Measurement of Gas Concentrations by Mass Spectrometers
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
Günter Peter, N. Müller, W. Neff, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Functional units of every Quadrupole Mass-Spectrometer:
Ion source
Mass filter
Electron multiplier
Focus
Extraction
Wehnelt
Filament
Formation
chamber
Faraday
U+Vcost
Ion source
supply
RF
generator
Electrometer
preamplifier
HV
supply
Quadrupole controller
An ion-current, amplified ion-current or a count rate is the original out-put of these
instruments.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Ion-Source
The amount of ions generated in the ion source depends on the partial pressure of
the 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.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
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.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Stability of Quadrupole Mass Spectrometers: (see*)
The cracking pattern and the resolution versus mass range may change.
peak position
The mass-scale may shift.
Signal/Ion Current
sensitivity
1E-12
1E-13
43.4
43.6
43.8
44.0
44.2
44.4
44.6
amu
The amplification of the SEM or C-SEM may drift.
Partial Pressure Sensitivity [arb units]
100
80
60
trend: aging of the
channeltron
QMG 422 with Faraday
Prisma with Faraday
Prisma with Channeltron
40
20
0
0
10
20
30
40
50
60
Time [hours]
*
8th European Vacuum Congress EVC-8 Berlin,. 23.-26. Juni 2003
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
change of the cracking pattern (due to the electron energy)
1.20E+03
1.00E+03
8.00E+02
6.00E+02
4.00E+02
Ar++
2.00E+02
0.00E+00
18
23
28
33
38
43
amu
-2.00E+02
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
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?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
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.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
nature is not always kind!
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Mass-Spectrum of CO2 recorded at an electron energy of 70 eV.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Model spectrum (Origin of peaks)
Hydrogen
Carbon dioxide
0
5
10
Nitrogen
Argon
15
Oxygen
Carbon monoxide
20
25
30
35
Water
40
45
50
m/e
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
How to determine concentrations within a gas mixture of N2, CO and CO2 ??
a)
b)
c)
d)
e)
Measure the concentration for CO2 at m/e = 44
Subtract the contribution to mass 16 from CO2 (need to know the cracking pattern)
Determine the concentration of CO at m/e = 16 then (hope there is no O 2 in the sample!)
Subtract the contribution of CO to the spectrum from the measured spectrum.
Determine the concentration of nitrogen either at m/e=14 or at mass 28 now.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Mathematics:
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 zero
for 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 precisely
known, 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?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
S12
S14
=
S16
SN2(12)
SCO(12)
SCO2(12)
SN2(14)
SCO(14)
SCO2(14)
SN2(16)
SCO(14)
SCO2(14)
PN2
X
PCO
PCO2
PN2
PCO
The vector
is the solution for the linear equation defined above.
PCO2
If we multiply the coefficients SN2, SCO and SCO2 above by a factor F, then
PN2
the vector
1/F X
PCO
is a solution of the modified system of linear equations.
PCO2
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
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.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
first have a look what’s
the residual gas composition
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
It’s not only the mass spectrometer which
determines the results.
The vacuum system may be as important as well.
Subtract backgronund, however how stable is the
background from the residual gas?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
air – inlet and calibration by the known
composition of air:
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
still there is no overlap!
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
calibrate CO2 to Ar more
precisely by using a gas
mixture of known composition
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
In this way an existing calibration matrix
can be expanded.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Example: gas-mixture specified as
CO2 17.2%
CO 18.5%
Ar
19.2%
N2
45.1%
There is spectral overlap with this mixture.
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
as there is no N2, CO, CO2, Ar in the sample, the assumption
that the sum of all is 100% is not justified, result not applicable.
pressure change within a measurement cycle
Question?:
re-calibrate with this mixture
was the previous mixture more precise?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
changes in process pressure do not
influence the result
real, or due to changing
background?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
MID-Cycle Time versus Pressure Fluctuations
+/- 1%
Total Pressure [arb. units]
100
98
Dwelltime Mass1
96
Dwelltime Mass2 ..... Dwelltime Mass n
MID-Cycle: preselected masses are measured cyclic during a
preprogrammed dwell time
94
The MID-Signal may be modulated by pressure fluctuations, therefore
a short cycle time is an advatage.
A too short dwell time however may lead to an inacurate measurement of
92
the ion current (10
-12
ampere).
90
0
2
4
6
8
10
time [arb. units]
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
proces pressure about 1*10-7 mbar, that’s too low for realistic
measurement, because much to near to base pressure
re-adjustment of background subtraction
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
How to check whether
re-calibration is required?
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
record a test-gas spectrum
from time to time
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Ar
1.00E+03
Test-Gas-Mixture
N2
He
1.00E+02
Kr
Xe
1.00E+01
1.00E+00
0
20
40
60
80
100
120
140
export to excel
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
Difference of two spectra, each normalized to 1000 at it’s maximum
4.00E+01
difference +/- 3 % ok
3.00E+01
2.00E+01
1.00E+01
0.00E+00
0
50
100
150
amu
-1.00E+01
-2.00E+01
-3.00E+01
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
difference of two spectra squared:
3.00E+05
2.50E+05
mass scale shift
2.00E+05
1.50E+05
1.00E+05
5.00E+04
0.00E+00
18
23
28
33
38
43 amu
-5.00E+04
Günter Peter, Bled April 2012
Measurement of Gas Concentrations by Mass Spectrometers
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
Günter Peter, Bled April 2012
35
April 15
file_name
Thank you for your attention!
Günter Peter, Bled April 2012