GC-MS analysis of reaction products in nitrogen and methane gas mixture
L. Polachova1,3, G. Horvath2,3, J. Watson4, N.J. Mason3, F. Krcma1, M. Zahoran2 and S. Matejcik2
1
Faculty of Chemistry, Brno University of Technology, Purkynova 119, 612 00, Brno, Czech Republic
2
Department of Experimental Physics, Comenius University, Mlynska dolina F-2, 842 48 Bratislava, Slovakia
3
Department of Physics and Astronomy, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
4
Planetary and Space Sciences Research Institute, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Abstract: Recent space missions have revolutionized our knowledge of planetary
atmospheres in the solar system, most notably those of Mars and Titan.
Simultaneously laboratory plasmas have been used to mimic the physical and
chemical processes within such planetary atmospheres both to benchmark
physico-chemical models and to interpret observations e.g. by providing
plausible candidates for both spectral and mass spectrometric studies. In this
paper we report the products formed in a gliding arc discharge fed by a 2 or 5%
N2-CH4 gas mixture which mimics Titan’s atmosphere. Gas samples from the
discharge exhaust were analyzed by GC-MS. Acetylene, hydrogen cyanide, and
acetonitrile were found to be the major products. Minor detected products were:
ethane, ethene, cyanogen, propene, propane, propyne, 1,2-propadiene, 1-butene3-yne, 1,3-butadiene, 1,3-butadiyne, 2-propenenitrile, 2-propanenitril, 2methylpropanenitrile, 2-methylpropanenitrile, benzene, and toluene. Whilst many
of these compounds have been predicted and/or observed in Titan atmosphere the
present plasma experiments provides evidence both of the chemical complexity
of Titan and the mechanisms by which larger species grow prior to forming the
dust that covers much of Titan’s surface
Keywords: reaction nitrogen-methane gas mixture, gliding arc discharge, cold
trap, Gas Chromatography-Mass Spectrometry.
1. Introduction
Titan is the largest moon in Saturn’s lunar system
and has been a subject of interest to astronomers and
planetary scientists for a long time because the
atmospheric conditions are thought to have
resembled those conditions on the Earth several
billion years ago [1]. Its atmospheric composition is
principally nitrogen with 2–6 % of methane and
some other gases that could be generated at low
power discharges in the methane clouds [1]. The
most important minor compounds detected by
Cassini-Huygens space mission are nitriles (HCN,
HC3N, HC5N, C2N2) believed to be formed as a
result of dissociation of nitrogen and methane either
by solar induced photolysis or by electron impact [2]
and hydrocarbons (C2H2, C2H4, C2H6, C3H8, C3H4
[2]). In order to understand the physical and
chemical processes leading to such observed
phenomena additional laboratory simulations are
required.
Discharges have been shown to be good mimics of
planetary atmospheres providing insights into both
physical and chemical processes. DBD, glow,
microwave, RF and corona discharges [3-11], have
been all used in order to study electron-molecule and
ion-molecule reactions in planetary atmospheres. In
particular in recent years several of these discharges
have been used to mimic Titan’s atmosphere and
shown that various complex compounds can be
formed, for example the higher hydrocarbons,
nitriles or even amino acids. This paper presents the
results of organic synthesis in a Titan like
atmosphere operated in a Gliding Arc Discharge
(GAD). We observe the production of C6 to C9
hydrocarbons from methane in the discharge. To
understand the molecular kinetics during the
discharge, the energy distribution as well as
knowledge of plasma composition is necessary.
In this paper we provide our results of a qualitative
and quantitative GC-MS analysis of gliding arc
discharges fed by a CH4 – N2 gas mixture.
2. Experiment
The apparatus used in our experiments is shown
schematically in Figure 1. The flow rates through the
reactor for both CH4 and N2 were regulated using
MKS mass flow controllers. The measurements were
carried out in a flow regime over a range from 100
and 200 ml min-1 at atmospheric pressure and
laboratory temperature. The discharge electrode
system had the standard configuration of a classical
gliding arc. A pair of stainless steel holders was
positioned in parallel to the iron electrodes, but in
this case the plasma was not gliding due to the low
flow rate. Astable abnormal glow plasma occurred
between the electrodes at their shortest distance of
2 mm, and a plasma channel with diameter of 1 mm
was formed. The electrical parameters were
measured by a Tektronix oscilloscope using high
voltage probe and Rogowsky current probe. The
reactor chamber had a volume of 0.3 L. The
discharge was powered by a DC HV source. The
discharge was ignited at 5500 V, then voltage drop
reached a value at about 400 V but with increasing
current (between 15 and 40 mA) the voltage was
found to slightly decrease from 400 to 350 V. The
present experiments were performed for different
N2:CH4 ratios in range from 2 % and 5 % methane in
nitrogen.
Gaseous samples of the products formed in the
discharge were taken out for GC-MS analysis
through a gas outlet into a cold trap. The cold trap
was subsequently heated and the resultant gas
sample for GC-MS analysis was taken using a lock
syringe before being immediately analyzed using
Gas Chromatography and Mass spectrometry. GCMS analysis was carried out using an Agilent
Technologies 6890 gas chromatograph coupled to a
5973 mass spectrometer. Separation was performed
on a J&W GS-Q PLOT column (30 m in length,
0.32 mm internal diameter). Helium with a column
flow rate of 2 sccm was used as the carrier gas.
