THE ORIGIN OF METASTABLE IONS

The term ‘metastable’ has been applied to those ions in a mass spectrometer that have just sufficient energy to fragment some time after leaving the ion source but before arriving at the detector. The excess of internal energy imparted to these ions during ionization is sufficient to give them a rate of decomposition such that the latter occurs during the ion flight-time. The product ions from such ‘in-flight’ fragmentation have less than the full kinetic energy originally imparted to the precursor metastable ion when it left the ion source because the initially imparted momentum must be shared between the products of decomposition. As one of these products is itself an ion that is necessarily of smaller mass than its precursor metastable species, it follows that the product ion must have less momentum than the precursor. It is this reduced momentum that leads to the products of metastable ions having an apparent mass different from the corresponding ‘normal’ product ions formed in the ion source. The difference is explained more fully below.

To understand the origin and decomposition of metastable ions, it is convenient to consider electron ionization in a conventional double-focussing, magnetic-sector mass spectrometer as depicted in figure 8.1.