Cooling Effect of Helium

The quadrupolar field is homogeneous only in the center of the ion trap. Ions should not approach the electrodes or their trajectories will become unstable and they will not be stored. To prevent this situation, one must introduce helium at a partial pressure of around 10 torr into the analyzer. The ions undergo multiple collisions with helium atoms. The collisions reduce the amphtude of their trajectories and confine them to the center of the analyzer. With ion traps using internal ionization, the carrier gas eluting from the chromatograph also plays the role of cooling gas. In any case, special attention must be taken to ensure that very high purity helium (and no other carrier gas) must be used. 

The helium flow rate in the analytical column must be set to a value providing a partial pressure of about 10 torr in the ion trap (i.e., about 1.0 ml..min for a 30-m capillary column with an internal diameter of 0.25 mm and about 1.5 mL.min- for a 60-m capillary column with the same internal diameter). Finally, calibration of the mass analyzer mnst be carried out each time the helium flow rate is modified in the chromatographic method. As a matter of fact, changing the partial pressure of 

FIGURE 4.13 Electron ionization mass spectra of a hexabromodiphenylether (formula on left) recorded with an internal ionization ion trap and a quadmpole. 

Generally speaking, analyzers of different kinds supply comparable mass spectra for a given molecule. This is fortunate because otherwise a given spectra database (refer to Chapter 8 dedicated to databases) would be useful only with a single type of analyzer which would limit experimentation. In reality, the characteristics of a spectrum depend on the fragmentation rates of molecular ions and therefore on the internal energy supplied to the ions. As seen previously, the source conditions determine the spectrum results. The analyzer functions only to separate the ions. Nevertheless, exceptions to this rule can be observed. 

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