An isotope dilution liquid chromatography-mass spectrometry method for detection of melamine in milk powder

Adulteration/unintentional contamination of milk with melamine could have negative health and economic implications especially in the developing countries due to insufficient laboratory support and surveillance. This paper presents an Isotope Dilution Liquid Chromatography-Mass Spectrometry (ID LC-MS) method developed for detection of melamine in powdered milk. The rapid sample preparation involved dissolution of 1g of milk powder in 2.5% formic acid, precipitation of protein with acetonitrile, spiking of samples with melamine (triamine-¹⁵N₃) at 200 µg L^-1 and detection of intrinsic ¹⁴N-melamine molecular ratio to the spike. The isotope dilution calibration procedure was free from matrix effects, unlike other methods where the detector sensitivity can fluctuate up to several orders of magnitude. Limit of detection of the method was 13 µg kg^-1, and the recovery of melamine at 50, 100, and 250 µg kg^-1 was 78.7-126.3%. The method was used to determine melamine levels in 22 milk powder products (local and imported) available in Sri Lanka. Melamine was detected in all the samples (range = 0.33-0.96 mg kg^-1). Full cream milk powders (both local and imported) contained melamine in the range of 0.39-0.84 mg kg^-1, and various health and pregnancy formulas contained <0.5 mg kg^-1 of melamine. Two imported infant formula samples contained the highest levels of melamine (0.96 and 0.94 mg kg^-1). Although these melamine levels are below the regulatory limit in Sri Lanka (1 mg kg^-1), a monitoring programme would ensure consumer safety.