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TOTAL ORGANIC CARBON
Total organic carbon (TOC) is an indirect measure of organic molecules present in pharmaceutical waters measured as carbon. Organic molecules are introduced into the water from the source water, from purification and distribution system materials, and from biofilm growing in the system. TOC can also be used as a process control attribute to monitor the performance of unit operations comprising the purification and distribution system.
A number of acceptable methods exist for analyzing TOC. This chapter does not limit or prevent alternative technologies from being used, but provides guidance on how to qualify these analytical technologies for use as well as guidance on how to interpret instrument results for use as a limit test. The Standard Solution is a theoretically easy-to-oxidize solution that gives an instrument response at the attribute limit. The analytical technology is qualified by challenging the capability of the instrument using a theoretically difficult to oxidize solution in the system suitability portion of the method.
Analytical technologies utilized to measure TOC share the objective of completely oxidizing the organic molecules in an aliquot of sample water to carbon dioxide (CO2), measuring the resultant CO2 levels, and expressing this response as carbon concentration. All technologies must discriminate between the inorganic carbon, which may be present in the water from sources such as dissolved CO2 and bicarbonate, and the CO2 generated from the oxidation of organic molecules in the sample.
Two general approaches are used to measure TOC. One approach determines TOC by subtracting the measured inorganic carbon (IC) from the measured total carbon (TC), which is the sum of organic carbon and inorganic carbon:
TOC = TC – IC.
The other approach first purges the IC from the sample before any carbon measurement is performed. However, this IC purging step also purges some of the organic molecules, which can be retrapped, oxidized to CO2, and quantitated as purgeable organic carbon (POC). The remaining organic matter in the sample is also oxidized to CO2 and quantitated as nonpurgeable organic carbon (NPOC). In this approach, TOC is the sum of POC and NPOC:
TOC = POC + NPOC.
In pharmaceutical waters, the amount of POC is negligible and can be discounted. Therefore, for the purpose of this methodology, NPOC is equivalent to TOC.
Apparatus Requirements— This test method is performed either as an on-line test or as an off-line laboratory test using a calibrated instrument. The suitability of the apparatus must be periodically demonstrated as described below. In addition, it must have a manufacturer's specified limit of detection of 0.05 mg of carbon per L (0.05 ppm of carbon) or lower.
Reagent Water— Use water having a TOC level of not more than 0.10 mg per L. [note—A conductivity requirement may be necessary to ensure method reliability.]
Glassware Preparation— Organic contamination of glassware results in higher TOC values. Therefore, use glassware and sample containers that have been scrupulously cleaned of organic residues. Any method that is effective in removing organic matter can be used (see Cleaning Glass Apparatus
1051). Use Reagent Water for the final rinse.
Standard Solution— Unless otherwise directed in the individual monograph, dissolve in the Reagent Water an accurately weighed quantity of USP Sucrose RS, to obtain a solution having a concentration of about 1.2 mg of sucrose per L (0.50 mg of carbon per liter).
Test Solution— [note—Use extreme caution when obtaining samples for TOC analysis. Water samples can be easily contaminated during the process of sampling and transportation to a testing facility.] Collect the Test Solution in a tight container with minimal head space, and test in a timely manner to minimize the impact of organic contamination from the closure and container.
System Suitability Solution— Dissolve in Reagent Water an accurately weighed quantity of USP 1,4-Benzoquinone RS to obtain a solution having a concentration of 0.75 mg per L (0.50 mg of carbon per liter).
Reagent Water Control— Use a suitable quantity of Reagent Water obtained at the same time as that used in the preparation of the Standard Solution and the System Suitability Solution.
Other Control Solutions— Prepare appropriate reagent blank solutions or other specified solutions needed for establishing the apparatus baseline or for calibration adjustments following the manufacturer's instructions, and run the appropriate blanks to zero the instrument.
System Suitability— Test the Reagent Water Control in the apparatus, and record the response, rw. Repeat the test using the Standard Solution, and record the response, rS. Calculate the corrected Standard Solution response, which is also the limit response, by subtracting the Reagent Water Control response from the response of the Standard Solution. The theoretical limit of 0.50 mg of carbon per L is equal to the corrected Standard Solution response, rS – rw. Test the System Suitability Solution in the apparatus, and record the response, rss. Calculate the corrected System Suitability Solution response by subtracting the Reagent Water Control response from the response of the System Suitability Solution, rss – rw. Calculate the response efficiency for the System Suitability Solution by the formula:
100[(rss – rw) / (rS – rw)].
The system is suitable if the response efficiency is not less than 85% and not more than 115% of the theoretical response.
Procedure— Perform the test on the Test Solution, and record the response, rU. The Test Solution meets the requirements if rU is not more than the limit response, rS – rw. This method also can be performed alternatively using on-line instrumentation that has been appropriately calibrated, standardized, and has demonstrated acceptable system suitability. The acceptability of such on-line instrumentation for quality attribute testing is dependent on its location(s) in the water system. These instrument location(s) and responses must reflect the quality of the water used.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
| Topic/Question | Contact | Expert Committee |
| General Chapter | Gary E. Ritchie, M.Sc.
Scientific Fellow 1-301-816-8353 |
(PW05) Pharmaceutical Waters 05 |
| Reference Standards | Lili Wang, Technical Services Scientist 1-301-816-8129 RSTech@usp.org |
USP32–NF27 Page 239
Pharmacopeial Forum: Volume No. 34(5) Page 1241