Sample preparation for XRF: how to prepare a sample for analysis

Sample preparation is an important step when using any analysis method, since the accuracy of the results obtained depends on the quality of sample preparation, and incorrect sample preparation can lead to their absolute incorrectness. Sometimes the preparation of a sample for measurement is a longer, laborious and expensive process than the measurement itself.

The method of X-ray fluorescence analysis is quite lenient in this regard, since it is an express method that does not require complex sample preparation, but this does not mean that it can be completely neglected. The main types of samples measured by XRF are solid samples (various metals, alloys, both ordinary and precious, scrap metal, etc.), powdered samples (usually these are crushed heterogeneous samples, such as soils, ores and autocatalysts) and liquids (petroleum products).

Solid samples

The requirements for solid samples are quite simple. The only condition is that there is a flat and clean surface to take the measurement. Preparation of solid samples is carried out as follows:

• Polish the sample to obtain a smooth, flat surface. Use a grinding tool for hard metals such as iron and steel, and a lathe or milling machine for soft metals such as copper and aluminum alloys. (this step can be skipped if a suitable smooth surface is already on the sample).

• Clean the sample surface with a file for metal. It is very important to use a separate file for each type of sample (for example, one for aluminum alloys, one for steels, etc.), otherwise particles of the previous cleaned sample can be brought onto the surface of the sample, which will lead to surface contamination.

• If there is any doubt about the surface cleanliness, you can compare the measurement results before and after grinding the sample, if they differ significantly, grind again, continue until the difference between the results before and after is within the measurement error.

Powder samples

There are several options for preparing powder samples of varying degrees of complexity, such as powder compaction, flux fusion, or direct measurement of finely ground powder. The last option is the simplest and most common method. In this case it is necessary:

• Prepare cuvettes and film for measurements. There are many types of films for XRF, to select the most suitable type for a specific task and device, it is important to consult with its supplier.

• Grind the sample. This is necessary to achieve a homogeneous mixture so that the result represents the characteristics of the entire sample, and not individual grains. The finer the grains, the better the result. The optimal grain size is <75 µm.

• Pour the powder into the cuvette and measure. At the same time, there should be no voids between the grains on the measuring side, and the powder should look like a flat, even surface. Otherwise, it means that the sample is not fine enough.

Liquids

The XRF method is not sensitive to the state of aggregation, and allows the measurement of liquid samples directly. Thus, to measure liquid samples, it is sufficient:

• Prepare cuvettes. For liquids it is necessary to use open cuvettes (which have a film on only one side), otherwise the film of the cuvette will swell due to the evaporation of the liquid.

• Select the correct film for the liquid to be tested. Polypropylene type film is used for acid and alkali solutions, and polyester type film is used for oil products.

• It is important to check beforehand for impurities in the film being used for analysis, as polyester-type film may contain impurities that may be of interest for analysis.

• Pull the film over the cuvette (the film must be tightly stretched and not sag), and pour the liquid, after which the measurement can be carried out.

Thus, sample preparation in XRF is not a very time-consuming process, but it should not be neglected, since the quality of the measurement results depends on it. The sample preparation steps described above will be sufficient to obtain good results using any XRF analyzer.


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