analysis of tapwater and groundwater matrices spiked at the 0.10 g L^–1, revealing the suitability of the method for determination of pyrethroid pesticides in real samples. The method was shown be reliable and sensitive and a small volume of sample was required to monitor pyrethroids at ultra-trace levels, in compliance with international regulatory directives on water quality.     

Use of a stopped-flow technique for investigation and determination of thiourea and its <Emphasis Type="Italic">N</Emphasis>-methyl derivatives as inducers of the iodine–azide reaction

A stopped-flow technique has been used to investigate the behaviour of 2-thiourea, 1-methyl-2-thiourea, 1,3-dimethyl-2-thiourea, and 1,1,3,3-tetramethyl-2-thiourea in the induced iodine–azide reaction. This technique enables the progress of the reaction to be followed by monitoring the decrease in the absorbance of the iodine–starch complex at 595 nm. The effect of the concentration of the reagents on the rate of the reaction was investigated and a kinetic method for determination of the compounds is proposed. 2-Thiourea, 1-methyl-2-thiourea, and 1,3-dimethyl-2-thiourea can be determined in the range 3–75 mol L^–1 and 1,1,3,3-tetramethyl-2-thiourea in the range 2–200 mol L^–1.     

New developments in the production and use of CRMs in Romania

 A lot of effort is being made in Romania to meet the present main strategic goal – EU integration. Since the confidence in measurements is of considerable importance in almost every field of activity, the National Institute of Metrology (INM) is involved in improving its calibration and measurement capabilities to provide services in accordance with the latest European Regulation. Within this framework the assurance of the required traceability of all measurements plays a most important role. As reliable analytical measurements depend largely upon reference materials and the assurance of the traceability of amount measurements is still developing in Romania, a new approach regarding the function of Certified Reference Materials (CRMs) may be emphasized. The experience of the INM as well as new developments in Romania in preparation and certification of Reference Materials (RMs) are described. A short review of the locally available RMs and CRMs is given. Some aspects regarding the use of RMs and CRMs, especially for calibration, are discussed for their applicability for analytical measurements. Received: 31 October 2002 Accepted: 24 January 2003 Presented at CERMM-3, Central European Reference Materials and Measurements Conference: The function of reference materials in the measurement process, May 30–June 1, 2002, Rogaška Slatina, Slovenia Correspondence to M. Buzoianu     

Method comparison study on soot-selective techniques

In a long-term field study at two locations with different air pollution levels several soot-selective measurement techniques were compared with a thermochemical method which measures non-extractable carbon (NEC) detecting the evolved CO₂ by means of coulometric titration. The attenuation measurement technique (aethalometer) and the aerosol photoemission method showed good correlations to NEC for concentrations ranging from 1.6g/m³ to 40.8g/m³. The specific mass absorption coefficient of black carbon with respect to NEC varied between 8.4m²/g and 13.7m²/g with respect to the measurement sites, but the value was found to remain constant at each site independent of seasonal or meteorological variations. The ratio of photoelectric signal to NEC varied between 16 fA·(g/m³)^–1 and 33 fA·(g/m³)^–1 depending on the age of the aerosol. The diurnal variations of that ratio showed strong similarities to the traffic patterns. Additionally a slight temperature dependence of this ratio was found for the aged aerosol with a proportionality factor of – 0.35 fA·(K·g/m³)^–1. With the reflectance measurement technique (smoke shade method) reliable NEC determination was not possible for NEC concentrations 5 g/m³.     

Determination of chloroethenes in environmental biological samples using gas chromatography coupled with solid phase micro extraction

Summary An analytical method has been developed to determine the chloroethene series, tetrachloroethene (PCE), trichloroethene (TCE),cisdichloroethene (cis-DCE) andtransdichloroethene (trans-DCE) in environmental biotreatment studies using gas chromatography coupled with a solid phase micro extraction (SPME) technique. The volatile chlorinated compounds in aqueous solution can be analyzed directly without solvent extraction, purge and trap, or thermal heating. The calibration curves have demonstrated good linear relationships within 50.0 to 3000.0 g L^–1 concentration range. Detection limits are 18.0, 5.0, 25.0, and 42.0 g L^–1, for PCE, TCE,cis-DCE andtrans-DCE, respectively. Factors which affect the SPME process, such as sample adsorption time, thermal desorption time, and concentration of salt in the matrix, have also been evaluated.     

