Axis BTA 2100D Instrukcja Operatora Pobierz pdf (Strona 203)

201

Specifying the Ion Preparation Parameters

The Ion Preparation Parameters control how the basic EI or CI scan functions

are modified to prepare the ions for scanning. You have control over which ions

are ejected, which are retained, and which will undergo dissociation (CID). Your

input and the Ion Preparation technique you are using determine how the scan

function is constructed and what custom waveforms are created to complete the

analysis.

Theory of Operation of the SIS Option

Using the Selected Ion Storage (SIS) option, you may selectively accumulate or

store specific ions in the ion trap. Success with this technique requires that there

be no excessive space-charge build up. Optimum mass resolution is achieved by

controlling the number of ions in the trap. When the concentration of background

matrix or co-eluting compound is large compared to the target compound,

Automatic Gain Control (AGC) maintains the correct number of ions in the trap

by reducing the ionization time. However, this also reduces the number of target

compound ions available for analysis.

SIS eliminates unwanted ions by ejecting them from the trap. Given the optimum

number of ions that can be stored in the trap, SIS enriches the sample ions

relative to the unwanted matrix ions by ejecting the latter throughout ionization.

The unwanted ions are ejected from the trap by a technique known as resonant

ion ejection.

Trapped ions exhibit a characteristic, or secular, frequency of oscillation. This

frequency depends on the mass of the ion and the amplitude of the fundamental

storage rf field. An ion's secular frequency increases with increasing storage rf

voltage, and decreases with increasing mass.

Normally, mass scanning results from the application of a single supplemental

dipole field to the end caps of the ion trap, and the application of a linear

frequency increase of rf voltage. As the rf voltage is increased, the ion's secular

frequency increases until it matches the frequency of the applied supplemental

field. At that point, resonance occurs and the ion absorbs energy from the

external field. The amplitude of the ion motion increases, and continues to

increase until the ion is ejected from the trap and the ion is detected.

Resonant ion ejection may also be used to eject multiple ions in a fixed rf storage

field. This is accomplished by applying multiple frequencies to the end caps of

the ion trap. One convenient method of applying multiple frequencies is to use a

broadband multifrequency waveform. The multifrequency waveforms used in SIS

include those frequencies required to eject the unwanted ions from the trap

without affecting the ions to be stored.

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Axis BTA 2100D Instrukcja Operatora Strona 203