Determination of Nitrogen/Protein in Feeds, Grains, and Pet Food (FP928)

Importance of the Topic

Protein quantification in animal feed, grains, and pet food is critical for nutritional assessment, regulatory compliance, and economic valuation. Accurate determination ensures product quality, informs feed formulation, and safeguards consumer interests.

Study Objectives and Overview

This application note describes the use of the LECO FP928 macro combustion analyzer to measure nitrogen content and calculate protein levels in various feed matrices. The method follows AOAC 990.03 and evaluates performance across different carrier gases (helium, argon) and gas aliquot loop sizes (3 cm3, 10 cm3).

Methodology and Instrumentation

The workflow involves sample grinding to a fineness below 0.5 mm, moisture adjustment or software correction per AOAC 930.15, and macro combustion in a high-temperature ceramic furnace under pure oxygen. Combustion gases pass through a thermoelectric moisture trap and a copper reduction tube, converting nitrogen oxides into molecular nitrogen, which is detected by a thermal conductivity cell. Carrier gas choice affects sensitivity and cost: helium delivers highest sensitivity, while argon offers lower operating expenses. Two aliquot loop sizes allow optimization of precision versus reagent usage. Calibration uses certified reference materials (EDTA and phenylalanine) with a linear calibration through the origin. Analytical sequences include system conditioning (blank runs), calibration or drift correction, and automated sample analysis using an autoloader.

Results and Discussion

Performance was tested on dry cat food, hay, corn silage, distillers grain, mixed feed, and corn grain. All sample types yielded consistent nitrogen and protein values across configurations. Using helium and the 10 cm3 loop produced the lowest standard deviations (≤0.01 %N). The smaller 3 cm3 loop reduced carrier gas consumption and extended component life with minimal precision penalty. Argon delivered acceptable results with slightly higher baseline noise. Moisture-corrected nitrogen data combined with a protein conversion factor of 6.25 produced protein contents tightly aligned with reference values.

Practical Benefits and Applications

The FP928 method offers rapid, accurate, and precise nitrogen/protein analysis ideal for QA/QC applications in feed mills, grain elevators, pet food manufacturing, and nutrition research laboratories. Automated operation, selectable carrier gases, and flexible loop sizes support high throughput and cost-effective workflows while adhering to recognized official methods.

Future Trends and Opportunities

Emerging developments may include integration of alternative detection technologies (such as infrared cells), miniaturized combustion modules, real-time moisture monitoring, and advanced software for data analytics and laboratory information management system integration. Expanding validated applications to novel feed ingredients, by-products, and alternative protein sources will address evolving industry demands.

Conclusion

The LECO FP928 macro combustion analyzer, operated in accordance with AOAC 990.03, provides a robust, versatile, and efficient solution for nitrogen and protein determination in feed and related matrices. Its adaptability to different carrier gases and aliquot loop sizes ensures optimal performance across diverse laboratory requirements.

References

• AOAC Official Method 990.03, Crude Protein in Animal Feed by Combustion
• AOAC Official Method 930.15, Moisture (Loss on Drying) for Feeds
• AOCS Ba 4f, Sample Preparation for Nutrient Analysis
• USDA Circular No. 183, Protein Conversion Factors