View source for Physics:Isotopologue

{{short description|Molecules which differ only in isotope composition}}

In [[Chemistry:Chemistry|chemistry]], '''isotopologues''' are [[Physics:Molecule|molecule]]s that differ only in their [[Physics:Isotope|isotopic]] composition.<ref>{{GoldBookRef|file=I03351|year=1994|title=Isotopologue}}</ref> They have the same [[Chemistry:Chemical formula|chemical formula]] and bonding arrangement of [[Physics:Atom|atom]]s, but at least one atom has a different number of [[Physics:Neutron|neutron]]s than the parent.

An example is [[Chemistry:Water|water]], whose [[Chemistry:Hydrogen|hydrogen]]-related isotopologues are: "light water" (HOH or {{chem2|H2O}}), "semi-heavy water" with the [[Physics:Deuterium|deuterium]] isotope in equal proportion to [[Physics:Hydrogen-1|protium]] (HDO or {{chem2|^{1}H^{2}HO}}), "[[Chemistry:Heavy water|heavy water]]" with two deuterium isotopes of hydrogen per molecule ({{chem2|D2O}} or {{chem2|^{2}H2O}}), and "super-heavy water" or [[Chemistry:Tritiated water|tritiated water]] ({{chem2|T2O}} or {{chem2|^{3}H2O}}, as well as {{chem2|HTO [^{1}H^{3}HO]}} and {{chem2|DTO [^{2}H^{3}HO]}}, where some or all of the hydrogen atoms are replaced with the radioactive [[Physics:Tritium|tritium]] isotope). [[Chemistry:Oxygen|Oxygen]]-related isotopologues of water include the commonly available form of heavy-oxygen water ({{chem2|H2^{18}O}}) and the more difficult to separate version with the {{chem2|^{17}O}} isotope. Both elements may be replaced by isotopes, for example in the [[Physics:Doubly labeled water|doubly labeled water]] isotopologue {{chem2|D2^{18}O}}. All taken together, there are 9 different stable water isotopologues,<ref>The nine stable isotopologues are {{chem2|H2^{16}O}}, {{chem2|H^{16}OD}}, {{chem2|D2^{16}O}}, {{chem2|H2^{17}O}}, {{chem2|H^{17}OD}}, {{chem2|D2^{17}O}}, {{chem2|H2^{18}O}}, {{chem2|H^{18}OD}}, {{chem2|D2^{18}O}}</ref> and 9 radioactive isotopologues involving tritium,<ref>The nine tritiated isotopologues are {{chem2|H^{16}OT}}, {{chem2|D^{16}OT}}, {{chem2|T2^{16}O}}, {{chem2|H^{17}OT}}, {{chem2|D^{17}OT}}, {{chem2|T2^{17}O}}, {{chem2|H^{18}OT}}, {{chem2|D^{18}OT}}, {{chem2|T2^{18}O}}</ref> 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 isotopically modified element, the term ''[[Physics:Isotopomer|isotopomer]]'', first proposed by [[Biography:Jeffrey I. Seeman|Seeman]] and Paine in 1992, is used.<ref name="Seeman">{{cite journal |last1=Seeman |first1=Jeffrey I. |last2=Secor |first2=Henry V. |last3=Disselkamp |first3=R. |last4=Bernstein |first4=E. R. |title=Conformational analysis through selective isotopic substitution: supersonic jet spectroscopic determination of the minimum energy conformation of o-xylene |journal=Journal of the Chemical Society, Chemical Communications |date=1992 |issue=9 |page=713 |doi=10.1039/C39920000713 }}</ref><ref>{{cite journal  |last1=Seeman |first1=Jeffrey I.  |last2=Paine, III |first2=John B. |title=Letter to the Editor: 'Isotopomers, Isotopologs' |journal=Chemical & Engineering News |date=December 7, 1992|publisher=American Chemical Society |volume=70 |issue=2 |doi=10.1021/cen-v070n049.p002 |url=https://pubs.acs.org/doi/pdf/10.1021/cen-v070n049.p002 |access-date=28 August 2020|doi-access=free }}</ref>
Isotopomerism is analogous to constitutional isomerism 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 {{chem2|C2H6O}}. Mono-deuterated ethanol, {{chem2|C2H5DO}}, is an isotopologue of it. The structural formulas {{chem2|CH3\sCH2\sO\sD}} and {{chem2|CH2D\sCH2\sO\sH}} are two isotopomers of that isotopologue.

