Isotope ratio mass spectrometry

Infobox chemical analysis
name = Isotope ratio mass spectrometry

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caption =Isotope ratio mass spectrometer with gas bench in foreground
acronym = IRMS
classification =mass spectrometry
analytes =
manufacturers =
related = Accelerator mass spectrometry
hyphenated =

Isotope ratio mass spectrometry (IRMS) is a specialization of mass spectrometry, in which mass spectrometric methods are used to measure the relative abundance of isotopes in a given sample.cite journal |author=Paul D, Skrzypek G, Fórizs I |title=Normalization of measured stable isotopic compositions to isotope reference scales - a review |journal=Rapid Commun. Mass Spectrom. |volume=21 |issue=18 |pages=3006–14 |year=2007 |pmid=17705258 |doi=10.1002/rcm.3185] cite journal |author=Stellaard F, Elzinga H |title=Analytical techniques in biomedical stable isotope applications: (isotope ratio) mass spectrometry or infrared spectrometry? |journal=Isotopes in environmental and health studies |volume=41 |issue=4 |pages=345–61 |year=2005 |pmid=16543190 |doi=10.1080/10256010500384333]

Introduction

The isotope ratio mass spectrometer (IRMS) allows the precise measurement of mixtures of stable isotopes. [cite book | author=Townsend, A. (ed)| year=1995 | title=Encyclopaedia of Analytical Science "Encyclopaedia of Analytical Science" | publisher=Academic Press Limited |location=London] This technique has two different applications in the earth and environmental sciences. The analysis of 'stable isotopes' is normally concerned with measuring isotopic variations arising from mass-dependent isotopic fractionation in natural systems. On the other hand, radiogenic isotope analysis involves measuring the abundances of decay-products of natural radioactivity, and is used in most long-lived radiometric dating methods. [cite book | author=Dickin, A.P. | year=2005 | title=Radiogenic Isotope Geology | url=http://www.onafarawayday.com/Radiogenic/ | publisher=Cambridge University Press]

Many radiogenic isotope measurements are made by ionization of a solid source, whereas stable isotope measurements of light elements (e.g. H, C, O) are usually made in an instrument with a gas source. In the latter case, dual gas inlets enable reliable repetition of measurements by supplying continuous streams of the reference and sample gases, which are sequentially switched by a changeover valve. The IRMS's collector also features an array of Faraday cups (conductive, metal vessels which neutralise ions that hit them whilst themselves becoming charged), or "multicollector", which allows the simultaneous detection of multiple isotopes. [cite journal |author=C. B. Bouthitt and K. Garnett|title= The Evolution of the Multicollector in Isotope Ratio Mass Spectromety|journal=Proceedings of the 18th AMZSMS Conference |pages=THO–07] Samples must be introduced as pure gases, achieved through combustion, gas chromatographic feeds, [W. Meier-Augenstein, "J. Chromarogr., A", 1999, 842, 351-371.] or chemical trapping. By comparing the detected isotopic ratios to a measured standard, an accurate determination of the isotopic make up of the sample is obtained. For example, carbon isotope ratios are measured relative to the international standard for CO₂. (The CO₂ standard is produced from a fossil belemnite found in the Pee Dee formation, which is a limestone formed in the Cretaceous period in South Carolina, U.S.A.; its ¹³C:¹²C ratio is 0.0112372.)

Operation

A Nier mass spectrometer is an instrument for analysing the isotope ratios. It was designed by Alfred Nier. In the most general terms the instrument operates by ionizing the sample of interest and subjecting the resultant ions to a physical process that separates them according to their mass to charge ratio (m/z).

Instruments have been developed based on several techniques for mass separation and tuned to a wide range of applications. This article describes one of these application areas, instruments adapted specifically to measure the relative abundance of masses up to around mass number 66.

This field is of interest because the relative variation in mass between isotopes in this range is large enough to give rise to variation in chemical, physical and biological behaviour. This leads to measurable effects on the isotopic composition of samples characteristic of their biological or physical history.

As a specific example, the hydrogen isotope deuterium (heavy hydrogen) is almost double the mass of the common hydrogen isotope. Water molecules containing the common hydrogen isotope (and the common oxygen isotope, mass 16) have a mass of 18. Water incorporating a deuterium atom has a mass of 19, over 5% heavier. The energy to vaporise the heavy water molecule is higher than that to vaporize the normal water so isotope fractionation occurs during the process of evaporation.

Thus a sample of sea water (Vienna Standard Mean Ocean Water, or VSMOW) will exhibit a quite detectable isotopic ratio difference when compared to Arctic snowfall (standard light Arctic precipitation, or SLAP).

It is critical that the sample be processed before entering the mass spectrometer so that only a single chemical species enters at a given time. Generally, samples are combusted or pyrolyzed and the desired species (usually hydrogen gas H₂, nitrogen (N₂), carbon dioxide, or sulfur dioxide) is purified by means of traps, filters, catalysts and/or chromatography.

