Carbon Measurement Methods for Cleaning Validation (Peer Reviewed)

The authors compare direct combustion with rinse and swab sampling methods.
Aug 02, 2012
Volume 36, Issue 8

Cleaning validation provides assurance that the quantity of residual substances collected from equipment surfaces are within permissible limits, helping to ensure quality control and safety in pharmaceutical manufacturing facilities. Three different cleaning validation methods for measuring the carbon in residual samples of various pharmaceutical substances were compared.

The challenges of conducting cleaning validation are documented in the literature. R. Baffi et al., for example, described the diverse analytical challenges arising in validating cleaning procedures for biopharmaceutical products produced by recombinant DNA, in which a broad range of potential residual cellular components and trace levels of detergents must be quantified (1). M.A. Strege et al. described the total organic carbon (TOC) analysis of swab samples for cleaning validation of bioprocess fermentation equipment and discussed accuracy, limits of detection, limit of quantitation, linearity, and precision (2). K.M. Jenkins et al. compared the advantages and disadvantages of multiple methods for cleaning validation, including high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), spectrometry, TOC, and conductivity (3). A.J. Holmes et al. described the TOC method for measuring residual aspirin on aluminum, stainless steel, painted carbon steel, and Plexiglas (4). The latter two authors describe the swab challenge as noted in the FDA guide to inspections of cleaning surfaces.

For cleaning validation using a TOC analyzer, the following types of sampling methods are available:

  • rinse sampling
  • swab sampling with aqueous extraction
  • swab sampling with direct combustion.

These methods were compared using a total organic carbon analyzer (TOC-LCPH, Shimadzu) to measure residual pharmaceutical products and their constituent substances.

Preparation of residue samples

Table I: Substances used for residue measurements.
Residue samples were prepared by applying various types of pharmaceutical products and their constituents to stainless steel pots. Compounds with varying levels of water solubility (i.e., soluble, insoluble, and very insoluble) were evaluated to determine how each method performed. The water-soluble substances were dissolved in water and the water-insoluble substances were dissolved in ethanol or acetone, as shown in Table I. Solution concentrations were adjusted to 2,000 mgC/L (i.e., carbon concentration of 2000 mg/L).

The carbon contents of tranexamic acid (C8H15NO2), anhydrous caffeine (C8H10N4O2), isopropylantipyrine (C14H18N2O), and nifedipine (C17H18N2O6) were estimated by molecular formula. Carbon contents of Gentashin ointment (aminoglycoside antibiotic) and Rinderon ointment (corticosteroid) were determined with the TOC analyzer by adding samples of the ointments directly into a solid-sample combustion unit (SSM-5000A, Shimadzu) since molecular formula for these compounds are unknown.

Each residue sample consisted of a 5-cm2 area on the surface of a pot to which 100 μL of each solution was applied and dried. Thus, there were 200 μg carbon in the sample at each application site.

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