A valve in a critical-point drier regulating the flow of liquid carbon dioxide was found to be the source of minute metal particles which contaminated the fragments resulting from freeze-fracturing of food samples.
Larocque, Gisele and Yang, A. F.
"Critical Point Drier as a Source of Contamination in Food Samples Prepared for Scanning Electron Microscopy,"
Food Structure: Vol. 12
, Article 9.
Available at: http://digitalcommons.usu.edu/foodmicrostructure/vol12/iss1/9