In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, isotopologues (also spelled isotopologs) are
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s that differ only in their
isotopic composition. They have the same
chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...
and bonding arrangement of
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s, but at least one atom has a different number of
neutron
The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s than the parent.
An example is
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
, whose
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
-related isotopologues are: "light water" (HOH or ), "
semi-heavy water" with the
deuterium isotope in equal proportion to
protium (HDO or ), "
heavy water
Heavy water (deuterium oxide, , ) is a form of water (molecule), water in which hydrogen atoms are all deuterium ( or D, also known as ''heavy hydrogen'') rather than the common hydrogen-1 isotope (, also called ''protium'') that makes up most o ...
" with two deuterium atoms ( or ); and "super-heavy water" or
tritiated water ( or , as well as and , where some or all of the hydrogen is the radioactive
tritium isotope).
Oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
-related isotopologues of water include the commonly available form of heavy-oxygen water () and the more difficult to separate version with the isotope. Both elements may be replaced by isotopes, for example in the
doubly labeled water isotopologue . Altogether, there are 9 different stable water isotopologues, and 9 radioactive isotopologues involving tritium, for a total of 18. However only certain ratios are possible in mixture, due to prevalent hydrogen swapping.
The atom(s) of the different isotope may be anywhere in a molecule, so the difference is in the net chemical formula. If a compound has several atoms of the same element, any one of them could be the altered one, and it would still be the same isotopologue. When considering the different locations of the same isotope, the term ''
isotopomer'', first proposed by
Seeman and Paine in 1992, is used.
Isotopomerism is analogous to
constitutional isomerism or
stereoisomerism
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
of different elements in a structure. Depending on the formula and the symmetry of the structure, there might be several isotopomers of one isotopologue. For example, ethanol has the molecular formula . Mono-deuterated ethanol, or , is an isotopologue of it. The structural formulas and are two isotopomers of that isotopologue.
Singly substituted isotopologues
Analytical chemistry applications
Singly substituted isotopologues may be used for
nuclear magnetic resonance experiments, where deuterated solvents such as
deuterated chloroform (CDCl or CHCl) do not interfere with the solutes' H signals, and in investigations of the
kinetic isotope effect.
Geochemical applications
In the field of
stable isotope geochemistry, isotopologues of simple molecules containing rare heavy isotopes of carbon, oxygen, hydrogen, nitrogen, and sulfur are used to trace
equilibrium and
kinetic processes in natural environments and in
Earth's past.
Doubly substituted isotopologues
Measurement of the abundance of
clumped isotopes (doubly substituted isotopologues) of gases has been used in the field of stable isotope geochemistry to trace equilibrium and kinetic processes in the environment inaccessible by analysis of singly substituted isotopologues alone.
Currently measured doubly substituted isotopologues include:
*
Carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
: COO
*
Methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
:
13CH
3D and
12CH
2D
2
*
Oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
:
18O
2 and
17O
18O
*
Nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
:
15N
2
*
Nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...
: NNO and NNO
Analytical requirements
Because of the relative rarity of the heavy isotopes of C, H, and O,
isotope-ratio mass spectrometry (IRMS) of doubly substituted species requires larger volumes of sample gas and longer analysis times than traditional stable isotope measurements, thereby requiring extremely stable instrumentation. Also, the doubly-substituted isotopologues are often subject to isobaric interferences, as in the methane system where CH and CHD ions interfere with measurement of the CHD and CHD species at mass 18. A measurement of such species requires either very high mass resolving power to separate one isobar from another, or modeling of the contributions of the interfering species to the abundance of the species of interest. These analytical challenges are significant: The first publication precisely measuring doubly substituted isotopologues did not appear until 2004, though singly substituted isotopologues had been measured for decades previously.
As an alternative to more conventional gas source IRMS instruments,
tunable diode laser absorption spectroscopy has also emerged as a method to measure doubly substituted species free from isobaric interferences, and has been applied to the methane isotopologue CHD.
Equilibrium fractionation
When a light isotope is replaced with a heavy isotope (e.g., C for C), the bond between the two atoms will vibrate more slowly, thereby lowering the zero-point energy of the bond and acting to stabilize the molecule. An isotopologue with a doubly substituted bond is therefore slightly more thermodynamically stable, which will tend to produce a higher abundance of the doubly substituted (or “clumped”) species than predicted by the statistical abundance of each heavy isotope (known as a stochastic distribution of isotopes). This effect increases in magnitude with decreasing temperature, so the abundance of the clumped species is related to the temperature at which the gas was formed or equilibrated. By measuring the abundance of the clumped species in standard gases formed in equilibrium at known temperatures, the thermometer can be calibrated and applied to samples with unknown abundances.
Kinetic fractionation
The abundances of multiply substituted isotopologues can also be affected by kinetic processes. As for singly substituted isotopologues, departures from thermodynamic equilibrium in a doubly-substituted species can implicate the presence of a particular reaction taking place.
Photochemistry occurring in the atmosphere has been shown to alter the abundance of O from equilibrium, as has
photosynthesis
Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. Measurements of CHD and CHD can identify
microbial processing of methane and have been used to demonstrate the significance of
quantum tunneling in the formation of methane, as well as mixing and equilibration of multiple
methane reservoirs. Variations in the relative abundances of the two NO isotopologues NNO and NNO can distinguish whether NO has been produced by bacterial
denitrification or by bacterial
nitrification.
Multiple substituted isotopologues
Biochemical applications
Multiple substituted isotopologues may be used for
nuclear magnetic resonance or mass spectrometry experiments, where isotopologues are used to elucidate metabolic pathways in a qualitative (detect new pathways) or quantitative (detect quantitative share of a pathway) approach. A popular example in biochemistry is the use of uniform labelled glucose (U-
13C glucose), which is metabolized by the organism under investigation (e. g. bacterium, plant, or animal) and whose signatures can later be detected in newly formed amino acid or metabolically cycled products.
Mass spectrometry applications
Resulting from either naturally occurring
isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
s or artificial
isotopic labeling, isotopologues can be used in various
mass spectrometry applications.
Applications of natural isotopologues
The relative mass spectral intensity of natural isotopologues, calculable from the fractional abundances of the constituent elements, is exploited by mass spectrometry practitioners in quantitative analysis and unknown compound identification:
# To identify the more likely molecular formulas for an unknown compound based on the matching between the observed isotope abundance pattern in an experiment and the expected isotope abundance patterns for given molecular formulas.
# To expand the linear dynamic response range of the mass spectrometer by following multiple isotopologues, with an isotopologue of lower abundance still generating linear response even while the isotopologues of higher abundance giving saturated signals.
Applications of isotope labeling
A compound tagged by replacing specific atoms with the corresponding isotopes can facilitate the following mass spectrometry methods:
# Metabolic flux analysis (MFA)
# Stable isotopically labeled internal standards for quantitative analysis
See also
*
Mass (mass spectrometry)
*
Isotope-ratio mass spectrometry
*
Isotopomer
*
Clumped isotopes
*
Isotopocule
References
External links
Fractional abundance of atmospheric isotopologues SpectralCalc.com
{{Authority control
Isotopes