Processing tanks may display residues on the tank surfaces after routine cleaning, typically appearing as faint whitish or dark stains that cannot be easily wiped off. In order to remove the residue without damaging the tank surface, it is necessary to identify the residue so that the correct solvent or cleaning method can be used. However, it can be challenging to obtain a large enough sample for analysis, due to the small amount of residue. For unambiguous identification of the residue, the sampling method should allow for complete removal of the residue from the sampling tool prior to analysis, without any attached material from the sampling tool.

Traditionally, white cotton or polyester swabs have been used as sampling tools, but they are not ideal because only a small amount of residue is transferred to the swab, the sample is difficult to remove from the swab for analysis, and light-colored residues cannot be seen on the white swabs. The purpose of this study was to investigate alternative sampling methods that would fulfill three objectives: 1) obtain a larger volume of residue than the swab method; 2) the residue should be easily removable from the sampling tool for micro-analysis; and 3) the sampling tool should not damage the tank surface.

Several traditional and non-traditional scraping tools were compared for their ability to fulfill our objectives:

  • Cotton and polyester swabs
  • Double-edge razor blade
  • Polypropylene spatula
  • Plastic polystyrene knife

Our "tank surfaces" consisted of a flat stainless steel sheet with a thin spotty coating of rust, and a stainless steel vessel having a faint coating of white-colored hard water deposits. The stained surfaces were vigorously rubbed with the dry swabs, or moderately to lightly scraped with the other sampling tools while holding each tool at approximately 45 degrees to the surface. The sampling tools were examined under a stereomicroscope, and a tungsten needle was used to transfer some of the residue to a glass slide. The transferred residue was examined under a polarizing light microscope at 500X magnification to see whether particles from the sampling tool had contaminated the residue.

Cotton or polyester swabs that were used to sample the rust-colored surface displayed a visible residue of fine brown particles trapped among the swab fibers. The brown particles were very difficult to remove from the swab without removing some cotton at the same time. When swabs were used to sample light-colored residue, no residue was visible on the swab with the naked eye. Under the microscope, a faint residue was seen on the swab, but could not be easily removed from the fibers if it was not clearly visible in the microscope. Swabs are most useful for heavy deposits of dark-colored residues.

The blade was held at about a 45-degree angle, and the tank surface was gently scraped. Light-colored and dark-colored residues were easily seen on the blade, and could be easily removed for analysis. The blade may scratch some tank surfaces, and fine metal wear particles can be dislodged from the blade and mixed with the sample, making it difficult to identify residues that consist of metal corrosion. The razor blade is most useful for hard or soft non-metallic residues.

The edge of the spatula was used to scrape the tank surface to remove the residue. Using a dark blue spatula, light-colored residues were clearly visible. For dark-colored residues, a light-colored spatula should be used. However, polypropylene is a somewhat soft material, and rolls of plastic form along the edge of the spatula with the particles of interest trapped within. The residue cannot be easily separated from the loose plastic filaments for analysis. Scraping tools made from soft plastics such as polypropylene are best suited for soft waxy residues that do not require hard scraping of the surfaces.

The knife was a standard disposable plastic knife available at any grocery store. We removed the handle from the knife, and held the blade at a 45-degree angle while the teeth were scraped over the tank surface. A large amount of residue was collected in the teeth of the knife, and additional loose residue clung to the surface of the blade adjacent to the teeth. The loose residue on the blade was easily removed for analysis. The residue on the teeth contained a few scattered fine polystyrene particles that were abraded during the scraping process. Polystyrene is readily soluble in many common solvents and can be easily dissolved if knife debris is suspected of contaminating the residue—leaving the residue intact and able to be re-tested.

The best scraping tool for process tank residues depends on the type of residue to be sampled. This brief study focused on hard residues on metal surfaces, as these are the most challenging residues for sampling. If the residue consists only of metal corrosion, any of these sampling methods, except the razor blade, are suitable. The polystyrene knives were the best sampling tools for both hard and soft tank residues in this study, combining ease of sample removal and minimal contamination from the sampling tool, without damaging the tank surface. The knives can be obtained in different colors to contrast with the color of the tank residue, enhancing the visibility of small amounts of sample. The knife, or any of the other sampling tools, can be attached to a glass slide with tape, and inserted into a standard 50mL plastic tube with a threaded cap for shipping or safe transport to the lab for analysis. The standard glass slide fits firmly into such a tube, and will not move during shipping.

Mary Stellmack is a senior research chemist at McCrone Associates, Inc., the analytical services division of The McCrone Group. Stellmack co-teaches a course in Infrared Microscopy at Hooke College of Applied Sciences.