Uncertainty of chemical oxygen demand determination in wastewater samples

 The measurement uncertainty of the result of chemical oxygen demand determination in wastewater was evaluated. The major sources of uncertainty of the result of measurement were identified as the purity of reagents, volumetric operations, gravimetric operations, bias, and the repeatability of the method. Identification and evaluation of uncertainty sources was followed by combined uncertainty calculations. The combined uncertainty was compared to the experimentally determined variation and good agreement was found, indicating that the major uncertainty sources had been identified. The results show that the major sources of uncertainty arose from repeatability at high concentration level and volumetric steps at low concentration level, thus revealing the target operations for reducing the measurement uncertainty of this determination. Received: 5 August 2002 Accepted: 5 November 2002 Acknowledgements This research was supported by the Ministry of Education, Science and Sport of the Republic of Slovenia (Project Z2–3530). 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 A. Drolc     

Simultaneous ion chromatographic determination of selenium and metal ions in waters at trace level

Summary An ion-chromatographic procedure is described for the determination of selenium (VI) at μg L⁻¹ level in the presence of anions and heavy metal ions. Maximum permissible concentrations and effects from each interfering substance were investigated for the Se concentration range 12.5–1,000 μg L⁻¹. The method, optimized for the detection of SeO ₄ ²⁻ , gives results suitable for speciation analysis. Total selenium can be determined after complete conversion to selenate ion by oxidation with KMnO₄. The detection limit of selenium is 4.8 μg L⁻¹ (0.96 ng for 200 μL sample). Paper presented at the 41st Pittsburgh Conference, New York, March 5–9, 1990.     

An estimation of the measurement uncertainty for an optical emission spectrometric method

 A validation procedure based on the ISO/IEC 17025 standard was used to demonstrate the long-term stability of a calibration process and to assess the measurement uncertainty of a standard test method for optical emission vacuum spectrometric analysis of carbon and low-alloy steel (ASTM E 415–99a). The validation was used to provide documented evidence that the selected method fulfils the requirements and that the method is ”fit for purpose”. A test for drift was applied to determine statistically whether the analytical results vary systematically with time. The accuracy and traceability of the optimised method were tested by an analysis of closely matched matrix certified reference materials (CRMs). The measurement uncertainty estimations took account of the precision study, the bias and its uncertainty, and the qualification of uncertainties not considered in the overall performance studies. Received: 2 November 2002 Accepted: 2 January 2003 Acknowledgement The author expresses gratitude to Dr. Aleš Fajgelj for helpful discussions during the 3rd Central European Conference on Reference Materials and Measurements. 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 T. Drglin     

A need for clearer terminology and guidance in the role of reference materials in method development and validation

 The fact that various definitions and terminology applied to measurements in analytical chemistry are not always consistent and straightforward, by not only answering the question ”what”, but also ”how”, leads to their various interpretations. This results in non-uniform implementation of very basic and essential metrological principles in chemistry. Such a diverse situation is not conducive to the endorsement of harmonised measurements all across the world, to serve as a tool for improving the quality of life in its broadest sense for all its citizens. The discussion in this paper is focused on problems associated with terminology and definitions of ’reference material’ and ’validation’. The role of reference materials in measurement processes for purposes other than calibration and validation principles in analytical chemistry are also discussed in this paper. Where possible, potential solutions are proposed, but more often, questions of essential importance are raised in order to initiate international discussion which will hopefully lead to equally understandable answers. Received: 2 November 2002 Accepted: 3 February 2003 Acknowledgements   The author is grateful to Aleš Fajgelj for his comprehensive comments on the topic described in this paper. Sincere thanks also to Philip Taylor, Ewa Bulska, Emilia Vassileva, Miloslav Suchanek and Margreet Lauwaars for their contribution during fruitful discussions on validation. Presented at the 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 N. Majcen     

Evaluation of measurement uncertainty in the determination of 210Pb and 210Po using beta counting and alpha spectrometry

 Measurement uncertainties in the determination of ²¹⁰Pb and ²¹⁰Po in Reference Material IAEA-300 (Baltic Sea Sediment) were evaluated. ²¹⁰Pb and ²¹⁰Po were separated from the matrix using an Sr resin column. The chemical yield of ²¹⁰Pb was determined gravimetrically in PbSO₄ form. Precipitation was followed by beta proportional counting after ²¹⁰Bi ingrowth. ²¹⁰Po was determined by alpha spectrometry after its spontaneous deposition on a Cu planchet. The major source of uncertainty was identified as the statistical counting uncertainty, which was also expected and is almost impossible to reduce without extension of the time required for an analysis. The expanded uncertainties were determined as 7.4% and 12.2% for ²¹⁰Pb and ²¹⁰Po, respectively. Received: 3 September 2002 Accepted: 3 December 2002 Acknowledgement This work was financially supported by Ministry of Education, Science and Sport, Republic of Slovenia (Project group PO-0106–0532). 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 P. Vreˇcek     

