SELM-1: Selenium enriched yeast certified reference material

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The following table shows those analytes for which certified values have been established for this selenium enriched yeast reference material (SELM-1).

Certified values are based on unweighted mean results from data generated by three independent sets of measurements in the case of selenomethionine (SeMet ) and methionine (Met) and two independent measurements for total selenium. The expanded uncertainty (U_CRM) in the certified value is equal to U = ku_c, where u_c is the combined standard uncertainty calculated according to the JCGM ^Footnote1 and k is the coverage factor. The value of u_c is determined from the combined uncertainties of the vairous analytical methods (u_char) as well as uncertainties associated with homogeneity (u_hom) and stability (u_stab). It is intended that U_CRM encompasses every aspect that reasonably contributes to the uncertainty of the measurand ^Footnote2. A coverage factor of 2 was applied.

The table below lists the certified property values in SELM-1.

Table 1: Certified quantity values for SELM-1

Measurand	Mass fraction, (mg/kg)
Total Selenium	2059 ± 64
Selenomethionine	3448 ± 146
Methionine	5758 ± 277

A comprehensive certification report is available upon request.

The analytical methods used/developed for the measurement of SeMet and Met in yeast matrix are documented in the peer-reviewed literature ^{Footnote4 Footnote5 Footnote6 Footnote7 Footnote8}.

Intended use

This certified reference material is intended for the calibration of instruments and evaluation of methods for the determination of SeMet, Met, and total selenium in yeast or materials of a similar matrix. The material is not intended for nutritional, medical or diagnostic use.

Storage and sampling

To ensure the stability of the SeMet and Met it is necessary to store this material at a temperature of -20 °C. Prior to use, the bottle should be rotated and shaken to ensure the contents are well mixed. The bottle should be tightly closed thereafter and returned to -20 °C storage.

Instructions for drying

A separate aliquot of the sample should be used to obtain a dry weight correction factor. Drying for 4 days in a freeze-dryer has proved to be a relatively simple method to achieve constant weight.

Preparation of SELM-1

A dry commercial selenized yeast sample (yeast grown in Se rich media) was used for the preparation of this CRM. No additional screening or blending was used. The material was bottled 'as is'.

The material was bottled in cleaned amber glass bottles at an 8 g nominal weight. During bottling argon gas was used to flush the headspace of the vials to displace air. After bottling the material was sterilized by subjecting it to a minimum dose of 25 kGy gamma irradiation at the Canadian Irradiation Centre, Laval, Quebec.

Certification

Property values were determined by isotope dilution GC-MS and isotope dilution LC-MS (SeMet and Met) and by isotope dilution ICP-MS and ICP-OES for total Se. All measurements contributed to this certificate were conducted by Chemical Metrology personnel at NRC Ottawa.

Uncertainty

Guidelines for CRM producers suggest all sources relevant to the user of the material should contribute to the uncertainty of the certified value ^{Footnote 2 Footnote 3}. Included in the overall uncertainty estimate are uncertainties associated with the batch characterisation (u_char), uncertainties related to possible between-bottle variation (u_hom) as well as instability derived from effects relating to long term storage and transport (u_stab). Expressed as standard uncertainties these components can be combined as:

Equation 1:  $u_{\mathrm{c(CRM)}}^2=u_{\mathrm{char}}^2+u_{\mathrm{hom}}^2+u_{\mathrm{stab}}^2$

The characterization uncertainties (u_char) were calculated from the standard deviation of the means. The calculated uncertainty components for SELM-1 are reported in Table 2.

Homogeneity

This material was tested for homogeneity using ANOVA based on results from randomly selected bottles. Results from different bottles, as determined by ID-GC-MS, resulted in uncertainty components reported in Table 2. The homogeneity is warranted for sub-samples of 250 mg or greater.

Stability

Based on a six year study of the stability of Met and SeMet in SELM-1, an uncertainty component was assigned. Measurements were conducted on samples stored for one month at -20 °C, +4 °C, +22 °C and +44 °C. An uncertainty component associated with the short term stability (transport) was evaluated but considered insignificant for Met and total Se. However, some loss of SeMet was observed and a short term stability uncertainty component was thus assigned.

Table 2: Uncertainty components for SELM-1

Element	Se, mg/kg	SeMet, mg/kg	Met, mg/kg
uchar	28	29	54
uhom	15	20	31
ustab	-	64	124
uc	32	73	138
UCRM (k=2)	64	146	277

Metrological traceability

Results presented in this certificate are traceable to the SI through gravimetrically prepared standards of established purity and international measurement intercomparisons. As such, they serve as suitable reference materials for laboratory quality assurance programs, as outlined in ISO/IEC 17025. This CRM is registered at the Bureau International des Poids et Mesures (BIPM) in Appendix C of the Comité International des Poids et Mesures database listing Calibration and Measurement Capabilities accepted by signatories to the Mutual Recognition Arrangement of the Metre Convention.

Accreditation

The Chemical Metrology laboratory is compliant to ISO 17025 and ISO Guide 34, with approval by The Inter-American Metrology System (SIM). The certificate of approval is available upon request.

Updates

Users should ensure that the certificate they have is current. The NRC web site will contain any new information.

Acknowledgements

The following staff members of the Measurement Science and Standards portfolio at the NRC contributed to the production and certification of SELM-1: C. Brophy, M. McCooeye, V.P. Clancy, P. Maxwell, S. McSheehy, Z. Mester, R.E. Sturgeon, S.N. Willie, and L. Yang.

The cooperation of the following is gratefully acknowledged: Thomas Tompkins, Lallemand, Montréal, Canada.

Date of issue: June 2005
Date of expiry: May 2015
Revised: December 2006
Revised: May 2010

Approved by:

Zoltan Mester, Ph. D.
Group Leader, Chemical Metrology
Measurement Science and Standards

Comments, information and inquiries should be addressed to:

National Research Council of Canada
Measurement Science and Standards
1200 Montreal Road
Building M-12
Ottawa, Ontario K1A 0R6

Telephone: 613-993-2359
Fax: 613-993-2451
Email: CRM-MRCInorganic-Inorganiques@nrc-cnrc.gc.ca