Sample Preparation of Films
Window (aka Sample Support) Film
A variety of window films are available to fill a variety of needs. Decisions on the type of film to use are made based on factors such as cost, x-ray transmission, chemical resistance, reproducibility, and ease of use.
The most common types of films are addressed below.
Mylar is often the first choice people make because it is low in cost and has a high tensile strength so it gives very reproducible results. It has worse transmission characteristics than most other films, but because it is so strong it is available in a thin, 1.5 um form. The 6.3 um material is often used when analyzing most high Z elements, while the 3.6 um form is used for elements between Al and Ca. Mylar’s weakness is that it has poor resistance to acids, which eliminates a broad class of solution applications. The 1.5 um film is difficult to handle, and is not widely used for routine analysis.
Polypropylene is the next most popular film. It is the next least expensive film after Mylar and is resistant to most acids. It also has better x-ray transmission than Mylar. It usually comes in a 6.3 um thickness and is useful for analyzing elements from Al up in the periodic table. Polypropylene’s weakness is that it has poor tensile strength and stretches readily, meaning it has poorer reproducibility than Mylar. Also, while it is resistant to hydrocarbons it absorbs them, softens and wrinkles over time, so it is a poor choice for use with oils and solvents.
Prolene and Spectrolene
These films have similar chemical resistance properties to polypropylene, and so are useful for acid analysis. They are also a little better than polypropylene for hydrocarbon analysis. They have superior x-ray transmission and are available in 6 um and 4 um thicknesses. These films are highly recommended when analyzing low atomic number elements such as Na and Mg in solutions. Because of these properties Spectrolene and Prolene have become the favored general-purpose film in many XRF laboratories. The 4 um film can still be handled reasonably well by an average technician. The major drawback is the comparatively high cost of these films.
Polycarbonate is similar in cost and transmission to polypropylene. Its advantage is that it’s tensile strength is similar to Mylar, but with better transmission characteristics. And it comes in 5 um and 2 um thicknesses. It has very good reproducibility, close to Mylar. It is also highly resistant to hydrocarbons, so it is the preferred film for measuring low atomic number, Na-Cl, elements in oils and solvents. This material is known to tear easily, particularly when assembling Somar style cups. The 2 um film, like other very thin films, is difficult to work with on a routine basis.
Kapton film has a distinct yellow appearance. It comes in a thick 7.6 um film and has poor x-ray transmission characteristics. It is also the most expensive of the XRF window films. With all these negatives it is not hard to imagine that it is rarely used. Kapton is however resistant to highly aggressive acids, such as concentrated SO4, HF, and aqua regia, that destroy other films.
Teflon, Tefzel, PFA
The most common use of Teflon is as a microporous film that is used to cover cups of loose powders when they need to be analyzed in a vacuum. It allows the air to escape without allowing powder to be sucked into the vacuum pump, and distributed everywhere in the analysis chamber.
Tefzel and PFA films are not available in very thin films, 12.5 um is the thinnest size. They are not very x-ray transparent, and are not a stock item for most XRF film venders. They can be used with some acids when even Kapton is not good enough, and they can be used as a window lining in some on-line applications.
Cups with solid Be windows are available for analyzing very light elements. They are very expensive costing a couple hundred dollars each. They must be cleaned between analyses, and can be destroyed by most acids. They are seldom used for general analytical work.
Numerous other specialty films have been used, but are generally limited to special laboratory methods.