4.1 Sample Preparation
For the Karl Fischer titration method utilizing an oven, the critical parameters during sample preparation are use of a dry box, proper evacuation of moisture from the sample vials prior to utilization, consistent sample cutting technique with
capability to cut small samples, and immediate sealing of samples into sample vials. The sample preparation takes only 5-‐10 minutes once the sample vials have been dried, but due to the elasticity and flexibility of the rubber, obtaining a consistent shape and small sized stopper sample may be difficult.
For the Karl Fischer titration method utilizing THF extraction, the critical parameters are the type, composition, and dryness of the caps, cap liners, and bottles and the method of THF aliquoting. Seemingly minor variations such as the use of septum caps versus non-‐septum caps during the extraction or the use of a funnel and graduated cylinder versus a glass pipette for THF aliquoting can have significant implications on the measured water content. The stoppers are tested whole, therefore manipulation of the sample is minimal, but a 24 hour extraction time must occur prior to titration initiation.
Both the THF extraction method and the oven method utilize a Karl Fischer coulometric titrator for water content analysis with a Hydranal AG analytical solution containing the necessary Karl Fischer reaction components. They also involve the use of a dry box either for sample preparation or analysis. The THF extraction method utilizes PTFE-‐lined non-‐septum caps and 125-‐mL bottles that are
reusable as long as they are properly washed and dried and preferably dedicated to the THF extraction test method. In addition, tetrahydrofuran (THF) must be used for the extraction process and a calibrated 250-‐μL air-‐tight syringe delivers the sample aliquot injections. The oven method, on the other hand, uses disposable PTFE-‐backed silicone septa caps and 6-‐mL KF sample vials, and a capper that hermetically seals the vials. More critically, the oven method requires the use of a specialized oven with a specific needle through which a dry carrier gas is passed. The oven must have the proper components to join with the Karl Fischer
coulometric titrator and may be expensive, especially if an autosampler is desired.
At the time of analysis, the Karl Fischer titration method with oven requires little analyst intervention. Once the vial has been placed into the oven and the needle inserted, the analyst must only press start and the titration is performed. As it is through a direct connection between the oven and the Karl Fischer titration cell that the stopper moisture is introduced, sample addition variation and titration cell changes are minimized. However, this titration may take thirty minutes or greater depending on the sample and test parameters chosen.
The Karl Fischer titration method with THF extraction requires more manipulations and analyst input at the time of analysis. Analyst must swirl the titration cell prior to sample addition, wait for the drift to stabilize, and then inject the sample aliquots into the center of the titration cell. Analyst cannot swirl the titration cell after sample aliquoting or large injection-‐to-‐injection variation is recorded. This analytical method is therefore more sensitive to analyst technique and analyst-‐to-‐
analyst variation. Three aliquots must be injected per sample and the total analysis time is typically not more than fifteen minutes. Due to the sensitivity of the test method to septum cap moisture, the sample aliquots cannot be made through a septum, but instead the sample lid must be removed. This analytical method
requires greater manipulation by the analyst and exposure to THF, which is volatile and flammable. However, all sample aliquoting is performed in a dry box, which lowers the safety concerns.
The calculations for both the Karl Fischer titration method with oven and the Karl Fischer titration method with THF extraction are comparable. Each test method’s calculations take into account the testing environment by subtracting out a blank and results are obtained in milligrams (mg) water per stopper. The oven method also calculates the percentage of water per stopper based on the initial weight of the entire stopper.
4.5 Test Method Development
The Karl Fischer titration method with oven must be specialized to each type and composition of stopper in terms of the sample shape, size, oven temperature and carrier gas flow rate. Small variations to any of these parameters will impact the moisture content results measured. In particular, experiment trials found that the diffusion path length of the water from the interior of the stopper sample to the surface is the rate-‐limiting step in identifying an accurate water content of the sample. Therefore test parameters must be identified for each type and
composition of stopper that will optimize the diffusion path length. This fact makes this analytical method more time consuming during development activities and could make it more difficult to validate.
For the Karl Fischer titration method with THF extraction, the testing parameters are not as specialized to the stopper characteristics, therefore the development time is shorter and less complex for a new stopper. The stopper is tested whole, so sample size and shape need not be evaluated. In addition, the THF extraction method’s main variables are the THF extraction time and the extraction volume. As a large percentage of the stopper’s water is extracted within the first six hours of extraction based on the performed trials, even if a new stopper required greater than six hours to remove most of the stopper’s moisture, the 24-‐hour extraction time is likely to encompass this increased time. In addition, the THF extraction volume may need to be modified based on the size or theoretical water content of the stopper. However, this change is addressed in the calculations performed and does not largely modify the test method, therefore extensive development and revalidation is not necessary.