Axis BTA 2100D Manual do Operador descarregar pdf (Página 213)

211

Note that at a storage level of 48 m/z and a CID bandwidth of 4 kHz, the mass

range increases from 3 to 40 m/z when the parent ion mass increases from 100

to 400 m/z. Given a parent ion mass of 400 m/z and a CID bandwidth of 4 kHz,

the mass range decreases from 40 m/z for a storage level of 48 m/z to a range of

12 m/z for a storage level of 143 m/z.

The amount of energy that is converted to internal energy in the parent ion

depends on the number of collisions (excitation time), the relative energy of the

collisions (CID excitation amplitude), and the rate that the internal energy is

removed by collisional deactivation (excitation method). Collision-induced

dissociation in the ion trap is always in competition with ion ejection. If the CID

excitation amplitude selected is too large, the parent ion will be ejected to the

trap electrodes before it can collide with background helium atoms. If the CID

excitation amplitude selected is too small, the energy of the parent ion will not

exceed the internal energy threshold required to break the chemical bonds and

form product ions. This is because energy is constantly being removed by low

energy collisional deactivation. Therefore, the CID excitation amplitude and the

excitation time are both used to optimize the CID process in conjunction with an

appropriate choice of excitation method.

Product Ion Mass Scanning

Following formation of the product ions by collision-induced dissociation, a single

rf ramp is used along with the axial modulation field to resonantly scan ions from

the trap into the electron multiplier. This process generates the product ion

spectrum.

Chemical Ionization

Chemical ionization (CI) may also be used to selectively fill the ion trap. In this

case, the ion formation and matrix ion ejection steps described above are

modified. Insertion of the three subsequent MS/MS operations, i.e., parent ion

isolation, product ion formation, and product ion mass scanning, into the basic CI

scan function (following the ionization and reaction steps) creates the CI-MS/MS

scan function.

EI/MS/MS and CI/MS/MS Automatic Space Charge

Control

Space charge control for EI/MS/MS and CI/MS/MS occurs by forming the ions

during a fixed ionization period. Ion formation occurs in the same manner as for

EI or CI. After ion formation, all ions outside of the isolation window specified by

the user are removed from the trap. The ions are subsequently scanned from the

trap at the normal scan rate.

Summary

The four operations described above, i.e., ionization, parent ion isolation, product

ion formation, and product ion mass scanning, are used in the analytical scan.

When automatic space charge control is used the number of parent ions in the

isolation window are used to determine the ionization time required to maintain

the optimum number of ions in the trap. Thus, the total ion space charge level is

held constant as the sample and matrix levels change. Maintaining a constant

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