Sample preparation for XRF analysis – food products, animal feed, premixes and related materials
Food samples, as well as animal feed samples, may present themselves as very heterogeneous materials with many of the same elements distributed into various constituents like protein-, fat- and carbohydrate-rich phases. One can only accurately determine reproducible and representative elemental content, with the proper sample preparation steps.
Typical operations such as grinding and pressing might be difficult or even impossible to perform due to oily or sticky sample characteristics. You might have to follow special procedures. Fortunately, these procedures are normally easy to implement using readily available specialized equipment.
Mineral premixes are typically inorganic samples that in many cases, can be easily prepared as pressed pellets or fused beads.
Loose powder/ liquid cups
You can analyze powdered, homogeneous materials directly in loose powder cups (liquid cups). This is a quick and practical solution when the sample must be recovered or when accuracy and reproducibility are not the main drivers.
Any liquid or semi-solid samples (solutions, emulsions, slurries, etc.) you can simply analyze in liquid cups. No further preparation is needed. However, you have to carefully ensure that these samples are stable and will not segregate during the analysis.
Malvern Panalytical offers dedicated, single-use, sample cups in 32 or 40 mm diameter, with corresponding lids. Carefully designed and with high standards in production control, these cups assure safety in operation and that you will get the best analytical conditions for any liquid (or loose powder) samples.
Grinding
Grinding of dry and brittle samples to a fine powder is required to minimize undesired particle size effects and to allow further processing like pressing or fusion.
Cutting, blending & homogenizing
Moist, fatty, or very soft samples, including fresh produce or other plants- or animal-derived substances may require cutting into fine particles or blending into a homogeneous slurry. You can easily achieve this with cutting or knife-mills or high-power specialized lab blenders. Samples processed in such a way can be conveniently pressed into pellets or presented to the XRF spectrometer in a liquid cup.
Pressing
As a general rule, pressed samples deliver more accurate and reproducible results than loose powders in cups. Detection limits are also lower, as there are no absorption losses due to the cup‘s foil.
Pellets can be pressed free, into Al cups or steel rings. Cellulose or protein-rich samples may be pressed as is, but more mineral-rich samples may require the use of binders to achieve the necessary mechanical stability and robustness.
Fusion
You can prepare mineral-rich samples or calcinated residues as fused borate beads. For applications where accuracy and reproducibility play a significant role, this is arguably the best presentation form for a sample. Using an automated system, you dissolve the sample into a molten glass-forming flux at high temperatures, ultimately resulting in a completely homogeneous bead. This way, particle size and matrix effects can be virtually eliminated.
It is not always easy to obtain suitable standards that have the same particle size, mineralogy, surface roughness and segregation characteristics as the production samples. Therefore, the role of fused beads is extremely important in setting up reference calibrations for determining in-house standards for use in production control calibrations.
Handling oily samples
Materials with high-fat content require special care during grinding and pressing to avoid the mobilization of the fat, clumping, and segregation of the sample. For this, you will need an additive for oily samples. It will guarantee that any fatty components are not mobilized during normal grinding and pressing, leading to increased sample preparation repeatability, accuracy, and representativeness of results.
Previously in the seven-part series on ‘Sample preparation for XRF analysis’:
Watch out for more blogs in the series on “Sample preparation for XRF analysis” in the course of this year.