Axis BTA 2100D Manual do Operador descarregar pdf (Página 210)

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MS/MS Scan Function

Parent Ion Isolation

The parent ions can be completely isolated from any remaining matrix ions in

another two-step process.

The first step ejects the ions with masses below the parent ion mass by ramping

the rf field amplitude with axial modulation applied to the end cap electrodes.

This step resonantly ejects all ions having masses up to, and including, the mass

just below the parent ion mass. Axial modulation is then turned off.

In the second step, a broadband multifrequency waveform (WF2) is applied to

the trap to simultaneously eject all masses above the parent ion mass. Isolation

occurs at elevated rf levels where mass resolution is optimal. Thus, it is possible

to achieve unit mass isolation over the entire mass range of the ion trap.

Achieving unit isolation is useful in isolating a single mass in an isotopic cluster,

or in separating the parent ion from interfering matrix ions. This high resolution

isolation method is further advantageous in that it is calibrated using standard

mass calibration information, and uses the same broadband waveform for all

masses. Thus, if the system is recalibrated, or if a new parent ion mass is

selected, recalculation of the waveform is not required.

Product Ion Formation

Product ions are formed from the parent ions by collision-induced dissociation

(CID). The parent ions constantly undergo collisions with helium gas in the ion

trap. Normally these collisions involve relatively small energies, but if the

translational energy of the parent ion is increased, the collisions may convert the

translational kinetic energy to internal vibrational energy.

If the parent ion acquires enough vibrational energy, one or more chemical

bonds in the ion may be broken, forming ions of lower m/z than the original

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