Laser Ablation ICP-MS Analysis at Oregon State University

[What have we got?]  [Sample Preparation]  [Documentation]  [Schedule Analysis Time]

[Quadrupole/Main Lab Web Camera] [Clean Lab/NuPlasma Web Camera]

General Information

Through the W.M. Keck Collaboratory for Plasma Spectrometry in the College of Oceanographic and Atmospheric Sciences (COAS) Oregon State University possesses world-class facilities for making Laser Ablation ICP-MS (LA-ICP-MS) measurements of a range of solid materials. These facilities may be used by internal or external users. The major users of the facility thus far are geologists and geochemists, interested in the chemical compositions of rocks, glasses and minerals, and fisheries biologists using otoliths to study fish life histories. However we have also used the equipment to study a wide range of other solid materials, including gels, plastics, alloys and more and would be happy to work with you to see if we can help you with your solids analysis needs.

Equipment

For ablation we use a NewWave DUV 193 ArF Excimer laser system (see right), typically with high purity He as the carrier gas.  The 193 nm wavelength produced by the LambdaPhysik ArF Excimer laser ablates silicates, carbonates and a range of other solid materials with minimal matrix dependence and elemental fractionation. The ArF Excimer laser is particularly well-suited to ablation of transparent glasses and minerals, such as quartz and some carbonates. There is also little or no thermal ablation at this wavelength. The spot size is controlled via an aperture wheel and with this we can vary the spot size between 10 - > 160 microns in diameter.  The sample can be viewed in reflected or transmitted light at a range of zoom magnifications during ablation. Sample movement and laser parameters are computer controlled, and can also be linked to the mass analyzer computer to enable automatic data acquisition.

For mass analyzers we have a choice of three different ICP-MS systems:

VG PQ ExCell quadrupole ICP-MS (web camera). Since 2001 this instrument has been a workhorse for a wide range of chemical and isotopic analyses in the Keck Collaboratory. For laser ablation it is used for rapid trace element analysis where relatively large mass tables are required (up to an over 30 elements). Sensitivity using a 80 micron laser spot and 5 hz pulse rate are ~ 1000 cps/ppm and detection limits are typically sub-ppm under normal ablation conditions. This machine excels (no pun intended) at measurement of element abundances in solids. Some of the current uses for this machine are:

• Analysis of lithophile trace elements (incl. Sc, V, Cr, Ni, Rb, Sr, Zr, Nb, Y, Ba, REE, Ta, Hf, Pb, U, Th etc.) in silicate minerals, melt inclusions and glasses.

• Measuring Sr/Ca ratios and trace element (Ba, Mg, Mn, Pb, Cu, Zn etc.) fish otoliths and scales.

• Elemental analysis of gels, plastics, alloys and other artificial materials

• Analysis of archeological materials

ThermoElemental Axiom Single Collector Sector Field ICP-MS. This instrument was also installed in 2001. For laser ablation the higher sensitivity (up to 4 Mhz/ppb ¹¹⁵In in solution mode) and high mass-resolving capacity (up to 25,000 with 2% transmission) coupled with the lower magnet scan rate make this machine best suited for analyses involving fewer analytes where high sensitivity, high peak/background and/or high mass resolving power is needed. To date this machine has been used for high sensitivity analysis of light (Li, Be, B), alkali (Rb, Cs) and REE elements in silicate glasses where low detection limits are required. Using normal ablation conditions detection limits are in the < 1 - 10 ppb range. Other uses of this machine are for analysis of platinum group elements in silicate glasses, and measurement of Pb isotope ratios in glasses and minerals and heavy rare earth elements at low abundances in minerals such as plagioclase and olivine.

NuPlasma Multicollector ICP-MS. (WeB camera) This machine, installed in the Keck Collaboratory in mid 2003, is well suited to the measurement of isotope ratios via laser ablation. Current development work is aimed at Sr and Pb isotope measurements in silicate glasses and minerals. Please contact me if you interested in these type of measurements, or check back here for regular progress reports.

Sample Preparation Guidelines:

Sample preparation requirements for Laser Ablation can be fairly minimal, and if required we can ablate non-polished surfaces and analyze vacuum-sensitive materials.  The only real limit is the size of object that can fit in the ablation chamber. However, for the best possible results it is often still best to conduct analyses on flat polished surfaces, which simplifies the interaction of the laser with the surface and makes it easier to document the exact location of each analysis point by reference to photomicrographs. Further, because LA-ICP-MS data is most commonly reduced by reference to an internal standard element that has an independently-known concentration (typically Si, Ca, Ti, Mg etc...), it may be best to prepare your sample so it can also be analyzed by electron microprobe in addition to laser ablation.

In general the sample chamber can fit two types of prepared sample materials:

1. Material mounted on American-size 2" x 1" (50 x 25 mm) petrographic thin sections. These should, if at all possible, be polished and above all should not have a cover slip. The overall thickness of material on top of the section can be up to ~1-2 mm. If you are preparing polished thin sections specifically for laser ablation analysis of rocks or other solids I would recommend making them ~ 100 microns thick to allow for extra ablation depth (although we can also analyze standard 30 micron thin sections if need be). Note that European-size 50 x 25 mm thin sections are slightly too large for the sample holder and need to be ground down (we can do this on site).

2. 25 mm diameter circular plugs similar to those commonly used for electron and ion microprobes. These can be up to 15 mm thick. The mounting media can be any of the commonly used mounting materials - I generally use epoxy.

Sample Documentation

I cannot overstress the importance of good sample documentation. It is the solution to efficient use of machine time (less time spent finding out where you are on your sample = more time using the machine for analysis). The optics of the viewing system for the laser are good, but not great, and a good-quality photomosaic map of the sample surface taken in reflected light is your key to efficiently locating sample analysis points. For locations where point location is critical (e.g. small mineral inclusions or defects) a larger scale photo is also worthwhile. The laser optical system also has transmitted light, but not with crossed polarizers.

If you are visiting OSU to to laser ablation work you can bring the photos with you or use the facilities in the Department of Geosciences to make them after you arrive - but give yourself enough time to do this (at least an hour per sample map).

Fee Schedule

Fees for the use of the laser ablation system (for ExCell, Axiom or NuPlasma mass analyzers) are:

Academic user $60-70/hour

Commercial user $100/hour

Schedule analysis time

To discuss your analysis needs or schedule analysis time please contact: