SRS Labs QMS 100 Series, Руководство пользователя

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Calibration 3-13
User’s Manual
(Slope), stored in the non-volatile memory of the RGA, to calculate the 8 bit settings of the DAC
according to the linear equation:
DAC8 (m) = DS
.
m + DI (DC_Tweek (m) = (DAC8(m) - 128)
.
19.6 mV)
where m is the mass in amu, and DAC8(m) is the 8 bit setting at that mass.
The purpose of the Peak Width Tuning Procedure is to determine the values of DI and DS so that all
the peaks in an analog spectrum have the desired peak width (typically Dm
10%
=1 amu). The calibrated
parameters are saved in the non-volatile memory of the RGA Head (DI and DS commands,
respectively) and used by the firmware to generate the internal scan parameters used to adjust
DC_Tweek during scans and single mass measurements. Please consult the Tuning Commands List in
the RGA Programming chapter of this manual for details on the DS and DI commands.
Note: The sensitivity of the peak widths to the DC_TWEEK voltage can accurately be approximated
to: -1 amu per 550mV (28 bits). In other words, a 550 mV increase in DC_Tweek voltage causes a 1
amu decrease in the width of any peak. The DC_Tweek voltages span from -2.5 to +2.5 V with
increments of 19.6 mV per bit (255 total bits). This corresponds to a peak width adjustment range of
+/- 4.5 amu and minimum increments of 0.036 amu per bit.
As described above, the peak width tuning procedure requires the introduction of two known gases into
the vacuum system. A low mass gas (1-20 amu recommended) is used to adjust peak widths at the low
end of the mass scale, a high mass gas, with a mass-to-charge ratio close to the upper limit of the
scanning range of the RGA, is used to adjust the widths at the high end of the mass scale. Several
analog scans are performed on the sample and the values of DI ( nominally 128) and DS (nominally 0)
are adjusted until all peaks appear at their correct setting. The formulae used to correct the calibration
parameters during peak position adjustment are described next:
Low Mass Peak Width Adjustment : To increase the low mass peak width by an amount
∆
m
AMU’s modify the value of DI from its original value, DI
0
, to:
DI = DI
0
-
∆
m * 28
Notes:
•
The new DI value must fall within the acceptable parameter range of the DI command.
•
A change in DI affects the width of all the peaks in the spectrum.
•
A decrease in DI results in broader peaks at a rate of 0.036 amu per bit removed.
High Mass Peak Width Adjustment : To modify the high mass peak width by an amount
∆
m
AMU’s, modify the value of DS from its original value DS
0
to:
DS = DS
0
- 28 * (
∆
m) / m
0
Notes:
•
The new DS value must fall within the acceptable parameter range of the DS command.
•
Modifying DS affects the width of peaks at the higher end of the spectrum. An increase in DS
results in the peaks getting sharper, and a decrease in DS results in the peaks getting broader