A TOC analysis is quantitative but not qualitative. In other words, TOC does not identify or distinguish among different compounds containing oxidizable carbon. Therefore, a manufacturer should limit the amount of background carbon (carbon from sources other than the target compounds) as much as possible. Established limits for target compounds must be corrected for background carbon.

An advantage of TOC is that a level has been established for purified water which represents a good target level for residual analysis. The United States and European Pharmacopeias require purified water and water for injection (WFI) to contain no more than 500 ppb of TOC. This represents a defensible standard level because it would be difficult for a regulatory agency to justify a cleaning level for a device which is lower than that required for purified water. By coupling TOC analysis with a conductivity and pH analysis, also required for purified water and WFI testing, device extracts may also be analyzed for ionized compounds such as acids, bases, or salts.

Gravimetric analysis

Another method used to validate cleaning procedures is a gravimetric analysis based on ASTM F2459. The procedure extracts and quantifies residual manufacturing materials on medical devices. One advantage of the method is that extraction solvents other than purified water may be used, allowing for detection of nonwater-soluble contaminants. However, this gravimetric method excludes residues more volatile than the extraction solvent.

The extraction solvent is chosen based on the solubility of the target residues and characteristics of the device materials. Several solvents may be required if more than one type of residue is present on the devices. Devices may be pooled for the analysis to increase the method's sensitivity, but there is an increased risk of adding non-soluble debris created through friction between devices during the extraction.

The devices are sonically agitated for a specified time and temperature. The extraction times and temperatures must be validated with each study based on the target residues. To meet criteria specified in ASTM F2459, extraction parameters must be adjusted for an extraction efficiency greater than 75%. After extraction, devices are rinsed and removed, and the extract reduced. The concentrated extract is then transferred to a crucible of known weight where it is evaporated. The crucible is reweighed. Theweight difference represents the amount of extractable soluble and insoluble residue from the device.

If the test quantifies a significant extractable residue, it may also be identified by infrared spectroscopy. A general analysis or interpretation of the sample spectrum can reveal the presence of certain compounds such as hydrocarbons and amines. This identification may also be made by comparing the sample spectrum to the spectra of target compounds.

Another option with this method is to separate the extractable residue into soluble and insoluble portions. The residue is dissolved in the appropriate extraction solvent and the solution filtered. The soluble and insoluble residues are then calculated based on the weight change of the filter. This information is invaluable if non-soluble debris is a primary concern.

Detergent residual analysis

A third method to validate cleaning operations focuses on compounds that absorb ultraviolet and visible light. It is most often used to detect detergents. Residuals are extracted from the devices in a known volume of purified water and the extracts are analyzed using a UV/VIS spectrophotometer. As with TOC, the extraction ratio must be controlled for accurate analytical results. If too much water is used, the residue may not be detectable even though present. If too little water is used, the residue may not be adequately removed from the device.

Linear regression and a standard curve are used to calculate the concentration of the target compound. Because each compound responds differently, each analyte must be validated for accuracy, precision, ruggedness, limit of detection, and limit of quantitation. A sample of the pure detergent must be available to perform the validation. Of course, some compounds are unsuitable for analysis using this method due to insufficient response to ultraviolet/visible light.

The method is also quantitative but not qualitative. If several extracted contaminants absorb ultraviolet/visible light, there is no way to distinguish one from another. Concentrations are calculated as worst-cases, assuming all resulting absorbance is due to a single contaminant. This method does not identify the absorbing substances in the extract.

The procedure has been used over 10 years, and many common detergents have been validated as suitable for the method. The “real life” positive controls for this method are devices which have been cleaned using the target compound but not rinsed. These devices must be cleaned beforehand to reduce background interferences.

Quantifying the residuals from manufacturing is crucial when validating a cleaning procedure for newly manufactured medical devices. The TOC, gravimetric, and detergent analyses are all practical techniques that may be used during the validation process.

For further reading

ASTM F2459-05. 2005, Standard Test Method for Extracting Residue from Metallic Medical Components and Quantifying via Gravimetric Analysis, ASTM International. West Conshohocken, PA.

LeBlanc, Destin A, Human Drug CGMP Notes Excerpt: 1st Quarter 2002, www.cleaningvalidations.com.

LeBlanc, Destin A, Cleaning Memo for July 2005: Revisiting Cleaning Validation for Medical Devices, www.cleaningvalidations.com.

Nelson Laboratories Inc. 2008. STP0028, Total Organic Carbon: Wet Oxidation Method. Nelson Laboratories Inc. 2008. STP0134 Quantification of Extractable Residue by Gravimetric Analysis.

Nelson Laboratories Inc. 2008, STP0023 Ultraviolet/Visible Spectroscopy HP 8453.