Analysis of Nitrite and Nitrate in Foods: Overview of Chemical, Regulatory and Analytical Aspects.

In this chapter, several factors that should be considered for selecting and developing suitable analytical methods for determining nitrite/nitrate are presented. Nitrite and nitrate occurrence and suitability are a controversial issue. Nitrite is an approved additive considered a foremost curing ingredient for the preservation of meat products. Nitrate is a natural constituent of the human diet that, however, raises fears for its suggested potential harmfulness related to carcinogenesis and environmental contamination. Chemical, regulatory, and analytical aspects are discussed in the light of the need to obtain reliable data of nitrite and nitrate for law enforcement purposes, exposure estimates, and investigation of their physiological role in the human body. In addition, current metrological aspects to ensure the "fitness for purpose" of the selected method are suggested and discussed.

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 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.

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.

Stability studies and purification procedure for nitrite solutions in view of the preparation of isotopic reference materials.

The lack of reference materials, accurately certified for nitrite, is a problem in view of the importance of this species for environmental and medical reasons. This work outlines a plan for the preparation of nitrite isotopic reference materials (IRMs) in the form of high purity solutions, certified for their nitrite-nitrogen isotopic composition and nitrite concentration. To achieve the desired accuracy (expanded uncertainty U with a coverage factor k=2 of

Fluorimetry of selenium in body fluids after digestion with nitric acid, magnesium nitrate hexahydrate, and hydrochloric acid.

A digestion procedure involving nitric acid, magnesium nitrate hexahydrate, and hydrochloric acid suffices for selenium determinations in whole blood, serum, and urine by molecular fluorescence spectrometry. To test the accuracy of the method we compared the results with those from hydride-generation atomic absorption spectrometry, and we also analyzed reference materials.

The Nordic Trueness Project 2002: use of reference measurement procedure values in a general clinical chemistry survey.

Up to 136 laboratories participated in a joint effort to assess the trueness of routine measurements for 14 serum components. An unmodified, fresh-frozen human serum ("IMEP-17 Material 1"), produced for an international inter-laboratory comparison, served as the "master material". The serum had assigned values of the highest available metrological quality, and is assumed to involve no or negligible commutability problems. The material was used in the assignment of traceable values to two other reference sera, "CAL" and "X", through parallel measurements on the three materials according to a common protocol. In this transfer process, uncertainty estimates were provided for all values. The material CAL had been supplied with reference measurement procedure values in 1997, and the two sets of assigned values agreed well. A lyophilized control serum "HK02" was also included in the routine analysis series. It, too, had assigned values based on reference measurement procedures. Significant matrix effects were found. The project has provided: Assigned traceable values for 14 components in a fresh-frozen serum, available to Nordic laboratories for the coming years as "NFKK reference serum X"; Confirmation of earlier assigned reference measurement procedure values for a number of components in CAL, the main calibrator in the Nordic Reference Interval project (NORIP). The transferred values will now serve as the primary reference.; Evidence of long-term stability ( > or = 5 years) of the fresh-frozen serum CAL when stored at -80 degrees C; Evidence of substantial matrix effects in the processed serum HK02. The findings should be used to discuss to what extent reference measurement procedure values are useful and cost-efficient for this type of material.