SAMPLE PREPARATION: Casting
Your sample type will determine the exact approach here, but the end result will be the same. The end product will be a 1-inch-diameter circular mount that has been polished and includes mineral standards and labels. The two types of mounts recommended for WiscSIMS analysis are epoxy rounds and thin-sections. Below, we describe the casting of epoxy rounds. If you will be preparing a thin-section instead, please follow this link to the decision tree for advice on how to proceed.
Mount dimensions and sample arrangement:
Each epoxy round must be 1-inch (<25.6 mm) in diameter and ideally is 5 mm thick, although a maximum thickness of 12 mm is allowable. Mounts with the maximum thickness of 12 mm may tilt in the holder, which can lead to significant analytical problems. Below is a schematic of the sample holder used in the SIMS so you have an idea of why this geometry is required.
Take note of the dashed blue line in the map view above (top panel). This line illustrates the portion of the epoxy mount within the "sweet spot" for analysis, which has a 5 mm radius in the classic sample holder shown above (Kita et al., 2009) and an 8 mm radius is possible with care using the updated design (Peres et al., 2013). The WiscSIMS lab has a number of each holder version. All sample and standard analyses must be within an 8 mm radius of the center of the epoxy mount.
Example mounts for your consideration:
Note for grain-mounts (micron- or mm-scale):
You will be arranging your sample(s) on the casting surface (described below) along with 2-3 grains of the appropriate mineral standard and any labels, paying special attention to the 8-mm radius sweet spot.
Note for other (larger) samples:
You will likely cut/saw your sample in order to: 1) fit within the analytical "sweet spot," and/or 2) expose the analytical surface of interest. Some considerations depending on your sample type:
- For samples with parallel layers, consider cutting diagonally across the layers so that you don't lose layers by cutting them out.
- For samples where you plan to make analyses along a linear traverse >15 mm in length (the diameter of the "sweet spot"), you will need to cut the sample into multiple sections. If possible, try to mount multiple sections of the traverse within a single mount. It will save you time (and money), but be sure to carefully document and label how the sections fit together. Take extra care to keep track of sample names/orientations while preparing multiple mounts!
Whenever the analytical target is on a surface that has been cut (e.g. by a rock saw, or preferably a jeweler's saw with a small-kerf diamond-embedded blade.), it is essential that the cut surface be pre-flattened before casting in epoxy. Otherwise, after casting, a significant amount of material may have to be removed by grinding in order to expose the analytical target. Excessive grinding can introduce large scratches, can grind through mineral standard grains, or worse, can remove important portions of your sample. To avoid these issues, please grind the cut analytical surface of your sample to flatness before casting in epoxy using a 6-μm (no greater than 9-μm) fixed-diamond grinding disc.
Materials used for casting by WiscSIMS:
- aluminum plate
- permanent marker
- washer (1-inch outer diameter, 10- or 16-mm inner diameter)
- double-sided sticky tape, 1.25-inch diameter (Kapton)
- teflon mold (1-inch inner diameter, ~2-cm depth)
- Buehler release agent (part #: 208185016), coated and dried on inside wall of teflon mold before casting
- sealant putty; wrap a single coil around the outer wall of teflon mold near its base
- hotplate, set to 60°C
- Struers EpoFix Resin (#: 40200030) and Hardener (#: 40200031).
- sample label (see below)
Note that some mounting cements are not appropriate for SIMS analysis. While we strongly recommend the Struers EpoFix product listed above, we understand that its use may not always be feasible. The Edinburgh Ion Microprobe Facility produced an excellent report of epoxy resins used in ion microprobe studies; see their Results Summary for other epoxy recommendations. Note that in the past we have used Buehler EpoxiCure® 2, but a recent (~2016) change in the formula led us to find an alternative.
Prepare the mold:
- Please review these illustrative slides (pdf).
- Use permanent marker to trace the outer and inner edges of the washer onto the middle of the aluminum plate. The outer circle will mark the edge of the epoxy mount, and the inner circle marks the corresponding analytical sweet spot. All analytical targets and standard grains need to end up within the inner circle.
