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 ∼200 pL was captured at the needle tip. After drying the sample, the needle tip was dipped into the solvent for ∼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.