Links to Specific Facilities Information:
The Radiogenic Isotope Lab includes clean chemistry (Room 375) and
mass-spectrometry (Room 372) labs. In addition, a general work area
is adjacent to the main labs (Room 371), and a mineral separation lab (Room 276)
and rock crushing lab (Room B168) provide sample preparation support. The lab
was originally built in 1987, and a
$ 1M renovation was completed in 2000. The clean chemistry lab was renovated
and expanded in 2005. The entire laboratory
facility has a value in excess of $ 3M.
Clean Chemistry Lab Lab (Room 375 Weeks Hall)
Because most elements that have long-lived radioactive isotopes (which allow
one to study processes that have occurred over the age of the Earth) have high
atomic numbers (Z), such elements commonly exist in only trace quantities (ppt
to ppm). Therefore, modern radiogenic isotope geochemistry often requires
isotopic analysis of picogram (10-12) to microgram (10-6) quantities of an
element (to an precision of +/- 0.001% sometimes!). These stringent
requirements (ultra-high precision analyses of samples you literally cannot see
in the bottom of your beaker!) require that chemical processing of samples be
done in a very clean environment, where sample handling, reagent preparation,
and chemical separation is all done in an environment where contamination is
minimized.
The clean-chemistry laboratories of
the Radiogenic Isotope Lab are divided into sub-labs/areas for clean sample
preparation, balance room, reagent preparation and cleaning, and chemical
separation in ultra-clean or non-metal environments.
Very clean water is directly produced from two Barnstead Nanopure ion-exchange
systems (a third is in the Mass Spectrometry Lab). A pure silica glass
still is used for preparing clean HCl, which is further purified in sub-boiling
Teflon stills. Other reagents, such as HNO3, HBr, and HF are purified in
sub-boiling Teflon stills. Chemical separation is usually accomplished by
a variety of ion-exchange chromatographic methods, all of which are done under
laminar-flow (HEPA-filtered), clean-air conditions. Evaporation of
separated "cuts" from the ion-exchange columns is also done in small
HEPA-filtered evaporation boxes. Current chemical
separation capabilities include the first-transition elements such as Fe, as
well as Rb-Sr, REEs, Sm-Nd,
Lu-Hf, common Pb, and U-Pb (geochronology).
Mass Spectrometry Lab (Room 372 Weeks Hall)
Modern, research in isotope geochemistry requires very high-precision isotope
ratio measurements, often at the limit of counting statistics for a given sample
size. Higher and higher precisions on smaller and smaller samples on more
and more difficult-to-analyze elements is the clear trend in isotope
geochemistry! High-precision isotope measurements generally require a
magnetic-sector based instrument, which provides the flat-topped peaks that are
required for high-precision measurements. Moreover, simultaneous
measurement of multiple isotopes is also a requirement for high-precision
analyses so that ion beam instabilities do not affect precision.
Thermal Ionization Mass Spectrometer (TIMS)
The VG Sector 54 (TIMS) instrument was originally installed in 1988,
and significantly upgraded in electronics and detector system between 1997 and
1999. TIMS instruments provide exceptionally precise and accurate
isotope ratios (the closes to "truth" of any method), and generate
ions by resistive heating of a filament that contains the sample. TIMS-based
isotopic analysis is ideally suited to elements that have moderate ionization
potentials and are moderately volatile; they are not well suited to isotopic
analysis of refractory elements, or those that have very high ionization
potentials. Moreover, control of instrument-induced isotopic fractionation
(which occurs in all mass spectrometers) is required, and this can be
accomplished by running standards under "similar" conditions,
normalizing the data to a non-radiogenic ratio (applicable only for elements
that generally have only one radiogenic isotope), or use of a double spike
(which requires four or more isotopes).
The U.W. Madison TIMS instrument has 7 movable Faraday collectors, a Daly
multiplier, a 16 sample turret, and is well-suited for Rb-Sr, Sm-Nd, and U-Pb
isotope analysis. Supporting equipment includes a clean sample loading
area, as well as a degas bench for filament preparation.
Multi-Collector Inductively-Coupled Plasma Mass Spectrometer (MC-ICP-MS)
Our newest instrument combines the "back-end" of a multicollector
TIMS instrument with the "front-end" of an ICP source, with the additional
modifications of a collision cell, laminated magnet, and improved Faraday and
multiplier detector array. ICP sources are excellent for both low- and
high-ionization potential elements; this is both a blessing and a curse, since
everything in your solution is ionized with equal efficiency, including
potential interferences, since the ICP source does not have the "ionization
discrimination" abilities that a TIMS source has.
The Micromass IsoProbe installed at U.W. Madison is a next-generation
multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) that
was installed in 2000. The MC-ICP-MS instrument is equipped with a
collision cell (hexapol) that provides very high sensitivity and is effective at
removing a range of polyatomic interferences, which is very important for a
number of elements. In addition, the instrument has 9 movable Faraday
collectors, four movable channeltron multipliers, an ion-counting Daly
multiplier, a wide-aperture retarding filter (WARP) for high-abundance
sensitivity work, and a laminated magnet that allows a 17% relative mass
dispersion. Sample introduction is done in a micro-clean environment,
commonly using an autosampler, and can be accomplished with either a
micro-concentric desolvating nebulizer (Cetac Aridus), direct-injection high
efficiency nebulizer (DIHEN), a variety of PFA low-flow nebulizers, or standard
“Meinhard-type” nebulizers. An adjacent laminar flow fume hood is used
for solution preparation. The IsoProbe is capable of very
high-precision isotopic analysis of a variety of geologically important
elements, including Li, B, Mg, S, Cl, Ca, Cr, Fe, Ni, Cu, Zn, Se, Sr, Nd, Hf,
Os, Hg, Pb, Th, and U, including combined multiple ion counting-Faraday analysis
and isotopic analysis of isotope that require exceptionally high-abundance
sensitivity.
