Determination of Carbon, Hydrogen, and Nitrogen in Flour and Plant Tissue

Determination of Carbon, Hydrogen, and Nitrogen in Flour and Plant Tissue — Summary

Significance of the topic

Accurate measurement of total carbon, hydrogen and nitrogen in flour and plant tissue is central for quality control, product uniformity and agronomic decision-making. In flour, C and N content inform on enrichment and compositional uniformity. In crop tissues, combined C/N data supports diagnosis of nutritional status, guides fertilization strategies and helps optimize yield and quality in high-value crops such as tobacco, cotton and fruit.

Objectives and overview of the study/article

• Demonstrate method performance for total C, H and N determination in representative flour and plant-tissue matrices using the LECO CHN828 combustion analyzer.
• Define sample preparation, operational parameters and calibration approaches appropriate for routine laboratory use.
• Present typical results and precision using helium or argon carrier gas and two aliquot loop sizes (3 cm and 10 cm) to highlight trade-offs between sensitivity, precision and consumable life.

Methodology and sample preparation

• Combustion: Samples are combusted in a pure oxygen atmosphere at high temperature (instrument setpoint 950 °C) in a vertical quartz furnace. Combustion gases pass through an afterburner (850 °C) and sulfur-scrubbing reagent prior to collection in a ballast volume.
• Gas handling and detection: Representative aliquots of the equilibrated ballast gas are introduced to a carrier gas stream (helium or argon) and analyzed by infrared (IR) cells for CO2 (C) and H2O (H) and by a thermal conductivity (TC) cell for N2 (N). Hydrogen analyses use a stop-flow technique.
• Sample prep: Samples are homogenized and typically dried at 80 °C for 2 hours (results reported on a dry basis). Dried samples are stored in a desiccator and analyzed within 24 hours. Typical sample mass is ~0.20 g placed in tin foil cups and sealed to minimize entrapped atmosphere.
• Calibration: Multi-point linear full-regression calibrations were used for C and H, and a linear force-through-origin calibration for N when appropriate. Certified reference materials (e.g., LECO LCRM EDTA and phenylalanine) were used for calibration and verification.

Used instrumentation

• Model: LECO CHN828 combustion Carbon-Hydrogen-Nitrogen determinator.
• Key features:

• Vertical quartz furnace with pure oxygen environment for complete combustion.
• Afterburner with sulfur scrubbing reagent and ballast gas equilibration prior to analysis.
• IR detection for CO2 and H2O; thermal conductivity detection for N2.
• Supports helium or argon carrier gas — helium provides higher sensitivity for nitrogen due to larger thermal conductivity contrast.
• Selectable aliquot loop sizes: 10 cm (optimized for lowest N range and best precision) or 3 cm (extends reagent life ≈3× and lowers cost per analysis).

• Nominal operational settings reported:

• Furnace temperature: 950 °C; afterburner: 850 °C.
• Typical nominal sample mass: 1.0000 g (instrument parameter) though analyses performed at ~0.20 g per sample.
• Burn profile: furnace flows 5.00 L/min → 1.00 L/min → 5.00 L/min with two 30 s steps before end.
• Ballast equilibrate time: 10 s; aliquot loop equilibrate time ~6 s; aliquot loop fill pressure drop 200 mm Hg.

Main results and discussion (typical outcomes summarized)

Overview of matrices analyzed: wheat flour, orchard leaves, corn gluten, alfalfa, and tobacco. Samples were dried (80 °C, 2 h) and analyzed in tin foil cups (~0.15–0.25 g fractional masses) using calibration materials (LECO LCRM EDTA and verified with LCRM phenylalanine). Results are reported on a dry-mass basis.

Representative average compositions (typical precision):

• Wheat flour: ~45.5% C, 6.45% H, 3.10% N. Observed standard deviations: C ~0.05–0.11, H ~0.02–0.03, N ~0.01–0.02 (depending on carrier gas and loop size).
• Orchard leaves: ~48.7% C, 6.18% H, 1.36% N. s(C) ~0.06–0.15, s(H) ~0.02–0.03, s(N) <0.01–0.02.
• Corn gluten: ~52.0% C, 7.02% H, 11.0% N. s(C) ~0.06–0.13, s(H) ~0.02–0.06, s(N) ~0.01–0.04.
• Alfalfa: ~43.9% C, 5.79% H, 2.91% N. s values comparable to other plant matrices (C s~0.09–0.14, H s~0.03–0.07, N s~0.01–0.04).
• Tobacco: ~48.2% C, 5.8–5.9% H, 3.29% N. Reproducibility similar to orchard leaves.

Analytical performance notes:
• Carrier gas choice influences sensitivity for nitrogen: helium offers the best sensitivity and lowest limits of detection; argon yields lower sensitivity due to smaller thermal conductivity difference with N2.
• Aliquot loop size trade-offs: the 10 cm loop improves precision especially at low N concentrations; the 3 cm loop conserves reagent life and lowers per-sample cost while providing adequate precision for many applications.
• Blank stability criteria: when using helium, the standard deviation of the last five blanks should be ≤0.001% (10 ppm) for each element; with argon the criterion is ≤0.005% (50 ppm). Additional blanks may be required to meet these thresholds.

Practical benefits and applications of the method

• Rapid, reproducible total C/H/N determinations for quality control of flour (uniformity, enrichment verification) and for agronomic monitoring (nutrient status, fertilization planning).
• Small sample mass requirement (~0.2 g) and straightforward tin-cup sample handling enable efficient throughput for routine laboratories.
• Flexibility in carrier gas and aliquot loop configurations permits optimization for sensitivity, precision or operating cost depending on laboratory priorities.

Future trends and potential applications

• Integration with laboratory information management systems and automated sample handling to increase throughput and traceability in high-volume testing environments.
• Development of greener, lower-cost carrier gas strategies and improved reagent formulations to reduce operational cost and environmental footprint.
• Coupling total-element combustion analyzers with isotopic ratio or speciation techniques (e.g., IRMS, GC) for expanded capabilities: provenance studies, detailed nitrogen-cycle investigations and product authentication.
• Improved data analytics for multivariate interpretation of C/H/N patterns in crop diagnostics and product quality assessment.

Conclusion

The LECO CHN828 application demonstrates robust, precise measurement of total carbon, hydrogen and nitrogen in both flour and diverse plant tissues. Proper sample drying, use of certified reference materials and attention to carrier gas and aliquot-loop selection allow laboratories to balance sensitivity, precision and cost. The method supports routine QC of food ingredients and informed agronomic decision-making for nutrient management.

Reference

• LECO Corporation. Application Note: Determination of Carbon, Hydrogen, and Nitrogen in Flour and Plant Tissue. Instrument: CHN828. St. Joseph, MI, USA. Form No. 203-821-686. 2/24—REV0. © 2024 LECO Corporation.
• Reference materials cited in the application note: LECO LCRM and LRM materials (e.g., LECO 502-896 EDTA; LECO 502-642 Phenylalanine), NIST or equivalent certified reference materials.