Atmospheric pressure chemical ionization (APCI) is a soft-gas phase ionization technique that is typically preferred for medium-polar compounds. The gas phase acidities and basicities of the analyte and solvent vapor play an important role in the APCI process. APCI provides molecular mass information for compounds of medium polarity that have some volatility. APCI is typically used to ionize small molecules with molecular masses up to about 1,000 Da.
Use APCI in either positive or negative ion polarity mode. For most molecules, the positive ion mode produces a stronger ion current. This is especially true for molecules with one or more basic nitrogen (or other basic) atoms. Molecules that generally produce strong negative ions with acidic sites, such as carboxylic acids and acid alcohols, are an exception to this general rule.
In general, APCI produces fewer negative ions than positive ions. However, the negative ion polarity mode can be more specific because it generates less chemical noise than the positive mode does. Consequently, the signal-to-noise ratio (S/N) might be better in the negative ion mode.
The upper limit for the system’s flow rate is 3,000 µL/min. However, for optimal APCI performance, set the flow rate1) into the MS over a range of 100–2,000 µL/min. This flow range is optimal for the following separation techniques: analytical LC, microbore LC, and semi-preparative LC.
APCI is a very robust ionization technique. It is not affected by minor changes in most variables, such as changes in buffer type or buffer strength.