==Singly substituted isotopologues==
===Analytical chemistry applications===
Singly substituted isotopologues may be used for [[Physics:Nuclear magnetic resonance|nuclear magnetic resonance]] experiments, where deuterated solvents such as [[Chemistry:Deuterated chloroform|deuterated chloroform]] (CDCl<sub>3</sub>) do not interfere with the solutes' <sup>1</sup>H signals, and in investigations of the [[Chemistry:Kinetic isotope effect|kinetic isotope effect]].

===Geochemical applications===
{{main|Chemistry:Isotope geochemistry}}
In the field of [[Chemistry:Isotope geochemistry|stable isotope geochemistry]], isotopologues of simple molecules containing rare heavy isotopes of carbon, oxygen, hydrogen, nitrogen, and sulfur are used to trace [[Chemistry:Chemical equilibrium|equilibrium]] and [[Chemistry:Kinetic isotope effect|kinetic]] processes in natural environments and in Earth's past.

==Doubly substituted isotopologues==

Measurement of the abundance of [[Physics:Clumped isotopes|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:
*[[Chemistry:Carbon dioxide|Carbon dioxide]]:<ref>Ghosh, Prosenjit, et al. "<sup>13</sup>C–<sup>18</sup>O bonds in carbonate minerals: A new kind of paleothermometer". Geochimica et Cosmochimica Acta 70.6 (2006): 1439–1456.</ref> <sup>13</sup>C<sup>18</sup>O<sup>16</sup>O
*[[Chemistry:Methane|Methane]]:<ref>Young E. D., Kohl I. E., Sherwood Lollar B., Etiope G., Rumble D. III, Li S., Haghnegahdar M. A., Schauble E. A., McCain K. A., Foustoukos D. I., Sutclife C., Warr O., Ballentine C. J., Onstott T. C., Hosgormez H., Neubeck A., Marques J. M., Pérez-Rodríguez I., Rowe A. R., LaRowe D. E., Magnabosco C., Yeung L. Y., Ash J. L., and Bryndzia L. T. (2017) "The relative abundances of resolved <sup>12</sup>CH<sub>2</sub>D<sub>2</sub> and <sup>13</sup>CH<sub>3</sub>D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gas". Geochimica et Cosmochimica Acta 203, 235–264.</ref> <sup>13</sup>CH<sub>3</sub>D<ref>{{cite journal|doi=10.1021/ac5010579 | pmid=24895840 | volume=86 | issue=13 | title=Measurement of a Doubly Substituted Methane Isotopologue,<sup>13</sup>CH<sub>3</sub>D, by Tunable Infrared Laser Direct Absorption Spectroscopy | journal=Analytical Chemistry | pages=6487–6494 | last1 = Ono | first1 = Shuhei| year=2014 | hdl=1721.1/98875 | hdl-access=free }}</ref><ref>{{cite journal | last1 = Stolper | first1 = D. A. | last2 = Sessions | first2 = A. L. | last3 = Ferreira | first3 = A. A. | last4 = Neto | first4 = E. V. Santos | last5 = Schimmelmann | first5 = A. | last6 = Shusta | first6 = S. S. | last7 = Valentine | first7 = D. L. | last8 = Eiler | first8 = J. M. | year = 2014 | title = Combined <sup>13</sup>C–D and D–D clumping in methane: methods and preliminary results | journal = Geochim. Cosmochim. Acta | volume = 126 | pages = 169–191 | doi=10.1016/j.gca.2013.10.045 | bibcode=2014GeCoA.126..169S}}</ref> and <sup>12</sup>CH<sub>2</sub>D<sub>2</sub>
*[[Chemistry:Oxygen|Oxygen]]:<ref>{{cite journal | last1 = Yeung | first1 = L. Y. | last2 = Young | first2 = E. D. | last3 = Schauble | first3 = E. A. | year = 2012 | title = Measurements of <sup>18</sup>O<sup>18</sup>O and <sup>17</sup>O<sup>18</sup>O in the atmosphere and the role of isotope-exchange reactions | journal = Journal of Geophysical Research | volume = 117 | issue = D18| page = D18306 | doi = 10.1029/2012JD017992 | bibcode=2012JGRD..11718306Y| doi-access =  }}</ref> <sup>18</sup>O<sub>2</sub> and <sup>17</sup>O<sup>18</sup>O
*[[Chemistry:Nitrogen|Nitrogen]]:<ref>{{cite journal | last1 = Young | first1 = E. D. | last2 = Rumble | first2 = D. III | last3 = Freedman | first3 = P. | last4 = Mills | first4 = M. | year = 2016 | title = A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O<sub>2</sub>, N<sub>2</sub>, and CH<sub>4</sub> and other gases | journal = International Journal of Mass Spectrometry | volume = 401 | pages = 1–10 | doi=10.1016/j.ijms.2016.01.006| bibcode = 2016IJMSp.401....1Y | doi-access = free }}</ref> <sup>15</sup>N<sub>2</sub>
*[[Chemistry:Nitrous oxide|Nitrous oxide]]: <sup>14</sup>N<sup>15</sup>N<sup>18</sup>O and <sup>15</sup>N<sup>14</sup>N<sup>18</sup>O<ref>Magyar, P. M., Orphan, V. J., and Eiler, J. M. (2016) Measurement of rare isotopologues of nitrous oxide by high-resolution multi-collector mass spectrometry. Rapid Commun. Mass Spectrom., 30: 1923–1940.</ref>

