Answer 4.7

a) Ion clouds in quadrupole ion traps are stored in a space that is rather confined. How is it achieved to keep the ions close to the center of the trap?

Different from all other types of mass analyzers, quadrupole ion traps are operated with a low-pressure buffer gas, typically helium at about 0.1 Pa. The gentle collision of the ions with the gas reduces the amplitude of their motion, and thus, prevents them from geting too close to the surfaces of the electrodes.

b) Why is it important to control the ion population in a QIT or LIT?

All ion traps face the problem of Coulombic repulsion of ions of the same polarity. As mentioned above, the ion cloud should not expand too much in order to avoid close proximity to the walls. The maximum population of a QIT is usually in the order of 1 million ions.

c) Which type of quadrupole ion trap is less susceptible to space charge effects, the QIT or the LIT?

The inner volume available for ion storage of LITs is larger than that of QITs. Thus, LITs can store a larger number of ions whilst maintaining the same concentration of ions per volume, or charge density. This is a major advantage of LITs over QITs.

d) So how do you control the number of ions admitted to a QIT or LIT?

Ion traps can be operated to scan extremely fast, although this is achieved at the cost of mass resolving power. The trick is to run a pre-scan to probe the actual sample ion fluence. For example, ions are accumulated for 10 ms and quickly ejected to use the resulting siganl intensity as a measure of the actual number of sample ions per unit time. The analytical scan then is based on a longer accumulation time calculated from the pre-scan. It now uses 10 to 20 times more ions and slower scanning for better signal-to-noise ratio and higher mass resolving power. The technology was once introduced by Thermo as „automatic gain control“.