From conventional Proton-Transfer-Reaction Mass Spectrometry
(PTR-MS) to a universal trace gas analyzer
Alfons Jordan1, Achim Edtbauer1, Eugen Hartungen1, Simone Jürschik1, Philipp Sulzer1, Lukas Märk1, Tilmann D. Märk1,2
Compound
Proton-Transfer-Reaction-Mass Spectrometry (PTRMS) is a well established technology for real-time
trace gas analysis in the fields of environmental
research, food and flavor science, medicine, homeland
security, etc. [1]. However, one limitation of
conventional PTR-MS is that only compounds
possessing a higher proton affinity than water can be
ionized. Also recent add-ons,, like the so-called
Switchable Reagent Ions (SRI) [2] feature, where O2+
(and NO+) can be utilized to ionize trace gas molecules
via charge transfer, are limited to compounds having
ionization energies below 12.1 eV, which still excludes
important substances like CO, CO2, SO2, CH4, etc.
Here we present a novel development (that we call
SRI+) avoiding these limitations while preserving all
advantages of conventional PTR-MS (fast response
times, high sensitivity, no sample treatment, etc.).
Formula
Methane
Carbon monoxide
Nitric oxide
Hydrogen sulfide
Carbon dioxide
Nitrous oxide
Nitrogen dioxide
Sulfur dioxide
Sulfuryl fluoride
Exact
mass (amu)
16.031
27.994
29.998
33.987
43.989
44.001
45.992
63.961
101.959
CH4
CO
NO
H2S
CO2
N2O
NO2
SO2
SO2F2
Ionization
energy (eV)
12.61
14.01
9.26
10.46
13.78
12.89
9.59
12.35
13.04
In the table above some compounds of the present study are listed including the exact
masses and the ionization energies.
g
All of these compounds
p
can be ionized via charge
g
transfer from Kr+.
N2
120
Experimental Setup
For the present study we utilized a high resolution (up to 8000 m/∆m)
and high sensitivity (up to 280 cps/ppbv) PTR-TOF 8000 which has
been described in detail elsewhere [3,4]. A schematic view of this
instrument is printed on the right.
We modified the ion source and the vacuum conditions in a way that
we can now produce H3O+, NO+, O2+, Kr+ and Xe+, respectively, at
very high purity levels without the need of a reagent ion mass filter
between the ion source and the drift tube. The ionization energy (IE)
off Xe
X + is
i similar
i il to
t the
th IE off O2+ (12.13
(12 13 and
d 12.07
12 07 eV,
V respectively),
ti l )
i.e. Xe+ can be used as a substitute in environments where oxygen
gas cylinders are prohibited (because of fire protection or ex-proof
laws). Therefore, here we only present data obtained with Kr+ (IE
14.00 eV) as reagent ions.
As with Kr+ also some of the main compounds of air get ionized, a
buffer gas possessing a higher IE than Kr has to be added to the
sample gas. In the present study we used He (IE 24.59 eV) and N2
(IE 15.58 eV), respectively, as a buffer gas and diluted the sample
gas utilizing a simple mixing setup.
setup
+
The screenshot above was taken from one of our in-house
programmed data processing software programs.
programs Human breath
(CO2 source) was mixed with pure N2O in a PTFE sampling bag and
subsequently analyzed with an SRI+ equipped PTR-TOF 8000. It can
not only be seen that the instrument is capable of detecting the two
isobaric compounds (mass difference 0.01 amu) separately but also
that the software can identify the two peaks automatically.
On the top right a section of a mass spectrum obtained from a
measurement of cigarette smoke is displayed. Three isobaric
compounds share the nominal mass 28 m/z.
m/z However,
However with the high
mass resolution of the instrument it is possible to clearly identify them
as CO, N2 (impurity from the ion source) and C2H4. Please note that
none of these molecules could be ionized via PTR from H3O+, thus
these results demonstrate the importance of the novel SRI+
development.
On the right a 3D graph of a benzene measurement is shown.