===Analytical requirements===
Because of the relative rarity of the heavy isotopes of C, H, and O, [[Chemistry:Isotope-ratio mass spectrometry|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. In addition, the doubly-substituted isotopologues are often subject to isobaric interferences, as in the methane system where <sup>13</sup>CH<sub>5</sub><sup>+</sup> and <sup>12</sup>CH<sub>3</sub>D<sup>+</sup> ions interfere with measurement of the <sup>12</sup>CH<sub>2</sub>D<sub>2</sub><sup>+</sup> and <sup>13</sup>CH<sub>3</sub>D<sup>+</sup> species at mass 18. A measurement of such species requires either very high mass resolving power to separate one isobar from another,<ref>{{cite journal | last1 = Eiler | first1 = John M. |display-authors=0 | year = 2013 | title = A high-resolution gas-source isotope ratio mass spectrometer | journal = International Journal of Mass Spectrometry | volume = 335 | pages = 45–56 | doi=10.1016/j.ijms.2012.10.014| bibcode = 2013IJMSp.335...45E }}</ref> 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.<ref>{{cite journal | last1 = Eiler | first1 = J. M. | last2 = Schauble | first2 = E. | year = 2004| title = <sup>18</sup>O<sup>13</sup>C<sup>16</sup>O in Earth's atmosphere | journal = Geochimica et Cosmochimica Acta | volume = 68 | issue = 23| pages = 4767–4777 | doi=10.1016/j.gca.2004.05.035 | bibcode=2004GeCoA..68.4767E}}</ref>

As an alternative to more conventional gas source IRMS instruments, [[Physics:Tunable diode laser absorption spectroscopy|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 <sup>13</sup>CH<sub>3</sub>D.

===Equilibrium fractionation===
When a light isotope is replaced with a heavy isotope (e.g., <sup>13</sup>C for <sup>12</sup>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.<ref>Urey, H. C., 1947. The thermodynamic properties of isotopic substances. J. Chem. Soc. London 1947, 561–581.</ref> 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.<ref>{{cite journal | last1 = Wang | first1 = Z. | last2 = Schauble | first2 = E. A. | last3 = Eiler | first3 = J. M. | year = 2004 | title = Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases | journal = Geochim. Cosmochim. Acta | volume = 68 | issue = 23| pages = 4779–4797 | doi=10.1016/j.gca.2004.05.039 | bibcode=2004GeCoA..68.4779W}}</ref> 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. [[Chemistry:Photochemistry|Photochemistry]] occurring in the atmosphere has been shown to alter the abundance of <sup>18</sup>O<sub>2</sub> from equilibrium, as has [[Earth:Photosynthesis|photosynthesis]].<ref>{{cite journal | last1 = Yeung | first1 = L. Y. | last2 = Ash | first2 = J. L. | last3 = Young | first3 = E. D. | year = 2015 | title = Biological signatures in clumped isotopes of O<sub>2</sub> | journal = Science | volume = 348 | issue = 6233| pages = 431–434 | doi=10.1126/science.aaa6284| pmid = 25908819 | bibcode = 2015Sci...348..431Y | doi-access = free }}</ref> Measurements of <sup>13</sup>CH<sub>3</sub>D and <sup>12</sup>CH<sub>2</sub>D<sub>2</sub> 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 N<sub>2</sub>O isotopologues <sup>14</sup>N<sup>15</sup>N<sup>18</sup>O and <sup>15</sup>N<sup>14</sup>N<sup>18</sup>O can distinguish whether N<sub>2</sub>O has been produced by bacterial [[Biology:Denitrification|denitrification]] or by bacterial [[Biology:Nitrification|nitrification]].

