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Uncertainty Evaluation for Determination of Biuret in Automotive Urea Solution by HPLC

Automotive urea solution (DEF urea) is a critical consumable for SCR systems in diesel vehicles, and biuret content is a strictly controlled impurity index. According to national standards, the biuret content in automotive urea solution must not exceed 0.3%. Excessive biuret can decompose into cyanuric acid, which adheres to SCR catalysts, causes nozzle blockage, reduces NOₓ conversion efficiency, and even damages the exhaust system.

To ensure accurate and reliable test results, it is essential to evaluate the measurement uncertainty of biuret detected by high-performance liquid chromatography (HPLC). This study establishes a mathematical model, analyzes major uncertainty sources, quantifies each component, and calculates the expanded uncertainty. The results provide a scientific basis for quality control in testing laboratories, manufacturers, and inspection institutions.

Why Biuret Detection Uncertainty Matters?

Biuret is a byproduct of urea production at high temperatures. Its content directly affects:

SCR catalyst service life
Injection nozzle anti-clogging performance
Compliance with GB 29518-2013 standards
Accuracy of product qualification judgments

The uncertainty evaluation reveals key error sources, helping laboratories optimize operation steps and improve data accuracy.

Experimental Conditions

Instruments & Reagents

Agilent 1260 HPLC system
METTLER TOLEDO ME204 electronic balance
Biuret standard solution: 2.0 g/L
Mobile phase: methanol + 10 g/L potassium dihydrogen phosphate (pH=6.0) = 5:95 (V/V)
Chromatographic column: ZORBAX SB-Aq 4.6 mm × 150 mm, 5 μm
Detection wavelength: 200 nm
Flow rate: 1.2 mL/min
Injection volume: 5 μL
Column temperature: 20 ℃

Test Method

Prepare biuret standard working solutions and establish a calibration curve.
Weigh approximately 0.6 g of the sample, dilute to 10 mL with mobile phase, filter, and inject.
Calculate biuret content using the linear regression equation.

Mathematical Model

Where:

x: biuret content (%)
C: concentration from calibration curve (mg/mL)
V: sample dilution volume (mL)
m: sample mass (g)

Sources of Uncertainty

The main sources of measurement uncertainty for biuret in automotive urea solution are:

Measurement repeatability
Purity of biuret standard material
Preparation of standard solution
Calibration curve fitting
HPLC instrument stability
Sample weighing
Sample volume fixing (dilution to mark)

Uncertainty Components Evaluation

1. Standard Stock Solution (uₐ)

From certificate: expanded uncertainty = 0.01, k=2
Relative standard uncertainty: uₐ = 0.005

2. Standard Solution Preparation (uᵦ)

Including pipette and volumetric flask errors:

Combined relative uncertainty: uᵦ = 0.01207

3. Calibration Curve Fitting (u꜀)

Linear equation: y = 7440.74742x + 8.36884
Correlation coefficient: 0.99983
Relative standard uncertainty: u꜀ = 0.006921

4. HPLC Instrument Stability (uᴅ)

Instrument quantitative repeatability: 0.28%
Relative standard uncertainty: uᴅ = 0.001616

5. Measurement Repeatability (uₑ)

8 parallel tests
Relative standard uncertainty: uₑ = 0.0004048

6. Sample Weighing (uբ)

Balance error and repeatability
Relative standard uncertainty: uբ = 5.76×10⁻⁶

7. Sample Volume Fixing (uɡ)

10 mL Class A volumetric flask error
Relative standard uncertainty: uɡ = 0.00311

Suggestions for Improving Accuracy

To reduce measurement uncertainty and improve data quality:

Use certified reference materials with clear uncertainty.
Strictly control the preparation process of standard solutions.
Optimize HPLC conditions to improve the linearity of the calibration curve.
Use Class A volumetric flasks and pipettes.
Strengthen control of constant-volume operation.
Maintain the HPLC in good calibration.

conclusion

Evaluating the measurement uncertainty of biuret in automotive urea solution by HPLC is of great significance for ensuring data accuracy and product compliance. The expanded uncertainty is 0.012% (k=2), and the main influencing factors are standard solution preparation, curve fitting, and sample volume fixing. By optimizing these key steps, laboratories can effectively improve the reliability of biuret detection results.

Tech Blog

Uncertainty Evaluation for Determination of Biuret in Automotive Urea Solution by HPLC

Why Biuret Detection Uncertainty Matters?