Page 193 - Tandem Techniques

Page 176

The ICP ion source has several unique advantages; the samples are introduced at atmospheric pressure,
the degree of ionization is relatively uniform for all elements, and singly charged ions are the principal
ion product. Furthermore, sample dissociation is extremely efficient and few, if any, molecular
fragments of the original sample remain to pass into the mass spectrometer. High ion populations of
trace components in the sample are produced, making the system extremely sensitive.

Nevertheless, there are some drawbacks. The high gas temperature and pressure evoke an interface
design that is not very efficient; only about 1% of the ions that pass the sample orifice pass through the
skimmer orifice. Furthermore, some molecular ion formation does occur in the plasma, the most
troublesome being molecular ions formed with oxygen. These can only be reduced by adjusting the
position of the cones, so that only those portions of the plasma where the oxygen population is low, are
sampled.

Although the detection limit of an ICP-MS is about 1 part in a trillion, as already stated, the device is
rather inefficient in the transport of the ions from the plasma to the analyzer. Only about 1% pass
through the sample and skimming cones and only about 106 ions will eventually reach the detector.
One reason for ion loss is the diverging nature of the beam, but a second is due to space charge effects
which, in simple terms, is the mutual repulsion of the positive ions away from each other. Mutual ion
repulsion could also be responsible for some non-spectroscopic inter-element interference (i.e. matrix
effects).

Figure 5.9
Modified Ion Lens for ICP-MS