利用台式高分辨Orbitrap LC-MS测定乳品中的三聚氰胺
简介
Generally used for industrial manufacturing, melamine, a nitrogen-rich white crystal, has been found as an adulterant used to falsify the protein levels in many milk products. In the 2008 Chinese milk scandal, thousands of young children who consumed melamine-contaminated milk products were reported to have developed sickness related to kidney stones and renal failure. More recently in January 2010, China reported another recall of melaminetainted condensed and powdered milk products. Contaminated milk products were also found during the 2008 scandal in many other countries and regions, causing widespread concern and demand for monitoring melamine in various milk products.
Different countries vary in setting the Maximum Residue Limit (MRL) for melamine, but generally follow the United States Food and Drug Administration (US FDA) MRL of 1 ppm for infant formula and 2.5 ppm for other milk products.1 Most advanced food testing labs employ mass spectrometry-based methods, particularly liquid chromatography tandem mass spectrometry (LC-MS/MS), for detecting sub-ppm to low ppb levels of melamine. Milk is a complex matrix containing soluble proteins, sugars and lipids, with additional enriched nutrients such as vitamins and minerals added to infant formula. Sample cleanup is critical and two approaches are generally used. First is the dilute-and-shoot approach in which the milk products are dissolved in diluted acid, followed by protein precipitation with acetonitrile. US FDA uses such a method for reporting a limit of quantitation (LOQ) of 250 ppb on LC-MS/MS.1 In the second approach, more timeconsuming and labor-intensive solid phase extraction (SPE) is used to remove the interferences and enrich the sample melamine for more sensitive quantitation at low ppb level by LC-MS/MS.
Another complication in analyzing melamine with LC/MS is that melamine, being a strong polar small molecule, cannot be retained in conventional reverse-phase HPLC. Ion paring or HILIC (Hydrophilic Interaction Chromatography) mode is used.
In this study, we evaluate a simple and rapid LC/ MS method to screen trace levels of melamine in milk products by utilizing a benchtop high resolution, accurate mass Orbitrap mass spectrometer. The sample preparation uses dilute-and-shoot. Analysis is fast and requires only a 1-minute LC separation.
实验仪器
结论
The high resolution, accurate mass Exactive massspectrometer was shown to be sensitive in detecting<0.1 ppb melamine (0.5 pg on column) in neat standard, and response is linear from 0.1 to 100 ppb. The error for mass accuracy is <2 ppm with lock mass. Milk samples prepared by dilute-and-shoot showed severe ion suppression that was reduced with a simple 1-minute isocratic HILIC LC separation, after which a consistent response factor of 0.3-0.5 for each sample matrix was obtained for quantitation. Quantitation limits were less than 44, 65, and 110 ppb for coffee cream, infant formula and 3-in-1 instant coffee mix, respectively, exceeding the requirement of 250 ppb LOQ as stated by US FDA for infant formula.
