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Melamine Detection by HPLC

Adulteration with melamine powder in food and dairy products has become a major food safety hazard worldwide. The traditional Kjeldahl method only measures total nitrogen content and cannot distinguish real protein from illegally added melamine, which leads to falsely high protein readings.

High Performance Liquid Chromatography (HPLC) is a reliable, sensitive and efficient technique for direct qualitative and quantitative detection of melamine. This article introduces the standard HPLC detection workflow for melamine in dairy samples, covers required instruments, reagents, detailed operating steps, calculation methods and essential precautions. It serves as a practical guide for food testing laboratories, dairy enterprises and quality control institutions.

Melamine Detection by HPLC Working Conditions

Required Laboratory Equipment

The full set of testing devices includes: High Performance Liquid Chromatograph, analytical balance, centrifuge, ultrasonic water bath, solid-phase extraction (SPE) device, nitrogen evaporator, vortex mixer, 50 mL plastic centrifuge tubes with stoppers, cation-exchange solid-phase extraction columns, organic microporous filters and qualitative filter paper.

Chemical Reagents

  • Chromatographic pure reagents: Methanol, acetonitrile, octanesulfonic acid sodium salt
  • Analytical reagents: Ammonia water, trichloroacetic acid, citric acid
  • Melamine standard sample: Purity ≥ 99%, prepared with 50% aqueous methanol solution
  • Ultra-pure water for laboratory use

Standard HPLC Parameters

Stable chromatographic conditions are the foundation of accurate detection:
  • Chromatographic column: C8 or C18 column
  • Column temperature: 40 °C
  • Flow rate: 1 mL/min
  • Detection wavelength: 240 nm
  • Injection volume: 20 μL

Complete Sample Pretreatment Process

Sample pretreatment directly affects extraction efficiency and final detection accuracy. Follow the standardized steps below:
  1. Weigh 2 g of the well-mixed sample and transfer it into a 50 mL sealed centrifuge tube.
  2. Add 15 mL of 1% trichloroacetic acid solution and 5 mL of acetonitrile, mix thoroughly.
  3. Place the tube in an ultrasonic water bath for 10 minutes of ultrasonic extraction, then vortex-shake for another 10 minutes.
  4. Centrifuge the mixture for 10 minutes at a speed above 4000 rpm.
  5. Filter the supernatant with qualitative filter paper, then dilute the filtrate to 25 mL using trichloroacetic acid.
  6. Take 5 mL of the filtrate and mix with 5 mL of ultra-pure water, then load the mixture onto a cation-exchange SPE column.
  7. Rinse the SPE column sequentially with ultra-pure water and methanol. Elute the target substance with ammoniated methanol (mixed by 5 mL ammonia water and 95 mL methanol). Control the flow rate to below 1 mL/min throughout the SPE process.
  8. Transfer the eluate to a test tube and dry it completely with nitrogen at 50 °C using a nitrogen evaporator.
  9. Re-dissolve the residue with 1 mL mobile phase and vortex evenly. Filter the solution through a microporous filter membrane before loading it into the HPLC system.

Blank Control Test

Perform a blank experiment synchronously with ultra-pure water. Keep all operating steps identical without adding any sample, to eliminate interference from reagents and the experimental environment.

Quantitative Calculation Method

Calculation Formula

Use the external standard method for quantitative analysis. Ensure the sample peak area falls within the linear range of the standard curve. Dilute and re-test samples that exceed the linear range.
Formula for melamine content in samples
Formula for melamine content in samples
Parameter Explanation:
  • X: Melamine content in the sample, unit: mg/kg
  • A: Peak area of melamine in the sample solution
  • C: Concentration of melamine standard solution, unit: μg/mL
  • V: Final constant volume of the sample solution, unit: mL
  • As: Peak area of melamine standard solution
  • M: Mass of the tested sample, unit: g
  • F: Dilution multiple of the sample

Critical Operational Precautions

To avoid test errors, false positives and equipment damage, strictly follow the requirements below in each step:
  1. Sample uniformity: Fully stir and mix samples before weighing to guarantee representative test results.
  2. Special treatment for milk powder: Completely dissolve the milk powder with trichloroacetic acid before adding acetonitrile. Properly extend the ultrasonic time to achieve full extraction. Combined ultrasonic and shaking treatment can improve extraction efficiency.
  3. Centrifugation speed: The national standard requires a minimum speed of 4000 rpm. Practical tests prove that 4000 rpm cannot obtain a clear supernatant. It is recommended to set the speed to 15000-16000 rpm for optimal separation.
  4. SPE column activation: Activate the cation exchange SPE column before use. Rinse with 3 mL methanol, followed by 5 mL ultra-pure water, with the flow rate controlled at < 1 mL/min.
  5. Flow rate control: Keep the flow rate below 1 mL/min throughout solid phase extraction. Excessively fast flow will weaken adsorption, raise column pressure and even collapse the filler inside the column.
  6. Prevent column dry-out: Never let the SPE column run empty during extraction, as this can cause severe deviations in detection data.
  7. Mobile phase treatment: Filter the mobile phase through a 0.20-0.45 μm microporous membrane, then degas by ultrasonication or vacuum before use.
  8. Daily equipment maintenance: Clean the chromatographic column and pipeline in accordance with specifications after daily testing. Re-equilibrate the column to maintain stable column efficiency.
  9. False-positive confirmation: HPLC has a certain probability of yielding false positives. Any sample with a positive melamine test result must be re-identified by professional, authoritative testing institutions for a final conclusion.

conclusion-Melamine Detection by HPLC

HPLC is widely used for melamine detection due to its high speed and sensitivity. Compared with the traditional Kjeldahl method, it can directly detect melamine content and effectively identify food adulteration.

For daily testing, operators need to standardize sample pretreatment, strictly control the centrifugation speed and the flow rate during solid-phase extraction, and regularly maintain chromatographic columns. Following the above procedures and precautions can ensure accurate, repeatable and reliable test data. This method is applicable to the quality inspection of dairy and other food products and provides reliable technical support for food safety supervision.

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