Swab-sampling with direct-combustion method
Swab sampling with direct combustion consists of wiping the inside surface of the production apparatus with a piece of quartz
filter-paper swab material, and then conducting measurement using a direct-combustion carbon-measurement system. The swab
material with adhering residue is measured directly (i.e., without first extracting with water) in a TOC analyser using a
connected solid-sample combustion unit or module (SSM).
Figure 3: Total organic carbon (TOC) concentrations for (a) tranexamic acid, (b) isopropylantirine and (c) Gentashin ointment
using swab sampling with direct combustion.
To evaluate the rate of recovery of the different types of substances using this method, paper swab material (45-mm diameter
Advantec quartz glass paper QR-100, heat treated at 600 °C for 15 min) was used to wipe the sample adhering to the stainless
steel pot and placed in the sample boat, which is then placed in the SSM (SSM-5000A, Shimadzu) connected to the TOC analyser
(TOC-LCPH, Shimadzu). Three replicates of each sample were run. The SSM uses 400 mL/min oxygen as a carrier gas. The calibration
curve is a 1-point calibration using 1% C glucose aqueous solution. The total carbon (TC) content on the swab was measured
directly by the TOC analyser. Selected measurement data are shown in Figure 3.
Table IV: Measurements using swab sampling with direct combustion.
Since the carbon content in each of the residue measurement samples is 200 µg, the TC value would be 200 µg if all of the
sample were wiped off. For the blank, measurement was conducted in the same way by wiping the stainless pot, which had no
sample applied. The measured blank value was subtracted from each TC value, and then divided by the theoretical value of 200
µg using Equation 1 to determine the rate of recovery. The results are shown in Table IV. A high recovery rate of about 100% was obtained for all the substances, regardless of whether they were water soluble or
Table V: Summary of measurement results.
The measurement methods used here and their respective recovery rates are summarised in Table V. When using the rinse- and swab-sampling methods, some of the water-insoluble substances had high recovery rates while others
had low recovery rates. It is thought that this may be due to differences in the affinity with which the substances adhere
to the stainless steel pot. Accordingly, it is possible that residue evaluation using these methods would be difficult for
substances with low recovery rates.
In contrast, high recovery rates were obtained for all the substances when using the swab sampling with direct-combustion
method, regardless of whether the substances were water soluble or water insoluble. Therefore, this method is considered to
be the most versatile measurement method for conducting cleaning validation, especially when multiple compounds are being
manufactured in the same vat, if the compounds are unknown, or if there is a possibility the known compounds will decompose
into other compounds.
Robert Clifford*, PhD, is industrial business unit manager at Shimadzu Scientific Instruments, 7102 Riverwood Drive, Columbia, MD 21046, tel. 800.447.1227,
. Minako Tanaka is a scientist at Shimadzu Applications Development Center, Kyoto, Japan, firstname.lastname@example.org
*To whom all correspondence should be addressed.
Submitted: 17 Nov. 2011. Accepted: 12 March 2012.
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