What is the standard block?
It is a 1″ round brass disk containing an array of polished, carbon coated mineral and glass grains. Most have relatively well known chemical compositions. NOTE: the polish and carbon coat are exquisitely fragile! Never touch the surface even with gloves!
Why a standard block?
Instrumental chemical analyses typically require reference standards, against which the instrument signal can be compared. Our current EDS system has standardless analysis capability which is remarkably good, but standards-based analyses are more accurate. While EDS analyses can never match the detection limits or energy resolution of wavelength dispersive electron probes, accuracy can be comparable for the more abundant elements.
Primary standard
Homogeneous in BSE, X-ray images, and spot analyses, characterized as standard reference materials by other labs, EDS analysis and stoichiometric constraints match analysis reasonably well. Alternatively, simple compounds of high purity with known, consistent stoichiometry are also included. We currently have primary standards for these elements at concentrations >5%: C, O, F, Na, Mg, Al, Si, P, S, K, Ca, Ti, Fe, Ni, Cu, Zn, Sr, Zr, and Ba, though some are not silicates in case you are worried about that.
Secondary standard
Homogeneous in BSD and X-ray images, EDS analysis and stoichiometric constraints give some confidence that the analyzed composition is good for major components. These might become primary standards with more information.
Questionable
Inhomogeneous in BSD and/or X-ray images, or fine-grained and polycrystalline, or otherwise of uncertain composition. These might be useful for practicing skills getting BSE images that show small atomic number differences, or in obtaining nice-looking X-ray maps. Some of these, if they contain homogeneous, modest size grains, might eventually become secondary standards.
How do you pick what goes into a standard block?
First, you decide what your analytical interests are. As a geoscientist, I’m interested in minerals and natural glasses, so that is what we have in our standard block. Next you want homogeneous grains that have been well characterized by other laboratories, and preferably in use as electron probe standards elsewhere (primary standards). Such things are precious and rare, though we do have a some from the USGS, NIST, Smithsonian, and a couple of friendly, generous, and saintly electron probe labs. The next best thing is to look for some of your own materials and characterize them yourself. This isn’t easy, which is why good standards are so precious. As a lad I made several attempts to find and characterize such things, but I rarely got past tests for homogeneity. A good example was diopside that I separated from a bimineralic calcite-diopside marble from the Adirondacks. I reasoned that, at metamorphic temperatures of over 700°C in a relatively inert calcite matrix, the diopside crystals would certainly be homogeneous. They weren’t. Every grain was chemically zoned. I only had two successes: high-purity magnetite octahedra from a chlorite schist (essentially no impurities), and tremolite from a calcite-tremolite-phlogopite marble in the Northwest Adirondack Lowlands. Alas, the magnetite is now lost, and I never managed find the time to separate enough tremolite from phlogopite to get an XRF or other analysis. Materials and entire pre-made blocks can also be purchased from commercial companies such as SPI Supplies.
How do you make your own standard block?
Most standard blocks are 1″ (25.4 mm) rounds, though there are other shapes and sizes, too. First pick the shape and size. Next, the idea is to get all the grains at the same physical level, so it is easy to polish down to them.
- The easiest is to take a plastic disk, spread some epoxy on it (sticky) and put on the grains in an array with tweezers. These can later be covered with more epoxy, ground, and polished. This works best if the grains are big enough to manipulate with tweezers (0.25-1 mm), but it doesn’t work so well with very tiny grains or powders (at least with my skill level).
- Another way is to drill shallow holes into a plastic or metal disk and put the individual grains there. This works best if the grains are relatively big, 1-2 mm.
- Holes can be drilled all the way through a plastic or metal disk, the disk mounted on a piece of double-sided sticky tape, and the grains dropped into the holes. Then the holes are filled with epoxy. That’s what I did, and it works well for any size grains or powders, though sometimes you have to use a little metal funnel to get them in. Unfortunately there are also risks. Bubbles in the epoxy sometimes float the grains to the wrong side of the metal disk, sometimes the grains don’t get all the way to the bottom despite pushing them down with a pin, and sometimes very thin grains land flat on the bottom of the holes and are then ground away during grinding and polishing. This is not mentioning the fact that this is all done with the block upside down. As the Fates would have it, all three happened to me. This standard block was just slightly beyond my limits of skill. Live and learn. Anyway, there are fixes for these problems: push the epoxy plug partway out to expose the stuck grains, push the epoxy plug all the way out and remount for the floated grains (epoxy can be rendered soft and brittle with concentrated nitric acid), and replace the ground-away grains with new material from somewhere else. Pushing out epoxy plugs is only possible with metal disks. It’s not easy to do but is best done cold, preferably freezer cold to shrink the epoxy away from the metal.
- Grains can be mounted in individual metal tubes that are fixed in an array in a metal disk with set screws. This is the kind I grew up with, and I even designed and had built a nice eight-position block. Blocks like this have the advantage of being able to change the standards to suit your project, but the disadvantages of more complexity, fewer standards in the block at any one time, and the nuisance of having to to polish the standards individually.
What have I learned from making this block?
First, 97 positions on the standard block was way too many. Spend more time selecting better materials and less time characterizing them. Second, avoid hydrous materials. These have another degree of compositional freedom that makes constraining their composition that much harder. Unfortunately, I’m a big fan of amphiboles and micas so we happen to have a lot of them. Oh, well. Third, try to get materials in grain sizes that are easy to manipulate with tweezers. Unless you have experienced it, you can’t imagine what electrostatic charges can do to tiny grains. Fourth, practice first with sand and silt to make sure your techniques and skills will work. Fifth, have fun and don’t worry. If you make a mess of this one, you can always make another! Just remember, everyone else is counting on you!
