USGS-GD-Scientific Capabilities - MICROBEAM INSTRUMENTS ION PROBE Technique

 

 

MICROBEAM INSTRUMENTS

ION PROBE

Technique

 

An ion microprobe provides in situ elemental and isotopic analyses of very small amounts of solid material at precisely located spots a few to tens of micrometers in diameter and 1-2 micrometers in depth. An ion microprobe uses an ion beam (typically oxygen or cesium) to produce a plasma of the target material. The ionic species in this plasma are introduced into an energy filter and a mass spectrometer and measured by a sensitive counting device. Commercial ion microscopes, produced for over 20 years and used in industry and academic research, use a small mass spectrometer with limited mass resolution to provide elemental, particularly trace element, analyses. The ion microprobe (called the SHRIMP for Sensitive High Resolution Ion MicroProbe) was developed by Dr. W. Compston at the Australian National University in the late 1970s. This instrument improved mass resolution by a factor of 10-20 and was able to provide for 30 micrometer analysis spots:

  1. true isotopic analyses for heavier elements allowing the determination of U-Pb ages of zircons,
  2. precise trace elemental analysis including the rare earth elements, and
  3. S, Pb, and Hf isotopic compositions. Stanford and the USGS jointly purchased a third generation version of this type of ion probe (the SHRIMP RG) that will have mass resolution improved by a factor of 3-4 over existing probes.

Material analysis capabilities:

  • In situ analysis
  • Quantitative elemental concentrations at ultra-trace levels and precise isotopic ratios
  • Analyses of very small areas or amounts of samples
  • Data for a wide range of solid materials (silicates, carbonates, oxides, sulfides, etc.)
  • A relatively non-destructive analysis
  • Analysis without chemical preparation, thus minimizing the possibility of contamination
  • The best detection limits (ppb) for many elements from an in situ analysis technique
  • High spatial resolution in order to identify and analyze different areas of heterogeneous or compositionally zoned materials

Applications:

High sensitivity, in situ, microanalysis of solid materials has tremendous potential in a wide range of sciences including chemistry, physics, and solid state electronics, in addition to the earth sciences. Ion microprobe analysis has yet to realize its full potential in the earth sciences because most of the available instruments have been dedicated mainly to problems in meteoritics and geochronology. An ion microprobe could be applied to important problems in many disciplines. Present and future applications include:

  • Environmental pollution-the location, identification, quantification, and finger-printing of metallic and non-metallic pollutants in, or adsorbed on, solid particles and surfaces (e.g., modern sediments, soils) and in biogenic materials.
  • Hydrologic tracers-constraints on reservoir processes and solid-water interactions through monitoring of tracers on particles, in cements, or any solid substrate
  • Paleoclimate-using stable isotope thermometry of macro- and microfossils, fine-scale determination of temporal changes in water chemistry recorded by tracers, Sr isotopic ratios of microfossils as indicators of water composition and geologic age
  • Metals in coal-determining the locations and concentrations of trace metals in coal and the stable isotopic compositions of host phases
  • Sedimentary rocks, diagenesis, weathering, soil science-monitoring trace constituents in carbonate and phosphate rocks from their formation to their destruction (e.g., natural weathering metallic pollutants, use of phosphate-based fertilizers, degradation of limestone building materials, etc.)
  • Volcanic ash correlation-precise trace element finger-printing of individual glass shards in ash beds for correlation of Quaternary and older sediments
  • Elemental diffusion, partitioning, movement- understanding partitioning of elements among various mineral and organic phases under different natural and experimental conditions
  • Petrology and economic geology-stable isotope ratios, isotopic tracers, trace element partitioning applied to magma crystallization and degassing, and to the genesis of economic mineral deposits
  • U-Pb determination

Operations:

The instrument will be housed in the Green Earth Sciences Building at Stanford University. Staffing and operational costs are to be shared by the USGS and Stanford. USGS staffing will consist of a senior geologist and an electronics technician. It is anticipated that a large number of scientists from Bureau operations across the country will actively participate in analytical research during 1-3 week visits. Some portion of the instrument will be sold commercially to provide a shared contribution toward operational costs. USGS personnel view the instrument as a Bureau level resource that will contribute to existing and future programmatic efforts and allow personnel to successfully attract and compete for funding from outside agencies and commercial sources and from NSF-funded cooperative research. The USGS is guaranteed fifty percent of the instrumentís analysis time. Access to the instrument during USGS time allotment will be controlled by a panel of USGS scientists who will review proposals, offer advice about instrument capabilities and efficient use, and set research priorities consistent with scientific and programmatic needs.

Additional Websites:

SIMS Tutorial -- Charles Evans and Associates tutorial developed with NSF fungind. Well illustrated, succinct, low on jargon.

 

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URL http://geology.cr.usgs.gov/capabilities/chema/microinst/iprobe/tech.html