SRS Labs QMS 100 Series, Руководство пользователя

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Calibration 3-5
User’s Manual
Calibration of Partial Pressure
All quantitative calculations performed with the RGA rely on the assumption that there is a linear relation
between the partial pressure and the corresponding RGA signals of the gases. Each gas ionizes
differently, and its ions make it through the mass filter with different efficiencies. As a result the
proportionality constant relating the ion current of a gas to its partial pressure is dependent on the
specific gas.
The partial pressure sensitivity of the RGA to a gas g,
S
g
, is defined as the ratio of the change
(H-
H
0
)
in principal mass peak height to the corresponding change
(P-P
0
)
in total pressure due to a change
in partial pressure of the particular gas species.
H
0
and
P
0
are background values.
S
g
= (H-H
0
) / (P-P
0
)
The units of
S
g
are of ion current per unit pressure (amp/Torr, for example).
The sensitivity of the RGA changes with time due to aging of the probe, and is a strong function of the
operating conditions of the instrument. Careful quantitative analysis requires that the sensitivity factor,
S
g
, be determined for every gas which may be a component gas in the system being analyzed. The
sensitivity factors must be obtained under the same operating conditions that will be used during general
partial pressure analysis since they depend on many instrumental parameters, including: ionization
energy, emission current, mass filter setting, type of detector, etc.
Important: In order to separate the gain of the electron multiplier from the intrinsic sensitivity of the
RGA head, the sensitivity factors of the RGA are defined for Faraday Cup detection. A separate
Electron Multiplier Gain Factor, is used to correct the ion signals when the electron multiplier is used.
See the Electron Multiplier Tuning section for details.
Two calibration factors are used in the QMS system: the RGA sensitivity and the pressure reduction
factor. The RGA sensitivity is the factor which converts the ion current that is measured by the
electrometer to partial pressure at the ionizer. The pressure reduction factor accounts for the large
pressure reduction performed by the two stage inlet (capillary and aperture). Determination of these
factors requires comparing the system with a known-accurate pressure gauge and calculating the factor
that makes the QMS agree with the standard. Calibration is not necessary on a frequent interval, but is
required whenever operating conditions change.
The RGA intrinsically measures an ion current, which is proportional to the partial pressure at its ionizer.
While the software can be set to report ion currents, most users will need to measure partial pressure at
the inlet of the capillary. To convert between the two, the partial pressure reported by the software is
calculated by the formula:
P = ion current (A)
pressure reduction factor
sensitivity factor (A Torr )
= ion current (A) overall factor (Torr A )
i
-1
-1
× ×
.
The pressure reduction factor is a function of the capillary dimensions, the performance curve of the
pumps, and the dimensions of the aperture. The sensitivity factor is a function of the precise dimensions
of quadruple and ion optics, the state of the detector, the ionizer filament, and the four parameters which