A new reagent system for the highly sensitive spectrophotometric determination of selenium

Highly sensitive and simple spectrophotometric determination of selenium is described for the determination of selenium(IV) using a new reagent leuco malachite green. The method is based on the reaction of selenium(IV) with potassium iodide in an acidic condition to liberate iodine, the liberated iodine oxidizes leuco malachite green to malachite green dye. The green coloration was developed in an acetate buffer (pH 4.2–4.9) on heating in a water bath (∼ 40 °C). The formed dye exhibits an absorption maximum at 615 nm. The method obeys Beer’s law over a concentration range of 0.04–0.4 μg mL⁻¹ selenium. The molar absorptivity and Sandell’s sensitivity of the color system were found to be 1.67 × 10⁵ L mol⁻¹ cm-¹ and 0.5 ng cm⁻², respectively. The optimum reaction conditions and other analytical parameters have been evaluated. The proposed procedure has been successfully applied to the determination of selenium in real samples of water, soil, plant material, human hair, and cosmetic samples. The results were compared to those obtained with the reference method. Statistical analysis of the results confirms the precision and accuracy of the proposed method. In addition, the developed method is cost-effective and involves easily accessible instrumentation technique which can be used by ordinary research laboratories.     

Selenomethionine content of candidate reference materials

Selenium has been identified as an antioxidant of importance in the diet. Accurate determination of its chemical forms depends on the availability of suitable reference materials (RMs). Two candidate reference materials for determination of selenomethionine (Semet) in food-related materials, a standard wheat gluten sample (NIST RM 8418 Wheat Gluten) and a commercial selenium enriched yeast, have been examined by use of a gas chromatography–isotope dilution mass spectrometry (IDMS) procedure, after treatment of the matrix with 0.1 mol L^–1 hydrochloric acid containing stannous chloride, addition of CNBr, and extraction with chloroform. This procedure results in cleavage of the CH₃Se group to form volatile CH₃SeCN. Addition of isotopically enriched ⁷⁴Semet to an analytical sample enables estimation of the naturally occurring protein-bound ⁸⁰Semet by IDMS without a protein-digestion process. We found that the Wheat Gluten RM contains a significant amount of Semet as a portion of its assigned value of 2.58 μg Se_total g^–1. Commercial selenium yeast tablets are labeled as containing an elevated level of “organic selenium”, usually as Semet. The sample we investigated contained 210 μg Se_total g^–1 sample as determined separately by IDMS, measuring elemental selenium after digestion. 73% of this total (153±21 μg Se_Semet g^–1; n = 23) was present as Semet. Thus, these two materials contain significant amounts of their total selenium content as Semet and would be good candidates for further study and characterization as reference materials for determining this important food component. The CNBr reaction used will also enable the determination of Se-(methyl)selenocysteine, the biological role of which is of recent interest. In addition to matrix RMs for Semet, it is important to have standard materials of the pure substance. We have examined a sample of a candidate standard material of selenomethionine being prepared by the USP. It was confirmed that this material is pure selenomethionine. Received: 13 December 2000 / Revised: 5 March 2001 / Accepted: 12 March 2001     

Estimation of measurement uncertainty for the determination of aflatoxin M1 in milk using immunoaffinity clean-up procedures

A measurement uncertainty estimated for aflatoxin M₁ determination in milk sample has been calculated using data generated from analytical method validation studies. The protocol adopted is described in detail in document LGC/VAM/1998/088. The uncertainty budget was based on precision, trueness and ruggedness data. The individual contributions are described in detail. The expanded uncertainty for aflatoxin M ₁ at a concentration of 20 ng L⁻¹ was estimated as 2.81 ng L⁻¹. This was calculated using a coverage factor of two which gives a level of confidence of approximately 95%. Presented at AOAC Europe / Eurachem Symposium March 2005, Brussels, Belgium     

A definitive RNAA method for determination of selenium in biological samples: uncertainty evaluation and assessment of degree of accuracy

This article describes work on the development of a highly accurate RNAA method for determination of selenium in biological samples. The analytical post-irradiation procedure is based on a combination of cation-exchange and extraction chromatography with final selective and quantitative fixation of selenium on a column packed with 3,3′-diaminobenzidine (DAB) supported on Amberlite XAD4, followed by gamma-ray spectrometric measurement. The suitability and accuracy of the method was demonstrated by analysing CRMs with certified selenium content. The uncertainty budget for Se determination in standard reference material Peach Leaves NBS 1547 was estimated; the combined standard uncertainty was calculated as 1.7%. The described method fulfils all the criteria for definitive methods. It was subsequently used for determination of selenium in biological materials intended as new CRMs and proficiency test samples.     