Injection was at a 5:1 split and injector temperature
was 220 °C. The GC oven temperature was held for
2 min at 35 °C and then programmed at 10 °C min−1
upto 220 °C with the final temperature held for 5
min. The MS was operated in electron impact
(70 eV) mode and scanned between 12-120 amu at
approximately 11 scans/sec.
Figure 1. Experimental set up: 1- storage bottle of nitrogen, 2storage bottle of methane, 3- MKS mass flow controllers, 4- DC
power supply, 5- oscilloscope, 6- electrode system, 7- reactor
body, 8- IR gas cell, 9- FTIR spectrometer, 10- cold trap for
GC-MS, 11- exhaust.
3. Results and discussion
The example of results from GC-MS analysis are
shown in Figure 2. Various compounds were
detected, for example the hydrocarbons: ethene,
acetylene, ethane, propene, propane, propyne, 1,2propadiene, 1-butene-3-yne, 1,3-butadiene and 1,3butadiyne. The following nitriles were also
identified: hydrogencyanide, acetonitrile, cyanogen,
2-propenenitrile, 2-propanenitrile, 2-methylpropenenitrile and 2-methylpropanenitrile. Benzene and
toluene were detected as the aromatic compounds.
The concentration of acetylene, hydrogen cyanide,
and acetonitrile were higher than expected but it
should be mentioned that only products having
quantitative (peak area) higher than 106 have been
considered in the analysis. Many of these
compounds have been detected or are predicted to be
in Titan’s atmosphere.
Figure 3. Results of quantitative analysis for major products
formed in the discharge at different concentration of CH4-N2 gas
mixture.
Figure 2. Sample GC-MS spectrum of analysed products
formed in a gas mixture CH4:N2 = 2:98 at flow rate of 200 ml
min-1.
Figure 3 shows the quantitative analysis of some
hydrocarbons formed under different experimental
conditions. The quantitative unit is represented by a
peak area. As is shown in Figure 3, the concentration
depends on the initial methane content. Figure 3
shows the concentration of major products (ethene,
acetylene, ethane, cyanogen, propene, hydrogen
cyanide, acetonitrile, propenenitrile, propanenitrile).
Since these experiments have been carried out for
different CH4-N2 mixture ratios and the same gas
flow rates, the evolution of different product
concentrations was found to depend strongly on the
initial gas ratios. The dependence of product
concentrations is strongly dependent on the different
CH4-N2 gas mixtures. An increase in the initial CH4
content from 2 to 5% caused a major increase in all
the product yields, however, there was a more
significant effect on some hydrocarbon-like products
than others: the yield of all N-containing products
increased rapidly but was less dramatic in the
hydrocarbons and hydrogen cyanide yields.
Figure 4 shows the quantitative unit (peak area) of
the major products formed at various flow rates and
the same CH4 content. Increasing the flow rate of the
gas mixture CH4-N2 content from 100 to 200 sccm
causes a decrease in the total product yield.
Furthermore, as it was expected, lowering the flow
rate of the gas mixture produced higher yields of
products. The highest yield for all products was
reached for 2% of CH4 at the lowest flow rate.
Figure 4. Results of quantitative analysis for major products
formed in the discharge with different experimental flow rates
and the same methane concentration.
Figure 5 shows the same situation as above but at
higher methane concentration. Comparing figures 4
and 5, mixtures prepared under identical conditions
but with different methane concentration in the gas
mixture showed that lowering the flow rate of the
gas mixture produced higher concentrations of the
products. The highest yield of all products was
reached for 5 % of CH4 at the lowest flow rate.
Figure 5. Results of quantitative analysis for major products
formed in the discharge at different experimental flow rates abut
with the same methane concentration.
4. Conclusion
In this contribution we present the results of a GCMS study of stable gaseous products formed in the
gliding arc discharge fed by different atmospheric
pressure mixtures of N2:CH4 (2 and 5 % of CH4)
operated in a flowing regime with a discharge
current of 30 mA at room temperature and
atmospheric pressure. Ex situ GC-MS analysis of the
gaseous products showed that HCN, C2H2, CH3CN
are the major products in our CH4/N2 non-thermal
plasma. The yields of these compounds are as
follows: HCN >C2H2> CH3CN. Minor detected
products were: ethane, ethene, cyanogen, propene,
propane, propyne, 1,2-propadiene, 1-butene-3-yne,
1,3-butadiene, 1,3-butadiyne, acetonitrile 2propenenitrile
and
2-propanenitrile,
2methylpropanenitrile,
2-methylpropanenitrile,
benzene and toluene. Such experiments can provide
information that could aid our understanding of
processes in Titan’s atmosphere.
Acknowledgements. This work has been supported
by the Grant UK/140/2010, Slovak Research and
Development Agency VEGA 1/0051/08, APVV 0365-07,
ESF COST Actions CM0601, CM0805 and
EUROPLANET TNA2. Research plan of Czech Ministry
of Education No. MSM 0021630501 and Czech Science
Foundation project No. 104/09/H080 supported this
research, too.
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