Performance of a new nebulizer system for simultaneous determination of Sb,Sn (hydride generation), V,and Zn by ICP–OES

A new nebulizer system is described that extends the analytical capability of the inductively coupled plasma technique to include the simultaneous determination of two elements Sb and Sn (hydride-forming), with two conventional elements, V and Zn. The main advantage of this system is its simultaneous determination of elements that form volatile hydrides and elements that do not, without any instrumental changes. Optimization of reaction and instrumental conditions was performed to characterize the new system. The performance of the new nebulizer system was evaluated by studying the effect of some transition metals (Ni, Cu, Co, and Fe, 1–1000 mg L^–1) on the Sb, Sn, V, and Zn emission signals (1 mg L^–1). Interferences from transition metal ions were found to be insignificant for determination of the four elements in presence of L-cysteine. Long-term and short-term stability was also evaluated. The precision, expressed as RSD for 15 replicate measurements was 0.7% for Sb, 1.7% for Sn, 2.5% for V, and 2.3% for Zn at 200 g L^–1 of each analyte. The detection limits obtained were 0.52, 1.3, 3.2, and 4.7 g L^–1 for Sb, Sn, V, and Zn, respectively. Spike and recovery experiments were performed on the NIST 1643c trace metals in water standard reference material and results were in agreement with the certified values.     

Determination of hexavalent chromium by using speciated isotope-dilution mass spectrometry after microwave speciated extraction of environmental and other solid materials

Precise and accurate determination of hexavalent chromium in different types of solid environmental sample is regarded as a technical challenge with significant potential error if historically accepted methods are used. Microwave-assisted alkaline extraction (0.5 mol L^–1 NaOH+0.28 mol L^–1 Na₂CO₃) followed by anion-exchange chromatographic separation and inductively coupled plasma mass spectrophotometric detection has been shown to provide accurate and precise results. To obtain a better understanding of potential species conversion during and/or after extraction steps, speciated isotope-dilution mass spectrometry (SIDMS) (EPA Method 6800) metrology has been successfully applied as a diagnostic tool with the modified accompanying extraction version of EPA Method 3060A. In our study, aggregate materials distributed over a large area of a major western US state were found to contain a high concentration of total chromium (195±13 to 709±19 g g^–1) and significant amounts of Cr⁶⁺ (141±6 to 341±29 g g^–1) which are at least three orders of magnitude higher than the US EPA threshold limit (0.5 g g^–1). Sediment samples from a major western US state, studied independently, were found to contain less (1.77±0.34 g g^–1) or no Cr⁶⁺ in the presence of significant total chromium.     

Characterization of airborne particulates by laser microprobe mass analysis

Summary Laser microprobe mass analysis (LAMMA) was applied to characterize aerosol particles collected and separated from 16m to 0.06m by a low pressure cascade impactor. Positive ion LAMMA spectra showed characteristic molecular peaks such as PbCl⁺, a series of Si₂O⁺–Si₂O₄ ⁺ and NaAl₂Si₂O₂ ⁺–NaAl₂Si₂O₅ ⁺, and TiO⁺ in 0.06–0.12m, 0.5–1m and 4–8m fraction, respectively. In the negative ion LAMMA spectra, it was observed that the fragment peaks of sulfate ions were deficient above 2m and those of nitrate ions were deficient under 2m. LAMMA allows remarkable insights into the chemical nature of aerosol particles.

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Charakterisierung luftgetragener Teilchen durch Laser-Microprobe-Massenspektrometrie

Zusammenfassung Laser-Microprobe-Spektrometrie (LAMMS) wurde zur Analyse atmosphärischen Aerosols herangezogen, welches im Korngrößenbereich zwischen 16m und 0.06m mit einem Niederdruckkaskadenimpaktor fraktioniert gesammelt wurde. Positive LAMMS-Spektren zeigten charakteristische molekulare Peaks, wie etwa PbCl⁺, eine Serie von Si₂O⁺–Si₂O₄ ⁺ und NaAl₂Si₂O₂ ⁺–NaAl₂Si₂O₅ ⁺, sowie TiO⁺ in der 0,06–0,12-m,- 0,5–1–m- bzw. 4–8-m-Fraktion. In den negativen LAMMS-Spektren konnten über 2m keine Fragmentpeaks für Sulfationen, unter 2m. keine für Nitrationen beobachtet werden. LAMMS ermöglicht eine bemerkenswerte Einsicht in die chemische Natur von Aerosolteilchen.

