Andreas Hilkert, Thermo Fisher Scientific, Bremen, Germany
Key Words
Compund Specific Isotope Analysis, Natural Gas, Methane,
GC Combustion, Isotope Ratio MS
Introduction
Natural gas is produced by biodegradation and by
thermal degradation of organic matter. The isotope ratios
of compounds in natural gas can hold information about
the substrate and the degradation processes. The 13C/12C
isotope ratios of all components in natural gas, including
methane, can be analyzed within the same GC-IRMS run.
In GC-IRMS, all carbon-bearing compounds eluting from
a GC are converted on-line into CO2 in the combustion
interface and are transferred on-line to the IRMS.
Methane (C1), a trace gas in air but the dominant
component in natural gas is difficult to combust.1, 2 The
high concentration of C1 versus the low concentration of
ethane to pentane (C2-C5) requires that the IRMS have a
large dynamic range. This usually leads to two separate
analyses of methane and of the minor components. The
Thermo Scientific™ combustion interfaces like the here
used GC/CII interface followed by GC/CIII and GC
Isolink in combination with Thermo Scientific IRMS
allow analyses of C1-C5 within one run by combining
highest sensitivity, linearity and stability with a wide
dynamic range and chromatographic integrity.
Results
Methane requires higher combustion temperatures which
usually result in a fast loss of oxygen from combustion
reactors containing only CuO. This creates uncertainties
for continuous methane analyses. A combination of nickel
and copper oxides in a capillary design, however, allows
higher temperatures without these negative effects. Above
940 °C > 99.97% of the methane is combusted (Figure 1);
beyond 960 °C no significant change in the δ-value can be
detected (Figure 2). The standard deviation within
960-1020 °C in this study is ± 0.026‰. The combustion
reactors in the GC/C II interface and its successors allow
to use these operating conditions routinely. Figure 3 shows a chromatogram for methane analysis in
natural gas. The peak intensities ranged from 60 mV to
9700 mV representing a dynamic range of 1 to 150. The
δ13C value of methane, at 9700 mV, was -49.050 ±
0.036‰ and the mean standard deviation of all the minor
components was 0.223‰. n-C5 and i-C5 were calculated
with an optimized background algorithm to account for
column bleed. These data clearly demonstrate the wide
dynamic range, linearity and stability of both the Thermo
Scientific IRMS and the combustion interfaces.
Similar results can be expected from a Thermo Scientific
Delta V™ IRMS or Thermo Scientific MAT 253™ IRMS
coupled with the Thermo Scientific GC Isolink™ II via the
Thermo Scientific ConFlo™ IV universal interface.
Method
Injector
split/splitness (split 1:70)
Cap. column
Poraplot Q, 25 m, 0.32 mm i.d.
GC program
4 min at 26°C, 5°C/min to 180°C, 5 min at 180°C
GC/C interface
Standard GC/C II interface oxidation
reactor at 980°C,
reduction reactor at 600°C
Table 1. GC-IRMS parameters.
Appli cat i on N ote 3 0 0 8 8
Investigating 13C/12C Isotope Ratios
of Methane-Pentane in Natural
Gas by GC-IRMS
13
100
12.6
δ13CRef [‰]
Combustion [%]
99
94,5
12.2
12.377‰
± 0.026‰
11.8
11.4
94
840
860
880
900 920 940 960
Temperature [°C]
Figure 1. Methane, % combustion vs. T [°C].
1.0000
x 10
0.9000
10.000
CH4
880
900 920 940 960
Temperature [°C]
980 1000 1020
Figure 2. Methane, δ13CRef [‰] vs. T [°C].
Methane
Ampl. = 9.7 V
Mean = -49.050‰
S.D. = ±0.036‰
Propane
Ampl. = 1.1 V
Mean = -30.669‰
S.D. = ±0.104‰
Ethane
Ampl. = 0.58 V
Mean = -32.595‰
S.D. = ±0.050‰
0.7000
m/z 44 [v]
m/z 44 [v]
860
0.8000
8.000
6.000
11
840
980 1000 1020
n-Butane
Ampl. = 0.38 V
Mean = -29.058‰
S.D. = ±0.243‰
0.6000
i-Butane
Ampl. = 0.28 V
Mean = -30.375‰
S.D. = ±0.278‰
CO2
Ampl. = 0.40 V
Mean = -15.382‰
S.D. = ±0.331‰
0.5000
0.4000
i-Pentane
Ampl. = 0.11 V
Mean = -27.223‰
S.D. = ±0.302‰
n-Pentane
Ampl. = 0.085 V
Mean = -27.041‰
S.D. = ±0.248‰
0.3000
4.000
Std.
Gas
1
0.2000
0.1000
2.000
200
C02
Et
400
Pr
600
i-Bu
800
1000
Time [s]
n-Bu
i-Pe
1200
1400
1600
1800
n-Pe
0.000
0
500
1000
Time [s]
1500
2000
Figure 3. GC-IRMS analysis of natural gas.
References
1 Merritt D.A. et al., Journal of Geophysical Research
Atmospheres, 100 (D1 PI.2), 1317-1326.
3 Hilkert A.W. et al., Rapid Commun. Mass Spectrom. 13,
1226–1230, 1999.
2 Brand W.A., Isotopes Environ. Health Stud., Vol. 31,
277-284.
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