The two most common types of IRMS instruments are continuous flowcite journal |author=Brenna JT, Corso TN, Tobias HJ, Caimi RJ |title=High-precision continuous-flow isotope ratio mass spectrometry |journal=Mass spectrometry reviews |volume=16 |issue=5 |pages=227–58 |year=1997 |pmid=9538528 |doi=10.1002/(SICI)1098-2787(1997)16:5<227::AID-MAS1>3.0.CO;2-J] and dual inlet. In dual inlet IRMS, purified gas obtained from a sample is alternated rapidly with a standard gas (of known isotopic composition) by means of a system of valves, so that a number of comparison measurements are made of both gases. In continuous flow IRMS, sample preparation occurs immediately before introduction to the IRMS, and the purified gas produced from the sample is measured just once. The standard gas may be measured before and after the sample or after a series of sample measurements. While continuous-flow IRMS instruments can achieve higher sample throughput and are more convenient to use than dual inlet instruments, the yielded data is of approximately 10-fold lower precision.

Moving Wire IRMS (MW-irMS)

This technique permits the analysis of small samples. Samples consisting of as little as 1μL are dried onto a nickel wire, which is passed through a furnace; the analysis occurs as the material combusts. The technique is typically used for Carbon-13 analysis, where samples containing as little as 1 nano-mole of Carbon can yield precise (within 1‰) results.cite journal
author = Sessions, A.L.
coauthors = Sylva, S.P.; Hayes, J.M.
year = 2005
title = Moving-wire device for carbon isotopic analyses of nanogram quantities of nonvolatile organic carbon
journal = Analytical chemistry(Washington, DC)
volume = 77
issue = 20
pages = 6519–6527
doi=10.1021/ac051251z] Sectstub|date=May 2008

Applications

The variation outlined above has applications in hydrology since most samples will lie between these two extremes.cite journal |author=Han LF, Gröning M, Aggarwal P, Helliker BR |title=Reliable determination of oxygen and hydrogen isotope ratios in atmospheric water vapour adsorbed on 3A molecular sieve |journal=Rapid Commun. Mass Spectrom. |volume=20 |issue=23 |pages=3612–8 |year=2006 |pmid=17091470 |doi=10.1002/rcm.2772] Given a sample of water from an aquifer, and a sufficiently sensitive tool to measure the variation in the isotopic ratio of hydrogen in the sample, it is possible to infer the source, be it ocean water seeping into the aquifer or precipitation seeping into the aquifer, and even to estimate the proportions from each source.cite journal |author=Weldeab S, Lea DW, Schneider RR, Andersen N |title=155,000 years of West African monsoon and ocean thermal evolution |journal=Science |volume=316 |issue=5829 |pages=1303–7 |year=2007 |pmid=17540896 |doi=10.1126/science.1140461]

Another application is in paleotemperature measurement for Paleoclimatology. For example one technique is based on the variation in isotopic fractionation of oxygen by biological systems with temperature.cite journal |author=Tolosa I, Lopez JF, Bentaleb I, Fontugne M, Grimalt JO |title=Carbon isotope ratio monitoring-gas chromatography mass spectrometric measurements in the marine environment: biomarker sources and paleoclimate applications |journal=Sci. Total Environ. |volume=237-238 |issue= |pages=473–81 |year=1999 |pmid=10568296 |doi=10.1016/S0048-9697(99)00159-X]

Species of foraminifera incorporate oxygen as calcium carbonate in their shells. The ratio of the oxygen isotopes oxygen 16 and oxygen 18 incorporated into the calcium carbonate varies with temperature and the oxygen isotopic composition of the water. This oxygen remains "fixed" in the calcium carbonate when the forminifera dies, falls to the sea bed, and it's shell becomes part of the sediment. It is possible to select standard species of forminifera from sections through the sediment column, and by mapping the variation in oxygen isotopic ratio, deduce the temperature that the forminifera encountered during life if changes in the oxygen isotopic composition of the water can be constrained.cite journal |author=Shen JJ, You CF |title=A 10-fold improvement in the precision of boron isotopic analysis by negative thermal ionization mass spectrometry |journal=Anal. Chem. |volume=75 |issue=9 |pages=1972–7 |year=2003 |pmid=12720329 |doi=10.1021/ac020589f]

In forensic science, research suggests that the variation in certain isotope ratios in drugs derived from plant sources (cannabis, cocaine) can be used to determine the drug's continent of origin.cite journal |author=Casale J, Casale E, Collins M, Morello D, Cathapermal S, Panicker S |title=Stable isotope analyses of heroin seized from the merchant vessel Pong Su |journal=J. Forensic Sci. |volume=51 |issue=3 |pages=603–6 |year=2006 |pmid=16696708 |doi=10.1111/j.1556-4029.2006.00123.x]

ee also

*Accelerator mass spectrometry
*Atom
*Table of nuclides
*Isotopomer
*Bainbridge mass spectrometer

References

Bibliography

*cite book |author=Goetz, A.; Platzner, I. T. (Itzhak Thomas); Habfast, K.; Walder, A. J. |title=Modern isotope ratio mass spectrometry |publisher=J. Wiley |location=London |year=1997 |pages= |isbn=0-471-97416-1 |oclc= 36461690|doi=
*cite book |author=Yamasaki, Shin-ichi; Boutton, Thomas W. |title=Mass spectrometry of soils |publisher=M. Dekker |location=New York |year=1996 |pages= |isbn=0-8247-9699-3 |oclc= 34473560|doi=