This parameter is used only in APCI mode, MMI-APCI mode, and MMI-ESI+APCI mode. It controls the current (in uA) from the corona discharge needle to the end plate. The field of free electrons that make up this current ionizes the mobile phase molecules. The ionized mobile phase molecules in turn react with, and ionize, the sample molecules.
| Minimum | Maximum | Step Size | Typical | |
|---|---|---|---|---|
| Positive Ions | 0 µA | 10 µA | 0.1 µA | 4 µA |
| Negative Ions | 0 µA | 100 µA | 1 | 4 µA |
| Positive mode (MMI-ESI, MMI-APCI, and MMI-ESI+APCI) | 0 µA | 10 µA | 0.1 µA | 5 µA (1 µA for MMI-ESI+APCI) |
| Negative mode (MMI-ESI, MMI-APCI, and MMI-ESI+APCI | 0 µA | 100 µA | 1 µA | 5 µA (1 µA for MMI-ESI+APCI mode) |
Optimum corona current is highly sample dependent. A current of 4 uA works well for the APCI calibrant. This parameter is not used by APPI.
In negative ion mode, the presence of an electron-capture reagent (oxygen, SF6, or chlorinated solvent) allows much lower Corona Current (approximately 4 µA). This results in more efficient ionization and less noise. If an electronegative solvent is not present, one can be added to the effluent. Chloroform at a concentration of 0.5 to 1% in the mobile phase works well for this purpose. One or two percent oxygen added to the nebulizing/drying gas performs the same function.
| A current of 4 µA works well for tuning because the APCI calibrant contains chloroform, an electron scavenger. |
| DO NOT exceed 2% oxygen in the nebulizing/drying gas. The presence of a volatile hydrocarbon (solvent), oxygen, and an ignition source (the corona discharge) in a confined space is potentially explosive. Oxygen content at 2% or lower should not cause combustion. |
See Also