LA-ICPMS Visit Preparation Guide
What elements can I analyze with LA-ICPMS?
A large number of elements can be analyzed with very high sensitivity by ICPMS. However, some elements that are poorly ionized in the plasma and/or have strong interferences cannot be analyzed effectively by ICPMS. Elements that are poorly ionized (and cannot be analyzed with ICPMS) include H, C, N, O, S, F and the ideal gases. Most other elements have at least one isotope that is free from interferences and allows concentrations to be determined by ICPMS. However, depending on the matrix composition in your sample, interferences may be a problem. To help remove the effect of interferences, the Agilent 7500ce instrument in the Fluids Research Laboratory (FRL) is equipped with an Octopole Reaction Cell (ORS). The ORS removes many common interferences and allows samples with complex matrices to be analyzed. A wide variety of geologic materials (silicates, carbonates, fluid inclusions, etc.) can be analyzed using the instrument in the FRL.
Here is a periodic table that will help explain which isotopes are used during analysis with the Agilent 7500ce.
What are the limits of detection for LA-ICPMS?
The limit of detection (LOD) for each element in an LA-ICPMS analysis depends on the intensity and duration of the signal and the background level. The LOD for most elements in mineral phases is < 10 ppm (and often < 1 ppm). However, for fluid inclusions the LOD is often higher (usually < 100 ppm) owing to the transient nature of the signal.
The Octopole Reaction Cell (ORS) in our ICPMS does allow for excellent LOD for both Fe and Ca (< 10 ppm). This is due to the removal of ArO and Ar interferences respectively.
Can I get isotopic ratios with LA-ICPMS?
The Agilent 7500ce in the Fluids Research Laboratory includes a quadrapole mass spectrometer that is designed to provide measurement of a large number of elements (isotopes) in a relatively short (transient) signal, as is often the case with fluid and melt inclusions. The ICPMS in the Fluids Research Laboratory does not have sufficient mass resolution for isotopic analysis of the light isotopes, and there is a laser induced fractionation/instrument bias for the heavy isotopes.
How should I prepare my samples for analysis with LA-ICPMS?
Fluid Inclusion Analysis: Fluid inclusion samples should be doubly polished chips of <= 10 mm in maximum dimension and <= 3 mm thick (including glass slide). The doubly polished sample can either be removed from the glass slide (preferred) or left mounted on the slide. The inclusions to be analyzed should be near the surface of the chip. A good rule of thumb is to observe the sample on a petrographic microscope using reflected light. Any inclusions that are close enough to the surface to be identified with reflected light should show good ablation behavior. Note, however, that inclusions that are too close to the surface may decrepitate explosively as soon as the laser is turned on.
Absolute element concentrations for fluid inclusions can be determined from bulk salinities and LA-ICPMS data (Heinrich et al, 2003). Bulk salinities (in weight % NaCl equivalency) for each inclusion should be determined by freezing point depression (Bodnar, 1993), halite disappearance (Sterner et al, 1989) or Raman analysis of the water peak (Mernagh et al, 1989). The data reduction process used in the FRL can accommodate either salinities, temperatures of ice melting or halite dissolution, or Raman spectra (as XY data file) as input.
Melt Inclusions: Melt inclusion samples should be mounted in either an electron microprobe mount (25.4mm round; 8 mm high) or on a glass slide, or can be contained in a doubly-polished chip that has been removed from the slide (preferred).
Homogeneous melt inclusions near the sample surface can be analyzed with a laser spot that is smaller than the melt inclusion. The data reduction process for such analyses is very straightforward as there is no host contribution to the signal. This method requires no independent knowledge of concentration of any elements in the inclusion before the analysis. For inclusions that are not exposed but are near the surface, a laser spot larger than the inclusion is used to expose the inclusion before switching to a smaller spot for the analysis. Electron microprobe data, while not required, are useful to compare and verify LAICPMS analyses of the major element concentrations of the host and melt inclusions.
Non-homogeneous melt inclusions are analyzed with a spot size that is slightly larger than the inclusion. The analytical volume includes all of the inclusion and part of the host material, and a host correction is used to determine the composition of the melt inclusion. It is necessary to know the concentration of at least one element in the melt inclusion to implement the host correction factor (Halter et al, 2002). This element should ideally be one whose concentration in the melt inclusion varies significantly from the host (e.g. 15 weight % Al in the inclusion vs 100 ppm in host). The known element concentration is usually determined by electron microprobe analysis of exposed inclusions or from whole rock analysis.
How should I prepare my mineral samples for analysis with LA-ICPMS?
Rock and mineral samples should either be mounted in an electron microprobe mount (25.4mm round; 8 mm high) or on a glass slide, or available as a loose, doubly polished chip. Samples other than probe mounts should be <= 10 mm in maximum XY dimension <= 3 mm tall.
What kinds of sample preparation can be done at the laboratory?
It is strongly recommended that all your sample preparation be completed before your visit. However, facilities for mounting and polishing of all types of samples are available if needed, although doing sample prep at the lab will significantly decrease the amount of time available for analysis.
What is the geometry of the ablation chamber?
How many LA-ICPMS analyses can be completed in a day?
The number of analyses that can be completed in a day depends on many factors, including: How easy it is to locate the exact spot (fluid inclusion, melt inclusion, mineral zone) where the analysis will be done. This in turn depends on how well characterized the sample is (drawings, photos, etc.). Remember that the quality of the optics in the ablation chamber is poorer than with a good petrographic microscope, and there are fewer magnification options available on the microscope used with the ablation chamber. How many samples you have. It will take significantly more time to analyze one inclusion in each of 40 samples than it will to analyze 40 inclusions in one sample. Loading a sample into the chamber takes time, so try to minimize the total number of samples. With a few samples (<4) that are well characterized, it is possible to analyze 40 or more spots in a day. The instrument setup/shutdown time is about 45 minutes. The time to perform data reduction is usually < 1 hour.