Page 401 - Tandem Techniques

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the total column eluent can be utilized without splitting the flow. However, even though all the solute
may enter the interface, not all the solute molecules are ionized, and not all the ions that are formed
enter the mass spectrometer. There are three alternative forms of the API source: one that uses a heated
nebulizer with a corona discharge, one that employs an atmospheric electrospray and one that uses an
ion spray. The interface that employs a heated nebulizer and a corona discharge ionization process is
that depicted in Figure 9.43. The liquid flow is nebulized by means of the gas flow which is then swept
by another stream of gas (the sheath gas or make-up gas) through a quartz tube heater that vaporizes the
solvent. The sample then drifts through a chamber containing a corona discharge, which is set up by a
potential difference of about 2000 volts, applied between a simple electrode arrangement. The charged
molecules of the solvent vapor are used as the ionizing agents.

The reactant ions that are formed in the corona discharge collide with the sample molecules and largely
give sample molecule plus a proton (hydrogen positive ions), i.e. [M+H]+. This is a process very
similar to chemical ionization but occurs at atmospheric pressure. The ions are drawn by means of an
electric field to a plate with an orifice, over which passes another flow of gas called the curtain or
barrier gas. The flow of barrier gas helps to prevent uncharged molecules from entering the ion source
but the charged ions pass through into the next chamber. The ions then pass through an aperture in the
next plate, the skimmer plate, and the space between the sampling plate and the skimmer plate is
connected to the first vacuum pump. After passing through the skimmer plate, the ions pass through an
aperture in a third plate into the mass spectrometer analyzer. The space between the skimmer plate and
the final plate is also connected to a second vacuum pump. By means of the differential pumping the
necessary pressure can be maintained in the mass spectrometer. As the sample entering the mass
spectrometer is virtually a parent ion already, the fragmentation pattern is very similar to that obtained
in MS/MS. In this way, the system differs fundamentally from the electrospray interface. The ionization
process is soft, very sensitive and gives good characteristic spectra, that can be used for both sample
identification and structural elucidation.