Status : progress report final report
1. Ref. No.:1185 2. Subject Field:Metrology in chemistry
3. Type of collaboration:
Comparison in measurement standards
3A. In the case of a comparison:
4.
4A
4B
Registered as Key comparison (KC) or Supplementary Comparison (SC) in the KCDB: no yes If yes: No. of KC/SC:
Participating Partners:
EURAMET members or associates (Institute’s standard acronym with country code in brackets)
LGC (UK), LNE (FR), PTB (DE), UME (TR)
Institutes not being EURAMET members or associates (Institute’s full name and name of the country)
5. Title:
Determination of Se-Met in human serum
6. Progress:
This international intercomparison was organised to validate the measurement procedures for the SI traceable determination of selenomethionine (Se-Met) in human serum developed within
WP2 of the project TRACEBIOACTIVITY.
The intercomparison was carried out using the matrix reference material BCR-637, where the total content of Se is certified but not the content of Se-Met, as the sample material and Se-Met enriched in 76Se as spike material for species-specific isotope dilution mass spectrometry (SS-
IDMS).
For the quantification of Se-Met in the serum LNE used single ID-ICP-MS including an inverse step to characterise the spike while LGC and PTB used double ID-ICP-MS. Expanded uncertainties associated with the results for the measurements at all three NMIs were below 8%
(k = 2), which is a remarkable achievement considering the difficulties arising from the quantification of Se species in serum at such low levels. The assessment of the expanded uncertainty for the measurement of Se-Met in serum with HPLC-ICP-IDMS was carried out on the basis of the GUM recommendations.
At LNE 5 replicates each of 5 independent samples were analysed for the determination of the uncertainty. LNE found a relative expanded uncertainty of approx. 5 %. The repeatability in the determination of the mass discrimination factor was the most predominant contribution with
20-40 % in case of direct IDMS and 60-70 % for the inverse step.
At LGC, the determination of Se-Met in serum was carried out using hydrolysis with proteolytic enzymes followed by double SS-IDMS with HPLC-ICP-MS. The concentration of the natural standard, the matching of the isotope ratios of sample and calibration blends (exact matching was found to be necessary for the low ppb levels of serum Se-Met) and the blend to blend variation were the major sources of contribution to the overall uncertainty. Having a natural Se-
Met standard of a known purity has also been proven essential to achieve traceability in the determination of Se-Met.
At PTB, the same measurement procedure as at LGC was used. As the purity of the Se-Met standard was determined using quantitative NMR resulting in a smaller uncertainty for the purity of the reference Se-Met, the major contributions to the uncertainty arise from the sample preparation, as well as the measured isotope ratios. The results for the uncertainty estimation emphasizes the importance to control the conditions for the enzymatic hydrolysis carefully.
The mass fraction of Se-Met (as Se) obtained by PTB, LGC, LNE and TUBITAK-UME in the BCR-
637 human serum were 12.5 +/- 0.57 µg kg-1 Se; 13.6 +/- 1.1 µg kg-1 Se, 13.1 +/- 0.66 µg kg-1 Se and 13.1 +/- 0.84 µg kg-1 Se, respectively. These mass fractions with their expanded uncertainties (k = 2) were found to agree reasonably well.
A joint scientific paper describing the uncertainty estimation, measurement challenges and

comparative data by these NMIs is in preparation
7. Coordinator´s name:Claudia Swart
Address: Physikalisch-Technische Bundesanstalt, Bundesallee 100,
38116 Braunscheig, Germany
Telephone: +49-531-592-3317 Fax:
E-mail: claudia.swart@ptb.de
+49-531-592-3015
8. Completion Date:
2011-06-30
9. Date
2011-07-04
Notes for completion of the form overleaf

(numbers refer to boxes overleaf)
IMPORTANT:
Forms are to be send to the EURAMET Secretariat ( secretariat@euramet.org
) as word or pdf file by
TC Chair or coordinator of the project with copy to TC Chair.
1 Ref. No. The project reference number which is assigned by the EURAMET Secretariat and on which progress is being reported.
2 Subject Field The field specified on the EURAMET Project Form “Proposal”
3 Type of collaboration The field specified on the EURAMET Project Form “Proposal”
4A
4 B
EURAMET members or associates Any institutes which have participated in the collaboration should be indicated in alphabetical order using their standard acronyms and country codes.
Institutes not being EURAMET members or associates should be indicated with full name in alphabetical order. See country codes in the last section of the directory. Every effort should be made to establish potential partners prior to completing a Proposal Form.
5
6
Title The title given in the EURAMET Project Form “Proposal”.
Progress A brief description of the progress should be entered in the space provided.
Comments on the advantages of undertaking the work collaboratively through EURAMET would be useful. Completion of this Report is not deemed as publication of the work.
Collaborators are encouraged to publish their work through normal channels, mentioning it was undertaken as a EURAMET collaboration.
7 Coordinator The Coordinator is the person who is appointed as the contact point for the project detailed overleaf. The name, full postal address, telephone and fax numbers and e-mail address of the coordinator should be given.
8 Completion Date If the progress of a project is being reported on this form then an estimate of the completion date should be made. If the project has now been completed then the actual date of completion should be given. For permanent agreements (e.g. development of primary standards) "ON-GOING" should be entered.
9 Date of transmission to EURAMET Secretariat.