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HOW TO SELECT THIN FILM WINDOWS FOR XRF ANALYSIS

An X-CELL®liquid sample cell must be covered by a thin film which serves as the sample window. The film must effectively contain the liquid while allowing uniform transmission of the X-rays to the sample. Your choice of a window film depends on your analytical priorities and the nature of the sample. Is strength more important than X-ray transmission, or vice versa? Are there certain impurities you cannot tolerate in the film? Is the sample chemically aggressive, capable of weakening or penetrating certain films? Is it hot when you pour it into the X-CELLS® or will it heat up during analysis and perhaps soften the film? Will it release volatiles and put pressure on the film from inside the cell?

Often the final choice of film balances several requirements. We recommend comparingfilms, and for that purpose we can supply on request samples of SPEX films and X-CELLS®. We also suggest strongly that if there is any question about the compatibility of a sample with certain films and/or X-CELLS®you test them together outside the spectrometer.

WHEN CHOOSING A THIN FILM WINDOW FOR YOUR APPLICATION, CONSIDER THE FOLLOWING FILM CHARACTERISTICS:

1.Strength: Leakage or rupture of the window during a run could cause damage to spectrometers with inverted optics. While SPEX SamplePrep accepts no responsibility for damage resulting from window leakage, the films offered by SPEX SamplePrep have been selected for their strength and durability. Kapton is the closest thing to a bombproof XRF window film. It is much stronger than any other SPEX SamplePrep film, heat-resistant to over 300° C, and chemically impervious to almost everything except strong alkali solutions. Six micron (6 µm) Mylar is also unusually strong for its thickness. The remaining films (5 µm Polypropylene, 4 µmUltralene, and 3 µmMylar ) have roughly comparable mechanical strengths, but differ in other key properties. Ultralene remains our best film in its balance of X-ray transmission, chemical resistivity, strength, and purity. All SPEX SamplePrep XRF window films except Kapton are weakened by heating much above 100° C.

2. Transmission: X-ray transmission through polymer films is affected by the thickness of the film as well as its composition. Of the SPEX SamplePrep XRF window films, 8 µmKapton is the least transparent to X-rays; 6 µm Mylar is more transparent, as is 5 µm polypropylene. The thinnest films (4 µmUltralene, and 3 µm Mylar) have the greatest degree of X-ray transmission, and should be considered for light element analysis in particular.

3. Uniform Thickness: The absorption of X-rays at a given wavelength is directly proportional to the thickness of the film. Thus you will get uneven fluorescent intensity with an uneven film. SPEX SamplePrep films are selected for uniform thickness so your results are reproducible.

4. Purity: No XRF window film is absolutely free of metallic impurities, but some are cleaner than others. While Mylar film makes a suitable window for most analyses, it may contain trace levels (ppm) of Ca, P, Fe, Cu, Zn, or Sb. Polypropylene film has been known to contain trace levels of Ca, Zr, P, Fe, Zn, Cu, Ti, and Al. Our cleanest film overall is Ultralene. Kapton is clean in most respects but now includes a phosphate-based surface coating to improve its handling characteristics. Our best advice is that you run a blank from every new roll or box you open since impurities can be different not only between types of film but also between different lots of the same material. SPEX SamplePrep cannot be held responsible for variation in film composition.

5. Chemical Attack: Your sample’s chemical characteristics may dictate your choice of window film. Some samples simply attack some films, exposing the spectrometer to possible damage if the window leaks. For each new sample, we strongly recommend that you test the window film by exposure to the sample for several times your longest anticipated running time. The tables which follow may be of some help in selection, but they are not meant to replace actual testing.

In general, Kapton is highly resistant to acids and organic chemicals and almost everything except strong bases. Mylar is vulnerable to strong acids and strong bases and resistant to organic chemicals. Polypropylene and Ultralene are reasonably resistant to acids, bases, and many organic chemicals, but can be attacked by aromatic and halogenated hydrocarbons. All of these films are attacked by strong oxidizing agents.

X-CELLS® are made of polyethylene, which is generally resistant to acids, bases, and most organic chemicals except halogenated hydrocarbons. Window films, being much thinner than X-CELL®bodies, are likely to perforate long before the X-CELL®is affected. However, polyethylene begins softening at temperatures well below 100°C, so if you are working with hot samples, pre-test the X-CELL®body as well as the window film.


CAUTION: SPEX SamplePrep window films and X-CELLS®are not intended to be used in vacuum-path XRF systems. An abrupt pressure change in an XRF spectrometer’s sample chamber can stretch or burst window film, or otherwise cause the X-CELL®to leak or come apart. It should also be noted that some volatile chemicals have the ability to generate pressure inside an X-CELLS®, and can stretch a film to bursting or (rarely) cause slow leakage by migrating through the joint between the film and the cell. Whenever there is a question about whether a SPEX SamplePrep X-CELLS®, window film, or combination thereof is likely to perform properly in a particular analytical situation, the analyst should first test these products in a way which does not risk contaminating the spectrometer.


 

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