Stable Isotope Principles

 

Stable Isotope Principles

An isotope is an atom whose nuclei contain the same number of protons but a different number of neutrons.  Isotopes are broken into two specific types: stable and unstable.  These unstable isotopes are more commonly referred to as radioactive isotopes.  There are approximately 300 known naturally occurring stable isotopes.  Most of the light elements contain different proportions of at least two isotopes.  Usually one isotope is the predominantly abundant isotope.  For example, the average abundance of ¹²C is 98.89%, while the average abundance for ¹³C is 1.11%.  Table 1 outlines the average isotopic abundances of elements that are most commonly measured for stable isotope measurements.

Table 1.  Natural Isotopic Abundances of light stable isotopes.

Hydrogen	Carbon	Nitrogen	Oxygen	Sulfur
1H – 99.984%	12C – 98.89%	14N – 99.64%	16O – 99.763%	32S – 95.02%
2D – 0.0156%	13C – 1.11%	15N – 0.36%	17O – 0.0375%	33S – 0.75%
			18O – 0.1995%	34S – 4.21%
				36S – 0.02%

Heavy isotopes undergo all of the same chemical reactions as light isotopes, but, simply because they are heavier, they do it ever so slightly more slowly.  These tiny differences in reaction rates cause the products of reactions to have different isotope ratios than the source materials.  Knowing the precise isotope ratios in plant and animal tissues allows us to know about the processes by which the materials were formed.  This can tell if a plant’s roots are tapping recent rain or deep groundwater, the water-use efficiency of whole forests, what an animal has eaten throughout its life and where it sits on the food chain, and the global sources and sinks for carbon dioxide in the atmosphere.  Historical materials, including those that may be many thousands of years old, can be analyzed in the same manner, allowing us to compare modern and ancient environments.

Before analysis can begin; however, it is important to have a good understanding of how a specific sample type can be affected by various processes; most notably, isotopic fractionation.  Isotopic fractionation causes stable isotopic abundance variations.  Fractionation is caused by the differences in the chemical and physical properties of a certain atomic mass and concerns the concepts of isotope exchange and kinetic processes in reaction rates. Changes in temperature are just one example of an isotope exchange process that can cause fractionation in an isotopic ratio.  This is why temperature stability is a priority in many instrumentation facilities.  Gas pressure can also have a significant role in determining the magnitude of fractionation effects.  Some examples of a kinetic isotope effects would be evaporation and condensation, diffusion, and dissociation reactions. 

Understanding the processes that may affect the isotopic relationship in a specific sample type is an important step toward understanding how isotopic delta values (d) are calculated.  An average difference in isotopic composition between the sample and the reference gas is determined using this equation: 

[(R_sample-R_standard)/(R_standard)] x 1000 = d_{sample-standard}

Table 2.  R_standard absolute ratio values for international standards. 

Standard	Rstandard	Atom
SMOW	0.00015575	2H
PDB	0.01119490	13C
SMOW	0.00200520	18O
NAIR	0.00361600	15N
CDT	0.04500450	34S

 

 

Primary Reference Scales

Note: “V” stands for Vienna, which is where the International Atomic Energy Agency (IAEA) is located.

 

V-SMOW (Standard Mean Ocean Water) – used as the “zero” for d²H and d¹⁸O isotope measurement.  The standard is an average of different ocean samples from around the world.

 

V-PDB (Pee Dee Belemnite) – used as the “zero” for d¹³C measurement.  The standard is a CaCO₃ from a belemnite from the Pee Dee formation in South Carolina.

 

Atmospheric Nitrogen – used as the standard reference for d¹⁵N measurement.  The air has a very homogeneous isotopic composition making this an ideal standard.

 

V-CDT (Canyon Diablo Troilite) – used as the standard for d³⁴S measurement.  It is an iron sulfide meteorite from Arizona.

 

Internal Standard Calibration

 

Internal standards are verified against internationally known reference materials; which are, in turn, calibrated against the primary reference scales.  COIL performs bi-annual calibrations to make sure that it’s quality control standards meet international specifications. 

 

Internal solid sample verifications are performed on various reference materials including: BCBG (cabbage), LOB (loblolly pine), CBT (brown trout), BBP (pine) and HCRN (corn).  COIL also has two specific soil standards. COIL uses methionine and sucrose as standards for linearity checks and percent element calibration.  Solid sample internal standards have gone through a rigorous process of being ground and reground and then checked against international standards such as NIST 2704, NBS 18, USGS 24, IAEA-C3, IAEA-C1, NBS 127, IAEA-CH6, NBS 22, IAEA-NO3, USGS 25, NBS 1575, NBS 1645, and NBS 19.  These standards have proven to be effective for continuous flow measurements. 

 

Internal water standards go through a similar process of calibration against international water references such as GISP, SLAP, and MISE.  COIL utilizes 3 specific internal standards including: COW (ocean water), MQ (deionized water), and CLW (lake water).  The internal water standards are also measured against specific gas references.  COIL uses Oztech gas reference tanks for calibration purposes on the dual inlet systems.   

 

COIL personnel will be happy to discuss any questions you may have regarding stable isotope measurements or instrument calibration.