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User Benefits Application News Instrument: TOC-L total organic carbon analyzer Oxidation Method: 680 °C combustion catalytic oxidation Catalyst: High sensitivity catalyst Measurement Items: NPOC (TOC by acidification and sparging) TOC (TOC by the TC-IC method) Calibration Curves: NPOC and TC: 2-point calibration curves based on potassium hydrogen phthalate aqueous solutions at 0 and 5 mgC/L IC: 2-point calibration curve based on sodium bicarbonate/sodium carbonate aqueous solutions at 0 and 5 mgC/L Injection Volume: 200 μL Sample: Commercial sodium dodecylbenzenesulfonate reagent was dissolved in pure water Samples and Analysis Conditions Four sodium dodecylbenzenesulfonate solutions were prepared as surfactant samples, at TOC concentrations of 0.5, 1, 2, and 3 mg/L, respectively (Table 1). TOC measurements of these samples were conducted under the conditions indicated in Table 2. The following two measurement methods were used. (1) TC-IC method: TC (total carbon) and IC (inorganic carbon) are measured separately, and then TOC is calculated as their difference (TOC = TC – IC). (2) NPOC method: TOC is measured as non-purgeable organic carbon (NPOC). After acidifying the sample to a pH < 3, IC is subsequently removed by sparging and the remaining carbon is measured as TC, which is regarded as TOC. The acidification and sparging steps are performed automatically by the instrument. Table 1 Samples Table 2 TOC Analysis Conditions Introduction Surfactants are important chemicals widely used in detergents and dispersants for pharmaceutical equipment cleaning processes, chemical manufacturing processes, and environmental water management. If surfactant residues remain after such processes, they can not only affect manufacturing processes and final product quality, but also increase the organic load in wastewater and environmental water. Therefore, quality control is also needed for organic matter derived from surfactants. Total organic carbon (TOC) analysis is effective for evaluating the quality of surfactants. In general, the NPOC (non-purgeable organic carbon) method is often selected for TOC measurement in terms of sample volume and analysis time requirements. On the other hand, because surfactants have both hydrophilic and hydrophobic groups in their structure, they tend to foam even at low concentrations. When such samples are measured by the NPOC method, which involves acid addition and sparging, foaming can cause the sample to overflow from the syringe vessel inside the instrument into the waste line. That can result in measurement values that are lower than the true value. This article compares TOC measurement results for surfactants obtained by the NPOC method and the TC-IC method using the Shimadzu TOC-L total organic carbon analyzer, which uses the combustion oxidation method. TOC can be evaluated accurately using the TC-IC method, even for foaming surfactants. The ASI-L autosampler enables automatic, continuous measurement of multiple samples, improving analytical efficiency. TOC Evaluation of Surfactants Arihiro Narita Total Organic Carbon Analyzer Sample Added Carbon Concentration (mg/L) in Sodium Dodecylbenzenesulfonate 1 0.5 2 1 3 2 4 3 Calibration Curves The TOC analyzer was calibrated using 2-point calibration curves for potassium hydrogen phthalate at 0 and 5 mgC/L and 2-point calibration curves for sodium bicarbonate/sodium carbonate aqueous solutions at 0 and 5 mgC/L. The potassium hydrogen phthalate calibration curve was used for NPOC and TC measurements and the sodium bicarbonate/sodium carbonate calibration curve was used for IC measurement. The calibration curves are shown in Figs. 1 (a) to (c). Fig. 1 (a) Calibration Curve Data (NPOC Measurement) Fig. 1 (b) Calibration Curve Data (TC Measurement) Fig. 1 (c) Calibration Curve Data (IC Measurement)