     

Adsorptive Voltammetric Study of Thorium–Alizarin Complexon Complex at a Carbon Paste Electrode

A new sensitive adsorptive voltammetric procedure is described for trace measurement of thorium. It is based on the cathodic stripping peak of the thorium–alizarin complexon (ALC) complex at a carbon paste electrode (CPE). The complex of Th(IV) with alizarin is adsorbed at a CPE in a mixed buffer solution (pH 5.0) which consists of 0.1mol·L^–1 sodium acetate and 0.04mol·L^–1 potassium biphthalate, yielding a sensitive cathodic voltammetric peak corresponding to the reduction of alizarin in the complex at –0.57V (vs. SCE). The second-order derivative peak current of the complex is linearly dependent upon the concentration of Th(IV) over the range of 3.0×10^–9 8.0×10^–7mol·L^–1. The detection limit is 1.0×10^–9mol·L^–1 for 180s accumulation. The molar ratio of each component in the complex was estimated as n_Th(IV):n_ALC=1:1 by a continuous variation method. The electrode processes of the Th(IV)–alizarin complex at a CPE were investigated. The procedure was successfully applied to the trace determination of thorium in ore and clay samples.     

Flow injection method for the fluorimetric determination of Zn with 8-(benzenesulphonamido) quinoline

The development of a fluorimetric method for the determination of zinc, based on the fluorescence of the zinc-8-(benzenesulphonamido) quinoline chelate in a micellar medium of sodium dodecylsulfate, is reported. The detection limit is 0.2 g l^–1, the working range is 0.5–700 g l^–1, and the sample throughput is 145 h^–1. The method was evaluated for the determination of zinc in food samples.A batch procedure for the simultaneous determination of zinc and cadmium, based on the synchronous and derivative spectra, is also proposed.     

Evaluation of a generalized regression artificial neural network for extending cadmium’s working calibration range in graphite furnace atomic absorption spectrometry

A generalized regression artificial neural network (GRANN) was developed and evaluated for modeling cadmiums nonlinear calibration curve in order to extend its upper concentration limit from 4.0 g L^–1 up to 22.0 g L^–1. This type of neural network presents important advantages over the more popular backpropagation counterpart which are worth exploiting in analytical applications, namely, (1) a smaller number of variables have to be optimized, with the subsequent reduction in development hassle; and, (2) shorter development times, thanks to the fact that the adjustment of the weights (the artificial synapses) is a non-iterative, one-pass process. A backpropagation artificial neural network (BPANN), a second-order polynomial, and some less frequently employed polynomial and exponential functions (e.g., Gaussian, Lorentzian, and Boltzmann), were also evaluated for comparison purposes. The quality of the fit of the various models, assessed by calculating the root mean square of the percentage deviations, was as follows: GRANN > Boltzmann > second-order polynomial > BPANN > Gauss > Lorentz. The accuracy and precision of the models were further estimated through the determination of cadmium in the certified reference material Trace Metals in Drinking Water (High Purity Standards, Lot No. 490915), which has a cadmium certified concentration (12.00±0.06 g L^–1) that lies in the nonlinear regime of the calibration curve. Only the models generated by the GRANN and BPANN accurately predicted the concentrations of a series of solutions, prepared by serial dilution of the CRM, with cadmium concentrations below and above the maximum linear calibration limit (4.0 g L^–1). Extension of the working range by using the proposed methodology represents an attractive alternative from the analytical point of view, since it results in less specimen manipulation and consequently reduced contamination risks without compromising either the accuracy or the precision of the analyses. The implementation of artificial neural networks also helps to reduce the trial-and-error task of looking for the right mathematical model from among the many possibilities currently available in the various scientific and statistic software packages.