Analytical Instrumentation The third consideration: Hydrocarbon quenching
As requirements for sulphur detection become more stringent, the importance of good chromatographic separation of the hydrocarbons from the sulphur compounds and the inertness of the analytical column increases. Detectors used for sulphur determination generally are specific (e.g., sulphur chemiluminescence detection, FPD, PFPD) and help eliminate positive response from chromatographic interferences. Unfortunately, when high levels of hydrocarbons elute through the detector simultaneously with sulphur compounds, the signal for sulphur is quenched and area counts are nonlinear. For a successful analysis, the analytical column must resolve the hydrocarbons from the sulphur compounds.
Quenching of the sulphur responses by the hydrocarbon combusted products, such as CO, CO2, H2O, S2O and not fully combusted hydrocarbons appears in sample matrixes with high hydrocarbon concentration and where the column separation is not optimal. The quenching is mostly seen using a FPD and PFPD detectors.
All detectors mentioned in this article have at least three orders of magnitude dynamic range, making them suitable for a relatively wide concentration band. A summary of
the detector characteristics used in this work is given in the table below. The values for sensitivity, selectivity, and dynamic range are those claimed by the respective suppliers.
Detector characteristics Detector
MDL sulfur Selectivity Dynamic Range Quenching
Equimolar response
Packed column compatible
Other elements
Approximate relative cost
15
FPD 20 pg/s 105 105 Yes No Yes P,Sn $
PFPD 5 pg/s 105 105 Yes Yes No P $$
SCD 1 pg/s 105 105 No Yes Yes N $ linear
AED 4 pg/s 105 104, linear No Yes Yes Total 26 $$$$
New Technologies: Analysis of sulphur species with conventional Gas Chromatography can extend to gasoline. Even then not all the sulphur species are identified. However sulphur speciation in more complex matrices like diesel, jet fuel and heavier is not possible due to limitation of the separation performance of the conventional one column configuration.
In combination with Simulated Distillation, sulphur fraction can be determined with boiling points cuts. However to separate sulphur species in complex matrices, powerful separation performance is necessary.
Figure 3: PE TOF MS
GCxGC is such a tool that increases peak capacity allows separation of relevant constituents that cannot be differentiated by one dimensional GC alone. This technology can be useful to be used with conventional detectors if the data speed of these detectors can handle the speed of the peaks coming from the GCXGC modulation. This new technology is best used with a TOF MS. As the data speed of the TOF MS is high enough to detect and identify the species. However there are no known
Figure 3: GCxGC separation of a diesel sample (part of a diesel spectrum)
libraries available specific for sulphur species, however there is a lot of work done to identify these species.
A standard GC-MS or tandem MS is not sufficient to identify the sulphur species that elute at the same time as many other isomers. With GCxGC combined with TOF MS and application knowledge makes these complex identification and quantification more feasible.
Remarks and conclusion
Gas Chromatography is a significant tool in sulphur analysis. The available detectors are sufficient to quantify sulphur species in low boiling range samples. For the heavy and more complex samples the separation is a limitation. If the matrix is reasonably simple and sulphur levels are in the low ppm to 50 ppb range then the FPD will usually be a good choice.
The PFPD also is capable of excellent sensitivity to low ppb levels. Because of quenching, it is best suited for light sulphur and hydrocarbon streams where good separation is achievable. Dynamic range must be considered as well. The AED and SCD can handle the widest concentration range from low ppb to high ppm levels. SCD is suitable for low boiling point ranges and for complex samples. However sulphur speciation is not possible in complex samples due to the difficulty of separation.
Many manufacturers try to improve their supplied applications for the sulphur analyses by taking into account all of the above considerations.
The use of dedicated detectors, sample handling, result interpretation is an art that only few master.
The development for the sulphur speciation in more complex samples are slow, even though attempts are made using conventional detectors to achieve this. As technology is improving and new technology takes a part in the petroleum laboratories including GC-MS, GC-MS-MS and GC- TOFMS. These technologies are more used in research and development and are complicated and not user friendly for routine analysis.
With thank to Mr. Jaap de Zeeuw, Restek corporation, Middelburg, The Netherlands
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