Sample Preparation for X-ray Fluorescence Analysis VIII.

---

The liquid solidification method converts a liquid sample into a stable solid specimen by mixing it with a solidifier that melts when heated and solidifies upon cooling. A typical preparation uses equal weights of sample and solidifier. The mixture is heated until completely liquefied, thoroughly mixed, poured into a mold, and cooled on a flat surface. The resulting specimen can then be analyzed by WDXRF as a solid rather than as a conventional liquid sample.

Solidification can eliminate the need for both a sample film and a helium measurement atmosphere when the sample is suitable for vacuum analysis. This improves sensitivity for light and ultralight elements whose fluorescent X-rays are strongly absorbed by films and helium. Solidification also immobilizes suspended particles, preventing them from settling and changing the measured X-ray intensity during analysis.

The liquid and solidifier are accurately weighed into a vial, with a 1:1 ratio such as 2.5 g sample to 2.5 g solidifier being typical. The mixture is heated at approximately 80–160°C until completely liquefied, which generally takes about 15 minutes. It is then thoroughly mixed, poured into a mold, and cooled to form a specimen with a flat measurement surface. The heating temperature must remain below the mixture's flash point and autoignition temperature.

Lubricating oils and greases are particularly suitable because their relatively high initial boiling points allow the solidified specimens to be measured under vacuum with little risk of contaminating the spectrometer. Heavy and light crude oils can also be solidified, but their more volatile components generally make helium measurement preferable. Suitability depends on initial boiling point, volatility during preparation, and miscibility with the solidifier.

Low-boiling petroleum products such as kerosene, gasoline, and diesel fuel are unsuitable because significant evaporation can occur while the solidifier is being melted. Water-containing liquids are also unsuitable because they do not mix adequately with the hydrophobic solidifier. These limitations mean that volatility and chemical compatibility must be considered before choosing solidification as the preparation method.

Suspended solids can settle during conventional liquid analysis, progressively changing their concentration in the X-ray measurement region and causing measured intensities to drift. Solidification locks the particles into position and produces much more stable measurements over time. For slurry oils containing catalyst fines, this approach stabilized measurements of elements such as aluminum and silicon. However, preparation reproducibility can still suffer if the original slurry cannot be sampled consistently because of high particle concentrations or unfavorable particle-size distributions.

Yes. Suitable solidified oils can be measured in vacuum without an analysis film, reducing absorption of low-energy fluorescent X-rays. This makes it possible to analyze elements that are difficult or impossible to measure effectively using conventional liquid cells. For example, sodium could be measured in a multielement oil standard, and a boron calibration covering 0–5000 μg/g was successfully established using solidified oil standards.

The solidifier dilutes the original sample, so its mass and composition must be included in the calculation. For semi-quantitative analysis, the solidifier can be defined as a binder and the actual sample and binder weights entered into the analytical software. Sample thickness should also be considered because the low-density solidified matrix may not provide infinite thickness for high-energy X-ray lines. The base composition of the original liquid, such as CH₂ for typical oils, should be specified as the balance component when calculating concentrations.