Reference materials for stable isotope analysis
By measuring a material of known isotopic composition, fractionation within the mass spectrometer can be removed during post-measurement data processing.
Without isotope references, measurements by mass spectrometry would be much less accurate and could not be used in comparisons across different analytical facilities.
Isotope reference materials are generated, maintained, and sold by the International Atomic Energy Agency (IAEA), the National Institute of Standards and Technology (NIST), the United States Geologic Survey (USGS), the Institute for Reference Materials and Measurements (IRMM), and a variety of universities and scientific supply companies.
Each of the major stable isotope systems (hydrogen, carbon, oxygen, nitrogen, and sulfur) has a wide variety of references encompassing distinct molecular structures.
For example, nitrogen isotope reference materials include N-bearing molecules such ammonia (NH3), atmospheric dinitrogen (N2), and nitrate (NO3−).
While the δ values of references are widely available, estimates of the absolute isotope ratios (R) in these materials are seldom reported.
This article aggregates the δ and R values of common and non-traditional stable isotope reference materials.
The δ values and absolute isotope ratios of common reference materials are summarized in Table 1 and described in more detail below.
Unlike reference materials, working standards are typically not calibrated across multiple analytical facilities and the accepted δ value measured in a given laboratory could reflect bias specific to a single instrument.
The broad evolution of reference materials for the hydrogen, carbon, oxygen, and sulfur stable isotope systems are shown in Figure 1.
[18] Originally SMOW was a purely theoretical isotope ratio intended to represent the mean state of the deep ocean.
Following the advice of an IAEA advisory group meeting in 1966, Ray Weiss and Harmon Craig made an actual solution with the isotopic values of SMOW which they called Vienna Standard Mean Ocean Water (VSMOW).
Additionally, Greenland Ice Sheet Precipitation (GISP) δ2H has been measured to high precision in multiple labs, but different analytical facilities disagree on the value.
These observations suggest GISP may have been fractionated during aliquoting or storage, implying that the reference material should be used with care.
In order to make measurements researchers use the reference material NBS-19, colloquially known as the Toilet Seat Limestone,[20] which has an isotopic ratio defined relative to the hypothetical VPDB.
To improve the accuracy of carbon isotope measurements, in 2006 the δ13C scale was shifted from a one-point calibration against NBS-19 to a two point-calibration.
[23] deviation Nitrogen gas (N2) makes up 78% of the atmosphere and is extremely well mixed over short time-scales, resulting in a homogenous isotopic distribution ideal for use as a reference material.
The Canyon Diablo Meteorite was chosen because it was thought to have a sulfur isotopic composition similar to the bulk Earth.
A meeting of the IAEA in 1993 defined Vienna Canyon Diablo Troilite (VCDT) in an allusion to the earlier establishment of VSMOW.
Like the original SMOW and VPDB, VCDT was never a physical material that could be measured but was still used as the definition of the sulfur isotopic scale.
For the purposes of actually measuring 34S/32S ratios, the IAEA defined the δ34S of IAEA-S-1 (originally called IAEA-NZ1) to be -0.30‰ relative to VCDT.
A summary list of non-traditional stable isotope systems is available here, and much of this information is derived from Brand et al.
[63] To generate a standard of known clumped isotope composition, current practice is to internally equilibrate analyte gas at high temperatures in the presence of a metal catalyst and assume that it has the Δ value predicted by equilibrium calculations.
Like most aspects of reporting isotopic compositions it reflects a combination of historical artifacts and modern institutions.
As a result, the details surrounding the certification of isotopic reference materials varies by element and chemical compound.
The isotopic composition of additional reference materials are either established through individual analytical facilities or through interlaboratory comparisons but often lack an official IAEA certification.
The agreed-upon isotopic composition of primary reference and the original calibration materials were generally not reached through interlaboratory comparison.
For dual-inlet and continuous flow mass spectrometry uncertainty in the raw isotopic ratio is acceptable because samples are often measured through multi-collection and then compared directly with standards, with data in the published literature reported relative to the primary reference materials.
However, the uncertainty in the raw isotopic ratio of reference materials is problematic for applications that do not directly measure mass-resolved ion beams.
To correct for such scale compression researchers calculate a "stretching factor" by measuring two isotopic reference materials (Coplen, 1988).
