HOW TO SELECT A THIN-FILM SAMPLE SUPPORT
WINDOW MATERIAL
Selection of a suitable thin-film sample support window is mostly based on satisfying primary important laboratory requirements:
- Convenience of use; contamination avoidance
- Analyte-line transmittance and intensity
- Chemical resistance to samples
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.
1. CONVENIENCE OF USE AND CONTAMINATION AVOIDANCE
Dispensing and handling a thin-film in attempting to prepare a sample cup with a sample support window is frustrating, inconvenient and a source for contamination. This is attributed to the static electrical charges inherent to the thin-films that create static cling. The thin-film tends to stick to any surrounding object and one’s person. This phenomenon presents the potential of contaminating the thin-film and affecting analytical x-ray data.
With the advent of SpectroMembrane® Sample Support Carrier Frames, there is no annoying static cling or potential contamination risk to the thin-film through handling or attraction of airborne particles. With the exception of the sample cup, the thin-film is never in intimate contact with anything else. Thin-film handling is performed by the use of the integrated carrier frames that automatically detach during the assembly process leaving taut thin-film sample planes.
2. 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, Ǻ) 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.
3. CHEMICAL RESISTANCE
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. The length of integration time, excitation potential and heat generation that a thin-film is exposed to are equally important to its integrity.
In addition to the following table Chemical Resistance Test Paper is also available to ascertain the chemical compatibility of thin-film windows to actual test samples. A Chemical Resistance Test Paper is placed underneath a test sample containing the thin-film sample support of interest and a test sample. After a given time has elapsed a visual indication of a stain insinuates the thin-film has been attacked by the sample substance. The absence of a stain is indicative of satisfactory usage of the thin-film with the sample substance. In some instances overnight testing may be necessary if a chemical attack is not immediately evidenced. Refer to “Chemical Resistance Test Paper” for full details.
CHEMICAL RESISTANCE OF THIN-FILM MATERIALS
| CHEMICAL | MYLAR | POLY- CARBONATE | ETNOM | POLY PROPYLENE | POLYIMIDE (KAPTON® | PROLENE | ULTRA POLY- ESTER™ |
|---|---|---|---|---|---|---|---|
| Acid, dilute or weak | G | G | G | E | N | G | G |
| Acids, conc. | G | G | G | E | N | E | G |
| Alcohols, aliphatic | N | G | G | E | G | E | N |
| Aldehydes | U | F | F | E | E | E | U |
| Alkalies, conc. | N | N | G | E | E | E | N |
| Esters | N | N | F | G | G | G | N |
| Ethers | F | N | F | N | U | N | F |
| Aliphatic Hydrocarbon | G | N | E | G | E | G | G |
| Aromatic Hydrocarbon | N | N | E | N | E | N | N |
| Hydrocarbon Halogenated | F | N | F | N | F | N | F |
| Ketones | N | N | G | G | G | G | N |
| Oxidizing agents | F | N | F | F | N | F | F |
E=Excellent, G=Good, F=Fair, N=Not recommended, U=Unknown
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."
CAUTION: All thin-film window materials affixed to sample cups present the risk of stretching or rupturing in the sample chamber and/or causing sample cup leakage through the ring and cell juncture or disassembly with potential contamination and damage to the system. These conditions are especially amplified when the sample chamber is subjected to a change in pressure attributed to evacuation, introduction of an inert gas flush or from pressure build-up within the sample cup from a sample substance out gassing or from volatility. Additionally, the possibility of pinholes, pores and depressions existing in any thin-film sample support substance regardless of form, configuration and packaging can present leakage of a sample with subsequent contamination and damage to the analytical instrumentation and it components. It is strongly recommended that the products used be subject to judicious testing, use and applications and user evaluation prior to actual use by a method that does not risk contamination or damage to the x-ray analytical spectrometer. The responsibility of product acceptance and performance resides totally with the user. Chemplex Industries, Inc. assumes no liability or guarantees whatsoever that the products will perform in accordance with their usage, advertisements or methodologies written, orally expressed, implied or insinuated.
® SpectroCertified, Prolene, Etnom and Chemplex are registered trademarks of Chemplex Industries,™ Ultra-Polyester is a trademark of Chemplex Industries, Inc.
® Mylar and Kapton are registered trademarks of E I DuPont de Nemours, Inc.