[meteorite-list] Instruments of Cosmochemistry: Ion Microprobe

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Mar 2 13:52:30 2006
Message-ID: <200603021850.k22IohU13125_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Feb06/PSRD-ion_microprobe.html

Instruments of Cosmochemistry
Ion Microprobe
Planetary Science Research Discoveries
February 28, 2006

Written by Linda M. V. Martel and G. Jeffrey Taylor
Hawai'i Institute of Geophysics and Planetology

In this series of articles, "Instruments of Cosmochemistry," PSRD
highlights the essential tools and amazing technology used by talented
scientists seeking to unravel how the solar system formed. You will find
information on how the instruments work as well as how they are helping
new discoveries come to light.lightbulb

------------------------------------------------------------------------
Precision Instrument

The ion microprobe is a Secondary Ion Mass Spectrometer (SIMS), a
precision instrument used to quantitatively analyze the elements and
isotopes of materials at microscopic
scales. This complicated instrument consists of an ion source, mass
spectrometer, detection system, associated electronics, and vacuum
system. A schematic diagram of an ion microprobe is presented later, but
first we'll look at the instrument's use of a primary ion beam to
bombard a sample of material and how that gives us information about the
material's composition.

In SIMS analysis, a primary, high-energy beam of ions (usually oxygen,
argon, or cesium) is aimed at a small area of a sample, such as a
mineral grain. The primary ions have energies on the order of 10,000
electron volts. (An electron volt is the energy given to an electron by
accelerating it through 1 volt of electric potential difference. The
electrons in a typical television screen ... not the new flat-screens...
have about 20,000 electron volts.) The primary ions dig a hole into the
sample causing sputtering of atoms and ions (called secondary ions) that
reveal the elemental and isotopic characteristics of the sample. See
graphic below.

[graphic of supttering]

Artist's rendition of an incoming beam of ions (colored purple)
impacting a sample and sputtering off the upper few layers of particles
(colored red, green, and blue) some of which are ions.

These secondary ions (approximately 1% of the sputtered material) are
accelerated into a mass spectrometer, where they are sorted by
mass/charge ratios with a large sector magnet. See diagram below.

[ions through mass spec]
[light through prism]

TOP: In a mass spectrometer, ions travel different paths through the
magnet to the detector due to their mass/charge ratios. A mass analyzer
sorts the ions according to mass/charge ratios and the detector records
the abundance of each ratio. BOTTOM: For an analogy, think of how a
prism refracts and scatters white light separating it into a spectrum of
rainbow colors.

A series of ion detectors counts the ions in different mass categories.
Analysts take these raw counts, apply corrections, and normalize them to
well-analyzed standards to arrive at the true abundances of elements and
isotopes in the original sample. (Read an account of SIMS in use in PSRD
article: A New Type of Stardust)

The diagram, below, shows the entire instrument scheme.

[schematic diagram of an ion microprobe]

The different components of an ion microprobe are numbered and
color-coded in this diagram. Ion beam paths are shown in white. Starting
from the left, the primary ion column (yellow) provides the
highly-focused ion beam produced from one of two sources (1,2). The
sample (5, red) is located where the primary ion column joins the
secondary ion extraction system (blue) wherein lenses focus the
sputtered secondary ions into the mass spectrometer entrance slit (9,
orange). The secondary-ion mass spectrometer (orange) is a double
focusing mass spectrometer with both electrostatic and magnetic sectors.
The electrostatic analyzer (10) bends lower energy ions more strongly
than higher energy ions. The ions then pass through the electromagnet
sector (13) where lower mass ions are bent more than higher mass ions.
Finally, the secondary ions pass to the sensitive detection systems
(green), which include an electron multiplier for counting individual
ions (19), and Faraday cup for measuring ion current (20). CamecaTM
ims-series ion microprobes also have the capability to operate in "ion
microscope" mode, providing elemental and isotopic imaging capability at
~1 micrometer spatial resolution. The images are captured by an imaging
device consisting of a channel-plate (17) and a fluorescent screen (18)
in the detection system.

SIMS analysts may choose to vary several features of the instrument: (1)
the polarity and species of the primary ion beam (O-, O2+, Ar+, Cs+ are
often used), (2) the impact energy (between ~3,000-20,000 electron
volts), current, and diameter of the primary ion beam, (3) the polarity
of the secondary ion beam, and (4) the initial kinetic energies of the
secondary ions detected.

------------------------------------------------------------------------
SIMS Virtues

SIMS allows the study of microscopic grains in their native habitat.
Unlike other types of mass spectrometers, the ion microprobe makes the
in situ measurements on polished natural samples, so the mineralogical
context of the grains remains intact. For a cosmochemistry example, see
PSRD article: Silicate Stardust in Meteorites
<http://www.psrd.hawaii.edu/June04/silicatesMeteorites.html>. In the
case of many extraterrestrial samples the grains are, in fact, so small
that it is physically impossible to separate them for standard mass
spectrometric analysis. So, the use of SIMS analysis eliminates the need
to physically segregate grains from a sample.

