||Ambient ionization mass spectrometry is a technique used for the analysis of samples under ordinary ambient conditions. It has many advantages, such as fast sample exchange, low operation time and few sample preparation which comparing with thus ion source operating in vacuum. Therefore, several different ambient ionization sources have been developed in the past decade. |
Direct Electrospray Probe ionization mass spectrometry (DEP-MS) was developed by Shiea, J in 1999, which providing an effective means of analyzing not only large biomolecules, but also small organic and inorganic compounds. It can rapidly generate electrospray from a droplet which was deposited on a probe. The DEP technique provides a number of unique analytical features containing: (a) low sample consumption and sample switching is immediate; (b) the capillary and pump are unnecessary; and (c) the probe is low-cost and easy to construct and clean, and is free to maintenance.
The first part of experiment in this research is based on the ideal of DEP. The leaf tissues were cut into a triangular shape, 15 mm long and 8 mm wide at the base, is held by a crocodile clip with the apex facing the inlet of the mass spectrometer and at a distance of 2 cm or more from it. Electrospray solutions are added onto the tissue to mix with the leaf sap, and a high voltage (4.5-5.5 kV) is applied to the leaf tissue through the crocodile clip, and generate ions for MS analysis. This method has been demonstrated to be applicable for the analysis of a wide variety of plant sample, such as the leaves tissue in different color, different growth, diseased and pesticide residue.
The second part of experiment is improved from the first research. Electrospray was produced via a metal probe which binding two fine stainless steel needle (0.27 mm diameter) together. This probe plays the role of sampling and ionization. An amount of liquid sample was loaded onto the probe when the tip of probe touched the plant tissue, then electropray solution are introduced by syringe pump into the probe and applied high voltage on the probe to produce electrospray ionization. Therefore, direct rapid analysis was achieved using this probe interfaced and this method can direct profile of phytochemicals in a section of plant tissue in different regions, including root, stem, leaf, outer and inner part of fruit. In addition, using this probe, solid samples are directed sampling equally. The solid materials can be observed on the tip of probe by scanning electron microscope (SEM). It can obtain reasonable strong ion signals rapidly for agrochemicals deposited on the surface of vegetables or fruits peel.