measurement of gas concentrations by mass spectrometers
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Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, N. Müller, W. Neff, Bled April 2012
Quadrupole Mass Spectrometers are
gas-specific instruments, the fragmentation pattern may lead to spectral overlap
several sources of instabilities
how to operate the instrument if the SEM or C-SEM is the major source of drift?
spectral overlap of gaseous components lead to
MCD-Measurements
how to calibrate
some (two) examples
check whether re-calibration is required
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Functional units of every Quadrupole Mass-Spectrometer:
An ion-current, amplified ion-current or a count rate is the original out-put of these instruments.
Faraday
U+Vcost
Electron multiplier
Ion sourcesupply
RFgenerator
HVsupply
Electrometerpreamplifier
Quadrupole controller
Ion sourceMass filter
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Ion-Source
The amount of ions generated in the ion source depends on the partial pressure ofthe individual species and on the energy of the electrons used for electron impact ionization.
As with every ionization gauge, the signal generated is gas-specific.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Ionization Probability compared to nitrogen (electron energy : 70 eV)
The same current for He and Ar makes a difference in pressure of a factor of 8.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Stability of Quadrupole Mass Spectrometers: (see*)
The cracking pattern and the resolution versus mass range may change.
The mass-scale may shift.
The amplification of the SEM or C-SEM may drift.
* 8th European Vacuum Congress EVC-8 Berlin,. 23.-26. Juni 2003
43.4 43.6 43.8 44.0 44.2 44.4 44.61E-13
1E-12
sensitivity
peak positionS
igna
l/Ion
Cur
rent
amu
0 10 20 30 40 50 600
20
40
60
80
100
trend: aging of the channeltron
QMG 422 with Faraday Prisma with FaradayPrisma with Channeltron
Pa
rtia
l Pre
ssu
re S
en
sitiv
ity [
arb
uni
ts]
Time [hours]
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
change of the cracking pattern (due to the electron energy)
amu
Ar++
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
There are different instruments on the market which are used for various applications.
Whether the mass scale, the resolution, the fragmentation pattern and the amplification may drift and at which time scale rather depends on the design of the instrument and the quality of the electronics used to drive the instrument.
No general figures and numbers can be given for all these “RGA’s”
Anyway: Calibration is required because the ionization is gas specific.
Fragmentation leads to an overlap of different species at the same mass.
If the drift of the SEM or C-SEM is the major (only) source of instabilities, is there a method to get rid of these drifts?
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
The following considerations are valid for Quadrupole Mass Spectrometers, in which the drift of the amplifier is the major source of instabilities, the other’s are neglegible. (see EVC-8 in 2003)
Calibration:To minimize the effort for calibration, gas mixtures which contain several components in a chemical inert carrier gas are used. The pre-condition is that there is no overlap between the components contained in the mixture.Automatically one then gets the sensitivity of the components relative to the carrier gas and the cracking pattern of each component.
If the overall sensitivity of the instrument changes, then there is no influence on the result as long as the change occurs slow compared to a measurement cycle.
Furthermore if a calibration library exists with Argon and Nitrogen as carrier gas the sensitivity for Ar and for N2 have to be determined only in order to combine both libraries.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
nature is not always kind!
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Mass-Spectrum of CO2 recorded at an electron energy of 70 eV.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Model spectrum (Origin of peaks)
0 5 10 15 20 25 30 35 40 45 50
m/e
Hydrogen Nitrogen Oxygen Water
Carbon dioxide Argon Carbon monoxide
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
How to determine concentrations within a gas mixture of N2, CO and CO2 ??
a) Measure the concentration for CO2 at m/e = 44b) Subtract the contribution to mass 16 from CO2 (need to know the cracking pattern)c) Determine the concentration of CO at m/e = 16 then (hope there is no O2 in the sample!)d) Subtract the contribution of CO to the spectrum from the measured spectrum.e) Determine the concentration of nitrogen either at m/e=14 or at mass 28 now.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
signal at mass 12 = PN2*SN2(12) + PCO*SCO(12) + PCO2*SCO2(12)
signal at mass 14 = PN2*SN2(14) + PCO*SCO(14) + PCO2*SCO2(14)
signal at mass 16 = PN2*SN2(16) + PCO*SCO(16) + PCO2*SCO2(16)
where PN2 = the pressure of N2 and SN2(12) is it’s sensitivity factor at mass 12 (which is zerofor mass 12) and so on for all the three components.