Application News Sample NPOC conc. (mg/L) Recovery (%) 1 0.2114 42.3 2 0.4796 48.0 3 0.8688 43.4 4 1.386 46.2 Sample TC conc. (mg/L) IC conc. (mg/L) TOC conc. (mg/L) Recovery (%) 1 0.5127 0.01482 0.4979 99.6 2 1.022 0.02819 0.9941 99.4 3 2.073 0.06437 2.008 100.4 4 3.062 0.02981 3.033 101.1 Measurement Results 1. TOC Measurement by the NPOC Method Table 3 indicates the results obtained when the sodium dodecylbenzenesulfonate solutions were measured by the NPOC method. The TOC recovery rates were below 50 % for all samples. The measurement data for each sample are indicated in Figs. 2 (a) to (d). Table 3 Summary of Measurement Results by the NPOC Method 2. TOC Measurement by the TC-IC Method Table 4 shows the results obtained when the sodium dodecylbenzenesulfonate solutions were measured by the TC- IC method. The TOC recovery rates close to 100 % were obtained for all samples. The measurement data for each sample are shown in Figs. 3 (a) to (h). Table 4 Summary of Measurement Results by the TC-IC Method Fig. 2 (b) Sample 2 (1 mgC/L, NPOC Measurement) Fig. 2 (a) Sample 1 (0.5 mgC/L, NPOC Measurement) Fig. 2 (c) Sample 3 (2 mgC/L, NPOC Measurement) Fig. 2 (d) Sample 4 (3 mgC/L, NPOC Measurement) Fig. 3 (a) Sample 1 (0.5 mgC/L, TC Measurement) Fig. 3 (b) Sample 1 (0.5 mgC/L, IC Measurement) Fig. 3 (c) Sample 2 (1 mgC/L, TC Measurement) Fig. 3 (d) Sample 2 (1 mgC/L, IC Measurement)
Application News www.shimadzu.com/an/ Shimadzu Corporation © Shimadzu Corporation, 2026 For Research Use Only. Not for use in diagnostic procedures. This publication may contain references to products that are not available in your country. Please contact us to check the availability of these products in your country. The content of this publication shall not be reproduced, altered or sold for any commercial purpose without the written approval of Shimadzu. See https://www.shimadzu.com/about/trademarks/index.html for details. Third party trademarks and trade names may be used in this publication to refer to either the entities or their products/services, whether or not they are used with trademark symbol “TM” or “”. Shimadzu disclaims any proprietary interest in trademarks and trade names other than its own. The information contained herein is provided to you "as is" without warranty of any kind including without limitation warranties as to its accuracy or completeness. Shimadzu does not assume any responsibility or liability for any damage, whether direct or indirect, relating to the use of this publication. This publication is based upon the information available to Shimadzu on or before the date of publication, and subject to change without notice. First Edition: Jun. 2026 01-01160-EN Conclusion Table 5 summarizes the results obtained by measuring sodium dodecylbenzenesulfonate solutions, used here as representative surfactant samples, by the NPOC method and the TC-IC method. With the NPOC method, the TOC recovery rates were below 50 % for all samples, whereas the TC-IC method produced satisfactory TOC recovery rates close to 100 % for all samples. Fig. 3 (e) Sample 3 (2 mgC/L, TC Measurement) Fig. 3 (f) Sample 3 (2 mgC/L, IC Measurement) Fig. 3 (g) Sample 4 (3 mgC/L, TC Measurement) Fig. 3 (h) Sample 4 (3 mgC/L, IC Measurement) Carbon conc. in sample NPOC method TC-IC method Conc. (mg/L) Recovery (%) Conc. (mg/L) Recovery (%) 0.5 mg/L 0.2114 42.3 0.4979 99.6 1 mg/L 0.4796 48.0 0.9941 99.4 2 mg/L 0.8688 43.4 2.008 100.4 3 mg/L 1.386 46.2 3.033 101.1 Table 5 Summary of Measurement Results Sodium dodecylbenzenesulfonate solutions (carbon concentration: 0.5 mg/L) is also used in Japanese Pharmacopoeia (JP) recovery tests . For surfactants that readily foam, such as in this example, NPOC measurements involving acid addition and sparging may affect measurement accuracy due to sample loss. For such samples, accurate TOC values can be obtained by using the TC-IC method. In addition, by using the ASI-L autosampler, multiple samples can be measured automatically and continuously.
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