Yet, the US Food and Drug Administration's Guide to Inspection of Validation of Cleaning Processes (3) and other literature (4) discount using a visible limit as the sole acceptance criterion for cleanliness. Conversely, Mendenhall concluded that visible cleanliness criteria were more rigid than quantitative calculations and are clearly adequate for determining cleanliness (5). Other recent articles describe the justification and application of a visible-residue limit (VRL) (6, 7).

Many research teams have established quantitative VRL levels. Fourman and Mullen determined a visible limit of ~100 μg per 2 X 2-in. swab area (8) or ~4 μg/cm² . Jenkins and Vanderwielen observed residues as low as 1.0 μg/cm² with a light source (9). Forsyth et al. determined <0.4- to >10-μg/cm² VRLs for active pharmaceutical ingredients (APIs) and excipients (6).

Visible-residue parameters

The following VRL variables were evaluated: formulation composition, surface material, background, solvent effects, light intensity, light angle, viewing distance, viewing angle, and observer subjectivity. These factors were expanded from a previous study (6).

Stainless steel was chosen for the surface material because most manufacturing equipment surfaces are made of this material. Representative stainless steel coupons (304 grade, 2B finish) were used for spotting purposes in the laboratory setting. Larger equipment size and increased viewing distance cause reflected and ambient light that affect visibility in the manufacturing facility. To simulate these conditions, a background of stainless steel was used when viewing the spotted coupons.

The type of solvent and its solubility were also factors that affected appearance. Methanol was chosen as the solvent for the sample preparations because it leaves no residue, minimizes residual sample rings, and provides adequate solubility for most substances tested. When solubility was not achieved, samples were immediately spotted using a suspension in methanol. The common solvent allowed for a tighter control over spot sizes and concentration. The sample volume was varied for concentration differences. This procedure obviated the need for serial sample dilutions, thereby saving time and solvent consumption. A complementary volume of methanol was added to each sample to achieve a constant spot volume.