Accurate calibration of mercury vapour measurements

Almost all measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Setting aside the uncertainty in the vapour pressure of mercury at a given temperature, the accuracy of vapour phase mercury determinations depends critically on fully understanding the operation and sensitivities of the mercury bell-jar apparatus. This paper discusses the thermodynamic and kinetic considerations that must be taken into account when using the bell-jar apparatus, provides the theoretical basis for understanding the operation of the bell-jar, and presents experimental data demonstrating the systematic biases which may be obtained if the bell-jar is used incorrectly. These biases depend on the temperature difference between the mercury vapour in the bell-jar and the syringe used to remove the mercury vapour from the bell-jar, but they may be well in excess of 10% under some operating conditions. The results from this study have been used to propose best practice solutions for mercury vapour calibrations using the bell-jar.     

A national traceability system for chemical measurements

Current developments in Germany for establishing a traceability system for chemical measurements are reported. The focus is on a dissemination mechanism which employs chemical calibration laboratories accredited within the framework of the German Calibration Service (DKD) and acting as "multipliers" between the national standards level and the user level by providing the user with calibration means which are traceable to the SI via national standards. At the national standards level, a network of high-level chemistry institutes coordinated by the national metrology institute, PTB, provides the primary references for chemical measurements.The use of the metrological dissemination system provided by the DKD also for chemical measurements is a logical extension of a traceability mechanism, successful for more than two decades in general metrology, to metrology in chemistry. In detail, traceability structures in clinical chemistry, electrochemistry, elemental analysis and gas analysis are described. This system has become an important part of the efforts made in Germany to support chemical laboratories in meeting the traceability requirements of the market and of legal regulations.     

The traceability of analytical measurements

 Traceability to the System International (SI) is an important prerequisite for international comparability and uniformity of chemical measurements to ensure mutual recognition of the results. In theory, all measurements can be traced back to the seven base units of the SI. Although the traceability system works well for most physical measurements, in many analytical and in some spectrophotometric measurements this system is not satisfactory. This paper describes the particular and practical problems and the contribution of the Romanian National Institute of Metrology in this field. The paper discusses the following concepts: clearly defined targets in the form of requirement specification, knowledge of trueness and/or measurement uncertainty, and traceability through an unbroken chain of calibration to primary standards. Traceability and uncertainty being two concepts inherently coupled, two examples of assessment of the uncertainty of measurement results are given for two spectrophotometric methods currently used in chemical laboratories. Received: 17 July 1996 Accepted: 2 September 1996     

Improved traceability of pH measurements

Traceability is a prerequisite for the comparability and uniformity of measurements. Although pH-measurements are carried out on a large scale in laboratory and industry, the problems involved in the traceability of pH values have not adequately been solved in the past. The comparability of pH measurements is limited, among other parameters, by the accuracy of the pH values of the standard buffer solutions used to calibrate the pH meter-electrode assemblies. The measured pH(X) value must be traceable to primary standard pH(PS) values through an unbroken chain of comparisons, all values having stated uncertainties. A new primary standard measurement device for pH is used to certify primary pH reference materials from which these secondary reference materials can be derived.     

A traceability protocol to the SI by gravimetric analysis

 An example is presented of a traceability protocol for the measurement of a single-element strontium reference material solution, executed by a "primary" method of measurement for certification. The method of measurement is briefly described together with the measurement equation and the associated calculations for the estimation of uncertainties. This is followed by a discussion and estimate of each component of uncertainty associated with the measurement, together with a final estimate of uncertainty. The final estimate of uncertainty compares well with observed uncertainties for two previous laboratory measurements of the reference material. Received: 21 February 1998 · Accepted: 17 August 1998     

