Purity, degree of substitution (DS), and moisture content of carboxymethyl cellulose (CMC)

Significance of the Topic

Carboxymethyl cellulose (CMC) is a versatile cellulose ether widely used in pulp and paper, pharmaceuticals, cosmetics and paints due to its binding, thickening and disintegrant properties. Quality attributes such as purity, degree of substitution (DS) and moisture content strongly influence its performance. Traditional laboratory assays are time-consuming, creating a demand for faster, simultaneous analytical methods.

Objectives and Study Overview

The goal of this application note is to establish visible-near infrared (Vis-NIR) spectroscopy as an alternative to conventional methods for CMC quality control. The study focuses on developing calibration models to quantify:

• Purity (97–100 %)
• Degree of substitution (DS, 0.5–0.9)
• Moisture content (2–7 %)

Sample sets representing typical production variability were analyzed and used for external model validation.

Instrumentation Used

• NIRS DS2500 Analyzer (reflection measurement)
• NIRS DS2500 Iris sample adapter
• Vision 4.03 chemometric software

Methodology

Vis-NIR spectra were acquired from 400 to 2500 nm with the NIRS DS2500 system. Samples in glass vials were centered by the Iris adapter and scanned in reflectance mode. Spectral pretreatment involved a second derivative (segment size 10 nm, gap size 0 nm). Partial least squares (PLS) regression models were developed for each parameter and assessed via external validation.

Main Results and Discussion

Purity: A PLS model with three factors covering 420–1080 nm and 1120–2480 nm achieved R2 = 0.9836 and SEP = 0.1054. External validation deviations remained within ±0.04 %.

Degree of Substitution (DS): A five-factor PLS model over the same wavelength ranges yielded R2 = 0.9817 and SEP = 0.0147, with prediction errors under ±0.02 DS units.

Moisture Content: The three-factor PLS model produced R2 = 0.8401 and SEP = 0.4992. External comparisons showed deviations within ±0.2 % moisture.

Benefits and Practical Applications

• Rapid simultaneous analysis of multiple quality parameters in a single measurement
• Minimal sample preparation and user training
• High throughput suitable for QA/QC in regulated industries
• Significant time savings compared to ASTM D1347 and D1439 methods

Future Trends and Opportunities

Future developments may include inline process integration, advanced chemometric algorithms for enhanced accuracy, portable NIR sensors for at-line analysis, and cloud-based models for real-time quality monitoring.

Conclusion

Vis-NIR spectroscopy has been validated as a fast, reliable alternative for quantifying CMC purity, DS and moisture content. The PLS models demonstrate strong correlation with standard laboratory methods, supporting their implementation in industrial quality control.

References

• ASTM D1347 – Standard Test Methods for Water in Cellulose Derivatives
• ASTM D1439 – Standard Test Method for Degree of Substitution of Cellulose Ethers by Titration