==Multiple substituted isotopologues==
===Biochemical applications===
Multiple substituted isotopologues may be used for [[Physics:Nuclear magnetic resonance|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-<sup>13</sup>C 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 [[Physics:Isotope|isotope]]s or artificial [[Physics:Isotopic labeling|isotopic labeling]], isotopologues can be used in various [[Physics:Mass spectrometry|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.<ref>{{Cite journal|last=Böcker|first=S.|title=SIRIUS: Decomposing isotope patterns for metabolite identification|journal=Bioinformatics|year=2009 |volume=25|issue=2|pages=218–224|doi=10.1093/bioinformatics/btn603|pmid=19015140 |pmc=2639009 }}</ref><ref>{{Cite journal|last=Wang|first=Yongdong|title=The Concept of Spectral Accuracy for MS|journal=Anal. Chem.|year=2010 |volume=82|issue=17|pages=7055–7062|doi=10.1021/ac100888b|pmid=20684651 }}</ref><ref>{{Cite journal|last=Bluck|first=Les|title=The Role of Naturally Occurring Stable Isotopes in Mass Spectrometry, Part I: The Theory|journal=Spectroscopy|volume=23|issue=10|pages=36|pmid=23772100}}</ref>
# 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.<ref>{{Cite journal|last=Liu|first=Hanghui|title=Expanding the linear dynamic range for multiple reaction monitoring in quantitative liquid chromatography–tandem mass spectrometry utilizing natural isotopologue transitions|journal=Talanta|year=2011 |volume=87|pages=307–310|doi=10.1016/j.talanta.2011.09.063}}</ref><ref>{{Cite journal|last=Bach|first=Thanh|title=Importance of Utilizing Natural Isotopologue Transitions in Expanding the Linear Dynamic Range of LC-MS/MS Assay for Small-Molecule Pharmacokinetic Sample Analysis – A mini-review|journal=Journal of Pharmaceutical Sciences|year=2022 |volume=111 |issue=5 |pages=1245–1249 |doi=10.1016/j.xphs.2021.12.012 | pmid=34919967 |pmc=9018470 }}</ref>

====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)<ref>{{Cite journal|last=Wang|first=Yujue|title=Metabolic Flux Analysis-Linking Isotope Labeling and Metabolic Fluxes|journal=Metabolites|year=2020 |volume=10|issue=11|pages=447|doi=10.3390/metabo10110447|pmid=33172051 |pmc=7694648 |doi-access=free }}</ref>
# Stable isotopically labeled internal standards for quantitative analysis<ref>{{Cite journal|last=Stokvis|first=Ellen|title=Stable isotopically labeled internal standards in quantitative bioanalysis using liquid chromatography/mass spectrometry: necessity or not?|journal=Rapid Communications in Mass Spectrometry|year=2005 |volume=19|issue=3|pages=401–407|doi=10.1002/rcm.1790|pmid=15645520 }}</ref>

==See also==
*[[Physics:Mass (mass spectrometry)|Mass (mass spectrometry)]]
*[[Chemistry:Isotope-ratio mass spectrometry|Isotope-ratio mass spectrometry]]
*[[Physics:Isotopomer|Isotopomer]]
*[[Physics:Clumped isotopes|Clumped isotopes]]
*[[Physics:Isotopocule|Isotopocule]]

==References==
{{Reflist}}

==External links==
*[http://www.spectralcalc.com/ Fractional abundance of atmospheric isotopologues], SpectralCalc.com


[[Category:Isotopes]]

{{Sourceattribution|Isotopologue}}