Sample Preparation of Loose Powders
The simplest approach to analyzing a loose powder sample is to simply fill a sample cup approximately ¾ full and analyze it without any additional preparation, or simply tapping the cup on a clean surface to pack it to a more consistent density.
This method is satisfactory in same cases where the reproducibility requirements are not very strict. The greatest advantage with this method is that it is easy. Spinning the sample may improve the consistency of the measurement by averaging over a larger area.
There are many shortcomings with this technique; the bulk density may not be consistent, grain size variation cause the readings to vary and finer grains can be forced to the surface during tap packing. Because of these problems this method works best with homogenous material that has been dried and ground to a uniform grain size. Of course, these recommendations apply to every sample presentation method.
Manually Pressed Powders:
Several venders offer a sample press, a modified arbor press, that is designed to press a sample within a sample cup. The sample cup is first filled ¾ full or more and then positioned under the press and compacted to 15 to 30 Newton-meters. Higher pressures can be achieved depending on the press. The cup must be held down firmly to prevent the film from bulging, and it is usually a good idea to place a Kimwipe or other clean disposable material under the cup. Place another Kimwipe over the cup before pressing if the sample material is likely to stick to the press. If the material compresses a lot it may be necessary to refill and press 2 or 3 times to achieve a reasonable sample depth.
Manually pressed samples can still be prepared quickly since pressing generally takes less than a minute. Because tap packing is unnecessary this method eliminates the problem with finer particles settling. Ultimately, by giving the samples more uniform bulk density it is possible to achieve analytical reproducibility that is often no more than 10% worse than with a hydraulically pressed sample.
Hydraulically Pressed Powders:
The preferred method for analyzing powders or samples that are usually ground to a powder to make them more homogeneous is the hydraulic pressed pellet. Several manufacturers make hydraulic presses that are capable of pressures ranging from 10 to 50 tons or more. The press uses a die set to contain and form the sample during pressing. There are a few types of indie sets and a lot of variations to the methods incorporating a lot of individual creativity, but one basic outline follows.
The sample is first dried and ground to a fine consistency, 400 mesh or better is recommended. Remember that the x-ray wavelengths are still substantially smaller than the particles. Next the sample is usually blended with a binder that helps hold the pellet together, although some materials don’t require it. Selection of the best binder for a given material is an art form itself and is discussed in more detail on the binder page. A ring and puck mill or mixer mill is very useful for both the grinding and blending steps.
To prepare the press, the die set must first be cleaned with methanol or other solvent. The backing, usually an aluminum cap, is inserted into the die. A specific weight of sample is then poured into the cap, usually 5-10 grams. It is important to keep the mass constant because the sample may not be infinitely thick at high x-ray energies. Next a polished pellet is placed over the sample to produce a smooth finish. The plunger goes in after that, and then the die set is positioned in the press. Follow the press instructions for pressing the sample to 10-20 tons, and holding it for a period of time usually from 10-100 seconds. Different materials produce better pellets at different pressures, so finding the best pressure may require some experimentation. Once the best pressure is determined however all samples of that type must be compacted to the same pressure and hold time to achieve optimal analytical results. The pellet is then removed from the die set, taking care not to crack it in the process.
The finished pellet is uniform in composition, density, and mass per unit area, and has a smooth finish. These are all highly desirable traits for an XRF sample, so many operators will spend the needed five to ten minutes it takes to prepare the sample. The only remaining disadvantage with pelletized samples is that there are still matrix affects due to the grain size being larger than the x-rays.