Starting at about cycle 200 a gas standard containing 1 ppmv of
benzene is step-wise
p
admixed to the buffer g
gas ((He in this case)) in
increasing amounts. Moreover, although C6H6+ possesses the same
nominal mass as 78Kr+ both compounds can be clearly separated, i.e.
the very important molecule benzene does not get "masked" by the
extremely abundant reagent ion.
Acknowledgement
We gratefully acknowledge that this work was financially supported by the FFG, Wien, Austria.
C2H4
+
CO
0
27.97
27.98
27.99
28.00
28.01
28.02
Mass (m/z)
28.03
+
28.04
28.05
28.06
Figures d) - f) show three sulfur containing
compounds, namely H2S, SO2 and SO2F2. All
samples were present as gas standards with N2
as a buffer gas in concentrations of about 1
ppmv for the first two compounds and about 15
ppmv for the latter one. On nominal mass 34
m/z there is also 18OO+ (impurity from the ion
source) present at a very low abundance, which
can be easily distinguished from H2S+ yielding
more than one order of magnitude higher (d)).
SO2 shows
h
a single
i l peakk corresponding
di to
t SO2+
in figure e). As SO2F2 (f)) was present in a much
higher concentration (15 ppmv) the signal
intensity on 101.96 m/z already saturated the
detector. However, this does not limit the
instruments quantification ability, as the isotope
present at 103.95 m/z (4.5% natural abundance)
can be utilized for quantification in this case.
Ion yield (arb. units)
Ion yield ((arb. units)
600
400
300
200
100
0
43.90
43.92
43.94
43.96
43.98
44.00
44.02
0
33.85
44.04
18
33.90
CO
30
N2
+
OO
+
33.95
34.00
34.05
34.10
Mass (m/z)
Mass (m/z)
40
d)
+
H2 S
400
800
200
Ion yield (arb. units)
40
R
e
s
u
l
t
s
Figures
g
a)) - c)) on the right
g display
p y the results of
the analysis of a gas standard containing about
1 ppmv of CO2, CO and CH4 in N2, respectively.
CO2 is detected exclusively on its exact mass in
a), whereas CH4 yields CH3+ and CH4+ in c). The
impurity N2+ (from the ion source) and CO+ are
isobaric compounds. However, as it can be seen
in b) both peaks are separated by a 25% valley.
b)
+
SO2
2800
+
e)
2400
20
10
2000
1600
1200
800
400
0
27.90
27.95
28.00
28.05
0
63.85
28.10
63.90
200
CH3
63.95
64.00
64.05
Mass (m/z)
Mass (m/z)
Ion yield (arb. units)
Ion yield (arb. units)
80
a)
+
CO2
1000
Mass (m/z)
+
Kr
Kr,
Xe
Ion yield (arb. units)
Abstract
1 IONICON Analytik GmbH, Eduard-Bodem-Gasse 3, 6020 Innsbruck, Austria
Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
c)
+
SO2F2
30000
Ion yield (arb. units)
2
160
120
CH4
80
+
f)
+
20000
10000
40
0
14.50
14.75
15.00
15.25
15.50
15.75
Mass (m/z)
16.00
16.25
16.50
0
101.5
102.0
103.5
104.0
Mass (m/z)
References
IE data of the substances were taken from: http://webbook.nist.gov
[1] R. S. Blake, P. S. Monks, A. M. Ellis; Chem. Rev., 109 (3) (2009), 861-896.
[2] A. Jordan, S. Haidacher, G. Hanel, E. Hartungen, J. Herbig, L. Märk, R. Schottkowsky, H. Seehauser, P. Sulzer, T. D. Märk, Int. J. of Mass Spec., 286 (2009), 32-38.
[3] A. Jordan, S. Haidacher, G. Hanel, E. Hartungen, L. Märk, H. Seehauser, R. Schottkowsky, P. Sulzer, T.D. Märk, Int. J. of Mass Spec., 286 (2009), 122–128.
[4] E. Hartungen, P. Sulzer, A. Edtbauer, S. Jürschik, A. Jordan, L. Märk, T. D. Märk, http://www.ionicon.com/downloads/IONICON_PTR-TOF_8000_2000_Performance-Data_White-Paper_2012.pdf.
104.5