This extraction replication technique is especially useful when the surface is rough and soft at the same time. The collodion film will separate the particles lightly held in crevices without removing any of the substrate as follows:

Step 1.  Two drops of the collodion solution are picked up on a tungsten needle and spread over about 10-20 mm² of the contaminated surface. More drops may be used to cover a larger area. A third drop may be necessary to obtain a final film thickness of about 20 µm. Thinner films are fragile and harder to remove. The solvent should evaporate in 5-15 minutes at room temperature. The preparation can be dried faster in an oven at about 60-80°C (see Figure 2).

Step 2.  The dry collodion film with the particles is pulled off the substrate and placed, particle side down, on a glass slide for analysis by polarized light microscopy. The contamination particles may be hard to characterize if, for example, the substrate was a roughly machined part. The replica of the machining marks may interfere with the microscopical characterization (see Figure 3).

Step 3.  A 3-5 nL drop of amyl acetate, delivered from a micropipette, is brought in with a tungsten needle and allowed to flow halfway under the collodion film. This area of film is referred to as the “cleared replica.” The particles in the cleared portion of the 20 µm film of collodion are clearly visible with a polarized light microscope. The film acts like a refractive index liquid with n_D=1.51. The other half of the film is used for comparison (see Figure 4).

Analysis of the Contaminants by SEM/EDS

To obtain the elemental compositions of the contaminants, cut out a square of the replica, measuring ~0.2 mm, transfer it to a polished carbon or beryllium substrate, and dissolve the collodion by quickly passing small drops of amyl acetate over the square. Allow each drop to dry before passing another drop over the replica (see Figure 5).