An ion microprobe can measure isotope ratios with a precision of a part
per thousand or trace-element abundances at the part per million to part
per billion level, while retaining the mineralogic context on a
microscopic scale. The newest instruments, such as the CamecaTM 1280 ion
microprobe, allow cosmochemists to image the micro-distributions of
elements and isotopes at a sub-micrometer spatial scale. This allows
isotope ratio measurements on very small samples, such as sub-micrometer
presolar grains or comet samples returned by NASA's Stardust mission.

One of the features that permits a modern SIMS to analyze and image
small grains in place in a rock is its impressively high mass resolving
power. This parameter is expressed as the ratio of the mass of an ion to
the difference between two masses that can be separated. The latest
version of the CamecaTM SIMS, for instance, has a mass resolving power
of 6000 routinely (which is required for separating 17O- from an
interference 16OH- peak), but may be capable of a mass resolving power
of up to 25,000. This minimizes the overlap between two peaks being
measured, allowing measurements of elements that were not previously
accessible by ion microprobe.

------------------------------------------------------------------------
Cosmochemical Applications using the Ion Microprobe

Whenever new instruments or sharp improvements in analytical techniques
have come along they have led to startling new discoveries in
cosmochemistry. The ion microprobe is no exception. In fact, it might be
the poster instrument for showing how a new technique can change the way
we look at the universe.

PSRD has described discoveries in several areas of cosmochemistry that
rely on use of the ion microprobe (see listing in next section). One of
the most amazing of these is the study of pre-solar grains in meteorites
and interplanetary dust particles--grains manufactured in other stars.
stardust grainSome of these stars formed alongside the Sun. Others no
longer exist, exploding as supernovae and depositing grains into
interstellar space to be included into the mix of material from which
the Solar System formed. Pre-solar grains are tiny, less than about 10
micrometers across. They were originally identified in acid-resistant
residues when whole chondritic meteorites were dissolved. The residues
had extreme isotopic compositions compared to normal Solar System stuff,
suggesting an origin outside our Solar System. The ion microprobe has
allowed cosmochemists to identify and study grains with anomalous
isotopic compositions in place in a sample, greatly expanding our
knowledge of these ancient specs. Studies of stardust have joined
cosmochemistry with astronomy and astrophysics.

The ion microprobe has also contributed to unraveling the timing of
events in the early Solar System. Studies of the decay products of
short-lived isotopes (half lives much shorter than the age of the Solar
System) have allowed us to distinguish events that took place less than
a million years apart, yet over 4.5 billion years ago.

The ion microprobe allows us to date individual chondrules in primitive
meteorites, simultaneously measuring other properties.chondrule It is
all leading to a thorough, if not complete, understanding of what
happened when planets were beginning to form around the young Sun.
Studies of extinct short-lived isotopes in chondrites will shed light on
the stellar sources and timing of their formation compared to the timing
of formation of the solar system.

Orion nebulaThe ion microprobe enhances studies of early solar system
processes through measurements of chemical compositions, trace-element
abundances, isotopic fractionations, and the unique variations exhibited
by oxygen isotopes. Analyses of oxygen isotopes in chondrites and
observations of star-forming regions such as the Orion nebula are
leading to major leaps forward in our understanding fundamental
processes of star and planet formation.

The ion microprobe has also contributed to our understanding of planet
formation and initial differentiation (separation into a metallic core
surrounded by a rocky mantle and crust). D/H ratio studies Studies of
the ratio of deuterium (heavy hydrogen) to hydrogen in cosmic dust,
carbonaceous chondrites, samples returned from the Stardust mission to
comet Wild 2, and samples of asteroidal meteorites may allow a
definitive determination of the source of water on the Earth. The ion
microprobe also contributed mightily to our understanding of the
distribution of trace elements in Martian meteorites, helping determine
that the mantle of Mars is heterogeneous in composition. Analyses of
trace elements such as thorium in tiny volcanic beads collected on the
Moon have allowed cosmochemists to improve our estimate of the bulk
chemical composition of the Moon, an essential bit of information to
understanding the origin of the Moon.

