Gas Source Mass Spectrometry: Stable Isotope Geochemistry
What is Gas Source Mass Spectrometry: Stable Isotope Geochemistry
Fundamental Principles of Gas Source Mass Spectrometry: Stable Isotope Geochemistry
Gas Source Mass Spectrometry: Stable Isotope Geochemistry Instrumentation - How Does It Work?
Gas source instruments utilize one of two types of inlet systems, either a "viscous" gas flow inlet, or a "continuous" gas flow inlet. Viscous gas flow systems use a dual-inlet system that allows alternation between a "standard" gas (of known isotopic composition) and a sample gas (isotopic composition to be determined). Multiple measurements of each gas are used to calculate the isotopic composition of the sample gas. Instrumental precision of such measurements is typically greater than the sample reproducibility. Continuous gas flow systems use a carrier gas such as helium to carry the gas into the ion source chamber. An "open-split" device allows equivalent inlet conditions for both standard gas and sample gas. Continuous flow systems permit coupling of a gas chromatograph to a mass spectrometer for compound specific isotopic analysis. Viscous flow systems allow high precision analyses, but require a relatively large sample size in order to ensure viscous flow, which inhibits fractionation processes in the flow system. Only a small fraction of the gas is actually analyzed. Continuous flow systems yield lower precision analyses, but allow measurement of much smaller sample size.
A new generation of large radius SIMS instruments (secondary ion mass spectrometers) optimized for light stable isotopes are now available to augment the traditional stable isotope work done with gas source mass spectrometers. Because these are expensive to build and operate, they are likely to remain few in number. Their chief advantage is the ability to analyze small spots on minerals in situ with the same precision as conventional gas source instruments. For example, visit the Wisconsin Secondary Ion Mass Spectrometer Laboratory.
Mass-dependent fractionations result in different isotopic ratios in different compounds in equilibrium. The magnitude of isotopic fractionations depends on temperature. This is really useful! For example, the isotopic composition of precipitation depends in part on the temperature. Seawater in the open ocean has close to the same isotopic composition everywhere, but as one travels north or south from the equator, the precipitation becomes isotopically lighter. The glacial ice that has built up on Greenland from precipitation over the past 100,000 years contains a record of isotopic variation that corresponds to fluctuations in climate. Quaternary geologists read the climate history of the ice ages in the isotope record of the ice (Alley, 2002).
Isotopic fractionations should decrease to zero, theoretically, as temperature increases to infinity. At infinite temperature, the vibrational energy and strain on bonds would be so great that it wouldn't matter which isotope is present1 (O'Neil, 1986). So isotopic fractionations tend to be largest at low temperatures. The temperature dependence of isotope fractionations allows stable isotopes to be used for geothermometry, that is, the isotopic composition of two phases assumed to be in exchange equilibrium can reveal the temperature of their equilibrium exchange (Chacko et al., 2001; Valley, 2001).
Stable isotopes are useful in two important ways. One is as a geochemical tracer. As elements cycle through various reservoirs of natural systems, the isotopic ratio of an element in rocks or other material can elucidate the movement of elements through these reservoirs. Secondly, stable isotope ratios of two substances can be used to constrain the temperature of their exchange, if one can assume equilibrium.
Applications of stable isotope geochemistry include paleoclimatology, igneous petrology, sedimentary petrology, metamorphic petrology, economic geology, biogeochemistry, hydrogeology, marine science, paleontology, extraterrestrial geochemistry, global geochemical cycles, and on and on. In addition, many new directions have been developed regarding stable isotope geochemistry of non-traditional elements such as Li and Fe, just to name a couple (for example, see Johnson et al., 2004; Tomascak, 2004).
1 This is also true of cation exchange thermometers, such as garnet-biotite. The distribution coefficient approaches a value of one (i.e., no fractionation) as temperature approaches infinity.
Strengths and Limitations of Gas Source Mass Spectrometry: Stable Isotope Geochemistry?
User's Guide - Sample Collection and Preparation
Data Collection, Results and Presentation
Stable isotope ratios are reported using delta notation. This notation compares the ratio of the heavier isotope to the lighter isotope in a sample to that of a standard. Because the variations are small, we multiply by 1000 and the resulting delta value is therefore in permil (???). Delta values are defined as follows:
An equivalent way to write the delta expression is:
The abundances of isotopes of the commonly used stable isotopes and the standards employed are given in Table 1.
