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The electrospray ionization source has become one of the more popular LC/MS interfaces that are
commercially available. However, more recently, another type of ionization source, that operates at
atmospheric pressure, has become a strong competitor, and provides very similar ionization
characteristics, but functions somewhat differently.
The Atmospheric Ionization Interface (API)
In atmospheric ionization the liquid flow is nebulized in the source region, but the ions are formed at
atmospheric pressure. Ionization at atmospheric pressure has a number of advantages. It avoids the
problems that arise when a liquid flows directly into a vacuum, and it allows the separation system to
operate under ambient conditions. This is particularly advantageous where low flow rates are employed,
such as with microbore columns and capillary electrophoresis. A diagram illustrating the basic design of
the atmospheric ionization interface is shown in Figure 9.41.
Figure 9.41
The Atmospheric Ionization Interface.
The atmospheric pressure ionization (API) process has some similarity to the electrospray ionization
source, and can cope with a wide range of column flow rates, up to a maximum of about 2 ml/min.
Consequently,