------------------------------------------------------------------------
PSRD Articles Featuring the Ion Microprobe

    * Moving Stars and Shifting Sands of Presolar History
      <../July97/Stardust.html>
    * Using Aluminum-26 as a Clock for Early Solar System Events
      <../Sept02/Al26clock.html>
    * Silicate Stardust in Meteorites <../June04/silicatesMeteorites.html>
    * Hafnium, Tungsten, and the Differentiation of the Moon and Mars
      <../Nov03/Hf-W.html>
    * Gullies and Canyons, Rocks and Experiments: The Mystery of Water
      on Mars <../April01/waterFromRocks.html>
    * QUE 93148: A Part of the Mantle of Asteroid 4 Vesta?
      <../Jan03/QUE93148.html>
    * A New Type of Stardust <../Aug03/stardust.html>
    * Making Sense of Droplets Inside Droplets
      <../May05/chondrulesCAIs.html>
    * From a Cloud of Gas and Dust to an Asteroid with Percolating Hot
      Water <../Feb99/fayalite.html>
    * Triggering the Formation of the Solar System
      <../May03/SolarSystemTrigger.html>
    * Low-temperature Origin of Carbonates Consistent with Life in
      ALH84001 <../May97/LowTempCarb.html>
    * Cosmochemistry from Nanometers to Light-Years
      <../Jan06/protoplanetary.html>

------------------------------------------------------------------------
Ion Microprobe Laboratories doing Cosmochemistry and Geochemistry
(partial listing, U.S.)

    * SIMS at Laboratory for Space Sciences
      <http://presolar.wustl.edu/work/what_is_sims.html>, Washington
      University in St. Louis, Missouri.
    * SIMS at Hawaii Institute of Geophysics and Planetology
      <http://www.higp.hawaii.edu/ion_microprobe.html>, University of
      Hawaii in Honolulu.
    * UCLA National Microprobe Facility <http://sims.ess.ucla.edu/>, Los
      Angeles, California.
    * Northeast National Ion Microprobe Facility (NENIMF)
      <http://www.whoi.edu/science/GG/nenimf/>, Woods Hole Oceanographic
      Institution (WHOI), Woods Hole, Massachusetts.
    * GeoSIMS lab at Department of Geological Sciences
      <http://sims.asu.edu/>, Arizona State University in Tempe.
    * WiscSIMS lab at the Department of Geology and Geophysics
      <http://www.geology.wisc.edu/facilities/wiscsims/wisc_sims.html>,
      University of Wisconsin-Madison.
    * SIMS facility <http://epswww.unm.edu/simslab/>, Sandia National
      Lab and University of New Mexico, Albuquerque.
    * NanoSIMS lab at Department of Terrestrial Magnetism
      <http://www.dtm.ciw.edu/content/view/244/2/>, Carnegie
      Institution, Washington DC.
    * Stanford University and USGS SHRIMP Lab
      <http://shrimprg.stanford.edu/index.html>, Stanford, California.
    * JSC NanoSIMS, Houston, Texas.

------------------------------------------------------------------------
ADDITIONAL RESOURCES

    * Benninghoven, A., R??denauer, F. G., and Werner, H. W. (1987)
      Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental
      Aspects, Applications, and Trends, Wiley, New York, 1227 p.

    * Cameca <http://www.cameca.fr/html/research_instruments.html>
      scientific instruments for research.

    * Ion Probe Technique
      <http://geology.cr.usgs.gov/capabilities/chema/microinst/iprobe/tech.html>
      described by the United States Geological Survey.

    * Ireland, T. R. (1995) Ion microprobe mass spectrometry: techniques
      and applications in cosmochemistry, geochemistry, and
      geochronology. In Advances in Analytical Geochemistry, edited by
      M. Hyman and M. Rowe. JAI Press, Inc., Greenwich, Connecticut, p.
      1-118.

    * Neal, C. R., Davidson, J. P., and McKeegan, K. D. (1995) Secondary
      Ionization Mass Spectrometry (SIMS)/Ion Microprobe
      <http://www.agu.org/revgeophys/neal00/node6.html>. In Geochemical
      analysis of small samples: Micro-analytical techniques for the
      nineties and beyond
      <http://www.agu.org/revgeophys/neal00/neal00.html>. U.S. National
      Report to IUGG, 1991-1994, Reviews of Geophysics, Vol. 33 Suppl.,
      ?? 1995 American Geophysical Union.

    * Shimizu, Nobu (2004) If Rocks Could Talk...The ion microprobe
      extracts hidden clues about our planet's history and evolution.
      Oceanus. http://www.whoi.edu/oceanus/viewArticle.do?id=2437

    * SIMS Theory Tutorial
      <http://www.eaglabs.com/en-US/references/tutorial/simstheo/caistheo.html>
      from Evans Analytical Group.

    * SIMS Workshop <http://www.simsworkshop.org/>.

    * Stardust Mission <http://stardust.jpl.nasa.gov/home/>, NASA's
      comet sample return mission.

    * Valley, J.W., Graham, C M., Harte, B., Eiler, J. M., and Kinny, P.
      D. (1998) Ion Microprobe Analysis of Oxygen, Carbon, and Hydrogen
      Isotope ratios. In: Applications of Microanalytical Techniques to
      Understanding Mineralizing Processes, McKibben, M.A., Shanks,
      W.C., III, and Ridley, W.I. (eds.). S.E.G. Review in Economic
      Geology, v. 7, p. 73-97.
Received on Thu 02 Mar 2006 01:50:42 PM PST


Help support this free mailing list:



StumbleUpon
del.icio.us
reddit
Yahoo MyWeb