IRMS

July, 2009 (old page), updated on Aug. 21, 2009

Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC-C-IRMS) system at NIAES

Mass: Delta V Advantage (Thermo Scientific)
GC: Trace GC Ultra (Thermo Electron Corporation)

GC-Mass interface: Finnigan GC Combustion Interface III (Thermo Scientific)
Gas concentration device: Finnigan PreCon (Thermo Scientific)

1. Methane 13C/12C analysis: > 500 ppm

GC-Injection → Capillary-loop → Separation column → Combustion → IRMS

Overview: Less than 1mL of sample gas is introduced via GC injection port and carried by a constant mass flow (2mL/min) into a chilled capillary-loop (CP-PoraPlot Q, Varian) which acts as a cryofocus for CH4 (also CO2 etc...). The capillary-loop is flushed with helium to remove the bulk of the sample components, i.e. N2 and O2, that interferes with the isotopic signal of CH4. The capillary-loop is then warmed to 25°C, releasing the trapped gases into a separation column (CP-PoraPlot Q) for chromatographic separation. The separated methane is then combusted at 960°C and the resulting CO2 is fed into an continuous flow mass spectrometer (Delta V Advantage). Our design is based on Rice et al (2001).

1.1. GC-Injection

Direct injection through GC (Trace GC Ultra) is more favorable rather than installing the sample into sample port of PreCon because of a shorter time required for gas-purification process. See the following table for required sample volume to get 3.5V ~~2.0 V~~ signal for mass 44. A constant carrier flow at a rate of 2.0 mL/min is maintained.

	Split ratio
	5	10
500-ppm	0.80 ml ~~(1.2-ml)~~	-
2500-ppm	0.16 ml ~~0.24-ml~~	0.32 ml ~~0.48-ml~~
5000-ppm	0.08 ml ~~0.12-ml~~	0.16 ml ~~0.24-ml~~

modified on Aug. 21, 2009

1.2. Capillary-loop (modified T3 trap of PreCon)

The Capillary-loop consists of 70 cm of CP-PoraPlot Q capillary column (0.32 mm i.d.) jacketed within 1.59 mm (o.d.) stainless steel tubing, about 15 cm of which is immersed in a LN2/n-pentane bath maintained at -130 °C. This step purges the sample gas of O2 and N2 while trapping CH4 in the loop. The removal of N2 and O2 dramatically reduces the background signal, improving the overall measurement precision. Required time for this step depends on the volume of gas entering the capillary line. As a guide, given that a syringe injection volume is ~0.5 ml, 150 s is sufficient for split ratio of 10, and c. 300 s for 5.

1.3. Separation column

CP-PoraPLOT Q (Varian) (0.32mm i.d., 10μm film thickness, 25 m-long, Max. temp. 250°C) successfully separates CH4 from other residual gas compounds, such as N2, O2, CO, Ar, CO2, N2O, nonmethane hydrocarbons, etc.

1.4. Combustion

Combustion reactor consists of a non-porous alumina tube (Al2O3, 0.5 mm i.d., 1.55 mm o.d., 32 cm length) inside of which CuO, NiO, and Pt wires maintained at a constant temperature of 960°C by a resistively heated Al2O3 furnace (Thermo scientific). Methane is oxidized when passing through this furnace into CO2 and H2O. To maintain its oxidative capacity, the NiO and CuO catalysts are reoxidized weekly with O2 for several hours at 560°C.

1.5. Precision

Precision (one standard deviation) of standard gas for 5 consecutive measurements is ~0.05‰.

