Facilities
The Isotope Cosmochemistry and Geochronology Laboratory (ICGL) is a state-of-the-art facility dedicated to the dating and characterization of meteorites through isotopic analysis. The ICGL was founded by CMS director Prof. Meenakshi Wadhwa in 2007 and is managed by Associate Research Scientist Philip Janney and Research Specialist (Sr.) Rebekah Hines.
Most chemical elements consist of more than one isotope (isotopes are atoms of a given element with a particular number of neutrons), commonly known examples include 12C, 13C and 14C-the three naturally isotopes of carbon and 235U and 238U-the two naturally occuring isotopes of uranium (which are both radioactive). We measure the abundance of the radiogenic "daughter" products of short- (e.g., 26Al, 60Fe, 146Sm) and long-lived radioactive isotopes (e.g., 147Sm, 176Lu, 238U), relative to stable, non-radiogenic isotopes of the same elements to determine the age of a mineral or rock. We also measure the mass-dependent fractionation of light from heavy isotopes as a tracer of processes that may have occurred during the early history of the solar system such as evaporation, condensation and alteration processes.
Isotope ratios of elements are measured with a mass spectrometer. This instrument does three things: (1) it ionizes the sample by stripping off electrons and directing the ions, using electrostatic lenses in a high vacuum, to (2) an electromagnet (or other type of mass analyzer), where the ions of different atomic weights are separated and then (3) to one or a series of detectors, that register the incoming ions. These signals are fed into a computer which calculates isotope ratios and analytical uncertainties. The ratio of an isotope produced by radioactive decay to one that is non-radiogenic (i.e., not produced by radioactive decay) and not radioactive itself can be used to determine the age of a sample. More precisely, it allows the time elapsed since the sample was last isotopically homogeneous (as in a magma, for example) to be determined.
The ICGL is one of only a few isotope laboratories in the U.S. that are dedicated primarily to the analysis of meteorites It consists of a 1000 sq. ft. clean chemistry laboratory and a neighboring mass spectrometer facility. Ultra-clean conditions are essential to the functioning of the ICGL. These are provided, first, by a dedicated air handler supplying class 10,000 air, meaning the air contains fewer than 10,000 particles per cubic foot (normal air contains millions of particles per cubic foot). The lab also contains over 30 linear feet of ultraclean laminar flow cabinets and hoods providing class 100 conditions. These are used to perform preparation and chemical processing of meteorite samples. Second, water used in our laboratory undergoes three different purification steps to bring the background concentrations of most elements down to the parts-per-quadrillion level (a quadrillion is 1,000,000,000,000,000). Third, the acids we use to dissolve meteorites and for chemical processing of samples are distilled multiple times and contain only a few parts per trillion of most elements. All of these precautions are necessary make sure that data we produce are free of artifacts caused by terrestrial contamination in the laboratory.
Once processed, samples are analyzed for their isotopic compositions using a Thermo Neptune multicollector plasma mass spectrometer (image, left). Our mass spectrometer facility has the ability to analyze both sample solutions and solid samples, the latter by way of a Photon Machines Analyte 193 laser ablation system.
If interested, you can find details on our analytical instrumentation here
Our Thermo Neptune multicollector ICP-MS is equipped with nine faraday collectors, an axial secondary electron multiplier with RPQ energy filter, as well as three off-axis ion counters (MIC channeltrons) configured for low-level Pb isotope analysis. For solutions, we mainly use an ESI Apex sample introduction system (with optional Spiro desolvation module), but we sometimes also use a conventional glass spray chamber. We have an ESI SC-2 autosampler for automated analysis.
For projects where spatial location/resolution is important, we have two techniques available:
Sampling via a New Wave Research MicroMill, which allows precisely located drilling of geological materials. The sample powder excavated from holes as small as 50 microns in diamter is collected and processed through normal elemental separation chemistry and analyzed in solution.
In-situ isotopic analysis is performed using our Photon Machines Analyte 193 laser ablation system, which ablates the sample into particle sizes on the order of a few nanometers and can be directly ionized by the Neptune's argon plasma. The Analyte 193 consists of an ultrashort pulse length (4 ns) 193 nm homogenized excimer laser, a 3 in. diameter sample cell, high and low magnification cameras and a built-in beam profiling system. The effective range of spot sizes is from 5 to 150 microns.
Although the presence of matrix elements in the sample ablated by laser does limit analytical precision, the fact that no chemical processing is necessary makes this technique extremely useful in cases where isotopic variations are expected to be significant and high resolution data coverage is needed.