Axis BTA 2100D Manual del operador descargar pdf (Pagina 99)

scanned out of the ion trap. Thus, when the multiplier gain is changed, there is

no change in the actual quantity of ions formed and measured in the prescan

measurement, but the peak area of the AGC peak will be changed. The AGC

software will misinterpret this change in peak area as a real change in the

number of ions created in the ion trap and will adjust the ionization time

accordingly. An incorrect ionization time may cause a change in mass resolution.

The AGC software is dependent on the multiplier gain remaining constant in

order to measure the quantity of ions in the ion trap correctly and set the proper

ionization time.

Under certain circumstances (low background, small signal) increasing the

electron multiplier gain by +100 or +200 volts above the 10

gain can decrease

the detection limit. As explained in the note above, the same multiplier gain must

be used for determining the calibration curve and analyzing unknown samples.

Axial Modulation

The number of ions stored in the ion trap cavity is proportional to the ionization

time and the filament emission current. Although the maximum ion-storage

capacity of the ion trap is approximately 10

to 10

ions, space-charge repulsion

occurs when the number of ions exceeds 10

to 10

. Space-charge interactions

cause a loss in unit mass resolution. To overcome this limitation, a small ac

voltage, the axial modulation voltage, is applied to the endcap electrodes.

The axial modulation voltage is applied at a fixed frequency and amplitude during

the ramp of the rf voltage. The frequency of the axial modulation voltage is 485

kHz, which is about one-half of the frequency of the rf voltage. Only when an ion

is about to be ejected from the ion trap cavity is it in resonance with this

frequency. When an ion comes into resonance with this frequency, it moves

away from the center of the trap, where the field generated by the rf voltage is

zero (and space-charge effects are strong), into a region where the field

produced by the rf voltage is strong (and space-charge effects are small). As a

result, the ejection of the ion is facilitated, and mass resolution is significantly

improved. The default axial modulation voltage is 4.0. Once the optimum axial

modulate ion amplitude has been set for a given instrument, it need not be

changed.

Emission Current

The number of ions formed in the trap is a function of both the ionization time

and the intensity of the electron beam. In general, increasing the filament

emission current increases the quantity of ions produced. The relationship

between the filament emission current and the measured TIC (i.e., the number of

ions generated in the ion trap) is not a strictly linear one. In particular, the

emission current and the measured TIC do not vary linearly for emission current

values above 40 µA and at high sample concentration.

In certain cases, a high emission current can be used to increase the sensitivity.

One of these is EI/MS/MS where the MS/MS isolation step eliminates the higher

background that would normally arise from a high emission current.

Ion Trap Temperature

After the system has been shut down and the ion trap vented to the atmosphere,

we recommend an initial trap temperature of 220°C for the rapid removal of water

vapor. After this period, a reduction in trap temperature to 150-175°C will extend

the turbomolecular vacuum pump lifetime. Some applications may require higher

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