2. Methane 13C/12C analysis: < 2 to 500 ppm

Sample installation into PreCon → T1 trap → Precolumn→ Capillary-loop → Separation column → Combustion → IRMS

Overview: A large volme of sample gas (e.g. 100 mL) can be introduced via PreCon sample port. The sample is swept through the first two traps by 60 mL/min He flow. The first trap, a stainless steel tubing (1.59 mm o.d.), placed in liquid-N2 (-196°C) ~~liquid-N2/n-pentane (-130°C)~~, removes H2O, CO2 and N2O (and other high boiling point contaminants) from the sample gas. The effluent of T1 trap flows into the second cold trap (= precolumn packed with Hayesep D (Restek)) where CH4 collects, while purging of the bulk of N2 and O2. The precolumn is then warmed to 25°C releasing the trapped gases. A helium carrier gas flowing at ~2.0 mL/min carries the released gases into the cryo-focus section (capillary-loop), trapping the gas again in a smaller section that serves to sharpen the peak. The capillary-loop is then warmed to 25°C, releasing the trapped gases into a separation column (CP-PoraPlot Q) for chromatographic separation. The separated methane is then combusted at 960°C and the resulting CO2 is fed into an continuous flow mass spectrometer (Delta V Advantage). Our design is based on Rice et al (2001).

2.1. Sample intoroduction into PreCon

For the analysis of atmospheric CH4, a 25mL ~~100 mL~~ of air collected in a glass bottle is connected to the PreCon sample port. The bottle is flushed with a volume of He equivalent to > 5 times the sample volume so that the majority of CH4 has been removed and trapped on the precolumn (see below). Higher CH4 concentration requires less sample volume necessary for obtaining sufficient signal intensity. Hence, syringe-type injection into the PreCon carrier stream (needle injection thorugh a rubber septum equipped to a small size container) is available for the high concentration samples (e.g. > ~20 ppm). This alternative injection can greatly reduce the run times as the necessity for purging air from the preconcentration traps is considerably dissipated.

	Required volume	Sample introduction	T2	T3
Atmospheric (~2-ppm)	25 mL	25ml in 20ml vial	300 s	400 s
50-ppm	1 mL	Syringe injection into 20 mL vial	150 s	400 s
500-ppm	0.1 mL	Syringe injection into 20 mL vial	150 s	400 s

2.2 Precolumn (modified T2 trap of PreCon)

The precolumn consists of 20 cm × 3.18 mm (o.d.) stainless steel tubing packed with 100/120 mesh Hayesep D (Restek). The precolumn is immersed in the liquid-N2/n-pentane bath and maintained at -130°C in order to preferentially adsorb CH4 while purging of the bulk of N2 and O2. This leads to a significantly lowered background signal, improving overall precision of the measurement.

2.3 Capillary-loop (modified T3 trap of PreCon)

The Capillary-loop is identical to that used for high-concentration CH4 analysis. Howerver, longer time is required to transfer the gas from the precolumn to this cryofocus trap because of the larger inside volume of the precolumn. Typical time required for this step is 400 s.

Separation column and combustion furnace are identical to those used for high-concentration CH4 analysis.

2.4 Precision

Typical precision (one standard deviation) for atmospheric CH4 (70 ml) for 5 consecutive measurements is ~0.08‰.

2.5 Remark (added on Aug. 21, 2009)

Liquid-N2 (-196°C) is necessary for removing CO2 in the first trap. Liquid-N2/n-pentane trap (-130°C) is not cold enough to devolatilize CO2 in the helium stream.

3. Atmospheric CO2

GC-Injection → Capillary-loop → Separation column → IRMS

Overview: About 1mL of sample gas is introduced via GC injection port and carried by a constant mass flow (2ml/min) into a chilled capillary-loop (CP-PoraPlot Q, Varian) which acts as a cryofocus for CO2. The capillary-loop is flushed with helium for 200 s to remove the bulk of the sample components, i.e. N2 and O2 and Ar, that interferes with the isotopic signal of CO2. The capillary-loop is then warmed to 25°C, releasing the trapped gases into a separation column (CP-PoraPlot Q) for chromatographic separation. The effluent is fed into an continuous flow mass spectrometer (Delta V Advantage).

See above (Methane analysis) for descriptions of each part.

3.1 Precision

Typical precision (one standard deviation) for atmospheric CO2 for 5 consecutive measurements is ~0.10‰.

References and links

A. L. Rice, A. A. Gotoh, H. O. Ajie, and S. C. Tyler (2001) High-Precision Continuous-Flow Measurement of δ13C and δD of Atmospheric CH4. Anal. Chem., 73 (17), 4104-4110 • DOI: 10.1021/ac0155106.

Earth System Research Laboratory (NOAA)