Axis BTA 2100D Instrukcja Operatora Pobierz pdf (Strona 211)

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(parent ion) m/z. The m/z distribution that results from the CID process depends

on the characteristics of the parent ion and the amount of energy that has been

converted into internal vibrational energy.

The translational kinetic energy of the parent ion can be increased using either of

two methods. Each method requires application of a waveform to the trap. The

waveform amplitude is called the CID excitation amplitude, and the length of time

that the waveform is applied is called the excitation time.

The first method involves nonresonant excitation. In this case, a low frequency

supplemental dipole field is applied to the end caps of the trap, resulting in an

instantaneous change in the potential energy of the ion in the trapping field.

The restoring force of the trapping field converts the newly increased potential

energy of the ion into increased translational kinetic energy. A portion of this

kinetic energy is then converted into internal vibrational energy upon subsequent

collisions. This process is repeated during each oscillation cycle of the low

frequency dipole field.

The second method for increasing the parent ion kinetic energy involves

resonant excitation. This method requires application of a high frequency

supplemental dipole field to the end caps of the ion trap. The frequency must

match the oscillation frequency of the trapped ion. The resonant frequency of the

trapped ion depends on ion mass, space charge, rf trapping field amplitude, and

other instrumental factors. Thus, it is difficult to precisely calculate its value. The

amplitude of the rf trapping field is therefore modulated over a specified range.

As the resonant frequency of the trapped ion depends on the magnitude of the rf

field, modulating the rf field amplitude results in a modulation of the resonant

frequency of the ion. Modulation of this frequency causes the frequency of the

ion to periodically match that of the applied supplemental dipole field. Thus, the

energy coupled to ion motion is maximized, and the effects of shifts in the ion

resonant frequency are minimized. The effectiveness of the resonant excitation

method depends on the mass range over which the rf field is modulated, and the

total time spent in resonance.

An advantage of using nonresonant excitation is that it is not critical to match the

applied dipole frequency to that of the ion. Consequently, the method is affected

neither by electronic drifts, space charge effects, nor sample concentration. This

results in reproducible product ion spectra that are unaffected by changes in the

trapping conditions and sample concentration. This method is often useful with

parent ions that fragment by the breakage of a single weak chemical bond to

form highly stable ions containing functional groups that do not undergo

significant rearrangements. A disadvantage of the method is that it is not

selective with respect to excitation of ions in the trap . Therefore, the method

cannot be selectively tuned to excite only ions having a particular m/z. The

method is also less useful with parent ions in which multiple chemical bonds are

broken and with parent ions that undergo complex rearrangements following

collision-induced dissociation.

The resonant excitation method is selective to the mass range that is excited. It

permits coupling of energy to the motion of an ion having a particular m/z value

in a very controlled way. Consequently, the rate that the amplitude of the ion

motion increases is just balanced by the rate that energy is removed by

collisions. The ion is not ejected and energy can be deposited as internal energy

by increasing the number of collisions. This is done by increasing the excitation

time. Thus, it is possible to fragment parent ions that require the breakage of

The dipole field is simply an electric field oriented along the axis of the trap.

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