Determination of selenium in soil by hydride generation AAS

Hydride generation AAS was applied for the determination of total selenium in soil. The influence of various anions and cations present in the sample solution on the generation of selenium hydride was investigated. Special attention was paid to the wet oxidation procedure for the sample dissolution. The proposed procedure involves microwave sample preparation by using a mixture of H₂O₂/HNO₃/H₂SO₄. The accuracy of the procedure was checked by the analysis of the standard reference material Buffalo River Sediment (SRM-2704, NIST). Satisfactory agreement of the results obtained with the values reported was achieved. The detection limit of the procedure was 25 ng Se/g of soil. The relative standard deviation of the measurements varied from 5.5% for SRM-2704 up to 10% for the real soil samples. Received: 20 January 1997 / Revised: 24 February 1997 / Accepted: 1 March 1997     

Determination of selenium in vegetables by hydride generation atomic fluorescence spectrometry

A digestion mixture of H₂SO₄/HNO₃/H₂O₂/HF/V₂O₅ was investigated for decomposition of plant samples and sensitive detection of selenium was achieved by hydride generation atomic fluorescence spectrometry (HG-AFS). The method was found to be accurate and reproducible, with a low detection limit (DL) (0.14 ng g⁻¹ solution). The repeatability of the determination was mostly around 10%, the reproducibility over a period of 8 months for determination of selenium in the standard reference material Trace Elements in Spinach Leaves, NIST 1570a, was 9% and the relative measurement uncertainty was 7% using a coverage factor of 2.3 at 95% probability. The average recovery of the whole procedure was 90%. The characteristics of this method are simple and inexpensive equipment, low consumption of chemicals and the ability to analyse many samples in a short time. The whole procedure was carried out in the same PTFE tube, and in addition only a simple cleaning procedure is needed. As a consequence of all these advantages, the described method is suitable for environmental and nutritional studies. The selenium content was determined in 44 vegetable samples from different regions of Slovenia and the contents found were in the range 0.3-77 ng g⁻¹ wet weight.     

Triphenyltetrazolium chloride as a new analytical reagent for molybdenum(VI): Application to plant analysis

Solvent extraction of molybdenum(VI) ion associate with triphenyltetrazolium chloride (TTC) has been studied. TTC was proposed as reagent for the spectrophotometric determination of micro amounts of molybdenum(VI) at λ_max 250 nm. The optimum conditions for extraction of molybdenum(VI) as an ionassociation complex with TTC has been determined. Beer’s law is obeyed in the range of 0.5–10 μg/mL molybdenum(VI). The molar absorptivity of the ion-pair is 1 × 10⁶ L/mol cm. The sensitivity of the method is 9.6 × 10⁻⁵ μg/cm². The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase are: distribution constant K _D = 32.64, extraction constant K _ex = 2.19 × 10¹⁰ association constant β = 6.71 × 10⁸. The interferences of different cations, anions on molybdenum(VI) determination were also investigated. A sensitive and selective method for the determination of microquantities of molybdenum(VI) has been developed. The determination was carried out without preliminary separation of molybdenum. A novel procedure of molybdenum(VI) extraction and spectrophotometric determination in different plant samples was examined.     

Study of the bioavailability of selenium in cows’ milk after a supplementation of cow feed with different forms of selenium

The purpose of the work described in this paper was to develop an easy and quick in-vitro method for comparing the bioavailability of selenium in cows’ milk after different cow feed. The study focuses on bioavailability differences resulting from the use of different selenium species (organic selenium as selenised yeast and sodium selenite) for supplementation of forage. A procedure for determination of selenium in cows’ milk and dialysates, by hydride-generation atomic-fluorescence spectrometry (HG-AFS) after microwave-assisted acid digestion, was optimised. The results show it is possible to obtain cows’ milk enriched with selenium at different concentration without altering the original composition of the milk. The bioavailability was statistically greater for cows’ milk obtained after supplementation of forage with organic selenium at levels of 0.4 and 0.5 μg Se g⁻¹ than for that obtained after supplementation with inorganic and organic selenium at levels of 0.2 and 0.3 μg Se g⁻¹.     

