Varian 4000 MS, Руководство пользователя

Страница: 27 / 39

27
from the fact that the NIST library reports molecular weights to the nearest
integer mass unit only. The Varian MS Workstation software must decide to
which mass to assign measured intensity. If the exact mass of an ion happens to
fall close to the dividing line between integer masses, the software may make an
incorrect mass assignment. This scenario is more likely for molecules with higher
molecular weights, since the mass defects for several atoms may add together to
produce a sizable mass defect. For example, the exact mass for the lightest
isotope form of C
2
Br
6
is 497.51002, which could be assigned as either 497 or
498.
The
Multiplier Offset
adjusts the EM voltage as by much as
±
300V relative to
the current multiplier setting in the Module Attributes tab dialog in Manual Control
(this is usually the 10
5
gain value from Auto Tune). Sometimes better sensitivity
is achieved, particularly in techniques such as MS/MS, when the multiplier
voltage is increased. NOTE that this adjustment can be made on a segment-by-
segment basis.
The
Count Threshold
is normally 1; a value of 2-3 counts will reduce the
number of low-level ions reported in the mass spectrum. This approach may
improve library searches and reduce data file size at the cost of somewhat less
detailed information in the mass spectra. The count threshold is shown only if the
Customize
button is active.
Ionization Control
The
Target Total Ion Current
, or TIC is specified here. The Automatic Gain
Control (AGC) algorithm uses the ion count from a prescan at fixed ion time,
along with this target value, to calculate an ion time necessary to fill the ion trap
with the target number of ions during the analytical scan. The objective is to fill
the trap with an optimal number of ions during each analytical scan. The Target
TIC is usually not set below 10,000 for full scan acquisitions, but it should also
not be set too high or spectral distortions due to space charge may result (loss of
MS resolution and/or shift in mass assignments for strong chromatographic
peaks). Typically, a Target TIC between 20,000 and 40,000 counts gives the best
results.
EI Ionization Parameters
The
Emission Current
is the current of electrons produced by the filament and it
controls the number of electrons that enter the trap. By increasing the emission
current so that the ionization time of the baseline is approximately the maximum
ionization time, the maximum number of ions will be present. In certain cases, a
high emission current can be used to increase the sensitivity of the analysis. An
example of this is in EI/MS/MS where the MS/MS isolation step eliminates the
higher background that would normally arise from a high emission current.