The Techniques

CI - Chemical Ionization

Chemical Ionization is a relatively soft ionization technique. This method is based on the ionization of the compound by a gas phase proton transfer from ionized reagent ions formed via a heated filament. We use both methane (CH4) and ammonia (NH3) on a routine basis for CI. When protonated in the ion source they form (CH5+ and NH4+) respectively. High sensitivity can be achieved with this technique, especially using MS/MS in the select reaction monitoring (SRM) mode.

EI - Electron Impact

Electron Impact is a technique where high energy electrons collide with the analyte producing a parent ion which then fragments in a particular pattern. The fragments can be used to help in the identification of the technique lends itself readily to SP or GCMS analysis. High sensitivity can be achieved by this technique.

ESI - Electrospray Ionization

In Electrospray Ionization the analyte is in solution (aqueous or organic) and is introduced to the ion source through a needle at a high electrical potential, with the aid of nitrogen gas as a drying agent the liquid sprays out in droplet form, these droplets diminish in size in there transport to the ion-source and ionize as the droplet explodes. Normally the ions are in protonated form but this is dependent on the chemistry of the solvent and its modifiers. Both positive and negative species can be generated, e.g. acids readily lend themselves to negative ion detection.

The sensitivity as with any method is analyte dependent but for most, the low nanomole or picomole range can be expected. Flow rates of 3ul to 1ml/min are common. We regularly analyse sample of pure compounds by directly infusing the sample at 3ul/min into the ESI via a syringe pump. Complex mixtures can be separated on LC using standard 4.6mm id columns at 1.0ml/min or microbore 1.0mm columns at 50ul/min achieved using our HP1090 LC system. One limitation is that only volatile buffers or modifiers can be used in the LC solvent system. Both infusion and LC are routine in our lab.

APCI - Atmospheric Pressure Chemical Ionization

In the Atmospheric Pressure Chemical Ionization mode the analyte is delivered in a liquid stream from an LC system. When the liquid enters into a heated region of the APCI probe (up to 500 deg C) it is desolvated and forced out of the probe in a nitrogen stream. At the exit is a high voltage corona discharge needle, and the analyte is protonated by the H30+ ions that are produced in the discharge region. We perform this technique regularly on plasma extracts, monitoring for glucocorticosteroids at the low ng/ml range.

It is possible to separate complex mixtures by HPLC using 4.6 mm id columns as well as 2.1 mm id columns. As with ESI this a routine operation in our laboratory. The limitation is that only volatile buffers or modifiers can be used in the LC solvent system.