The following literature can be used to further explore Gas Source Mass Spectrometry: Stable Isotope Geochemistry
- Alley, R.B., 2002. The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton University Press. ISBN 13: 978-0-691-10296-2.
- Chacko, T., Cole, D.R., and Horita, J., 2001. Equilibrium oxygen, hydrogen, and carbon isotope fractionation factors applicable to geologic systems, in Stable Isotope Geochemistry (Valley, J.W. and Cole, D.R., Eds.). Reviews in Mineralogy and Geochemistry, Vol. 43, p. 1-81. Mineralogical Society of America, Washington, D.C.
- de Groot, P.A. (Ed.), 2004. Handbook of Stable Isotope Analytical Techniques, Volume 1. Elsevier, San Diego, USA.
- Faure, G. and Mensing, T.M., 2005. Isotopes: Principles and Applications, Third Edition. John Wiley & Sons, New Jersey.
- Hoefs, J., 2004, Stable Isotope Geochemistry, 5th ed. Springer, New York.
- Johnson, C.M., Beard, B.L. Roden, E.E., Newman, D.K., and Nealson, K.H., 2004. Isotopic Constraints on Biogeochemical Cycling of Fe. In: Geochemistry of Non-traditional Stable Isotopes (Johnson, C.M., Beard, B.L., and Albarede, F., Eds.). Reviews in Mineralogy Geochemistry, Vol. 55, 359-408. Mineralogical Society of America, Washington, D.C.
- Kendall, C. and McDonnell, J.J. (Eds.), 1998. Isotope Tracers in Catchment Hydrology. Elsevier Science B.V., Amsterdam, 839 p.
- Ohmoto. H. 1986. Stable isotope geochemistry of ore deposits. In: Stable Isotopes in High Temperature Processes (Valley, J.W., Taylor, H.P., and O'Neil, J.R, Eds.). Reviews in Mineralogy, Vol. 16, p. 491-570. Mineralogical Society of America, Washington, D.C.
- O'Neil, J.R., 1986. Theoretical and experimental aspects of isotopic fractionation. In: Stable Isotopes in High Temperature Processes (Valley, J.W., Taylor, H.P., and O'Neil, J.R, Eds.). Reviews in Mineralogy, Vol. 16, p. 1-40. Mineralogical Society of America, Washington, D.C.
- Shanks, W.C. III, 2001. Stable isotopes in seafloor hydrothermal systems: vent fluids, hydrothermal deposits, hydrothermal alteration, and microbial processes. In: Stable Isotope Geochemistry (J.W. Valley and D.R. Cole, eds.), Reviews in Mineralogy and Geochemistry, Vol 23: 469-525.
- Sharp, Z., 2007. Principles of Stable Isotope Geochemistry. Prentice Hall, New Jersey.
- Tomascak P.B., 2004. Developments in the understanding and application of lithium isotopes in the Earth and planetary sciences. In: Geochemistry of Non-Traditional Isotope Systems (Johnson CM, Beard BA, and Albarede F., Eds.). Reviews in Mineralogy Geochemistry, Vol. 55, 153-195. Mineralogical Society of America, Washington, D.C.
- Valley, J.W., Stable isotope thermometry at high temperatures. In: Stable Isotope Geochemistry (Valley, J.W. and Cole, D.R., Eds.). Reviews in Mineralogy and Geochemistry, Vol. 43, p. 365-413. Mineralogical Society of America, Washington, D.C.
For more information about Gas Source Mass Spectrometry: Stable Isotope Geochemistry follow the links below.
- Kendall and McDonnell, 1998 - Fundamentals of Isotope Geochemistry: Isotope Tracers in Catchment Hydrology
- Check out these excellent resources from the European Association of Geochemistry Nuts and Bolts of Mass Spectrometry Short Course 2012 -- Great tutorials on the basics of mass spectrometry!
Teaching Activities and Resources
Teaching activities, labs, and resources pertaining to Gas Source Mass Spectrometry: Stable Isotope Geochemistry.