7 Must Know Points to Understand Atmospheric Pressure Chemical Ionization
CHROMacademy delivers key knowledge quickly in bite-size modules, and to prove this here is our 7 point guide to understanding Atmospheric Pressure Chemical Ionization.
1) Atmospheric Pressure Chemical Ionization (APCI) uses gas-phase ion-molecule reactions at atmospheric pressure. It relies on the formation of a plasma of ions comprised, mainly, from the HPLC mobile phase components.
2) In APCI the eluent is introduced into the interface using a capillary of similar design to the ESI source. However, no potential is applied to the capillary, instead the liquid emerges from the capillary surrounded by a flow of inert nebulizing gas into a heated, vaporizing region.
3) The combination of nebulizing gas and heat forms an aerosol with the sprayed eluent that begins to rapidly evaporate
Aerosol droplet consisting of analyte and mobile phase molecules
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4) A pin is placed within or at the end of the heated region that has a high potential
(5 – 10 kV) applied to it and produces an electrical discharge (1-5 μA) which ionizes molecules within the aerosol. A combination of molecular collisions and charge transfer processed cause an ionized gas plasma to be formed, primarily from the eluent molecules that are in vast excess.
If nitrogen is the nebulizing and auxiliary gas with atmospheric water vapor present in the source then the following primary and secondary reaction cascade will occur in the corona discharge region.
APCI Cascade Reaction
5) The cascade reactions which occur in the corona discharge region result in the production of water cluster ions which predominate in the mass spectrum. Declustering of analyte ions may be achieved using one, or a combination, of the following approaches:
Using a counter current gas flow at the nozzle plate, also known as a curtain gas
Using a heated transfer capillary between the API region and the nozzle-skimmer region
Using a drift voltage between the nozzle and skimmer plates to promote intermolecular collisions between the ion clusters and background gas molecules (collision induced dissociation CID)
6) Sample molecules that elute in the gas phase into this plasma may be ionized via transfer of protons to give either positive or negative ions depending on the proton affinity of the analyte species relative to the solvent gas plasma molecules.
Negative Ion Mode
If the solvent (S) has a larger proton affinity than the analyte (M), then a proton is abstracted from the analyte by the solvent.
Also anions (A-) can be attached
The formation of ions by associative electron capture
Dissociative electron capture occurs in the presence of good leaving groups (X) like halogens
Positive Ion Mode
If theanalyte (M) has a larger proton affinity than the solvent (S) then the analyte will take a proton from the protonated solvent
If both species present similar proton affinities adducts are formed
Charge exchange or electron transfer occurs with reagent gases which do not contain available hydrogen atoms.
The analyte (M) has a lower proton affinity than the solvent (S)
7) A less well documented mechanism for APCI is the triboelectric effect. As the mobile phase and analyte species exit the nebulizer the shear forces generated by the nebulizing gas tear the liquid stream into droplets. The friction generated by this process generates an electric charge (the triboelectric effect) at the liquid-gas interface. This can cause ion formation in some non-volatile analytes. Triboelectric APCI does not depend upon the electrons generated by the corona discharge electrode and ions may be generated (with their associated mass spectrum signals) when the corona discharge is very low or turned off. Triboelectric APCI is most common for analyte molecules that are moderately polar and/or non-volatile.
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