Page 405 - Tandem Techniques

Page 389

Huang et al.[38], in their article on atmospheric pressure ionization, demonstrated the use of the API
interface in the LC/MS analysis of some benzodiazepines. They employed a Zorbax-Rx column, 25 cm
long, 4.6 mm I.D, which was connected by a standard API interface to the mass spectrometer. The mass
spectrometer was scanned between m/z values of 100 and 350 at a scan rate of 3 s/scan.

The separation was developed isocratically using a mobile phase composed of 40% v/v acetonitrile,
25% v/v methanol, and 35 % v/v water, containing 10 nM ammonium formate buffer. 25 ng of each
benzodiazepine was present in the sample mixture. All the benzodiazepines were well separated and the
analysis was complete in less than seven minutes. The positive ion mass spectra for each component
+
exhibited clear and unambiguous (M+H) ions for each benzodiazepine and that there was little
fragmentation of the parent ions. This lack of smaller fragments confirmed the gentle nature of the API
ionizing process. The API interface offers great promise for extended use in LC/MS tandem
instruments, and may well in time become as popular, if not more popular, than the electrospray
interfaces. Its great advantages are that it can operate at ambient pressures and temperature, and the
gentle nature of the ionizing process. Furthermore, the device does not need the extensive pumping
support that is required by the electrospray interface, which simplifies the apparatus and reduces its
cost.

Inductively Coupled Plasma LC/MS Interfaces

The most common application of LC/MS tandem instruments with the ICP interface is in the
determination of element speciation. In principle, the liquid chromatograph separates the different
compounds present in a given mixture, and the mass spectrometer picks out those separated
components that contain the specific element or elements of interest. The IC ionization interface is
particularly useful for this type of analysis, as it will provide charged atoms of all heavy elements
present, and it is the speciation of such elements that is important. The details of the ICP torch, that is
used to produce the charged atoms, has already been described and so some examples of its use will
now be given.