Total selenium and selenomethionine in pharmaceutical yeast tablets: Assessment of the state of the art of measurement capabilities through international intercomparison CCQM-P86

LGC Limited, Queens Road, Teddington, TW11 OLY, Middlesex, UK.
Analytical and Bioanalytical Chemistry (Impact Factor: 3.44). 02/2008; 390(2):629-42. DOI: 10.1007/s00216-007-1654-8
Source: PubMed


Results of an international intercomparison study (CCQM-P86) to assess the analytical capabilities of national metrology institutes (NMIs) and selected expert laboratories worldwide to accurately quantitate the mass fraction of selenomethionine (SeMet) and total Se in pharmaceutical tablets of selenised-yeast supplements (produced by Pharma Nord, Denmark) are presented. The study, jointly coordinated by LGC Ltd., UK, and the Institute for National Measurement Standards, National Research Council of Canada (NRCC), was conducted under the auspices of the Comité Consultatif pour la Quantité de Matière (CCQM) Inorganic Analysis Working Group and involved 15 laboratories (from 12 countries), of which ten were NMIs. Apart from a protocol for determination of moisture content and the provision of the certified reference material (CRM) SELM-1 to be used as the quality control sample, no sample preparation/extraction method was prescribed. A variety of approaches was thus used, including single-step and multiple-step enzymatic hydrolysis, enzymatic probe sonication and hydrolysis with methanesulfonic acid for SeMet, as well as microwave-assisted acid digestion and enzymatic probe sonication for total Se. For total Se, detection techniques included inductively coupled plasma (ICP) mass spectrometry (MS) with external calibration, standard additions or isotope dilution MS (IDMS), inductively coupled plasma optical emission spectrometry , flame atomic absorption spectrometry and instrumental neutron activation analysis. For determination of SeMet in the tablets, five NMIs and three academic/institute laboratories (of a total of five) relied upon measurements using IDMS. For species-specific IDMS measurements, an isotopically enriched standard of SeMet (76Se-enriched SeMet) was made available. A novel aspect of this study relies on the approach used to distinguish any errors which arise during analysis of a SeMet calibration solution from those which occur during analysis of the matrix. To help those participants undertaking SeMet analysis to do this, a blind sample in the form of a standard solution of natural abundance SeMet in 0.1 M HCl (with an expected value of 956 mg kg(-1) SeMet) was provided. Both high-performance liquid chromatography (HPLC)-ICP-MS or gas chromatography (GC)-ICP-MS and GC-MS techniques were used for quantitation of SeMet. Several advances in analytical methods for determination of SeMet were identified, including the combined use of double IDMS with HPLC-ICP-MS following extraction with methanesulfonic acid and simplified two-step enzymatic hydrolysis with protease/lipase/driselase followed by HPLC-ICP-IDMS, both using a species-specific IDMS approach. Overall, satisfactory agreement amongst participants was achieved; results averaged 337.6 mg kg(-1) (n = 13, with a standard deviation of 9.7 mg kg(-1)) and 561.5 mg kg(-1) (n = 11, with a standard deviation of 44.3 mg kg(-1)) with median values of 337.6 and 575.0 mg kg(-1) for total Se and SeMet, respectively. Recovery of SeMet from SELM-1 averaged 95.0% (n = 9). The ability of NMIs and expert laboratories worldwide to deliver accurate results for total Se and SeMet in such materials (selensied-yeast tablets containing approximately 300 mg kg(-1) Se) with 10% expanded uncertainty was demonstrated. The problems addressed in achieving accurate quantitation of SeMet in this product are representative of those encountered with a wide range of organometallic species in a number of common matrices.

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    ABSTRACT: The development of a reference method in analytical chemistry is presented. Liquid chromatography with inductively coupled plasma mass spectrometry is employed to perform speciation analysis. Applications are developed for the determination of selenomethionine in nutritional supplements. The use of isotope dilution, a primary method, is required to enable measurement traceability. Method validation is ensured by the study of a certified reference material.
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    ABSTRACT: This paper demonstrates for the first time that enzymatic hydrolysis of protein-bound selenium can be significantly accelerated by microwave energy. Extraction of selenomethionine from selenised yeast, used as the model sample, was performed using two-step enzymatic hydrolysis with protease/lipase/driselase in a focused microwave reactor. HPLC-ICP-MS with species-specific isotope dilution analysis (IDA) was used for species quantitation. To investigate enzymatic hydrolysis assisted by microwave energy the effect of parameters such as microwave power and extraction time on the extraction efficiency of SeMet from the Se-yeast CRM SELM-1 was investigated. The main chemical variables affecting enzymatic extraction of SeMet, such as pH, enzyme type, sample to enzyme mass ratio and sample to solvent mass ratio were set at the conditions used for conventional enzymatic hydrolysis (in a hybridisation oven). Using a power of 60 W and two consecutive extraction cycles, each of 15 min (total extraction time of 30 min) at 37 °C, the recovery of SeMet obtained by the proposed methodology and calculated based on the certified value of 3389 ± 173 mg kg−1 SeMet was 99.6 ± 1.9% (n = 3). The major advantage of the newly proposed method is the dramatic reduction of extraction time in comparison with that of conventional enzymatic hydrolysis, which requires a two-step extraction, each cycle of approximately 20 h. This was achieved without compromising extraction efficiency. Application of the proposed method to the accurate quantification of SeMet in pharmaceutical yeast tablets with a total Se concentration of approximately 300 mg kg−1 was also addressed. The recovery of SeMet, which was calculated based on the mean of results [561.5 mg kg−1 (n = 11, with a standard deviation of 44.3 mg kg−1)] obtained for the yeast tablets in the international intercomparison CCQM-P86,1 was found to be 100.1 ± 3.3%.
    Journal of Analytical Atomic Spectrometry 01/2008; 23(4). DOI:10.1039/b717854a · 3.47 Impact Factor
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