Isotopic Reference Materials

In science, isotope ratio measurements are often performed in a differential mode by comparing the measured isotope amount ratio in a samples to that of a standard. The ratio of both measurements, the isotope delta, allows small isotopic differences to be expressed unambiguously beyond the exact knowledge of absolute isotopic abundances.

Reference materials for use in differential measurements of stable isotope amount ratios have been used since the early 1950s. Since then, the number of international measurement standards for anchoring stable isotope delta scales has mushroomed from three to more than thirty, expanding for more than 25 chemical elements.

To provide an up-to-date comprehensive overview, the Commission has evaluated all isotopic composition reference materials that have played a significant role and/or were available to a wider scientific audience. This also applies to materials whose supply is now exhausted or that have been superseded by newer materials for various reasons. Below is a table of elements for which the Commission has evaluated isotopic composition reference materials. Click on individual elements to find out more information.

Z	Symbol	Element	Common delta reference material
1	H	hydrogen	VSMOW, SLAP
3	Li	lithium	LSVEC
5	B	boron	NIST SRM 951a
6	C	carbon	VPDB, LSVEC
7	N	nitrogen	Air N₂, USGS32
8	O	oxygen	VPDB, VSMOW, SLAP
12	Mg	magnesium	DSM3
14	Si	silicon	NBS28
16	S	sulfur	VCDT, IAEA-S-1
17	Cl	chlorine	SMOC, NIST SRM 975
20	Ca	calcium	NIST SRM 915a
24	Cr	chromium	NIST SRM 979
26	Fe	iron	IRMM-014
28	Ni	nickel	NIST SRM 986
29	Cu	copper	NIST SRM 976
30	Zn	zinc	IRMM-3702
31	Ga	gallium	NIST SRM 994
32	Ge	germanium	NIST SRM 3120a
34	Se	selenium	NIST SRM 3149
35	Br	bromine	SMOB
37	Rb	rubidium	NIST SRM 984
38	Sr	strontium	NIST SRM 987
42	Mo	molybdenum	NIST SRM 3134
47	Ag	silver	NIST SRM 978a
48	Cd	cadmium	NIST SRM 3108, BAM-I012
75	Re	rhenium	NIST SRM 989
76	Os	osmium	IAG-CRM-4
78	Pt	platinum	IRMM-010
80	Hg	mercury	NRC NIMS-1
81	Tl	thallium	NIST SRM 997
82	Pb	lead	NIST SRM 981, ERMM-3800
92	U	uranium	NIST SRM 950a
Z	Symbol	Element	Common delta reference material

Nomenclature

Isotope delta is defined as

δ_ref(ⁱE, sample) = R_sample/R_ref – 1,

where E is the element of interest, R_sample and R_ref are the measured isotope amount ratios in the sample and reference, whereas i is the mass number of the numerator isotope in both isotope ratios (denominator isotope is the same for both the sample and the reference).
For example, isotope delta for boron is commonly measured against NIST SRM 951 isotopic reference material of boric acid and, for substance P, it has a symbol δ_SRM951(¹¹B, P).

For certain elements (such as hydrogen, oxygen, and carbon) the isotope delta scale is defined using two reference materials.

In 2014, the Commission published a report on international isotopic reference materials.
Assessment of international reference materials for isotope-ratio analysis by W.A. Brand, T.B. Coplen, J. Vogl, M. Rosner, and T. Prohaska. Pure Appl. Chem. 86, 425-467 (2014)