Ulf Örnemark

Preconcentration and separation of inorganic selenium on Dowex 1X8 prior to hydride generation—atomic absorption spectrometry

Exchange reactions between inorganic selenium species and chloride were studied on Dowex 1X8. The concentration exchange constants were determined with the batch technique at room temperature and used to predict the chromatographic enrichment and separation of tetra- and hexavalent selenium. A procedure for the determination of total selenium after digestion with permanganate and anion exchange preconcentration was also developed. The enrichment techniques were applied to the determination of Se(IV) and total dissolved selenium in drinking water and fresh water using flow injection hydride generation-atomic absorption spectrometry (HG-AAS). Results agreed with those obtained in a HG-AAS system where selenium was preconcentrated as hydrogen selenide in a trap at liquid nitrogen temperature.

Determination of total selenium in water by atomic-absorption spectrometry after hydride generation and preconcentration in a cold trap system

A cold trap system for the determination of selenium by hydride generation-atomic absorption spectrometry (HG-AAS) is described. For a 30-ml sample the limit of detection is <2 ng/l. and the precision is better than 4% at the 30 ng/l. level. A number of digestion procedures for the destruction of organic matter prior to the determination of total dissolved selenium in water has been tested and compared. Concordant results were obtained except for oxidation by peroxodisulphate in strongly acidic solutions with a high content of organic material. The selenium concentrations found were in agreement with those obtained by HG-AAS after preconcentration by evaporation and dry ashing with the magnesium nitrate-nitric acid-hydrochloric acid method.

Determination of selenium in freshwaters by cathodic stripping voltammetry after UV irradiation

An analytical method was developed for the determination of total dissolved selenium in fresh waters, using linear sweep cathodic stripping voltammetry (CSV) in combination with UV photolytic digestion. Both the CSV method, based on the electrodeposition and stripping of Cu(2)Se, and the UV irradiation procedure were investigated in detail. In the presence of dissolved organic substances, as in freshwaters, Se(VI) is reduced to Se(IV) by UV irradiation in 0.1M hydrochloric acid. Glucose can be used as the carbon source in samples low in natural dissolved organic carbon (DOC). The photolytic yields of Se(IV) were about 90% in both cases. Five freshwater samples were analysed for total selenium by CSV after UV photolysis, and by hydride generation atomic absorption spectrometry (HG-AAS) after oxidative digestion followed by reduction with hydrochloric acid. The results agreed well and the concentrations were in the range 70-190 ng/l., well above the detection limit of the CSV method at 2 ng/l.

Evaluation of Uncertainty of Measurement in Routine Clinical Chemistry. Applications to Determination of the Substance Concentration of Calcium and Glucose in Serum.

Abstract We studied the uncertainty of measurement for the calcium and glucose (amount of) substance concentrations in serum. The evaluation follows a four-step procedure, which complies with the ISO document Guide to the Expression of Uncertainty in Measurement (GUM). The applications were chosen to represent commonly used measuring systems in medical laboratories. The uncertainty components are quantified using observations of the measuring system, and information from calibration certificates, instrument specifications and literature. The evaluation focuses on the measurement step but empirical terms are used to illustrate how the pre-analytical phase and patient-related issues can be accounted for. The software GUM Workbench® was used to facilitate calculations and to visualize the importance of each uncertainty component. The combined standard uncertainties (uc ) for the measurands were ≤2% including the pre-analytical uncertainty sources. The patient-related source is discussed in relation to clinicians diagnosis and decision-making. The evaluation, as carried out here for calcium and glucose substance concentration measurements, can easily be applied to many other measurands in clinical chemistry. This work emphasizes that the internal quality control can provide much of the information needed in the uncertainty evaluation, and that external quality assessment (EQA) schemes are important in the control of the uncertainty evaluated by the individual laboratories. Due to statistical and metrological limitations routine EQA schemes should themselves not be used as a means of uncertainty evaluation.

Determination of dissolved selenium(VI) in freshwater

A procedure is proposed for the determination of selenate in freshwaters with a high content of dissolved organic material. After passage through an XAD-8 column, selenate is collected on a strong anion exchanger and subsequently eluted with hydrochloric acid. Following conversion into the tetravalent state, selenium is determined using atomic absorption spectrometry after hydride generation and preconcentration in a cold trap system. The two-column procedure effectively separates selenium(VI) from possible organic interferents and allows quantification at the low ng/l. level. Results from the investigation of waters from lakes and streams indicate that selenium(IV) and (VI) may constitute a very small part of total dissolved selenium.

Wood WG. Questionable results – who directs the EQAS organisers?

In his letter, Professor Wood points to two different approaches used by organizers of external quality assessment (EQA) schemes to handle gross errors (‘‘blunders’’, ‘‘obvious mistakes’’) of their participants. In brief, either treat data as reported or correct them prior to further evaluation. Professor Wood calls for clear answers to the questions to be included in future guidelines. In the discussion of reporting blunders due to mixing of units, we would like to distinguish between mixing of quantities and mixing of prefixes or other multipliers of units. Regarding mixing of quantities, our opinion is that the EQA organizer should clearly indicate to the participants the kind-of-quantity (and unit) the results concern, e.g., in the case of thyroxine, either mass or substance concentration. The distinction between different kind-of-quantities is even more important for, e.g., peptide hormones, where results might be reported either as arbitrary substance concentration (unit ‘IU/l’) or mass concentration (unit ‘ng/l’). When participants are invited to report data for components with different kind-of-quantities (and units), which might be necessary in international schemes, the EQA organizer can decide to convert results by predefined factors for well defined measurands and display these in the same unit in order to demonstrate more easily the comparability. However, if the molecular entity is not obvious, and the magnitude of a conversion factor therefore less evident (e.g., for the peptide hormone prolactin), the EQA organizer may decide to maintain original data and treat them as representing different measurands. For a given quantity, blunders due to mixing of prefixes or multipliers of units might occur. This can be obvious, e.g., ‘mmol/l’ instead of ‘mmol/l’ for substance concentration of uric acid, and might, if suitable, be safely corrected by the organizer. Other cases, e.g., mixing of ‘mg/l’ and ‘mg/ml’, are impossible to discover from the inspection of reported numbers alone. We can see no arguments for specific guide-