X-ray Fluorescence Analysis of Liquid Samples without Helium Gas

Helium replaces the vacuum normally used in WDXRF instruments because liquids cannot be measured directly under vacuum without evaporating or causing measurement issues. Helium provides high X-ray transmission while maintaining a stable measurement environment, particularly for low-energy X-rays emitted by light elements. When helium is unavailable, converting the liquid into a solid enables measurements under vacuum.

Aqueous samples can be prepared using the droplet method, in which a measured volume of solution is deposited onto specialized filter paper and thoroughly dried. The resulting solid residue is analyzed under vacuum. This approach requires only a small sample volume and often provides improved sensitivity for light elements because there is no sample film to absorb low-energy fluorescence.

The droplet method allows vacuum measurements without helium, improves detection of light elements such as sodium and magnesium, requires only a few hundred microliters of sample, and eliminates concerns about liquid splashing during measurement. The dried sample also produces a higher peak-to-background ratio, making trace-element peaks easier to distinguish.

Highly concentrated solutions that crystallize after drying, solutions with low surface tension such as many organic solvents or surfactants, and samples that cannot produce a uniform residue are generally unsuitable. Concentrated plating solutions often require substantial dilution before preparation to achieve consistent analytical results.

The oil sample is mixed with a solidifying agent, heated until completely liquefied and homogeneous, and then molded into a solid specimen for analysis under vacuum. This preparation produces a stable sample while eliminating the need for helium during measurement.

Solidifying the sample prevents suspended wear particles or metal debris from settling during analysis, improving sample uniformity. It also enables vacuum measurements and can improve the detection of light elements by avoiding the sample film required for conventional liquid analysis. Standardless fundamental parameter calculations can account for the dilution introduced by the solidifying agent.

Samples with high water content may not solidify properly because the water separates from the solidifying agent. Highly volatile oils or fuels may lose components during heating or storage, potentially introducing analytical errors. Careful control of preparation temperature is also necessary to avoid exceeding the flash point of the sample mixture.

Selecting the appropriate sample preparation method based on sample type is the most effective strategy. Dilute aqueous solutions can often be analyzed using the droplet method, while lubricating oils and greases are good candidates for oil solidification. When the sample chemistry and preparation conditions are appropriate, these approaches enable high-quality vacuum measurements while substantially reducing helium usage.