Selection of a suitable XRF thin-film is mostly based on satisfying primary important laboratory requirements: - Convenience of use - Contamination avoidance - Chemical resistance to samples - Analyte-line transmittance and intensity
The information presented entails the main packaging forms thin-films are offered, the various types of thin-films, a methodology for evaluating an appropriate thin-film for maximizing analyte-line percent transmittance and the resistance to sample chemical attack. Other issues of laboratory concern may include sample weight retention strength, performance under differential pressure conditions, integration time, excitation potential and related heat generation. TRANSMITTANCE OF SPECTRAL LINES
The transmittance properties of a thin-film are functions of gauge thickness, density and mass attenuation coefficients of the constituent chemical elements in accordance with the following formula:
I / Io = exp [-(µ/d) (d)]
Where: I / Io = percent transmitted spectral line energy µ/d = mass attenuation coefficient dt = area concentration
The following graphic representation of the above formula is the most frequently used visual aid in comparing % transmittance values of various thin-film substances and gauges. Simply refer the spectral line of the element in the sample of analytical interest having the lowest energy, KeV, (or shortest wavelength, Ångstrom) to each of the curves representing thin-film materials and gauges. Compare their transmittance percentages and select the thin-film offering the highest % Transmittance value.
In conjunction with selecting a thin-film providing a high % Transmittance value for an element analyte-line of interest, evaluation of resistance to chemical attack by a sample is an appropriate consideration. Liquid samples are generally more prone to thin-film degradation and rupture than other sample forms. Refer to the "Resistance of Thin-Film Substances to Chemical Attack" table. Other factors equally influential to the integrity of a thin-film substance are: - Integration time extended analysis time - Excitation potential intense x-ray exposure - Heat generation from the sample
It is strongly urged that thin-film substances are subjected to judicious testing and evaluation prior to actual use to avoid inadvertent mishaps during analysis and potential damage to the x-ray instrumentation and costly ensuing clean-up.
IMPORTANT: The chemical resistances of thin-films contained in the above table are provided as a matter of information purpose only. They are not intended to preclude actual testing and suitability of use and applications. IMPORTANT: Some window materials may not be suitable for analyzing sulfur in diesel fuel, gasoline and other petroleum products containing aromatic hydrocarbons. ASTM D-6445-99 (Reapproved 2004) e1: "Samples of high aromatic content may dissolve polyester and polycarbonate films. In these cases, other materials besides these films may be used for X-ray windows, provided that they do not contain any elemental impurities. An optional window material is polyimide film. While polyimide film absorbs sulfur x-rays more than other films, it may be a preferred window material as it is much more resistant to chemical attack by aromatics and exhibits higher mechanical strength." ASTM D 4294-08a: "Any film that resists attack by the sample, is free of sulfur, and is sufficiently X-ray transparent can be used. Film types can include polyester, polypropylene, polycarbonate, and polyimide. However, samples of high aromatic content can dissolve polypropylene, polycarbonate and polyester." Chemplex Industries Inc. has always been the reference company for the preparation of samples for XRF analysis. All the major manufacturers of XRF instrumentation recommend products Chemplex. A constant research and innovation, continuous attention to the needs of the market and an unrivaled quality of products make Chemplex the ideal partner to ensure consistency and safety in the preparation of the samples for XRF analysis. The range Chemplex is complete and includes, in addition to the popular films and sample cups, several accessories ingenious and very useful for the preparation of the samples, or simply, to facilitate and improve the assembly of the parts constituting the sample holder finished.
For many years Nordtest has been the official distributor of Chemplex, and together they work to ensure their customers a high level of technical support, based on the extensive experience and expertise that have both in the XRF technique.