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 commonly referred to as radioactive isotopes. There are approximately 300 known naturally occurring stable isotopes.

Here in the Cornell Isotope Laboratory we focus on four specific light elements for isotopic ratio measurements: hydrogen, carbon, nitrogen and oxygen.

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.

 

 

Heavy isotopes undergo all of the same chemical reactions as light isotopes, but, simply because they are heavier, they do it ever so slightly slower. 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.

However, before analysis can begin 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.  

 

 

 

Primary Reference Scales

 

V-SMOW (Standard Mean Ocean Water) - used 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 for d¹³C measurement. The standard is a CaCO₃ from a belemnite from the Pee Dee formation in South Carolina.

 

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

 

 

Internal Standard Calibration

 

Internal (in-house) laboratory standards are verified for accuracy against internationally known reference materials, whose values are determined by the International Association of Atomic Energy (IAEA) in Vienna, Austria. These in turn, calibrated against the primary reference scales for the specific isotope of interest. COIL performs bi-annual calibrations for each peripheral device to make sure that it's quality control standards meet international specifications for each type of measurement performed within the facility.

 

EA-IRMS Analysis (Combustion)

Solid samples sent in for carbon and/or nitrogen measurements are analyzed using an elemental analyzer plumbed into an isotope ratio mass spectrometer. We try to specifically match samples with one of several in-house standards we have for checks on instrument precision and linearity.  These in-house standards include plant materials (corn, rice, cabbage, lettuce, pine needles, wood, cellulose); animal tissues (fish, mink); soils and sediments (low nitrogen desert soil, high elevation forest soil, organic river sediments); and chemical standards (methionine, sucrose, caffeine, acetanilide).  In addition two standards are used to perform normalization (linear regression) of data once the analysis is complete.  Precision on elemental results meet vendor specifications.

TC/EA-IRMS Analysis (Pyrolysis)

Solid samples sent in for hydrogen and/or oxygen measurements are analyzed using a Temperature Conversion elemental analyzer interfaced into an IRMS.  In-house standards are matched closely with the matrix of the sample being analyzed for checks on instrument precision and linearity.  We have several internal standards including plant material (rice, wood, cellulose); animal material (hair, feather); several carbonate and soil standards; and chemical standards (benzoic acid, acetanilide, methionine).  A linear regression of data is performed once analysis is complete.  Precision on elemental results meet vendor specifications.

Gasbench Analysis

Gas samples are measured in exetainer vials that are placed into the GC PAL autosampler.  These, in turn, are injected into the Gasbench II for analysis.  The Gasbench is plumbed directly into the Delta V IRMS.  Currently we have the capability to measure both N₂ and CO₂ gas samples.

CO₂ gas samples are accurately calibrated using known isotopic and concentration gases.  Initial calibration of gases is performed using two standards (1%CO₂ in a background of helium and 1%CO₂ in a background of nitrogen) at concentration levels ranging from ambient (approximately 400ppm – 10,000ppm).  These standards are used for the checks on instrument precision and linearity, as well as checks on concentration.

N₂ gas samples are calibrated using air samples corrected to the Atmospheric Air Primary Reference Scale.  Typically N₂ gas sample projects involve tracer or enriched ¹⁵N experiments.  Internal error is calculated based on the ¹⁵N carryover into the standards.  Normal concentrations for these types of experiments range from ambient (78%N) to ultra high purity (approximately 100%N).