Axis BTA 2100D Instrukcja Operatora Pobierz pdf (Strona 192)

190

GC peaks) the maximum ionization time of 65 milliseconds is selected, which

offers maximum sensitivity. As the amount of analyte increases, the ionization

time is decreased automatically in order to prevent saturating the ion trap with

ions.

The ion signals in each microscan are scaled automatically to correct for the

variation in ionization time. Microscans are averaged to improve the spectral

quality.

Ion Formation

In the EI mode electrons are passed through the ion trap cavity during the

ionization period. These electrons collide with the neutral sample molecules

present and transfer some of their energy to the molecules. These energetic

molecules decompose through a series of reactions producing ions and neutral

fragments. This set of fragment ions, characteristic of the sample molecule,

makes up the mass spectrum. The degree of fragmentation is a function of the

internal energy of the molecules, which is a function of the energy imparted by

the electrons, as well as the ion trap temperature and the axial modulation

voltage amplitude.

The ionization time is the time (between 10 µs and 65 milliseconds) during which

the electron gate voltage is positive, and the electron beam from the filament is

permitted to enter the ion trap. The electron beam generates ions by colliding

with sample and/or background molecules which are present within the ion trap.

The AGC software provides control of the ionization time so that the quantity of

ions can be maintained at the optimum level.

The number of ions formed in the trap is also a function of 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 A and at high

sample concentration.

Keep in mind that both the ionization time and the intensity of the electron beam

directly control the number of ions formed, and that the ion trap performs best

(producing the highest quality spectra with good resolution) when an optimum

quantity of ions (or “charge”) exists within its chamber.

AGC Prescan

The 0.1 millisecond ionization is followed by a ramping of the rf voltage to eject

the permanent background from 35 m/z to an upper mass limit selected by the

user. This upper mass limit is defined as the background mass. The default value

for the upper mass limit is 45 m/z; as a result, all masses below the background

mass (i.e., those masses corresponding to the chemical background) are ejected

from the trap.

The rf voltage is then ramped to its maximum value, and all ions with masses

above the background mass (i.e., those masses corresponding to sample ions)

are ejected from the ion trap and are detected by the electron multiplier. The

electron multiplier produces a signal proportional to the number of ions detected.

1 2 ... 187 188 189 190 191 192 193 194 195 196 197 ... 596 597

Komentarze do niniejszej Instrukcji

Brak uwag

Axis BTA 2100D Instrukcja Operatora Strona 192

Komentarze do niniejszej Instrukcji

Powiązane produkty i podręczniki dla Oprogramowanie Axis BTA 2100D