Atmospheric-pressure chemical ionization
Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry which utilizes gas-phase ion-molecule reactions at atmospheric pressure (105 Pa), commonly coupled with high-performance liquid chromatography (HPLC). APCI is a soft ionization method similar to chemical ionization where primary ions are produced on a solvent spray. The main usage of APCI is for polar and relatively less polar thermally stable compounds with molecular weight less than 1500 Da. The application of APCI with HPLC has gained a large popularity in trace analysis detection such as steroids, pesticides and also in pharmacology for drug metabolites.
A typical APCI usually consists of three main parts: a nebulizer probe which can be heated to 350-500oC, an ionization region with a corona discharge needle, and an ion-transfer region under intermediate pressure. The analyte in solution is introduced from a direct inlet probe or a liquid chromatography (LC) eluate into a pneumatic nebulizer with a flow rate 0.2–2.0mL/min. In the heated nebulizer, the analyte coaxially flows with nebulizer N2 gas to produce a mist of fine droplets. By the combination effects of heat and gas flow, the emerged mist is converted into a gas stream. Once the gas stream arrives in the ionization region under atmospheric pressure, molecules are ionized at corona discharge which is 2 to 3 kV potential different to the exit counter-electrode. Sample ions then pass through a small orifice skimmer into the ion-transfer region. Ions may be transported through additional skimmer or ion-focusing lenses into a mass analyzer for subsequent mass analysis.
Ionization in the gas phase by APCI follows the sequences: sample in solution, sample vapor, and sample ions. The effluent from the HPLC is evaporated completely. The mixture of solvent and sample vapor is then ionized by ion-molecule reaction.
The ionization can either be carried out in positive or negative ionization mode. In the positive mode, the relative proton affinities of the reactant ions and the gaseous analyte molecules allow either proton transfer or adduction of reactant gas ions to produce the ions [M+H]+ of the molecular species. In the negative mode, [M−H]− ions are produced by either proton abstraction, or [M+X]− ions are produced by anion attachment. Most work on the APCI-MS analysis has been in positive mode.
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