What is LA-ICP-MS?
- Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is an analytical technique used to determine the trace-element composition of a solid sample (e.g. rocks).
- Laser-ablation (LA) is the process of using a laser beam to remove matter from the surface of a sample. The fine particles thus generated can then be transported through a tube to be ionized by:
- Inductively Coupled Plasma (ICP) is a type of plasma generated by inductively heating a gas (we use argon) with an electromagnetic coil. The particles generated by LA are ionized when they pass through the plasma torch. These ions can be identified and measured with:
- A Mass Spectrometer (MS) is a tool to identify which isotopes/elements are present in an ionized sample (and their relative amounts). The ions moving from the ICP towards the detector of the MS are deflected by a magnetic field. The lighter ions are more easily deflected than the heavier ions, causing them to be “sorted” into different paths, each with a slightly different end point. The detector can then be set to focus on one of those end points at a time. The MS spends a specific amount of time focusing on each end point, counting the number of times in a second that an ion of that mass-to-charge ratio is detected before moving on to the next position.
What kinds of projects does this lab focus on?
The LA-ICP-MS laboratory at LTU is primarily interested in the trace element compositions of sulfide minerals in rocks associated with ore deposits. However, other types of analytical work are possible, on a case by case basis.
What is the difference between spot analysis and a composition map?
The Laser can be used to ablate a single spot on a sample to determine the composition of just that location, in which case one will need an appropriate standard reference material and information about an internal standard (see below) to convert the Counts Per Second (CPS) reported by the ICP-MS into more useful units, such as wt%, ppm, or ppb. Alternatively, we can use the laser to ablate a series of lines across a sample and convert the changes in CPS into false-colour images that show which areas are higher or lower in concentration for each element considered in the session.
How long will it take to make a trace element composition map of my sample/what settings should I use?
Mapping the composition of your sample with LA-ICP-MS can take very different amounts of time, depending on which settings you use:
- Size of map area: We can map areas at a variety of sizes on your sample, but, of course, the larger the area the longer it will take to map (if the other settings remain the same). Therefore, we can adjust this size up or down based on the remaining settings to achieve a “reasonable” amount of time for the scan.
- Laser Spot Size: This determines the size of the “pixels” in the final map. Choose a size that is smaller than the phases you are interested in. E.g. a 50 µm spot size gives very nice resolution if you are mapping a single crystal that is 5 mm in diameter, but if your crystals are only 100 µm in diameter you get only 4 pixels per mineral grain. Spot size also controls the number of lines needed to map a target area. Smaller spots = more lines needed = more time needed to run the map.
- Scan Speed: This is how fast the sample stage moves under the laser, which, in part, determines how many laser shots are fired per map row.
- Laser Rate: This is how many times each second that the laser fires, which directly controls how much of your sample is ablated during the mapping process, and therefore how many counts per second can be obtained for full list of the elements you choose to consider in this session.
- ICP-MS Dwell time: This is how much time is spent counting each of the elements considered in this session. This number should be low enough that there is time to count each of the elements on your list before the laser starts ablating the next area on your sample.
We have a spreadsheet available which calculates the total time needed for each map based on the above parameters so that you may choose the settings that are most appropriate for your needs. It is also possible to do a preliminary “fast” map, wherein the sample stage moves quickly and the laser doesn’t fire often per line to quickly (~20-30 minutes) obtain a low-resolution overview of the element distribution in your sample, from which you can select a smaller area of interest for second, much higher resolution, scan that takes several hours to run. This is especially useful for answering questions about which phase(s) contains a specific element.
What is data reduction for LA-ICP-MS?
Data reduction is the process of converting the CPS reported by the mass spectrometer into values with more useable units, such as parts per million (ppm), or weight percent (wt%). It is normal practice to measure an external reference standard (a sample of a known composition) in addition to the unknown sample in order to make this conversion. It is also necessary to make a correction by subtracting the “baseline” or “background” data (what appears to be present when there is no sample traveling through the system) from the measured totals. Further refinement of the results can be obtained by carefully selecting which portions of the data “peaks” are and are not considered in the calculations (e.g. one might wish to exclude the data from an inclusion of a very different mineral) or by appropriate selection of an internal standard, etc.
What is an internal standard and why do I need one?
An internal standard is used to correct for differing ablation conditions between the sample and the external standard reference material. The term “internal standard” is a holdover from the analysis of solutions via an ICP-MS, were the “internal standard” is an additional solution, of known composition and concentration, which is added to the unknown solution to be analyzed. Since the amount of that added element is known one can now convert the “counts per second” (CPS) measurement that the ICP-MS generates into a “weight %” (or other unit of choice) value and the relationship between the CPS and weight % for the internal standard can be applied to the other elements to convert their data from CPS to weight %.
However, for Laser-ablation ICP-MS one is analyzing solids, so one cannot simply add an element to the sample in the same manner. Therefore it is common for researchers to first take their samples to a microprobe (or other analytical equipment) to obtain the major element concentrations of the minerals present. One can then compare the relationship between the reported CPS and the known concentration of the internal standard element in the standard reference material with the concentration of the internal standard element in the sample to determine how much of an effect the differences in ablation conditions from the standard to the sample has upon the results.
Why do I want to use LA-ICP-MS if I already have microprobe results?
The microprobe is normally used to determine the major-element concentrations of minerals, while Laser-ablation ICP-MS is generally used to measure the trace-element composition. Used together one can obtain a much more detailed understanding of the mineral composition, which can be used to answer a variety of geological questions.
What if I don’t have the budget to obtain microprobe analyses to be used as an internal standard?
If you don’t have microprobe analyses of your sample from which you could choose values for an internal standard before your LA-ICP-MS sessions there are still options available:
- If your mineral is one for which one can assume stoichiometric values for the major elements then we can calculate an appropriate internal standard for use with that mineral.
- Alternatively, some researchers are having good success using a “sum-normalization calibration technique” to obtain robust quantitative results for their samples. See van Elteren et al. (2009) for a particularly well written and clear description of how geo-archeologists are doing this.
I already have high-resolution photos (or SEM images, etc.) of my sample—can we use these to drive directly to the mineral grains of interest for today’s session?
Yes! Bring your photo of the entire thin section or rock sample, with the area of interest marked, and your close up photos or SEM images with you to your LA-ICP-MS session. We can import the images and align them with the on-screen view of the sample, so long as you are able to recognize landmarks on screen and in the photos.
van Elteren, J. T., N. H. Tennent and V. S. Šelih (2009). "Multi-element quantification of ancient/historic glasses by laser ablation inductively coupled plasma mass spectrometry using sum normalization calibration." Analytica Chimica Acta 644(1-2): 1-9.