Determination of Nitrogen/Protein in Grains, Feeds, and Pet Food

Significance of the Topic

Protein analysis in grains, livestock feeds, and pet food is fundamental for nutritional evaluation, quality control, and commercial value determination. Fast, accurate nitrogen measurement underpins compliance with international standards and informs formulation and pricing decisions.

Objectives and Study Overview

This application note demonstrates the performance of the LECO FP628 nitrogen/protein determinator across various matrices, comparing two carrier gases (helium and argon) and two analytical protocols (High Precision and High Throughput). The goal is to assess sensitivity, precision, analysis time, and practical applicability in routine laboratory settings.

Methodology

Samples were prepared in accordance with AOAC Methods 990.03 and 992.23 to ensure uniform consistency. Combustion of 0.2–0.3 g samples took place in a vertical quartz furnace at 950 °C with an 850 °C afterburner under pure oxygen. Moisture was removed via a thermoelectric cooler without chemical reagents. Combustion gases were collected in either 3 cc or 10 cc loops and analyzed by a thermal conductivity detector. Calibration employed EDTA or phenylalanine standards, with drift correction and atmospheric blank compensation. Two methods were evaluated:

• High Precision: 4 min total analysis time optimized for lowest measurement uncertainty.
• High Throughput: 3.5 min cycle time for maximum sample throughput while maintaining performance.

Used Instrumentation

The LECO FP628 determinator features:

• Vertical quartz combustion furnace with pure oxygen environment.
• Thermoelectric cooler for moisture removal.
• Combustion gas handling with 3 cc or 10 cc aliquot loops.
• Thermal conductivity detector compatible with helium or argon carrier gases.

Key Results and Discussion

Testing on Grain A, Grain B, Dry Pet Food A, and Dry Pet Food B showed consistent nitrogen percentages and calculated protein values (using factor 6.25). Helium and argon models produced comparable averages (within 0.05 % N difference). Precision (standard deviation) ranged from 0.007 to 0.031 % N and 0.03 to 0.20 % protein. The High Throughput method reduced analysis time by 12.5 % with minimal impact on accuracy and precision.

Benefits and Practical Applications of the Method

The FP628 system offers:

• Rapid analysis with cycle times as low as 210 s (3.5 min).
• Elimination of metal oxidizers and reduced reagent consumption.
• Low cost per analysis via efficient gas handling and loop selection.
• Flexibility to switch between helium and argon based on gas availability.

This makes it well suited for routine QA/QC, research laboratories, and industrial quality control in agriculture, feed mills, and pet food production.

Future Trends and Potential Applications

Emerging directions include integration with automated sample preparation systems, development of alternative non-helium carrier gases, miniaturized combustion modules, and advanced data analytics. Expansion into novel food matrices such as alternative proteins or by-product streams and coupling with complementary detectors may further enhance nutritional profiling capabilities.

Conclusion

The LECO FP628 nitrogen/protein determinator delivers reliable, fast, and flexible analysis for a wide range of sample types. Both helium and argon models achieve high sensitivity and precision, supporting diverse laboratory requirements and ensuring compliance with regulatory standards.

References

• AOAC Official Method 990.03, Association of Official Analytical Chemists.
• AOAC Official Method 992.23, Association of Official Analytical Chemists.
• USDA Circular No. 183: Nitrogen-to-Protein Conversion Factors, United States Department of Agriculture.
• LECO Corporation. Application Note: Determination of Nitrogen/Protein in Grains, Feeds, and Pet Food. Form No. 203-821-485. 2022.