Richard Hoyt Lee 1
We usually assume that the compressed gas duster cans we purchase and use are of good quality. But beware of unexpected residue and contamination from additives mixed into the compressed gas used in the cans as the source of the gas for dusting!
I recently experienced serious contamination of a polished metal sample for analysis when I dusted it off with a can of commercial duster just before placing it on the microscope stage. As I focused on the sample, I saw spots of white stain and residue that should not have been there. Removing the sample, I gently huffed it and wiped it clean with a soft lens tissue. Having experienced this once before with a "lowest bidder" quality product (sorry, no brands), I did an experiment to test the amount of residue.
Since most commercial dusters use a liquefied gas, like a chlorofluorocarbon, inverting the can will expel a burst of liquid which will cover an exposed surface with a thick film and then evaporate. Using a clean glass slide for a test substrate for the liquid allows one to quickly see the amount of residue after evaporation. This also works well for simple compressed air dusters, which have no liquid source. Just give the clean glass slide an extra long dose of the propellant.
What is leaving the residue? In the case of liquefied gas dusters, probably a light oil or other form of lubricant is added to the liquid to help it extend the life of refrigeration units, which are the main end use of the product. With compressed air, light oil vapors always contaminate the air from the compressors, unless a special oil trap is used in the line. That is the reason most electron microscopes vent their chambers with compressed nitrogen rather than compressed air which is cheaper. Oil in the air will contaminate the vacuum systems of the electron microscopes. Oil can be trapped out, but it is difficult to remove all of it because most of the filters are just paper or polymer spun fiber types that will eventually become saturated.
Another application of dusters is the cleaning of microscope optics where no residue or deposits are tolerated due to image degradation. The incident that led to this article is the presence of a fiber visible in all the photos taken with a reflected light microscope. Disassembly of the microscope part by part eventually led to the troublesome fiber attached to the polarizing slider.
Electron microscopes are more critical because the vacuum system usually cannot be baked out to remove adsorbed deposits. There is an old "rule of thumb" about fingerprints and vacuum systems; that is, it takes 5 years to eliminate outgassing.
To the right is a photo of a clean glass slide sprayed with a large blast of a low cost and unnamed product. Notice the small droplets of liquid remaining.
FOOTNOTES
1 Argonne National Laboratory, Argonne, IL 60439