Determining phosphate concentration with Raman spectroscopy

Importance of the Topic

Phosphates are essential across agriculture, biology, and industry, serving roles from fertilizer components to corrosion inhibitors. Traditional wet chemical and chromatographic methods for phosphate determination involve reagents, lengthy preparation, and generate chemical waste. Raman spectroscopy offers a reagent-free, nondestructive alternative, delivering rapid, accurate phosphate quantification suited for real-time process monitoring.

Objectives and Overview of the Study

This study evaluates the feasibility of using handheld Raman spectroscopy, combined with Partial Least Squares (PLS) modeling, to rapidly and quantitatively determine total phosphate concentration in phosphoric acid samples. The goal is to compare speed, accuracy, and operational simplicity against standard wet chemical and ion chromatography methods.

Methodology and Instrumentation

Samples were prepared by serial dilution of a 68% phosphoric acid production sample to concentrations between 0.14% and 28%. Each 1 mL aliquot was measured in glass vials using a MIRA XTR handheld Raman spectrometer with 785 nm laser excitation at 50 mW. Spectra were acquired for 6 seconds and averaged over five consecutive scans. Raman data were processed in Vision software to develop a PLS calibration model correlating peak intensity and position to total phosphate concentration.

• MIRA XTR handheld Raman spectrometer (785 nm, 50 mW)
• Vial Holder Attachment and standard glass vials
• Vision software for PLS calibration and quantification
• Serial dilution protocol for sample preparation

Main Results and Discussion

The characteristic phosphate Raman band between 850 cm⁻¹ and 950 cm⁻¹ shifts from 890 cm⁻¹ to 897 cm⁻¹ as concentration increases. The PLS model exhibited a high coefficient of determination (R²) and low standard error, enabling quantification across a 0.14%–28% range. Validation with five independent dilutions yielded prediction errors below 2.4%. Method sensitivity could be further improved by increasing integration time, signal averaging, or employing immersion probes for higher laser power delivery.

Benefits and Practical Applications of the Method

• Reagent-free analysis eliminates chemical waste and disposal costs
• Minimal sample preparation and nondestructive measurement
• Rapid results within minutes enable real-time process control
• High accuracy over a wide concentration range supports diverse industrial needs

Future Trends and Opportunities

Integration of handheld Raman systems into automated production lines can enhance in-line monitoring of phosphate levels. Advances in probe design and data algorithms, including AI-driven spectral analysis, promise lower detection limits and improved robustness. Applications may expand to environmental monitoring, fertilizer quality assurance, and regulatory compliance in detergent and food manufacturing.

Conclusion

Handheld Raman spectroscopy combined with PLS modeling provides a fast, accurate, and eco-friendly method for total phosphate quantification. Its minimal sample preparation, nondestructive nature, and broad dynamic range make it an attractive alternative for industrial quality control and on-site analysis.

Reference

• Reimagining pH Measurement: Utilizing Raman Spectroscopy for Enhanced Accuracy in Phosphoric Acid Systems. Analytical Chemistry. DOI:10.1021/acs.analchem.9b05708