What is a stable isotope?
Stable Isotope Overview
Isotopes are atoms of the same element that have a different number of neutrons, and therefore differ in their atomic mass. For example, oxygen atoms all have 8 protons, and therefore an atomic number of 8, but there are three different kinds of oxygen found in nature:
16O (8 protons and 8 neutrons)
17O (8 protons and 9 neutrons)
18O (8 protons and 10 neutrons)
Elements may have two or more naturally occurring isotopes. The mass presented on the periodic table of elements represents the average mass for all isotopes based on the average natural abundance.
There are two types of isotopes, people are usually more familiar with radioactive isotopes, such as uranium. There are 65 naturally occurring radioactive isotopes and over 1000 manufacture radioactive isotopes. These radioactive isotopes are unstable and they spontaneously decay over time. The second type is a stable isotope, there are 260 stable isotopes found naturally, and they do not spontaneously decay over time. They were formed during the big bang and the relative amounts are considered to be stable. Usually an element will be found in both light and heavy isotopic forms, with the light isotope being more common.
In our lab we have capabilities to analyze samples for carbon and nitrogen isotopes.
How can we use stable isotopes?
Stable isotopes have multiple uses in biology, environmental science, and other disciplines. The following questions can be answered by using stable isotopes:
- What is the primary producer in this food web (13C)?
- What trophic level does this animal feed at (15N)?
- How long is this food chain (15N)?
- Does this aquatic organism originate from a marine or freshwater source (13C)?
- What are the rates of carbon and nitrogen turnover in this soil ( 13C, 15N)?
- What was the food source for this prehistoric human society ( 13C)?
- Was this animal feeding on C3 or C4 plants?
How do we measure stable isotopes?
We measure stable isotopes using Elemental Analyzer-Isotope Ratio Mass Spectrometer or EA-IRMS. This system consists of two separate systems an Elemental Analyzer (EA) and the Isotope Ratio Mass Spectrometer (IRMS). The EA can be configured to analyze a variety of stable isotopes, here we discuss the set up for carbon and nitrogen stable isotope analysis.
It is in the EA that a sample will be combusted, converting the sample to a gaseous state. This gas will be made of CO2 and N2 and various oxides of NOx, after passing through a series of tubes, will emerge as CO2 and N2. These gases now pass through a column where they are separated so that they reach the mass spectrometer at different times. The isotope ratio mass spectrometer consists of three main parts an ion source, a magnet and a detector. The source generates a beam of electrons which are used to ionize the N2 or CO2. These ions are then accelerated and focused into a beam that passes into a chamber where the magnet creates an electromagnetic field. This electromagnetic field is perpendicular to the flight path of the accelerated ions, and therefore the ions will be deflected from their path. The heavy ions will be deflected less, and the lighter ions will be defected more. The ions are deflected into series of detectors which pick up the intensity of the ion signal, this signal is recorded by a computer for processing and analysis.
Figure showing a simple diagram of EA-IRMS for δ¹³C and δ¹⁵N analysis taken from Carter J. F, and Barwick V. J. (Eds), Good practice guide for isotope ratio mass spectrometry, FIRMS (2011)