Study of the uncertainty in gravimetric analysis of the Ba ion

The determination of barium by the gravimetric method, in which the precipitation of BaSO₄ was formed and weighed, coupled with instrumental measurement of trace constituents was studied. The analyte’s remaining in the filtrate and washes, mechanical loss, contaminants in the precipitate are the main influencing factors of uncertainty. A series of condition tests have been done, to reduce the effect of the factors mentioned above and the optimum test condition was found. The determination was carried out with a strictly defined operational procedure. The trace amounts of barium in the filtrate, washes and mechanical loss were determined by ICP-AES, the chloride occluded in the precipitate was determined by ion chromatography (IC), calculated as BaCl₂ and barium, and sodium by FAAS, calculated as Na₂SO₄. The average mass of barium in the filtrate contributes about 0.06% relative to that of the total barium, in washes about 0.09%, mechanical loss about 0.06%, contaminants of BaCl₂ about 0.08% and Na₂SO₄ about 0.05%. All the trace constituents were determined and corrected on a sample-by-sample basis. Sources of uncertainty were assessed thoroughly. The uncertainty of this combined gravimetric-instrumental method was improved remarkably compared with that of gravimetric method alone. The expanded uncertainty (k =2) is 0.08%. Received: 15 June 2000 Accepted: 25 December 2001     

Interlaboratory comparisons – demonstrating degree of equivalence

 The role of matrix reference materials in the process of demonstrating the degree of equivalence of measurement results obtained from intercomparisons is outlined, reviewing exemplary selected experience gained at BAM regarding the determination of organic contaminants in environmental matrices. The specific characteristics of reference materials employed in the process of demonstrating equivalence between laboratories in the course of proficiency testing as well as the development, comparison and validation of methods are elaborated. The demand is for series of appropriately characterised samples which are fit for the purpose and it is seen from representative examples that the utilisation of such tailor-made RM designed to tackle the specific need of an analytical problem dominates over certified matrix reference materials in this context. Concluding, the role of certified matrix reference materials in quality assurance is briefly looked at both from the user’s and providers’ points of view. Received: 9 September 2002 Accepted: 16 December 2002 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 R. Becker     

Determination of 19-norandrosterone in human urine by isotope dilution gas chromatography-high resolution mass spectrometry

An accurate and precise method for the determination of total 19-norandrosterone (19-NA), a major metabolite of nandrolone, in human urine was developed based on isotope dilution gas chromatography-high resolution mass spectrometry (ID GC-HRMS). The 19-NA glucuronide, together with deuterated 19-NA (d₄-19-NA) (as the internal standard, IS), was subjected to enzymatic hydrolysis using β-glucuronidase, followed by solid phase extraction (XtractT, mixed-mode column) and liquid–liquid extraction cleanup. The native and deuterated analogues of 19-NA were then derivatised to the corresponding bis(trimethylsilyl) derivatives using N-methyl-N-trimethylsilyltrifluoroacteamide (MSTFA):NH₄I:Dithiothretitol (1,000:2:3 w/w). Identification was achieved under selected ion monitoring of the respective trimethylsilyl derivatives at ion masses m/z 405.26450 and 420.28800 for 19-NA and m/z 409.28920 and 424.31270 for d₄-19-NA within the specific time windows (±1% of the relative retention time to the calibration standard). A linear calibration curve (r ² > 0.9995) was obtained based on seven calibration points (five replicates at each level) in the range 0.05–10 ng/g. The detection limit for 19-NA was found to be 4 pg/g. The method has been applied for the determination of 19-NA by fortifying 19-NA glucuronide at three concentrations (0.2, 2.1 and 7.2 ng/g) in blank urine samples with excellent accuracy and reproducibility. To circumvent the iterative process of exacting matching, a single-point calibration procedure was adopted, where the acceptance criteria for the isotopic ratio in the sample (RF _s) and calibration blends (RF _c) was set close to unity (0.95–1.05). This method was successfully applied in a pilot inter-comparison study, with results in good agreement with the fortified value and other participants’ results (relative standard deviation, RSD < 2.0%) with an expanded relative uncertainty (coverage factor of 2 at 95% confidence level) of 4.7%. It was found in our determination that the main contributors to the uncertainty budget originated from the measurements of the purity of the reference material and the response factor of the calibration standard.     

