Determining phosphate concentration with Raman spectroscopy

Significance of the Topic

Phosphates are critical inorganic ions in agricultural, biological and industrial processes. Accurate and timely determination of phosphate concentration supports fertilizer production, environmental monitoring and industrial quality control. Traditional wet chemical and chromatographic techniques involve reagents, lengthy sample preparation and generate chemical waste. Raman spectroscopy offers a reagent-free, rapid alternative capable of routine or real-time analysis without extensive sample handling.

Study Objectives and Overview

This study evaluates the performance of handheld Raman spectroscopy combined with Partial Least Squares (PLS) modeling to quantify total phosphate concentration in phosphoric acid solutions. By preparing serial dilutions from a concentrated industrial sample and applying PLS regression to Raman spectra, the work aims to demonstrate accuracy, speed and operational simplicity compared to conventional methods.

Methodology

• Sample Preparation: A 68 % phosphoric acid production sample was serially diluted to concentrations between 0.14 % and 28 % dihydrogen phosphate.
• Measurement Parameters: 1 mL aliquots in glass vials were measured with a MIRA XTR handheld Raman spectrometer at 785 nm excitation and 50 mW laser power. Each spectrum was acquired over 6 seconds and averaged over five replicates.
• Data Analysis: Spectral data focusing on the 850–950 cm−1 phosphate band were processed in Vision software. A PLS calibration model correlated Raman peak intensity and frequency shifts with known phosphate concentrations.

Main Results and Discussion

Raman spectra revealed a concentration-dependent shift of the phosphate band from 890 cm−1 at higher concentrations to 897 cm−1 at lower levels. The PLS model achieved an R2 value exceeding 0.99 and low standard error, indicating excellent linearity and predictive precision. Validation with five independent samples yielded relative errors below 2.5 % across the tested range. The method demonstrated a limit of detection of 0.14 % phosphate, with scope for further improvement via increased integration time or immersion probe sampling.

Benefits and Practical Applications

• Reagent-Free Analysis: Eliminates colorimetric reagents and associated waste.
• Minimal Sample Preparation: Direct measurement of industrial samples.
• Rapid Results: Total analysis time under one minute per sample.
• Non-Destructive: Leaves samples intact for further use or analysis.
• Wide Dynamic Range: Accurate quantification from 0.14 % to 28 % phosphate.

Future Trends and Potential Applications

• Enhancing Sensitivity: Increasing laser power or using immersion probes to lower detection limits.
• Inline Process Monitoring: Integration into production lines for real-time quality control.
• Broader Speciation: Extending the technique to distinguish multiple phosphate species and related ions.
• Data-Driven Optimization: Applying advanced chemometric approaches for improved calibration robustness.

Conclusion

Handheld Raman spectroscopy combined with PLS modeling provides a reliable, fast and sustainable approach for phosphate quantification in industrial and environmental samples. Its minimal reagent use, rapid turnaround and high accuracy position it as a compelling alternative to traditional wet chemical and chromatographic methods.

Used Instrumentation

• MIRA XTR Basic handheld Raman spectrometer with 785 nm laser and Orbital Raster Scanning.
• Vial Holder Attachment for glass vial measurements.
• Vision software for spectral processing and PLS model development.

References

1. Reimagining pH Measurement: Utilizing Raman Spectroscopy for Enhanced Accuracy in Phosphoric Acid Systems. Analytical Chemistry, ACS Publications.