Micro analysis – chemical characterisation using microprobe
The microprobe, also known as EMPA or EPMA, is an instrument specifically designed for quantitative chemical characterisation and imaging of minerals, rocks and meteorites, bones, metals and alloys, ceramics, glasses, concrete, as well as other artificial substances.
The microprobe relies on the principle that a focused electron beam interacting with the surface of a substance emits characteristic X-rays that correspond to the elements present in the substance. By focusing the electron beam on the desired location, the microprobe analyses the chemical composition of micron-sized volumes of material.
The Cameca SX100 microprobe at the Open University is equipped with 5 WDX spectrometers which can detect most elements of the periodic table, allowing greater spectral resolution and better sensitivities than EDX-equipped SEMs. Each element is calibrated using standards, making this a fully quantitative micro-analytical technique.
Sample preparation for micro analysis
The microprobe analyses solid sample surfaces. The sample surface needs to be flat, polished and free from covering (glass slide, epoxy or similar). Conductive samples can be left without conductive coating whereas non-conductive samples such as silicate rocks need to be coated with a nm-thick conductive layer of carbon. Since a wide range of materials can be analysed with the microprobe, sample preparation for micro analysis may differ. Typical samples are:
- polished 25 mm round blocks,
- material fixed in epoxy resin and polished (e.g. particles), or
- thin sections mounted on glass slides (25 x 48 mm standard size).
Sample preparation for micro analysis (thin section / block / epoxy fixing and polishing) can be arranged in-house or elsewhere; carbon coating is done in-house prior to analysis. In some cases that depend on the scope and nature of the analysis, electron imaging of the sample prior to chemical investigation is advised (for navigation and targeting of analysis).
For fully-quantitative chemical micro analysis the microprobe needs to be calibrated using standards. The microprobe at the Open University is equipped with a vast selection of element and mineral standards used for calibration, and different standards can be used for different materials and measuring conditions.
Microprobe chemical micro analysis
The samples, as blocks or glass slides, are mounted on designed holders and placed in the microprobe chamber under vacuum. A task-file is prepared with the elements to be measured and the conditions to be used. The sample surface in the microprobe is then imaged using light-microscopy and/or using the electron beam (giving backscatter electron images, as in standard scanning electron microscopes). These imaging modes can be used for navigation to desired regions of the sample.
The microprobe can work in automated mode, meaning that it can be programmed to access specific coordinates and analyse µm-size spots in sequential mode. In addition to spot analyses, the microprobe can produce elemental distribution maps and line profiles, giving insights on the spatial distribution and variation of elements within the area of interest.
The beam size and acquisition conditions depend on the nature of the sample and scope of the analysis; usual beam diameters range from 1-10 µm. Depending on the interest (quantification of one particular element, rather than a batch of elements) and measuring conditions, each spot analysis can take 1 to 10 minutes.
An elemental map is an image of an area, whereby each pixel of the image corresponds to a quantified amount of the measured chemical element. For example, elemental maps of the elements Fe and Ni in an iron-nickel alloy can give insights on the quantity and distribution of the two elements. The resolution of elemental maps is a trade-off between the size of the area to be analysed, its resolution (given by the distance of each analysed point) and time. As a rule of thumb, high resolution elemental maps, measuring 10 elements, would normally take more than 6 hours to complete.
Results are provided in the form of oxide-weight% (for spot analysis), raw data files (for elemental maps) and image files (for electron images). Please note that the microprobe cannot discern between the valence states of elements (e.g. cannot distinguish Fe2+ from Fe3+).
Please feel free to contact us if you wish to know more specific case-related details.
Chemical data post-processing
Microprobe spot analysis can be presented in tables or graphs, or they can be processed for specific purposes. In the field of earth sciences, for example, oxide weight% are used to reconstruct mineral compositions. Relative element abundances can be used to estimate the pressure and temperature conditions of formation of specific minerals.
Elemental maps need to be processed to convert pixel values in oxide weight%, and thus extract the desired information. We can offer expert advice should you need help processing chemical maps and making a conclusion from your data.
What is the cost of one day at the microprobe?
