Effective Degreasing of Samples for EPMA

[AndrewLocock]

Preparation of polished surfaces may involve the use of an oil (either to suspend/disperse the polishing media, or to lubricate the process). Such oil is often not fully removed by rinsing or ultra-sonicating with alcohol. This can lead to problems during electron-probe microanalysis.

In advance of carbon-coating a sample in preparation for such analysis, Long (1977) recommended cleaning the sample with the use of petroleum ether in conjunction with an ultrasonic bath, as quoted below:
“It is perhaps worth noting here that the cleanliness of the specimen surface is of considerable importance in achieving an effective coating: careful degreasing in Analytical Reagent-grade petroleum ether in an ultrasonic cleaning bath, followed by washing in ethanol, is recommended.”
Excerpted from: Section III. Specimen Preparation, B. Surface Coating (page 299) of:
Long, J.V.P., 1977. Electron probe microanalysis. Chapter 6 in Physical Methods in Determinative Mineralogy, second edition, J. Zussman, ed., Academic Press, New York, pp. 273-341.

Petroleum ether (CAS Registry Number 8032-32-4, boiling point 35-60 Celsius) has several synonyms or near synonyms: ligroin, ligroine, naphtha solvent, naphtha petroleum, petroleum spirit, petroleum benzin, rubber solvent, etc. General details can be found at:
https://en.wikipedia.org/wiki/Petroleum_ether
http://www.cdc.gov/niosh/docs/1970/77-192.html

Petroleum ether is an effective degreasing agent. Unlike ethanol, it does not dissolve “permanent marker”. Also, unlike acetone, it does not appear to have any effect (in the short term) on the epoxies commonly used for mounting samples for electron-probe microanalysis. Petroleum ether is insoluble in water, but is both highly volatile and highly flammable. It should be kept away from any rubber (O-rings etc.). It is probably preferable to undertake handling of small volumes of petroleum ether in an appropriate fume hood.

As petroleum ether is highly volatile, and can have negative health effects, the following procedure has been adopted in our laboratory:
1)   Up to 3 thin sections or up to 5 one-inch diameter round mounts are placed (polished face up) in a transparent hard-plastic hinged box.
2)   Sufficient petroleum ether is added to cover the samples (usually 10 to 15 mL).
3)   The closed box is placed in a small water-filled ultrasonic bath, floating on the water.
4)   The ultrasonic bath is covered by a hard plastic hood attached to an open vacuum line.
5)   The ultrasonic bath is run for about 60 seconds.
6)   The box is removed from the ultrasonic bath, and the samples withdrawn with the use of tongs. The samples are allowed to dry in air, and can now be rinsed with ethanol.
7)   The remaining petroleum ether is decanted as chemical waste.

This text, and a photograph of our setup is given in the attached PDF.

[Probeman]

Hi Andrew,
In agree Pet ether is nasty stuff.  We've used hexane also to degrease samples that were polished using oil based polished compounds, but now that we use all water/alcohol based polishing agents, we simply wipe with a little *pure* ethanol and warm them for a few minutes in a drying oven and we're ready to carbon coat.

I should mention that one should never use denatured ethanol or isopropyl alcohol as they will dissolve the tiny edge of the epoxy where it meets the grain, resulting in tiny "moats" around each grain which may or may not be bridged by the carbon coat.
john

[AndrewLocock]

Hello,
Yes, petroleum ether consists largely of aliphatic hydrocarbons, especially pentane and isohexane.
Although it would be nice to just use water/alcohol based polishing agents, our prepartion lab continues to use oil-based compounds.
I understand that some labs that work with metallographic samples also use oil during polishing.
Thus the ongoing need for some of us to degrease the samples.

Thanks for the tip about epoxy - also stay away from acetone!
Cheers,
Andrew

[Probeman]

Thanks for the tip about epoxy - also stay away from acetone!

Hi Andrew,
God yes. Acetone is a nuclear bomb at that scale!
john

[Beamsys]

Wondering if anyone tried Novec-7100DL or similar (methoxy-nonafluorobutane) degreasers with SEM or electron probe samples? I've been using it for over a year on high-voltage components (electrostatic lenses, feedthroughs, and connectors) for HV and UHV environments with consistently positive outcomes, finding to be highly effective (subjective opinion). Residue specifications in single-ppm range suggests possible suitability for sample degreasing, while low volatility and low toxicity are also attractive. Being fully aware that "too good to be true usually is" - maybe these degreasers could be of interest for someone to try.

Valery

[crystalgrower]

My experience with petroleum ether is that it is very effective at removing oil without any damage to epoxy or mounted grains. I bought the commercial "hexanes" solvent (the grade commonly used for fat analysis).   Yes you need a fume hood. 

I would never do degreasing in any container other than precleaned dedicated borosilicate glassware.  The beaker can be covered with a petri dish while it is in the ultrasonic bath.  The solvent used to degrease after final polish can be reused once, to degrease drilling oil from mount blanks. 

Hexane can be predried using anhydrous Na2SO4 if one has water-sensitive grains to degrease.  You add some solid anhydrous Na2SO4 to a closed bottle of solvent, and rock  gently for a minute.  The anhydrous Na2SO4 absorbs all trace of water present and does not dissolve.  The "dried" solvent is decanted just before use. 

I have used fluorinated solvents.  I would not choose to use them when any lower cost option exists.  My experience in cleaning ion sources for mass spectrometers is that water in the initial stages might not be a good choice.  In that protocol, it was necessary to use several different solvents to eliminate the azeotropes that would form.  My personal choice for all polishing of stainless steel for UHV would be alumina in "dry" isopropanol followed by hexane.