DETERMINATION OF SULFUR COMPOUNDS continued
Table 1 – Instrument configuration and operating conditions S-Pro Select GC
Permeation oven 35 °C Helium dilution gas
Permeation devices
Dilution gas flow rate = 200–3600 mL/min H2
S wafer device—permeation rate = 809 ng/ min at 40 °C
COS wafer device—permeation rate = 1586 ng/min at 40 °C
MeSH wafer device—permeation rate = 441 ng/min at 40 °C
Automated injection system
Four-port selection valve
Six-port gas sampling valve (GSV) with 1-mL, Sulfinert-coated sample loop Automated, air-actuated valves All lines Sulfinert coated Valve oven temperature = 110 °C
Volatiles interface 200 °C Split mode
Split ratio = 10:1 Sulfinert coated
GC column
Agilent J&W (Santa Clara, Calif.) Select Low Sulfur column 60 m × 0.32 mm i.d.
Helium carrier gas = 1.2 mL/min Oven program
40 °C for 10 min 30 °C/min to 180 °C Hold for 10.5 min* Total run time = 25.2 min
Sulfur detection PFPD
2-mm combustor, BG-12 filter, R1924 PMT (photomultiplier tube)
/air ratio tuned for optimum sulfur emission 6–24 msec sulfur gate (linear mode) 1–3 msec hydrocarbon gate Square root on
Detector base temperature = 250 °C H2
*Oven hold time may be shortened depending on the target com- pounds.
The difficult separation of H2S from ethylene/ethane was achieved using
the Select Low Sulfur column. An H2
S peak typically elutes between the ethylene and ethane peaks for
many matrix concentration levels found in gaseous fuels. Some coelu- tion of H2
S may occur as the matrix concentrations increase above 50%
ethylene. COS was well separated from the propylene peak. These separa- tions are important because they demonstrate that the column is flexible
AMERICAN LABORATORY 22
Figure 3 – Natural gas sample 2 (obtained from DCG Partnership, Pearland, Texas).
enough to be used in several matrices, with no need to make significant changes to the chromatographic conditions. Ultimately, the matrix con- centration ranges can be balanced with the required sulfur sensitivity and GC split ratios to optimize performance for various types of analyses. Calibration was performed with the FlexStream gas standards generator (Figure 4). Linearity was established with a correlation coefficient of 0.999.
Repeatability study A repeatability study was performed by analyzing 20 replicate injections
of the same sulfur standards used for the calibration, with a flow rate of 1000 mL/min through the permeation oven.
Concentrations and %RSD were as follows: • H2
S at 0.376 ppmv—%RSD of 3.88%
• COS at 0.418 ppmv—%RSD of 3.34% • MeSH of 0.145 ppmv—%RSD of 5.84%
MARCH 2016
Figure 2 – Natural gas sample 1 (obtained from DCG Partnership, Pearland, Texas).
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