The Direct Analysis of Milk Powder on the Liberty Series II ICP-OES with the Axially-Viewed Plasma

Importance of the Topic

The analysis of milk powder is critical in both food safety and environmental monitoring. As a widely consumed dietary staple, milk powder can reflect the presence of toxic metals in the environment and directly impact human health. Rapid multi-element analysis supports quality control, regulatory compliance, and nutritional assessment without extensive sample preparation or hazardous reagents.

Objectives and Study Overview

This application note demonstrates a direct quantification method for major, minor, and trace elements in milk powder using an axially-viewed inductively coupled plasma-optical emission spectrometer (ICP-OES). The approach employs standard aqueous calibration, scandium internal standardization for viscosity correction, and caesium as an ionization suppressant. Validation against NIST SRM 8435 Whole Milk Powder assesses accuracy and stability.

Methodology and Instrumentation

The study avoids traditional wet digestion or dry ashing by preparing slurry solutions of milk powder at 0.5% and 4% w/v in 18 MΩ·cm water with 0.002% Triton X-100 and 0.5 mg/L Sc internal standard. Calibration standards cover nine concentration levels for target analytes (Ba, Mn, Zn, Sr, Mg, Fe, Na, Ca, K, P, S). A subset of solutions included 1% w/v CsCl to suppress ionization interference.

Instrumentation:

• Agilent Liberty Series II ICP-OES with axial plasma view
• RF power: 1.30 kW; plasma gas: 15.0 L/min; auxiliary gas: 1.5 L/min
• Spray chamber: Sturman-Masters; torch: 2.3 mm i.d. axial injector; V-groove nebulizer at 240 kPa
• Pump speed: 15 rpm; uptake rate: 1.5 mL/min; rinse pump at 50 rpm
• Integration times: 1 s for major elements and many minor elements; 3 s for trace elements and Sc
• Sample delay: 25 s; stabilization time: 20 s; polynomial background correction; PMT 600 V
• Auxiliary Gas Module 2 for S determination down to 175 nm

Key Results and Discussion

Measurement precision over 1.5 h for a 4% milk powder slurry showed 1.2–2.0% RSD with internal standard correction, compared to poorer stability without it. Moisture content of samples was low (1.3–1.9%). Without Cs, ionization interferences from easily ionized elements (Na, K, Ca) led to signal suppression or enhancement, causing deviations from certified values. Incorporating 1% CsCl minimized these effects, yielding results for SRM 8435 within certified uncertainties for all major, minor, and trace elements. Scandium internal standardization effectively corrected matrix viscosity and partial ionization suppression and compensated for injector fouling over long runs.

Benefits and Practical Applications

The direct ICP-OES method enables rapid, multi-element quantitation in milk powder with high throughput and minimal sample handling. Avoiding acid digestion reduces chemical hazards and sample preparation time. Using aqueous calibrations and internal standardization simplifies workflow and lowers operational costs compared to ICP-MS. The approach supports routine quality control in dairy processing, nutritional labeling, and environmental monitoring programs.

Future Trends and Potential Applications

Advances may include automated slurry preparation, on-line matrix matching, and further optimization of ionization suppressants for diverse high-matrix samples. Coupling axial ICP-OES with mass spectrometric detection or high-resolution optics could extend sensitivity and specificity. Portable or low-power plasma sources may enable field deployable screening of dairy products and environmental samples.

Conclusion

A direct analysis method for milk powder on an axially-viewed ICP-OES delivers accurate, precise multi-element data without wet digestion. Sc internal standardization and CsCl ionization suppression yield results consistent with SRM values and maintain stability over extended analysis. The streamlined protocol enhances laboratory efficiency and safety for routine dairy quality assessments.

Reference

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