Establishing measurement traceability in clinical chemistry

Measurement traceability is probably the most important tool for achievement of comparability in clinical chemistry. As stipulated by the In Vitro Diagnostica Directive of the European Union and several ISO standards, values assigned to calibrators and control materials must be traceable to reference materials and/or reference procedures of higher order. In the German proficiency testing system, statutory use of reference measurement procedures for several measurands has been in force since 1988. As a result, reference procedures are now regularly applied for the setting up of target values in the control samples of internal and external quality assessment and for assigning values to the manufacturers calibrator and control materials. Noticeably, the comparability of results obtained by different diagnostic tests has greatly improved for the measurement of many metabolites and substrates, e.g. creatinine, cholesterol, uric acid, total glycerol and urea. For many measurands in laboratory medicine the implementation of the concept of traceability proves to be much more difficult; this mainly concerns the measurement of proteins, in particular enzymes, proteo-hormones, tumour markers and cardiac markers. For such measurands the analyte must first of all be distinctively defined before a reference system can be established which comprises reference procedures, reference materials and networks of reference laboratories.Presented at the CCQM Workshop on Traceability 16–17 April 2002 at the BIPM, Sèvres, France     

Validation steps for traceability of linear calibrated chemical measurements

Based on the new draft of the EURACHEM/CITAC Guide Traceability in Chemical Measurement, this publication describes how traceability can be achieved for chemical measurements using a linear calibration function. Traceability can be accomplished without larger expenditure, if the measurement is calibrated on the basis of appropriate reference standards and the linear regression employed is selected and validated statistically in a suitable form. The determination of nickel in aqua regia eluates of sediment samples, employed for an ICP-OES measurement, is used as a practical illustration of this approach.     

Stated references for ensuring traceability of chemical measurements for long-term environmental monitoring

Traceability is now considered to be a key concept in chemical and biological measurement sciences. While this concept is increasingly used in many areas, it is still prone to misunderstandings with respect to more ‘classical' terms such as e.g. accuracy. Traceability implies that measurement data are linked to stated references through an unbroken chain of comparisons, all with stated uncertainties. What does the word ‘stated references' mean when it is applied to complex environmental analysis? This paper discusses how traceability can be conceived in the context of environmental monitoring, in particular the various stated references (documented standards, reference materials, environmental specimens) to which chemical environmental data may be linked to.     

The detection of mercury vapour by magnetically induced optical rotation

Equations to describe magnetically-induced optical rotation signals obtained from mercury atoms in the vapour phase are described and tested. The method gives good analytical sensitivity and selectivity. Its advantages over the conventional techniques of analytical spectrometry include the automatic tolerance of continuum background absorption and of source drift.     

Liquid chromatographic--cold vapour atomic fluorescence spectrometric determination of mercury species

Solvent extraction, sonication, and microwave-assisted extractions in the presence of extraction agents (thioacetic acid, citric acid, cysteine, 2-mercaptoethanol, HCl + NaCl, etc.) were tested for the isolation of mercury species. A mixture of 6 M HCl and 0.1 M NaCl was selected as the most suitable extraction agent. The extraction efficiency was about 10% higher and the RSD below 3.3% when microwave-assisted extraction was applied instead of sonication. The liquid chromatography-cold vapour atomic fluorescence spectrometry (LC/CV-AFS) method was optimised and used for separation and determination of inorganic mercury cations and alkylated and arylated mercury species. Isocratic elution at a flow rate of 0.15 mL/min (with a mobile phase containing 0.05% 2-mercaptoethanol (pH = 5) and 7% methanol and with a stepwise increase of methanol content up to 100% MeOH in the 15th min) was used for separation of mercury species on a Hypersil BDS C18 RP column. The limits of detection of the LC/CV-AFS system were estimated as 0.2 microg/L (3%) for MeHg+, 0.07 microg/L (5.3%) for inorganic Hg, 0.06 microg/L (3.4%) for PhHg+, and 0.12 microg/L (4.4%) for EtHg with the corresponding RSDs at 5 microg/L (n = 10) given in parentheses. The concentrations (2-10 mg/kg fresh weight) of total mercury and methylmercury (90-99% of the total mercury) in selected fish obtained by HPLC/CV-AFS were in good agreement (absolute deviations 0.05 mg/kg) but more precise (RSDs <5.4% at 5 mg/L, n = 10) than those determined by GC coupled to an electron capture detector. The RSDs (3.1-8.2% and 4.1-9.0%) of the overall analytical procedure for the determination of total mercury (AMA 254) and methylmercury (HPLC/CV-AFS) were determined for intra-day and inter-day assays, respectively.     

