Dipping probe electrospray ionization/mass spectrometry for direct on-site and low-invasive food analysis

Rapid, direct, on-site and noninvasive food analysis is strongly needed for quality control of food. To satisfy this demand, the technique of dipping probe electrospray ionization/mass spectrometry (dPESI/MS) was developed. The sample surface was pricked with a fine acupuncture needle and a sample of similar to 200 pL was captured at the needle tip. After drying the sample, the needle tip was dipped into the solvent for similar to 50 ms and was moved upward. A high-voltage was applied to the needle to generate electrospray when the needle reached the highest position, and mass spectra were measured with a time-of-flight mass spectrometer. For evaluation of the method, the technique was used to analyze foods such as vegetables, salmon flesh, cow's milk, yogurt, and soy-bean milk. The detected major ions for cow's milk and yogurt were [(Lac)(n) + Ca](2+) with n = 1-6 (where (Lac) is lactose), indicating that Ca2+ is tightly bound by Lac molecules.