Key Considerations for XRF Pelletising
Determining a material’s elemental composition requires instruments’ sensitivity and stability to achieve accurate results. Previously, those two factors were the most common source of error in XRF analysis, but as technology has advanced, so have the XRF instruments, which has meant they are no longer the primary source of error in an XRF sample analysis. Instead, the most common problems are found in sample preparation techniques. This blog post will look at the key considerations when using XRF pelletising as a preparation method.
What is XRF Pelletising?
Pelletising is the process of making pressed pellets for XRF analysis. Pressed pellets are a standard method of preparing samples, as they are cost-effective, quick, and offer high-quality results. Because the outcome of the XRF analysis is heavily dependent on sample preparation, pressed pellets are used to ensure accurate quantification of sample composition.
The pelletising process involves grinding the sample into a fine particle size, mixing it in a vessel with a binder or grinding aid, then adding the mixture to a die and pressing under a press machine with 15 to 35Ts of pressure. Although the process is relatively simple, there can be variations depending on the size of the pellets you need, which include the binder used and the amount of pressure applied to the sample. As a result, a few key considerations should be taken into account before starting, which we will discuss in the following section.
Key Considerations for XRF Pelletising
Sample preparation is crucial for achieving the right XRF results in your analysis, and many different factors can impact the outcome. Understanding why each factor is important and what issues they can cause is vital in preventing errors. Let’s take a look at those factors.
Binder
During the pelletising process, powders must be mixed with a binding agent to bind together correctly. If not, loose powder can contaminate the spectrometer and skew the results. Using the most suitable binder for your sample will hold the powders together and reduce the risk of loose powders breaking away. Typically, a cellulose/wax mixture is used. This binder will homogenize with the sample and bind the powders together during the mixing and pressing stages.
Contamination
One of the biggest problems in any scientific experiment is contamination, which can come from various sources. In XRF sample preparation of pressed pellets, contamination is most likely to occur during the grinding process. External components from the sample preparation instrument can be passed into the sample or through cross-contamination from other samples.
Dilution
Not only is the type of binding agent important, but also the quantity of it. The amount of binder used must stay the same for every sample to ensure accuracy levels and avoid over-diluting a sample. For most samples, a large amount of binder is used as weak pellets can break, damaging the spectrometer and destroying a sample. The most common sample dilution ratio is 20/30% binder to sample ratio is recommended.
Particle Size
Producing pressed pellets requires accuracy and specific measurements, and pelletising works well when samples are ground to a particle size of <50µm, although <75µm is also acceptable. Working with small particle sizes is important because this impacts how well the sample binds together under pressure, which impacts the analytical results.
Pressure
After the sample has been mixed with the binding agent, it is added to a die to be pressed. The sample is usually pressed with a hydraulic sample press, and the pressure applied through this instrument needs to be enough for the binder to recrystallize and for the sample to be completely compressed. Most samples will require pressing for 1-2 minutes, under 25-35T of pressure to ensure no void spaces are present in the pellet.
Thickness
Aside from particle size, the thickness of pressed pellets is also crucial for the effective analysis of a sample. If a sample is too thick, the X-rays will not be able to penetrate it up to a certain point. Because XRF analysis relies on emitted X-rays reaching the detector, the sample must be infinitely thick to the X-rays.
XRF Scientific: Pelletising Products
XRF Scientific supplies a wide range of pelletising products, from pellet binders, dies and hydraulic presses to ensure your XRF analysis applications are carried out to the highest standard. If your laboratory would benefit from high-quality pelletising products, contact us today for more information.
