Non-Resonant Activation

FIGURE 5.22 Principle of non-resonant activation in ion trap mass spectrometer. 

The non-resonant mode is sometimes compared to a shaker. We shake an ion trap violently to fragment the contents. It is generally used for dosage methods because activation by collisions is independent of the secular frequency of the precursor ions. The precursor ions are therefore not affected by the interfering compounds of the matrix eluted in the ion trap. 

Developing MS/MS for structural identification does not require a particular strategy. We isolate the ion whose affiliation we want to establish and dissociate it by collisions with different activation energy values to establish the order of appearance of the fragment ions. The radiofrequency is scanned at a large amplitude to detect all the fragment ions. Each one provides information about the structure of the analyzed molecule. 

Developing MS/MS with an ion trap that can be efficient for dosing imposes a few technical requirements. The first development step is the choice of the precursor ion from the source spectrum of the molecule. Obviously we want to choose the most abundant precursor possible after ensuring that it is characteristic of the analyte (and not among the ions that contribute to chromatographic background noise). 

The second step is the optimization of CID parameters. Optimization is the same technique whether the activation mode is resonant or non-resonant. This step requires optimization of two parameters the collision energy and the qz value at which the precursor ions are trapped. The qz corresponds to the component of q according to the z axis of the ion trap (axis through the center of both endcap electrodes (see Chapter 4). This value of qz, often neglected by novice users, is important because it determines the value of the m/z ratio beyond which the daughter ions will be stored in the ion trap. It also conditions the energy that must be supplied to fragment the precursor ion. 

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Among all non-resonant activation techniques, BAD has been shown to have unique advantages for the formation of product ions. Due to the necessity to utilize an additional power supply for generating the DC component, such an approach has not been used in any commercial mass spectrometer. Conversely, in the DIT, variation of the duty cycle of the rectangular waveform is controlled at software level and it allows readily introduction of the DC component for BAD experiments. 

The ejection of the ions of m/z ratios above 400 results in the application between the endcap electrodes of a combination of frequencies that make all these ions enter in resonance. When the isolation phase of the precursor ions concludes, the amplitude of the radiofrequency of ion trapping is brought back to a value corresponding to m/z 70 (in the example in Figure 5.20). This causes the ions of m/z ratios above m/z 70 to be trapped in the ion trap during the activation phase by collision. The principle of isolation of the precursor ions is the same whether resonant or non-resonant activation is used. 

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