The practical realization of the traceability of chemical measurements standards

 Metrology is based on the concept of traceability. Traceability provides a means of relating measurement results to common standards thereby helping to ensure that measurements made in different laboratories are comparable. Good progress has been made in the application of metrological principles to chemical measurement, but there remains confusion about how you actually achieve traceability in a practical way.

This paper elaborates on the meaning and application of much used phrases such as 'the value of a standard', 'stated references', 'unbroken chain of comparisons', and 'stated uncertainties'. It also explains how traceability can be established in a practical way for different types of stated references, namely pure substance reference materials, matrix reference materials, and primary and reference methods. Finally, traceability chains for some typical examples of chemical measurement are described.

     

The practical realization of the traceability of chemical measurements standards

Metrology is based on the concept of traceability. Traceability provides a means of relating measurement results to common standards thereby helping to ensure that measurements made in different laboratories are comparable. Good progress has been made in the application of metrological principles to chemical measurement, but there remains confusion about how you actually achieve traceability in a practical way. This paper elaborates on the meaning and application of much used phrases such as 'the value of a standard', 'stated references', 'unbroken chain of comparisons', and 'stated uncertainties'. It also explains how traceability can be established in a practical way for different types of stated references, namely pure substance reference materials, matrix reference materials, and primary and reference methods. Finally, traceability chains for some typical examples of chemical measurement are described.     

Basic equations and uncertainties in isotope-dilution mass spectrometry for traceability to SI of values obtained by this primary method

A current interest in chemistry concerns traceability of analytical measurements to the International System of Units (SI) and the estimation of their uncertainties in accordance with principles of metrology, that is, measurement science. “Primary methods of measurement” achieve traceability to SI directly without intermediate reference standards or materials and without significant empirical correction factors. Isotope-dilution mass spectrometry should be regarded as such a method. It has the potential of smallest presently achievable uncertainties for analytical measurements directly or for the certification of reference materials including those with abnormal isotopic composition. A simple explanation of the method including its basic equations is given. Full uncertainty estimation is emphasized in terms of these equations. The wider use of concepts of metrology in chemistry is discussed.     

Determination of mercury in seawater by cold vapour atomic absorption spectrophotometry

Summary To avoid systematic errors in the sampling and determination of mercury in seawater an integrated sampling-analyzing system has been developed. A 500 ml PTFE-bottle attached to a sampling gear is used as sampling vessel for the cold vapour atomic absorption Spectrophotometry, using nitrogen aeration, SnCl₂ reduction, and preenrichment/ purification by amalgamation on finely dispersed gold. The detection limit of this procedure was found to be 0.5 ng·l^–1 and the coefficient of variation 4% for 1.5 ng·l^–1.

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Bestimmung von Quecksilber im Meerwasser durch Kaltdampf-Atomabsorptionsspektralphotometrie

Zusammenfassung Zur Vermeidung von systematischen Fehlern bei der Ultraspurenanalyse von Quecksilber im Meerwasser wurde ein integriertes Probenahme-Analysensystem entwickelt. Eine 500 ml PTFE-Flasche dient in Verbindung mit einem Greifarm als Probenahmegefäß für Oberflächenwasser und wird anschließend als Reaktionsgefäß in die Analysenapparatur gesetzt. Die Bestimmung erfolgt mit der Kaltdampf-Atomabsorptionsspektralphotometrie nach Reduktion des Quecksilbers durch SnCl₂ und Anreicherung sowie Reinigung mittels Amalgamierung an feinverteiltem Gold. Die Nachweisgrenze dieses Verfahrens beträgt 0,5 ng·l^–1, der Variationskoeffizient 4% bei 1,5 ng·l^–1.

     

Determination of mercury traces by differential-pulse stripping voltammetry after sorption of mercury vapour on a gold-plated electrode

Trace mercury is reduced with tin(II) to mercury metal, which is volatilised by bubbling air through the solution. A certain fraction of this mercury is sorbed on a rotating gold disk electrode and stripped in a thiocyanate solution. The detection limit is about 30 ng Hg(II) in solution; the relative standard deviation is 6% for 100 ng Hg(II) (n = 7). The detection limit for mercury in air is 1.7 ng l^?1 with a preconcentration time of 10 min.