![]() |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
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.
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: [(Rsample-Rstandard)/(Rstandard)] x 1000 = dsample-standard Rsample is the ratio of the heavy isotope to light isotope in the sample. Rstandard is the ratio of the heavy isotope to light isotope in the working reference gas, which is calibrated against an internationally known IAEA or NBS standard. dsample-standard is the difference
in isotopic composition of the sample relative to that of the reference,
expressed in per mil (). Table
2. Rstandard absolute ratio values for
international standards.
Primary
Reference Scales Note:
V stands for V-SMOW
( V-PDB
( 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 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 its 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.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||