A Rapid Spectrophotometric Method to Resolve Ternary Mixtures of Propyphenazone, Caffeine, and Acetaminophen in Tablets

Summary.  A simple and rapid derivative spectrophotometric assay procedure is described for the analysis of caffeine (1), acetaminophen (2), and propyphenazone (3) in tablet formulations. The concentration range of application is 5.0–25.0 μg·cm⁻³ for 2 and 3 and 1.0–5.0 μg·cm⁻³ for 1. The method involves the extraction of the drugs from tablets with 0.1 N H₂SO₄, filtration, appropriate dilution, and measurement of the fourth derivative absorbance values at zero crossing wavelengths of 230.0, 263.2, and 256.6 nm for 1, 2, and 3. As a reference method, a reversed phase HPLC procedure was developed. Commercially available tablets were analyzed; statistical comparison of the results with those obtained from the reference method showed good agreement. The derivative spectrophotometric method has the advantage of being simple, rapid, inexpensive, and easy to perform. Received April 18, 2001. Accepted (revised) June 5, 2001     

A Rapid Spectrophotometric Method to Resolve Ternary Mixtures of Propyphenazone, Caffeine, and Acetaminophen in Tablets

 A simple and rapid derivative spectrophotometric assay procedure is described for the analysis of caffeine (1), acetaminophen (2), and propyphenazone (3) in tablet formulations. The concentration range of application is 5.0–25.0 μg·cm⁻³ for 2 and 3 and 1.0–5.0 μg·cm⁻³ for 1. The method involves the extraction of the drugs from tablets with 0.1 N H₂SO₄, filtration, appropriate dilution, and measurement of the fourth derivative absorbance values at zero crossing wavelengths of 230.0, 263.2, and 256.6 nm for 1, 2, and 3. As a reference method, a reversed phase HPLC procedure was developed. Commercially available tablets were analyzed; statistical comparison of the results with those obtained from the reference method showed good agreement. The derivative spectrophotometric method has the advantage of being simple, rapid, inexpensive, and easy to perform.     

Sorption-spectroscopic determination of selenium using a PANV-AV-17 fibrous ion-exchanger

The possibility of determining selenium on the solid phase of polyacrylonitrile fibers impregnated by an AV-17 anion-exchanger ([PANV-AV-17]) were examined by virtue of diffuse reflectance spectroscopy. Reactions of a complex formation between selenium(IV) and organic reagents on solid phase as well as the formation of an elemental selenium sol both on the solid phase and in solution followed by absorption were studied. The best analytical parameters were achieved in the adsorption of selenium sol formed in solution upon the addition of ascorbic acid. The comparison of the batch and dynamic adsorption modes revealed greater advances of the dynamic one. The optimal conditions for determining selenium were proved as follows: the formation of a selenium sol in a 1% ascorbic acid solution at pH 2 and dynamic adsorption at a flow rate of 5 mL/min. A procedure was developed for determining selenium with a limit of detection of 0.1 mg/L. The procedure was validated by the added-found method in the analysis of river and well natural waters and also the standard specimen OSO-200-90, which also contained As, Bi, Sb, and PO₄³⁻. The duration of analysis for 5–6 samples of the volume 100 mL was 30 min approximately, RSD < 20%.     

Uncertainty of determination of palladium in road dust sample by inductively coupled plasma mass spectrometry

The determination of palladium in a road dust sample taken close to the highway (w _Pd=450 ng g⁻¹) was carried out by the ICP-MS method after sample decomposition by aqua-regia. Analyses were evaluated by two methods: external calibration accompanied with mathematical correction of spectral interferences (EC) and isotope dilution measurement after separation of Pd by extraction to dibutyl sulfide solution (ID). In both cases, the uncertainties and accuracy of results were investigated. Although in the case of ideally homogeneous sample the repeatability of EC results (11 ng g⁻¹ Pd) was somewhat lower than those of ID results (16 ng g⁻¹ Pd), the uncertainties of results of both techniques were almost the same and they reached the level of 19 ng g⁻¹ Pd. The main uncertainty source of the EC method is represented by the correction of spectral interferences. In case of real non-homogeneous sample, the main uncertainty component represents the soil sampling. The uncertainty of results (approx. 75 ng g⁻¹ Pd) only slightly exceeded the repeatability (approx. 70 ng g⁻¹ Pd). The accuracy of results was proven by analyses of CRM TDB–1 Diabas Rock (in case of ID) and by the standard addition method (in case of EC).