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Thermal Ionization Mass Spectrometry (TIMS)

Paul Mueller, University of Florida
Jeff Vervoort, Washington State University

What is Thermal Ionization Mass Spectrometry (TIMS)

A TIMS is an instrument that measures isotopic ratios that are used in geochemistry, geochronology, and cosmochemistry.

Fundamental Principles of Thermal Ionization Mass Spectrometry (TIMS)

A TIMS is a magnetic sector mass spectrometer that is capable of making very precise measurements of isotope ratios of elements that can be ionized thermally, usually by passing a current through a thin metal ribbon or ribbons under vacuum. The ions created on the ribbon(s) are accelerated across an electrical potential gradient (up to 10 KV) and focused into a beam via a series of slits and electrostatically charged plates. This ion beam then passes through a magnetic field and the original ion beam is dispersed into separate beams on the basis of their mass to charge ratio. These mass-resolved beams are then directed into collectors where the ion beam is converted into voltage. Comparison of voltages corresponding to individual ion beams yield precise isotope ratios.

Thermal Ionization Mass Spectrometry (TIMS) Instrumentation - How Does It Work?

Modern instruments are composed of three primary components: 1) ion source, the region in which ions are produced, accelerated, and focused; 2) analyzer, the region in which the beam is separated based on mass/charge ratios; and 3) collector, a region in which the ion beams are measured either sequentially (single collector) or simultaneously (multi-collector). The electronics of these instruments must operate to very close tolerances in order to produce isotope ratios that are precise to 0.01-0.001%.

Applications

The primary application of TIMS is to measure the isotope ratios of elements used in geochronology and tracer studies. Geochronology refers to the use of radioactive decay in closed systems to obtain the time of a specific geologic event, which is referred to as an age. Tracer applications refer to the use of the growth of daughter isotopes from radioactive decay to evaluate the interaction between geochemical systems and/or reservoirs. This application provides only general chronologic information, often referred to as model ages, which more loosely constrain the timing of geologic processes and the development of, and interaction between, geochemical reservoirs.

For terrestrial systems, common applications in geochronology and tracer Studies involve the following radiometric systems

In cosmochemical systems, the measurement of isotopic compositions is primarily as tracers of nucleosynthetic processes and constraining the evolution of the solar system. This involves measurement of the systems noted above, but also includes the decay of short lived radionuclides, as observed principally in meteorites. In addition to the systems noted above, systems of cosmochemical interest include:
Non-radiogenic (stable) isotope-isotope ratios are typically used to characterize exchange processes, track reservoir interactions, and evaluate biologic and kinetic processes:

Strengths and Limitations of Thermal Ionization Mass Spectrometry (TIMS)?

Strengths

The advantage of TIMS compared to other isotope ratio techniques include:

Limitations

The disadvantages include:

User's Guide - Sample Collection and Preparation

As for all geochemical analyses, care must be taken to preserve sample integrity from the time of collection through analysis in all steps of physical and chemical preparation. Most applications require complete dissolution of the sample followed by liquid chromatography to isolate elements of interest, which is usually done in a "clean" laboratory (typically Class 100-1000). For geochronologic and many tracer applications, it is necessary to "spike" samples with an artificially enriched isotopic tracer in order to determine concentrations and parent-daughter ratios by isotope dilution. Elements are loaded directly as acid solutions on pre-cleaned metal ribbons for analysis.

Data Collection, Results and Presentation

Measured isotope ratios must be properly corrected for all instrumental biases, including mass fractionation. Once corrected, these ratios are suitable for plotting in any diagrams requiring atomic ratios (e.g., isochron, concordia, etc.) or for calculating model ages and initial isotopic ratios.

Literature

The following literature can be used to further explore Thermal Ionization Mass Spectrometry (TIMS)

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Teaching Activities and Resources

Teaching activities, labs, and resources pertaining to Thermal Ionization Mass Spectrometry (TIMS).


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