Sample Preparation for X-ray Fluorescence Analysis VII.

---

Two primary approaches are direct liquid measurement and the droplet method. In direct measurement, liquid is placed in a sample cell with a thin polymer film forming the X-ray measurement window. In the droplet method, a measured volume of solution is deposited onto filter paper or a specialized support, dried, and then measured as a solid specimen. Direct measurement is straightforward, while the droplet method offers advantages for light-element and trace-element analysis.

Selection should consider chemical resistance, mechanical strength, X-ray transmission, resistance to X-ray irradiation, and elemental impurities in the film. Polypropylene is generally suitable for acidic and alkaline solutions, while polyester is often suitable for lubricating and fuel oils. Because chemical resistance depends on the specific sample, compatibility should be tested with the actual solution before routine measurement.

The film absorbs part of the fluorescent X-ray signal, with the effect becoming especially important for long-wavelength X-rays from light elements. Film absorption therefore requires correction in fundamental-parameters-based semi-quantitative analysis. Polymer films can also contain additives such as Si, P, Ca, and Al that generate their own XRF signals. Blank measurements are useful for identifying and correcting these contributions, and film impurities should be checked for each lot because additive levels can vary.

The film should first be checked for chemical compatibility with the sample. Risk from X-ray irradiation can be reduced by lowering X-ray tube output, shortening measurement time, using a primary beam filter, or selecting a thicker or more irradiation-resistant film. Suggested maximum measurement times are approximately 10 minutes for 6 μm polypropylene, 15 minutes for 6 μm Mylar, and 20 minutes for 7.5 μm Kapton. Highly reactive, volatile, or unfamiliar samples require additional caution because film failure can release liquid inside the spectrometer.

Strong acids can damage the instrument if leakage or film breakage occurs, so short measurement times, reduced X-ray power, primary beam filters, and appropriately resistant films should be considered. Samples containing volatile components or high levels of F, S, or Cl may generate gases, creating risks of film expansion, leakage, and cross-contamination. Sealing the sample can reduce these problems. Highly volatile liquids such as light oil and gasoline should generally be measured for shorter periods, and samples should be removed promptly after measurement.

Because the liquid is dried before measurement, the prepared specimen is easier and safer to handle than a liquid-filled cell. The absence of a sample film permits measurement under vacuum and improves sensitivity for light elements such as B, F, Na, and Mg. Specialized filter supports can also provide high sensitivity for trace elements; with prior concentration, UltraCarry can enable analysis at concentrations of several tens of ppb.

A defined volume of solution is transferred with a micropipette onto a suitable filter support and dried. Recommended volumes are approximately 50–100 μL for MicroCarry and up to 500 μL for UltraCarry. Drying at a controlled temperature of about 40–60°C accelerates preparation while limiting damage to the support. High-density solutions that form crystals after drying should be diluted because nonuniform crystal deposition can cause substantial analytical error.

Because the filter material is thin, X-rays from the sample holder or support can contribute unwanted signals. Elements already present in the filter can also appear in the spectrum. Blank-filter measurements allow these contributions to be identified and corrected. Poor reproducibility can also occur when low-surface-tension liquids, such as some surfactant solutions and organic solvents, spread outside the intended deposition area.