NIMS-1: Natural abundance inorganic mercury isotopic certified reference material

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Certified values for isotope amount ratios, isotopic abundances and atomic weight of mercury have been established for this natural abundance inorganic mercury isotopic reference material (NIMS-1). These are listed in the table below.

Certified values are based on the unweighted mean of thirty-seven sets of measurement results generated at NRC. The expanded uncertainty (U_CRM) in the certified values is equal to U_CRM = ku_c, where u_c is the combined standard uncertainty calculated according to the JCGM Guide ^{Footnote1 Footnote2} and k is the coverage factor. The value of u_c is determined from the combined uncertainties associated with the analytical method (u_char) and with homogeneity (u_hom). A coverage factor of two (2) was applied. It is intended that U_CRM encompasses the uncertainty of every factor that reasonably contributes to the total uncertainty of the measurand.

Table 1: Certified quantity values for NIMS-1

Isotope mass number, A	Isotope amount ratio, n(AHg)/n(198Hg)	Isotope abundance, n(AHg)/n(Hg)
196	0.0154(4)	0.001 55(4)
198	1 (exact)	0.100 38(10)
199	1.6873(11)	0.169 38(9)
200	2.3050(24)	0.231 38(6)
201	1.3120(24)	0.131 70(12)
202	2.9629(39)	0.297 43(9)
204	0.6792(12)	0.068 18(6)

Table 2: Certified quantity values for NIMS-1

Atomic weight, Ar(Hg)
200.5924(8)

The analytical methods used/developed for the measurement of isotopic composition and the atomic weight of mercury are documented in the peer-reviewed literature ^{Footnote3 Footnote4}.

All raw data pertinent to the certification are also available ^{Footnote 4}.

Intended use

This certified reference material is intended for the calibration of instruments and evaluation of analysis methods for the determination of isotope ratios of mercury.

Storage

It is recommended that the material be stored at room temperature and the vials opened immediately prior to use.

Preparation

The reference material was prepared by dilution of NIST SRM 3133 (Mercury Standard Solution) with high-purity water to yield a mass fraction of inorganic mercury of approximately 5 mg kg^-1. An appropriate volume of concentrated BrCl, prepared according to reference ^Footnote5 was added as a stabilizer to result in a volume fraction of 0.5%. The material was bottled and flame sealed in cleaned amber glass ampoules in approximately 2 mL aliquots. The ampoules were gamma irradiated to a minimum dose of 25 kGy at the Canadian Irradiation Centre, Laval, Quebec to inhibit microbial activity.

Uncertainty

Guidelines for CRM producers suggest all sources relevant to the user of the material should contribute to the uncertainty of the certified value ^{Footnote6 Footnote7}. Included in the overall uncertainty estimate are uncertainties associated with the batch characterization (u_char) as well as uncertainties related to possible between-bottle variation (u_hom) ^{Footnote 4} .

Potential instabilities due to long-term storage and transport were not considered, as it is believed that the effects of such event would not be isotope specific.

Characterization

Property values were determined by multicollector ICP-MS. All measurements contributing to this certificate were conducted by Chemical Metrology personnel at NRC in Ottawa.

The calibration approach chosen here, also known as the empirical external normalization, is based on a complete understanding of the measurement process as described elsewhere ^{Footnote 3}. According to this strategy, the mass-bias corrected isotope ratios of mercury are obtained from the log-linear temporal drifts in the measured Hg and Tl isotope ratios. This model avoids the erroneous assumption regarding the equality of Hg and Tl fractionation functions. The certified values for isotope amount ratios, isotopic abundances and atomic weight of mercury are traceable to the SI via the known isotope ratio for thallium, as certified by NIST ^{Footnote 8}.

Homogeneity

This material was tested for homogeneity using analysis of variance (ANOVA) based on results from fourteen randomly selected bottles ^{Footnote 9 Footnote 10}. The contribution to combined uncertainty arising from bottle-to-bottle inhomogeneity was found to be insignificant.

Stability

The material is deemed stable with respect to the certified values for at least ten years.

Expiration of certification

The certified values are valid until July 2020 within the measurement uncertainty specified, provided the CRM is handled and stored in accordance with instructions herein. The stability of this CRM will continue to be monitored.

Metrological traceability

Property values presented in this certificate are traceable to the SI through NIST SRM 997 (Isotopic Standard for Thallium). 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 NIMS-1: C. Brophy, J. Meija, Z. Mester, R.E. Sturgeon, S. Willie, L. Yang.

The cooperation of the following is gratefully acknowledged:
D. Baxter (Analytica AB, Sweden),
L. Halicz (Geological Survey of Israel),
H. Hintelmann (Trent University, Canada).

Date of issue: July 2010
Date of expiry: July 2020

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