Desorption sources and Fast atom bombardment:

Desorption sources:

In these sources the solid or liquid sample is directly converted into the gaseous ions. These are used for high molecular weight (> 1000 Da) samples which are thermally unstable (i.e., decompose on heating) and are non volatile. A number of sources belonging to this category are available but we shall take up just one of them, namely the fast atom bombardment (FAB) method.

Fast atom bombardment (FAB):

In this method the anal yte is dissolved in a liquid matrix like glycerol and a small amount of this is placed on a target. The target is then bombarded with a beam of fast atoms (e.g., xenon or argon atoms at several keV). These desorb positive and negative analyte ions from the sample.

As the sample heating is achieved extremely quickly the fragmentation of the analyte ions is greatly reduced.

Thus, the ionisation chamber generates a stream of ions (primarily positive ions) which is accelerated by applying suitable potential and is sent to the analyser. Let us understand how the analyser separates these ions.