In this application note we determine the melamine contamination of water samples by UPLC-MS/MS.
The analyses can be fulfilled in 1.0 min with a recovery of 101.8±10.7% and a precision of 3.2%. With the range covering three magnitudes, the linear coefficient is greater than 0.99 and the detective limit is up to 0.4 ng/L.
The melamine contamination of milk that occurred in 2008 was undoubtedly a serious food safety situation in China as safe drinking water is an important part of food safety. Since both the drinking water and the food chain may cause a direct or indirect impact on human health, it is urgent to carry out the study on the analysis method of melamine residue in water samples.
An Ultra Performance Liquid Chromatography/tandem mass spectrometry (ACQUITY/Quattro Premier); MassLynx 4.1 workstation; solid-phase extraction (Zymark); Waters Oasis MCX SPE column (200 mg, 6 mL); pressure blowing concentrator (Organomation).
Methanol and acetonitrile are both in liquid chromatography pure (Fisher); water is obtained from a Millipore water purification system; ammonium acetate is in guaranteed grade (Merck), hydrochloric acid and ammonia are in analytical pure (purchased from East China Pharmaceutical Company); and the standard reference is purchased from J&K.
Solid-phase extraction: Add 5 mL of methanol and 5 mL water, at the speed of 5 mL/min to the column to make it active. Acidification of water sample: Add 0.25 ml of concentrated hydrochloric acid to 500 ml of water. Add the water sample with the speed of 4 mL/min and the volume of 500 ml. Rinse the sample with 5 mL of water and methanol respectively. Elute with 5 mL of methanol (containing 5% of ammonia) twice and concentrate the eluent with a pressure blowing concentrator until it is almost dried up. Dilute to 1 mL with mobile phase and filter it. Inject.
UPLC-MS method: column: UPLC column (ACQUITY UPLC BEH HILIC, 1.7 μm, 2.1 50 mm); column temperature: 40 °C; mobile phase: a mixture of acetonitrile and water (containing 10 mM of ammonium acetate) (90:10); flow speed: 0.4 ml/min.
The target contaminator is quantitatively analyzed with the MRM mode. Detect the target with ESI+. Capillary voltage: 3.0 KV, source temperature: 120 °C, the temperature of desolvent gas: 400 °C, flow speed: 800 L/h, cone gas flow: 50 L/h; argon flow speed: 0.38 mL/min at the MRM mode; cone current: 40 V, CID: 17; characteristic ions pair: 127>85, as in the following Figure 1.
Add 6.0 μg of target substance to 500 ml of testing water, and the recovery of a series of pre-sampling is 101.8±10.7% (n=6), as showed in Table 1.
Inject 1.0 μg/ml of standard solution for 6 times repeatedly and the RSD of peak area is 3.2%.
The signal/noise ratio is 15 for 1.0 ng/mL of standard solution and 3 for 0.2 ng/mL of standard solution, as shown in Figure 2. Taking the pre-sampling procedure into consideration, the quantitative detective limit of this method is 0.4 ng/L.
Prepare 4 standard solutions with different concentrations ranging from 1.2 ng/mL to 2400 ng/mL, the equation for the standard curve is y=1350.3x+161822,R=0.9925.
The blank is lower than the detective limit.
The water sample is obtained from a reservoir in Zhejiang province with a concentration of melamine of <0.4 ng/L. Add some standard solution to this water sample and analyze the content of melamine. The concentration of melamine is 0.03 μg/L and the total ion flow is showed in Figure 3.
The UPLC-MS/MS method used for the determination of melamine residue in water sample has been proved to be practicable. The analyses can be fulfilled in 1.0 min with a recovery of 101.8±10.7% and a precision of 3.2%. With the range covering three magnitudes, the linear coefficient is greater than 0.99 and the detective limit is up to 0.4 ng/L. All of these indicated that this method meets the requirement of practical analyses.
720003440, April 2010