The cost of the microprobe is calculated on instrument use, as an hourly or daily basis.
Daytime and night-time prices differ as the latter is in automated mode and does not require staff time.
Prices for academic institutions are discounted, please contact us to find out more.
Sample preparation (thin section / block / epoxy fixing and polishing) is not included in the price.
How much data can I generate in a day at the microprobe?
This depends on a number of factors, such as what elements you want to analyse, how many elements, the time the beam dwells on each spot (which depends on the element and its expected concentration), the time spent on setup and navigation, and the type of data (spot analysis, line profiles, elemental maps or a combination of these). As a generic estimate, expect around 10-15 spots per hour, for measuring 10 major elements. The time required for elemental maps greatly depends on the size of the area and the elements analysed, so an area can take between a couple of hours to up to twelve hours to map. The quality of the data obviously improves with time, but best map conditions can be discussed beforehand on a case to case basis. Generally speaking, a day of microprobe (24 h, i.e. daytime set up and overnight run) can result in 100 spot analyses and 2 maps.
How many samples can the microprobe take?
The microprobe can hold up to 4 thin section glasses, or 6 round 25 mm blocks simultaneously.
How long does it take to prepare sample(s) for micro analysis and get results?
Sample preparation varies considerably depending on the nature of the sample. Please discuss the nature of the sample with us if you wish it to be prepared here. Once the sample is in the form of block or thin section, it can be analysed in the microprobe repeatedly with minimal sample damage.
The same block or thin section can also be used for other micro-analytical investigations: it can be placed in the Scanning Electron Microscope (SEM) for imaging, cathodoluminescence (CL) or electron backscatter diffraction (EBSD) techniques, or it can be used for laser ablation.
If the samples are already prepared, only carbon-coating is needed and this procedure takes 15 minutes per sample.
Microprobe results are available as soon as the micro analysis is completed, so the data will be provided on the same day of analysis. Results are standardised and corrected, so no further corrections are generally needed. For particular data processing, results can be provided within a week, with prior arrangement.
How are microprobe samples analysed? Do I need to be physically present?
You may wish to be present whilst your samples are analysed by a member of our staff, or you can post the samples to us with detailed instructions about what and how you wish to have them analysed. The more we know about your sample, the aim and objectives you have, the better we can help you achieve the quality of results you need.
What should I know about my sample before micro analysing the sample with the Microprobe?
For best results, you should have a clear aim and objective to follow. Consider which elements you wish to measure, and for what reason. What is the detection sensitivity you are after? What should you analyse, and where within the sample? Is there an established analytical routine for samples of your type, or similar studies being published similar to the one you are after?
An electron image or an optical image of the sample might be of help in identifying your target areas. Spot analyses and elemental maps can serve different purposes, and perhaps an elemental map could be more useful than a spot analysis, or vice-versa.
Why choose the OU EM Suite for analysing your sample using the Microprobe?
We take pride in what we do, highest quality of our services whilst keeping to your timescale is paramount to us. Our members of staff and academics work on a range of different disciplines and we can therefore provide a range of expertise that spans the fields of earth sciences, planetary sciences, material sciences and life sciences. We can advise on sample preparation, data analysis and processing, and in case of unusual samples we are happy to test them with you and find a solution, communicating with you any difficulties or options we may discover. Please get in touch with us to discuss the options.
The microprobe at the Open University is an established instrument, well maintained over the years and with a vast range of standards to meet all sort of needs. We work regularly with a range of researchers or industrial customers.
An example of a micro analysis with microprobe
These set of images show a chemical reaction in a rock, consuming the mineral garnet and forming the minerals orthopyroxene + plagioclase + magnetite.
The top left image is a microprobe backscatter electron image, where the different minerals can be identified by different grey tones. The other images are microprobe elemental maps, coloured according to the concentration of the elements Si, Ca and Fe present in the different minerals.
The colour scale in these maps goes from blue: low concentration, to red: high concentration, of the element.
These microprobe elemental maps have been processed using XMapTools toolbox (Lanari et al., 2014), but the colour scales have not been converted into weight% quantities.