Dye, diethylene glycol, water, and surfactant content in ink

Importance of the Topic

The quality and consistency of ink formulations directly impact print performance, writing smoothness and drying characteristics in ball-point pens. Traditional wet-chemistry assays for determining dye, solvent, surfactant and water content are laborious, time-consuming and require skilled operators. Implementing visible to near-infrared (Vis-NIR) spectroscopy offers a rapid, non-destructive, single-scan approach capable of quantifying multiple ink components simultaneously, thus enhancing quality control throughput in ink production and R&D environments.

Objectives and Study Overview

This study aimed to develop and validate quantitative Vis-NIR spectroscopic methods for measuring four critical ink parameters—dye concentration, diethylene glycol (DEG) content, surfactant level and water fraction—in commercially available blue ball-point pen inks. A total of 20 ink samples, spanning the typical concentration ranges encountered in industry, were used to build predictive multivariate models. External validation was performed on eight independent samples to assess method robustness.

Used Instrumentation

• NIRS DS2500 Analyzer (Metrohm order 2.922.0010) in transflection mode
• Optically flat quartz glass transflection vessel (1 mm path length; Metrohm order 6.7401.000)
• Gold diffuse reflector (Metrohm order 6.7420.000)
• Vision 4.03 Chemometric Software (Metrohm order 6.6069.102)

Methodology

Ink samples were placed in the quartz transflection vessel, covered by the gold reflector, and scanned over selected wavelength ranges. Spectral pretreatments (second derivative, standard normal variate or none) were applied to reduce baseline variation and enhance analyte signals. Partial least squares (PLS) regression models were then developed to correlate Vis-NIR spectra with reference values obtained by established wet-chemistry methods. Model performance metrics included coefficient of determination (R2), standard error of calibration (SEC), standard error of cross-validation (SEV), F-value and prediction residual error sum of squares (PRESS).

Main Results and Discussion

• Dye Content
  • Wavelength range: 420–800 nm
  • Pretreatment: none
  • PLS factors: 2
  • R2=0.9961, SEC=0.0835%, SEV=0.0949%, PRESS=0.171
  Exceptional linearity and low error demonstrate high sensitivity to triphenylmethane and azo dye variations.
• DEG Content
  • Wavelength range: 2220–2300 nm
  • Pretreatment: second derivative + SNV
  • PLS factors: 2
  • R2=0.9934, SEC=0.5037%, SEV=0.5888%, PRESS=6.5865
  Model effectively captures O–H and C–H overtone features of diethylene glycol despite potential overlapping water signals.
• Surfactant Content
  • Wavelength range: 1350–2350 nm
  • Pretreatment: second derivative
  • PLS factors: 5
  • R2=0.9774, SEC=0.0368%, SEV=0.1316%, PRESS=0.2772
  Despite low overall concentration (0–1%), the method reliably quantifies surfactant through broad CH2/CH3 absorption bands.
• Water Content
  • Wavelength range: 1300–1550 nm
  • Pretreatment: second derivative
  • PLS factors: 3
  • R2=0.9909, SEC=0.5571%, SEV=0.9614%, PRESS=17.5618
  Strong water overtone signals allow accurate determination over the typical 63.6–82.1% range.

External validation on eight independent samples confirmed prediction errors within acceptable limits for all four parameters, demonstrating method robustness and transferability.

Benefits and Practical Applications

• Single-scan multi-parameter quantification reduces analysis time from hours to minutes.
• Non-destructive measurement preserves sample for further testing.
• Minimal user training required; ideal for at-line quality control.
• Eliminates hazardous reagents and waste associated with wet-chemistry methods.

Future Trends and Opportunities

Advances in miniaturized NIR spectrometers, combined with handheld or online implementations, will enable real-time monitoring of ink production. Integration with automated sampling systems and cloud-based chemometric updates can further streamline process analytical technology (PAT) workflows. Expanding spectral libraries to include a wider variety of ink colors and additives will enhance method versatility across the printing industry.

Conclusion

Vis-NIR spectroscopy, coupled with robust chemometric modeling, provides a rapid, accurate and user-friendly alternative to traditional wet-chemistry assays for ink analysis. The developed models demonstrate excellent performance across key formulation parameters, supporting efficient quality control and process optimization in ink manufacturing.

References

• No external references were cited in the source document.