- Cut a 1.5-inch length of Kapton tape, keeping the backing on top. In order to eliminate air pockets under the tape while laying it across the circles on the aluminum plate, begin by gently adhering one edge of the tape to the plate with your finger in a side-to-side motion and then gradually moving towards the opposite edge. If you trap an air bubble between the tape and the plate, start over with new tape. It's important that marker is only used on the aluminum plate and not on top of the tape – marker on top of the tape will bleed into the epoxy.
- Next, with the backing still attached to the tape, you can experiment with different arrangements of samples and standards within the analytical sweet spot that you can see through the tape, making sure there is still room for labels in the outer ring. Label plates with permanent marker (away from the tape) so you can keep track of sample names, orientations.
- When you have decided on a layout, remove the tape backing and adhere your analytical targets, standards, and labels into place. Consider archiving an image of the layout.
- Place the teflon mold on its mark, with dried release agent coated on inside wall, and sealant putty coiled around outer wall near its base.
- Push the coil of sealant putty into the corner between the side of the mold and the plate to prevent epoxy from leaking under the mold.
- Place the aluminum plate (with mold and samples in place) on a hotplate at 60°C for 2 hours before proceeding. Avoid using an oven, if possible.
Mix the epoxy:
- For Struers EpoFix, measure the resin and hardener at a ratio of 6.25 : 0.75 into a small disposable cup.
- Immediately begin gently mixing with a small metal or plastic stirrer (e.g. drink stirrer). Avoid wood or hollow stirrers as they will introduce air bubbles to the epoxy. It is critical to avoid introducing air bubbles to the mixture; stir gently without lifting the tip of the stirring rod out of the epoxy.
- You will see stringy polymers begin to form. Continue stirring until the strings disappear.
- Slowly pour the epoxy into the teflon mold. Pouring the epoxy onto your stirring rod and letting it stream off the tip into the mold helps to aim, limits how fast you pour, and may help reduce the amount of air bubbles. Pouring too fast may disrupt your samples or introduce air bubbles.
- If there are any air bubbles in the mold – particularly at the bottom, on what will be the analytical surface – you can try to carefully remove them by fishing them out or pushing them up the side of the mold with the tip of your stirring rod. If you have access to a vacuum container/oven made for impregnation, proceed with caution and test on non-precious materials. When operating correctly, these systems can remove air bubbles as the mount cures, but may also pull more bubbles from porous materials. Vacuum systems should never be used for top-mounting standards.
- Set the aluminum plate back on the hotplate at 60°C overnight to allow the epoxy to cure. Avoid using an oven; the hotplate will encourage air bubbles to rise away from the analytical surface as the epoxy cures.
- Next morning, remove the sealant putty and carefully pry your hardened epoxy plug from the plate and mold.
Eventually you will want to cut the epoxy mount to its final thickness (4-5 mm), but it is much easier to handle thicker mounts while lapping and polishing so we recommend waiting until a mount is polished before cutting the back off.
Label your mount with as much infomation as feasible including sample names, localities, name(s) of the included standard material, orientation markers, etc. We commonly print labels on standard printer paper (with font: Arial; size: 5), and cut out the label while leaving an extra ~2 mm of paper on two opposite sides to form tabs (illustrative slide, pdf). You can fold these tabs down to create legs for your label that can be now be placed on the double-sided sticky tape near, but not touching, the mold walls. It is important to make the legs >1 mm long (to avoid polishing away the label), but <4 mm (to avoid cutting off the label when you cut the mount to final thickness). Text on the label should face the back of the mount (face-up in the mold) so that you can read it when your sample is coated. If your label distinguishes multiple samples in a single mount, be sure that it is oriented correctly!
You could also epoxy a paper label to the back of a cured mount after it is cut to 4-5 mm thickness, but it's better to avoid this if possible. You could also engrave information on the back of your mount, but remember to cut it to the final thickness (4-5 mm) before engraving. For both of these cases, recall that cutting the mount to final thickness before polishing is not recommended, so you'd need to take special care to keep track of each sample's ID/orientation before they are finally labeled.
Markers should not be used as a final label on your mounts as we will use ethanol to clean your sample before SIMS analysis.