Authors
Wenmin Liu
Agilent Technologies Co. Ltd.412 Ying Lun Road
Waigaoqiao Free Trade Zone Shanghai, 200131China
Mario Morales
Agilent Technologies, Inc. 2850 Centerville Road Wilmington, DE 19808USA
Abstract
An Agilent dual plasma sulfur chemiluminescence detec-tor (DP SCD) combined  with an online dilutor was used for the analysis of sulfur compounds. By using this method, the detection limits of the sulfur compounds achieved the ppb level. The stability of the DP SCD was also investigated. The long-term and short-term stability show that the performance of DP SCD is stable, and no hydrocarbon interference was found during the analysis of natural gas samples.
Introduction
Many sources of natural gas and petroleum gases contain varying amounts and types of sulfur com-pounds. The analysis of gaseous sulfur compounds is difficult because they are polar, reactive, and present at trace levels. Sulfur compounds pose problems both in sampling and analysis. Analysis of sulfur compounds many times requires special treatment to sample pathways to ensure inertness
Detection of Sulfur Compounds in Natural Gas According to ASTM D5504 with Agilent's Dual Plasma Sulfur
Chemiluminescence Detector (G6603A) on the 7890A Gas Chromatograph Application
Hydrocarbon Processing
to the reactive sulfur species. Sampling must be done using containers proven to be nonreactive.Laboratory equipment must also be inert and well conditioned to ensure reliable results. Frequent calibration using stable standards is required in sulfur analysis [1].
GC SCD configuration with inert plumbing is one of the best methods to detect sulfur compounds in different hydrocarbon matrices. Sulfur compounds elute from the gas chromatographic column and are combusted within the SCD burner. These com-bustion products are transferred to the SCD detec-tor b
reactive diluent
ox via vacuum to a reaction cell for ozone mixing. This detection technique provides a highly sensitive, selective, and linear response to volatile sulfur compounds.
Agilent Technologies DP technology is the detector of choice for sulfur analysis when dealing with a hydrocarbon matrix. The burner easily mounts on the 6890 and 7890A GCs and incorporates features for easier and less frequent maintenance. In this application, the Agilent 355 DP SCD was used to analyze the gaseous sulfur compounds in natural gas. Detection limits, stability and linearity were investigated.
Experimental
An Agilent 7890A GC configured with a split/splitless inlet (Sulfinert-treated), and an Agilent 355 DP SCD were used. Sample introduction was through a six-port Hastelloy C gas sample valve (GSV) interfaced directly to the sulfur-treated inlet with Sulfinert tubing. An online dilutor was used for preparation of ppb-level sulfur compounds in
Heater150 °C Array Pressure14.5 psi
Septum purge flow  3 mL/min
Mode Splitless
Gas saver20 mL/min after 2 min
Sample loop  1 mL
Oven30 °C (1.5 min), 15 °C/min 200 °C
(3 min)
Column HP-1 60 m ×0.53 mm ×5 µm
Injection mode Static flow and dynamic flow modes
SCD Conditions
Burner temperature800 °C
Vacuum of burner372 torr
Vacuum of reaction cell  5 torr
H240 mL/min
Air53 mL/min
Results and Discussion
From the comparative results of the sulfur detec-
tors’ sensitivity, it could be seen that SCD is the
best detector for sulfur components, especially at
low levels [3]. The Agilent DP technology is the
most sensitive and selective detector for sulfur-
containing gaseous hydrocarbon samples.
Figure 2 is the chromatogram of low-level sulfur
compounds at 1.35 ppb (H2S), which is prepared
by the point-of-use gas blending system. Table 2 is
the calculated signal to noise (S/N) of each com-
pound, from the achieved data. It can be seen that
DP SCD can detect low-level sulfur compounds.
Figure 1.Diagram of online dilutor GC-DP SCD.
2
3
Figure 2.Chromatogram of sulfur compounds in helium at 1.35 ppb. (Refer to Table 1 for peak identification.)
Because the low-level sulfur components were pre-pared by the online dilutor system, which was pre-pared by adjusting the aux EPC to get appropriate diluent flow, high diluent flow could have the potential to cause high pressure in the sample loop, which results in the amount of the sample in the loop being different when the diluent flow changes from low to high. In this application, two sample injection modes, static and dynamic, were investigated. The mode is actuated by the on/off valve installed prior to GSV. When using static
injection mode, the valve is switched to the off position, the pressure in the sample loop balances to ambient pressure, and then the sample is injected into the GC.
Table 3 shows the linear ranges of the two injec-tion modes. The two injection modes have no dif-ference from a linearity perspective, which means that the two injection modes are both suitable when using the 1-mL sample loop. The 1-mL sample loop’s resistance is not high enough to cause variation in the sample injection amount.
Table 4 shows the long-term (72 hours) and short-term (8 hours) stability of the SCD at different concentration levels.
In an effort to investigate the coelution of hydro-carbon and sulfur, the same sulfur standards in natural gas were analyzed on the SCD. Figure 3
shows the chromatogram; no quenching was found.
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© Agilent Technologies, Inc. 2008
Printed in the USA
August 12, 2008
5989-9234EN

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