Three unknown and three equations.
Today’s computers solve such a system of linear equations in fractions of seconds.
Provided there are only the three components present and the sensitivity factors are preciselyknown, one get’s also a reasonable result.
However, here we still need to know the absolute sensitivity factors for the individual components.
Do we really?
Mathematics:
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
S12 SN2(12) SCO(12) SCO2(12) PN2
S14 = SN2(14) SCO(14) SCO2(14) X PCO
S16 SN2(16) SCO(14) SCO2(14) PCO2
The vector is the solution for the linear equation defined above.
PN2
PCO
PCO2
If we multiply the coefficients SN2, SCO and SCO2 above by a factor F, then
the vector 1/F X is a solution of the modified system of linear equations.
PN2
PCO
PCO2
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Assume that the ratio of the individual sensitivity factors is precisely known,
then the correct ratio of the partial pressures will be achieved:
The result will be F*PN2, F*PCO and F*PCO2 with an undetermined factor F.
The assumption that only these three components are present in the gas leads to
[N2] + [CO] + [CO2] = 100%
which can be easily computed by the measured ratio of the partial pressures.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
first have a look what’sthe residual gas composition
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
It’s not only the mass spectrometer whichdetermines the results.The vacuum system may be as important as well.
Subtract backgronund, however how stable is thebackground from the residual gas?
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
air – inlet and calibration by the known
composition of air:
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
still there is no overlap!
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
calibrate CO2 to Ar moreprecisely by using a gasmixture of known composition
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
In this way an existing calibration matrixcan be expanded.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Example: gas-mixture specified as
CO2 17.2%CO 18.5%Ar 19.2%
N2 45.1%
There is spectral overlap with this mixture.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Question?:re-calibrate with this mixturewas the previous mixture more precise?
pressure change within a measurement cycle
as there is no N2, CO, CO2, Ar in the sample, the assumptionthat the sum of all is 100% is not justified, result not applicable.
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
changes in process pressure do notinfluence the result
real, or due to changingbackground?
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
0 2 4 6 8 1090
92
94
96
98
100
The MID-Signal may be modulated by pressure fluctuations, thereforea short cycle time is an advatage.A too short dwell time however may lead to an inacurate measurement of
the ion current (10-12
ampere).
MID-Cycle: preselected masses are measured cyclic during a preprogrammed dwell time
Dwelltime Mass1 Dwelltime Mass2 ..... Dwelltime Mass n
MID-Cycle Time versus Pressure Fluctuations
+/- 1%
Tot
al P
ress
ure
[arb
. uni
ts]
time [arb. units]
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
proces pressure about 1*10-7 mbar, that’s too low for realisticmeasurement, because much to near to base pressurere-adjustment of background subtraction
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
How to check whether
re-calibration is required?
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
record a test-gas spectrumfrom time to time
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Ar
Test-Gas-Mixture
export to excel
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
amu
Difference of two spectra, each normalized to 1000 at it’s maximum
difference +/- 3 % ok
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
amu
mass scale shift
difference of two spectra squared:
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
How to operate the instrument if the SEM or C-SEM is the major source of drift
spectral overlap of gaseous components lead to
MCD-Measurements (system of linear equations solved on line)
how to calibrate
examples
Suggestion how to check whether re-calibration is required
Apr 20, 202335 file_name
Günter Peter, Bled April 2012
Thank you for your attention!
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