The rate constants of exothermic proton transfer reactions in the gas phase are close to the collision rate constants, so chemical ionization can efficiently generate ions. At atmospheric pressure, the rate of chemical ionization reactions is greater and the ionization efficiency should be higher. A major difficulty in designing an atmospheric pressure chemical ionization (APCI) ion source is the transfer of ions generated at atmospheric pressure into a mass analyzer in a state of high vacuum (<10-6 Torr).
The structure of an earlier APCI ion source is: a small volume (1cm3) ionization box is connected to the mass analyzer through a micropore (~25μm), and the sample (such as chromatographic effluent) enters the ionization box and is subjected to 63Ni. Beta-ray radiation ionizes. Its allowable carrier gas flow rate is 10-100ml/min. The ionization process is carried out at atmospheric pressure, and the mobile phase of the chromatography acts as a reagent gas. Due to its small size, the ion source is always heated, which reduces adsorption on the source walls.
Another design uses corona discharge ionization. The ionization chamber does not have a strictly defined edge, and the ionization zone is relatively large in volume from the corona point to the sampling micropore. The high-speed vacuum pump can maintain the vacuum of the analysis chamber, and the pore size of the sampling micropore is also increased to 100μm, and the allowable carrier gas flow rate can be as high as 9L/s. One interference with atmospheric pressure ionization is that solvent molecules (eg, water) form cluster ions with sample molecules. In the corona discharge ionization design, a layer of curtain air flow is added between the sampling micropore and the ionization reaction zone, which not only prevents the micropore from being blocked, but also declusters the cluster ions.
