In general, contamination of aperture surfaces is normally originated from deposition of organic compounds. The contamination sources are:
        i) Vacuum pump oil used in the pumping system.
        ii) Gaskets.
        iii) O-rings used to seal the vacuum system.
        iv) Greases used to lubricate O-rings.
        v) Specimen itself.

To reduce specimen contamination, viton O-rings and apiezon grease should be employed instead of silicon grease for vacuum seals.