iCAP PRO Series ICP-OES Determination of Impurity Elements in Battery-Grade NMP

Determination of Impurity Elements in Battery-Grade NMP Using iCAP PRO Series ICP-OES

Importance of the topic

N-Methylpyrrolidone (NMP) is widely used as a solvent in cathode coating and conductive slurries for lithium-ion batteries. Battery-grade NMP must meet stringent purity requirements because trace metal contaminants at low µg/L levels can degrade electrochemical performance, cycle life and safety. Rapid, reliable methods for quantifying metallic impurities in neat organic solvents are therefore critical for quality control in battery materials manufacture.

Objectives and overview of the study

This application note describes a direct, rapid ICP-OES method for multi‑element determination in battery-grade NMP using the Thermo Scientific iCAP PRO Series. The goals were to: minimize sample manipulation (no aqueous dilution or digestion), achieve µg/L detection limits, control carbon-related spectral background, demonstrate linear, matrix‑matched calibration, and verify precision and recovery suitable for industrial QA/QC.

Used instrumentation

• Thermo Scientific iCAP PRO Series ICP-OES (model: iCAP PRO XP Duo)
• 24‑element mixed standard solution (1000 mg/L, Spex)
• Semiconductor‑grade blank NMP
• Analytical balance, LDPE volumetric flasks, calibrated pipettes

Key instrument operating parameters (summary)

• Observation mode: Axial
• RF power: 1300 W; variable‑frequency RF generator for stable plasma with organic matrices
• Oxygen flow (added to plasma): 50 mL/min (optimized)
• Nebulizer: Concentric; nebulizer gas 0.4 L/min; pump speed 20 rpm
• Auxiliary gas: 1.0 L/min; cooling (plasma) gas: 14.5 L/min
• Center tube: 1.0 mm alumina; spray chamber: cyclonic with baffle

Methodology and optimization

Calibration and standards

Calibration standards were matrix‑matched by preparing intermediate and working standards directly in semiconductor‑grade NMP. A 20 mg/L intermediate stock was prepared from the 1000 mg/L mixed standard, and working calibrants were prepared at 0, 0.02, 0.05, 0.10 and 0.50 mg/L (i.e., 0–500 µg/L) in NMP. Matrix matching eliminates artefacts from aqueous calibration and reduces contamination risk.

Oxygen addition and plasma stability

Addition of controlled oxygen flow to the plasma was essential to limit carbon deposition and the molecular C2 spectral background produced by organic solvents. Optimization showed 50 mL/min oxygen provided a balance between suppressing carbonaceous interference and maintaining stable plasma (higher O2 risked plasma extinguishment; lower O2 left residual molecular background). The instrument’s variable‑frequency RF generator contributed to plasma robustness under these organic‑rich conditions.

Detection limit and blank handling

Method detection limits were estimated from 11 consecutive blank NMP measurements (MDL = 3 × standard deviation of blanks) and were found at the µg/L level for the target elements. Unspiked NMP blanks produced non‑detect results for the surveyed elements under the conditions used, demonstrating low background contamination in the blank solvent.

Recovery and precision testing

Spike recovery experiments were performed at 0.05 and 0.10 mg/L (50 and 100 µg/L) in two NMP samples. Recoveries for the target elements ranged broadly between approximately 87–108%, with most values near 95–105%, consistent with acceptable analytical accuracy for industrial QC. Repeatability was assessed using seven replicate injections of a 0.10 mg/L spike; relative standard deviations (RSDs) were typically well below 2% (many elements <1%), indicating excellent precision for routine monitoring.

Main results and discussion

Calibration linearity and sensitivity

All measured wavelengths for the suite of elements produced excellent linear calibration responses with correlation coefficients (R²) > 0.999 (several at 0.9997–1.0000). Reported method detection capabilities reached the µg/L level, adequate to monitor impurity concentrations well below typical battery‑grade specifications (commonly 10–20 µg/L).

Matrix and interference management

Matrix‑matched calibration in neat NMP minimized errors associated with differing plasma loading between aqueous standards and organic samples. Oxygen addition effectively reduced carbon molecular background (C2) and carbon buildup on torch and injector surfaces, thereby preserving analytical stability and signal integrity. The study notes that oxygen must be tuned carefully: too much extinguishes the plasma; too little fails to suppress the molecular background.

Analytical performance summary

• Linearity: R² > 0.999 for Al, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Na, Ni, Zn (wavelengths specified per element)
• Recoveries: mostly between ~92% and 108% across two spike levels (lowest observed ≈87.7% for Cu)
• Precision: RSD typically <1% for most elements; worst‑case values ≈1.07% (Al) and ≈1.00% (Na), all <2%
• Throughput: Each sample measured in approximately one minute (direct injection)

Benefits and practical applications

Advantages of the direct‑injection iCAP PRO ICP‑OES approach for battery‑grade NMP include:

• Minimal sample preparation—no aqueous dilution or acid digestion reduces contamination and sample handling time
• High throughput—data acquisition and injection completed in about one minute per sample
• Sensitivity suitable for battery QC—µg/L detection limits and linear, matrix‑matched calibration
• Robust plasma operation with organic matrices enabled by variable‑frequency RF and optimized oxygen addition

This method is well suited for incoming material control, batch release testing and routine monitoring of metallic impurities in solvents used for electrode manufacture.

Future trends and potential applications

• Broader adoption of direct‑injection ICP techniques for other battery‑relevant organic solvents (e.g., N‑methylacetamide, high‑boiling glycols) as instrument RF generators and plasma control continue to improve.
• Integration with automation and sample handling systems for high‑throughput production QC environments.
• Further refinement of oxygen/auxiliary gas strategies and torch/injector materials to extend runtimes between maintenance in highly organic matrices.
• Coupling results with elemental speciation workflows or hyphenated techniques to link elemental contamination to specific precursor sources in the supply chain.

Conclusion

The study demonstrates a fast, accurate and reproducible direct‑injection ICP‑OES method using the Thermo Scientific iCAP PRO Series for multi‑element determination in battery‑grade NMP. Matrix‑matched calibration, optimized oxygen addition and a variable‑frequency RF source enabled µg/L sensitivity, excellent linearity and repeatability (RSD <2%), and acceptable recoveries (≈88–108%). The approach reduces sample handling, increases throughput and meets the analytical demands of the battery industry for solvent quality control.

References

• Thermo Fisher Scientific. Application Note AN23055: iCAP PRO Series ICP‑OES — Determination of Impurity Elements in Battery‑Grade NMP. 2023.