How To Succeed In The Search For Nothing:
Assuming the surface is free of visible residue (i.e., that the cleaning stage is done), the challenge is now to sample that surface in a reproducible manner so that any (invisible) residues, present in extremely small amounts, are collected and delivered to the instrument for measurement.
The best type of swab for sampling is one with a head made of laundered polyester knit fabric, since that material provides the lowest levels of releasable particles, the highest recovery, and the lowest background when total organic carbon (TOC) measurements are employed as the analytical technique. To sample the surface, the swab is moistened then drawn across the surface in a thorough and reproducible manner to collect any residue into the interstices of the polyester knit fabric. The swab is then deposited into a suitable collection vial, then the residues extracted from the swab head for subsequent anaysis.
There might be a temptation to simply saturate the swab head with high-quality (e.g., TOC-grade) water to do the residue collection. This will cause problems, since the excess liquid on the swab head will simply spread the residue over the surface to be sampled and will not allow the residue to be picked up reproducibly into the swab fabric. For best results, the swab should be damp, but not saturated. This is best accomplished by immersing the head into a container of high-quality water, and pressing both sides of the swab head against the side of the container a few times to expel any air trapped in the fabric and allow the water to fully penetrate the fabric. Then the swab head is raised out of the water and the flat sides of the swab are drawn across the rim of the container to expel excess water and leave the swab head moist. The degree of moistness of the swab head (otherwise known as the percent wetting level) need not be identical from run to run, since residues will be picked up over a fairly wide range of swab head moistness.
The manner in which the swab is used to sample the surface (i.e., the swabbing pattern) is critical to ensure accurate and reproducible collection of residues. For easily accessible surfaces, a template with a 5 cm opening can be used to sample the same surface area each time. As with wiping, linear overlapping strokes over the surface to be sampled will ensure that the residue is collected into the moist swab head. The first side of the first swab is swiped horizontally ten times over the template opening, then the swab is flipped over and the second side is swiped vertically ten times over the same surface. This swab is deposited into the collection vial. The first side of the second swab is swiped diagonally upwards ten times, then flipped over and the second side swiped diagonally downward ten times. The second swab is deposited into the same collection vial. In this manner, the surface has been swabbed a total of 40 times, and there is a reasonable expectation that any residue on the surface has been transferred into the two swab heads. It is not required to use two swabs; often one will do.