Accurate Testing of Complicated Food Matrices

The total offline sample preparation time was approximately 30 to 40 minutes, including homogenization, centrifugation, and calibrator preparation. Compares representative standard high performance liquid chromatography (HPLC) and turbulent flow chromatography method chromatograms of 500 ng/kg (parts per trillion) tetracycline in a fish matrix. Using turbulent flow chromatography technology, the limits of quantitation (LOQs) were significantly lower (two- to 10-fold) for all four analytes using online extraction followed by LC/MS/MS compared to standard HPLC. This indicates that the LC/MS system can remove endogenous interferences, thus reducing ion suppression effects and improving detection limits.

2) Antibiotics in honey: Antibiotics are commonly used in beehives to control bacterial disease in honeybees, although caution is required to prevent persistent residues in food-grade honey. If antibiotic residues are present in high enough quantities, allergic reactions and bacterial resistance may develop. The conventional sample preparation for LC/MS/MS analysis of antibiotics in honey is time- and labor-intensive, often involving pH modification, hydrolysis, liquid-liquid extraction, SPE, solvent evaporation, and pre-concentration, and suffering, therefore, from low throughput. In addition, it is always an analytical challenge to deal with a large number of antibiotics belonging to different classes and often requiring multiple LC/MS methods.

Ten representative antibiotics used in honey, belonging to four different structural classes, were selected: sulfonamides, tetracyclines, aminoglycosides, and macrolides.3 The only offline sample preparation step required was the aqueous buffer dilution of raw honey to reduce the sample viscosity, which took less than 10 minutes. The online extraction clean-up was accomplished using a turbulent flow chromatography method involving two TurboFlow columns placed in tandem—a mixed mode anion exchange column and a polar-capped polymer-based column. Simple sugars were un-retained and moved to waste during the loading step while the analytes of interest were retained on the extraction column set. This was followed by organic elution to an end-capped silica-based mixed mode reversed-phase analytical column and gradient elution to a triple stage quadrupole mass spectrometer (Thermo Scientific TSQ Quantum Ultra). The total LC/MS/MS method run time was less than 18 minutes. A representative chromatogram of the 10 analytes at 100 ng/mL in 1:1 honey/ buffer was developed.

The results indicated that using two online turbulent flow chromatography extraction columns with different chem-istries extended the affinity range—further facilitating the separation and quantification of all of the representative compounds that have different chemical properties—in a complex honey matrix in a single analysis.

Quinolones, including fluoroquino-lones, in honey were also investigated.4 Instead of an SPE method, an online extraction method using turbulent flow chromatography was developed. The sample preparation time for the entire batch, including 16 compounds, dropped from five hours to 40 minutes, eliminating 80% of sample preparation time. The LOQs for the majority of analytes were 1 µg/kg (ppb) with no matrix interference. Representative selective reaction monitoring chromatograms at 20 µg/kg were developed and showed the selected ion transitions and retention times for the studied analytes.

3) Melamine in dairy products: The most notorious food safety incident in 2008 was the Chinese milk scandal involving melamine-tainted milk and infant formula. This incident triggered the largest global recall of Chinese-made diary products to date and prompted much stricter food safety regulations worldwide.

Consequently, scientists have developed many methods to analyze melamine residues in dairy-based products. Most of these approaches employ offline, disposable, cation exchange, SPE cartridges to prepare samples, coupled with LC/MS (MS/MS) analysis.

The goal in this experiment was to measure melamine with minimal sample preparation and high sample throughput. Using an LC/MS system, no offline sample extraction was required. Liquid or powdered dairy products were mixed with a solution of ammonium acetate in water and acetonitrile. After centrifugation, the supernatants were injected into the system. The quick-elute method took four minutes, with a one-minute data window. The lower detection limit was at least 50 ppb. Ion suppression caused by the co-elution of matrix components was minimal due to use of an atmospheric pressure chemical ionization source instead of the commonly used electrospray ionization source. The carryover level was also well-controlled and below 1%.5