What are stable isotopes? Colorado Plateau Stable Isotope Laboratory

What are stable isotopes?

Isotopes are atoms of the same element that differ in atomic mass, due to differences in the number of neutrons contained in the atoms' nuclei. For example, the three most abundant isotopes of carbon are carbon-12 (¹²C), which contains 6 protons, 6 electrons, and 6 neutrons; carbon-13 (¹³C), which also has 6 protons and electrons, but has 7 neutrons; and carbon-14 (¹⁴C), which also contains 6 protons and electrons, but has 8 neutrons. Having too few or too many neutrons compared to protons causes some isotopes, such as ¹⁴C, to be unstable. These unstable 'radioisotopes' will decay to stable products. Other isotopes, such as ¹²C and ¹³C do not decay, because their particular combinations of neutrons and protons are stable. These are referred to as stable isotopes.

How do you measure stable isotopes?

Stable isotopes of carbon, nitrogen, sulfur, oxygen, and hydrogen - those most commonly used in ecological and environmental research - are measured by gas isotope-ratio mass spectroscopy. The sample is converted into a gas (such as CO₂, N₂, SO₂, or H₂), the gas molecules are ionized in the Ion Source (Figure 1) which strips an electron from each of them (causing each molecule to be positively charged), and then the charged molecules enter a flight tube. The flight tube is bent, and a magnet is positioned over it such that the charged molecules separate according to their mass, with molecules containing the heavier isotope bending less than those containing the lighter isotope. Faraday collectors are present at the end of the flight tube to measure the intensity of each beam of ions of a given mass, after they've been separated by the magnet.

For CO₂, three faraday collectors are set to collect ion beams of masses 44, 45, and 46. Several masses are collected simultaneously, so that the ratios of these masses can be determined very precisely.

Ion Source

Magnet

mass 46

mass 45

mass 44

Flight tube

Figure 1. Dispersion pattern in the flight tube of a mass spectrometer for the three most common masses of CO₂, masses 44 (¹²C, ¹⁶O, ¹⁶O), 45 (¹³C, ¹⁶O, ¹⁶O), and 46 (¹²C, ¹⁶O, ¹⁸O). When CO₂ is introduced into the ion source, each molecule is stripped of 1 electron, becoming a positively charged ion when it enters the flight tube.

In the flight tube, the magnet causes the ions to be deflected, with a radius of deflection that is proportional to the mass-to-charge ratio of the ion. Heavier ions are deflected less (larger radius) than lighter ions. For example, for CO₂, mass 46 has the largest radius of deflection, mass 44 has the smallest, and mass 45 is intermediate. Charge also affects the radius of deflection, but - for the most part - this is held constant because the Ion Source strips only 1 electron from most molecules.

Element	*d value*	Ratio Measured (R)	International Standards	R, International Standards
Hydrogen	d D	2H/¹H	Vienna Standard Mean Ocean Water (VSMOW)	0.00015575
		²H/¹H	Standard Light Antarctic Precipitation (SLAP)	0.000089089
Carbon	d ¹³C	13C/¹²C	Vienna Pee Dee Belemnite (VPDB)	0.0112372
Nitrogen	d ¹⁵N	15N/¹⁴N	Air	0.003676
Oxygen	d ¹⁸O	18O/¹⁶O	Vienna Standard Mean Ocean Water (VSMOW)	0.0020052
		¹⁸O/¹⁶O	Vienna Pee Dee Belemnite (VPDB)	0.0020672
		¹⁸O/¹⁶O	Standard Light Antarctic Precipitation (SLAP)	0.0018939
Sulfur	d ³⁴S	34S/³²S	Canyon Diablo Triolite (CDT)	0.045005

How are stable isotopes used in ecological and environmental research?

Some of the most exciting advances in ecology and environmental sciences in the past decade have relied on stable isotopes. Stable isotopes can be used to address many questions in ecology and environmental sciences, including:

    -What is the base of this food web (¹³C, ³⁴S)?
    -How long is this food chain (¹⁵N)?
    -Where does this nitrate and sulfate pollution come from (¹⁵N, ³⁴S, ¹⁸O)?
    -How efficiently do these plants use water (¹³C)?
    -Where in the soil do these plants get their water (¹⁸O, ²H)?
    -How much nitrogen did a plant get from nitrogen fixation vs. NH₄⁺ or NO₃^- uptake (¹⁵N)?
    -What are the rates of carbon and nitrogen turnover in this soil (¹³C, ¹⁵N)?
    -Was this CO₂ produced by plant roots or by soil microorganisms (¹³C, ¹⁸O)?
    -Was this N₂O produced by nitrifiers or denitrifiers (¹⁵N, ¹⁸O)?
    -Where did this butterfly migrate from (¹⁸O, ²H)?
    -Did this prehistoric human society rely on corn as a food source (¹³C)?

Stable isotope techniques allow the investigation of these questions quantitatively and non-intrusively (without the environmental hazards of radioisotopes), and thus offer considerable advantages over other techniques. Indeed, many of these questions can only be addressed by using stable isotopes.

Many of these techniques rely on natural differences in the ways that 'heavy' and 'light' isotopes are processed in the environment through chemical, biological, and physical transformations. These are referred to as natural abundance isotope techniques. For example, plants preferentially take up carbon dioxide containing the lighter carbon isotope (¹²C-CO₂) in photosynthesis, but the degree of preference depends on water availability and on the photosynthetic pathway, which is a major distinguishing characteristic of plants from hot, xeric environments versus more mesic environments. Thus, the ¹³C composition of plants provides a time-integrated measure of the efficiency with which plants use water. It is also possible to tell how much a plant depends on surface water compared to deep sources of water by measuring the ²H and ¹⁸O composition of water in a plant's stem, because 1) 'heavier' water will evaporate at the soil surface more slowly than 'lighter' water, causing surface soil water to be isotopically enriched compared to deeper water, such as groundwater, and 2) the rain and snow that falls during winter (precipitation that contributes to deeper water sources in the soil) will be isotopically 'lighter' than summer precipitation.

Other stable isotope techniques rely on adding trace amounts of compounds that are artificially enriched in the rare (heavy) isotope of the element of interest. These are referred to as isotope tracer techniques. For example, without isotopes, measuring independently the processes of microbial production of ammonium (NH₄⁺) through mineralization and the consumption of NH₄⁺ through immobilization and nitrification was not possible, because all these processes occur simultaneously. By adding ¹⁵NH₄⁺ to soil and monitoring the rate at which it is 'diluted' by the more abundant ¹⁴NH₄⁺, one gets a measure of the rate of mineralization of soil organic matter, a rate that is independent of nitrification and immobilization (the NH₄⁺-consuming processes). By adding ¹⁵N as NH₄⁺ or NO₃^- and monitoring both ¹⁵N and ¹⁴N in the soil, it is possible to quantify each of these microbial transformations, in situ.