  
The Radiogenic Isotope Lab houses two magnetic-sector based mass spectrometers,
which are distinct primarily in their means for sample introduction. The
thermal ionization mass spectrometer (TIMS instrument) introduces the sample
through evaporation and ionization off of a filament (commonly Re or Ta metal),
whereas the multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS)
introduces the sample through a nebulizer (to produce a very fine aerosol),
followed by introduction into a hot Ar plasma (~ 8,000 oC) as the
means for ionization.
Magnetic-sector based mass spectrometry is essential for providing
"flat-topped" peaks for the masses, which is required for
high-precision isotope ratios; the flat top removes any changes in the relative
ion intensities if small amounts of drift occurs in the instrument during
analysis. In addition, simultaneous collection of all isotopes of interest
is also important for high-precision analyses, since ion beams (particularly
those generated by ICP sources) always have small variations in intensities due
to small changes in source conditions during analysis. These features
distinguish magnetic sector-based instruments that have multiple collection
from, for example, the common quadrupol-based ICP-MS instruments. In
addition, because the sources used in the mass spectrometers are either solid-
or solution-based, we must measure absolute ratios, and this contrasts with
gas-source mass spectrometers, which can inlet sample and standard gases under
identical source conditions, allowing isotope compositions to be determined on a
relative basis.
After passing the ion beam through a magnetic field to separate the masses,
isotope ratios are generally measured by passing the very small ion currents (10-16
to 10-10 Amps) through large specialized resistors (1010
to 1011 Ohm), which produces a relative range of voltages;
essentially these instruments are just really expensive voltmeters!. The
highest precision work requires use of a detector such as a Faraday collector,
where essentially one charge in is one charge out. However, much smaller
ion beams (smaller samples) may be analyzed on a number of multiplier detectors,
which include a robust Daly multiplier, as well as channeltron ion-counting
multipliers; there is no free lunch here, however, so the increased sensitivity
one obtains with a multiplier is balanced by a decrease in precision.
Click on the thumbnail images below for a larger view; use your browsers
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An example of a "flat-topped" peak
(over 232Th) that is produced by a magnetic-sector-based
instrument. |
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Our VG Instruments
Sector-54 TIMS mass spectrometer, being run by graduate student Merideth
Rhodes for her project on Sr isotope variations in the Green River Basin.
The source is out of the picture to the left, and the magnet is just
visible on the far left. The collector block lies just to the right
of the magnet, and the main electronics rack is on the right. |
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Detail of the TIMS source, which can hold a 16
sample turret. Tom Lapen's hand is shown for scale, where he is
adjusting the oxygen gas bleed for ultra-low-level Nd isotope analysis as
NdO+. |
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Our Micromass IsoProbe MC-ICP-MS mass
spectrometer. Tom Lapen (far left) is adjusting the sweep gas, while
post-doc Rene Wiesli (left) watches graduate student Nancy Mahlen (right)
set up files for an analytical session. The main part of the
instrument that is visible is the collector block. |
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A general schematic of the Micromass
IsoProbe (courtesy of the manufacturer). Samples are introduced
on the left, and ion beams traverse from left to right in the instrument. |
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A view of the collector block of the
Micromass
IsoProbe, our MC-ICP-MS instrument. The ion beam would come in
from the top of the photo. The drives that move the collectors to
various positions are seen on the sides of the block. The movable
channeltron multipliers are on the left side of the block. In the
lower part of the photo is the Daly multiplier, which has better noise and
stability specifications than the channeltron multipliers; the Daly is
fixed in the axial position. Between the Daly and the main block is
a wide-angle energy retarding filter (WARP), which is essential for
high-abundance-sensitivity work, such as is required for 230Th/232Th
measurements. |
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Another view of the collector block for the MC-ICP-MS
instrument. The collector block for the TIMS instrument is similar,
although it has fewer Faraday collectors, no movable channeltron
multipliers, and no WARP filter. |
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A schematic of the collector configuration of
the MC-ICP-MS instrument. The variety of movable and fixed detectors
provide an opportunity to analyze just about any element on the periodic
table that exists as a liquid or solid at STP. Of course, we will
ignore the monoisotopic elements! |
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Pre-sample handling for the MC-ICP-MS is more
complicated than for the TIMS instrument, requiring careful aliquoting of
solutions in a clean environment. Shown here is one of the laminar
flow fume hoods in the lab, this one located in the Mass Spectrometry Lab
(Room 375). |
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Sample introduction into the MC-ICP-MS
instrument starts with nebulization of the sample solution. Shown
here is one of the nebulizers we use, in this case, the Aridus
micro-concentric desolvating nebulizer, as well as an autosampler.
Both are housed in a clean bench. |
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One of the special features of the new
IsoProbe is the collision cell (hexapol) that is employed to
tighten the energy spread (ie: make flat-topped peaks), increase
sensitivity, and remove polyatomic interferences. In this photo, the
ICP source is on the left, and the ion beam would be passed from left to
right. |
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