Collisionally Induced Dissociation of Ions in a Strong Drift Field and its use as an Analysis Tool
Drift fields of 100–1000 Td. were used to co11isiona1ly dissociate ions (which were directly sampled from the lower atmosphere) to aid in their chemical idenitfication. The ions being studied make up only one part in 1017 of the gas in which they are found, and their mass spectrometric analysis is accomplished near the high sensitivity limit of the present apparatus. Since the chemical identity of the sampled ions can often not be determined from their mass alone, additional identification methods are needed. They must not, however, cause ionization in the far more abundant neutral gas or reduce the number of ions being detected. Under the appropriate conditions collisional dissociation of the ions being studied can accomplish both of the above goals, and also make available a continuously variable range of collision energies. At low field strengths ligand detachment from the parent ion can be observed. while fragmentation of the more strongly bound core ion is accomplished at fields near 1000 Td. An example of ligand removal is the gradual conversion of an atmosphericion family believed to be NH 4+·(H2O)n (mass 36, 54, 73…) into a single mass peak at mass 18 at 300 Td. in a nitrogen buffer gas. At 800 Td. an ion believed to be NO3– is seen to be largely dissociated into a fragment at mass 46 (probably NO2–) thus helping to confirm the identification of the parent ion as NO3–.