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Analytical Instrumentation
Gas Chromatography: an Accurate, Fundamental Tool in Sulphur Analysis
Khalid Tafrasti, Era Analytics FZCO Dubai Airport Free Zone Building 5EA, Office G01,PO BOX 293619 Dubai, United Arab Emirates Email:
Khalid.tafrasti@era-analytics.com • Web:
www.era-analytics.com
Sulphur Analysis is an essential analysis for refineries, petrochemical, food and environmental industries. Gas Chromatography is widely used nowadays in all petroleum, petrochemical and refinery laboratory for the analysis of various components, group of components, impurities and of course sulphur. A Gas Chromatograph is in general a very accurate tool for qualification and quantification for sulphur species and total sulphur.
Khalid Tafrasti
Several ASTM methods are available for the identification and quantitation of sulphur species in petrochemical and refinery products by means of Gas Chromatography. Depending on the sample matrix, ASTM methods are available using different detectors. The ASTM methods describing analysis with GC recommend sulphur specific detectors including PFPD, FPD, SCD and AED. Another suitable detector is the Mass Spectrometer.
Many Gas Chromatograph’s manufacturers (PE, Shimadzu, Thermo, Agilent Technologies, Bruker, etc) and Value Added Resellers such as Era Analytics (VARs) offer applications to determine sulphur in various matrices. To meet the customer’s requirements many factors have to be taken in to consideration to be able to deliver a gas chromatograph specific for the customer’s application: sensitivity, selectivity, linearity, repeatability, equimolarity and robustness are just a few of the requirements.
There are also many limitations that need to be taken into consideration where other improvements can be applied to enhance the GC and detector performance.
Calibration and sample handling systems are an integral part of the sulphur measurement system. Due consideration must be given to the reactive nature of sulphur compounds when designing sulphur measurement systems and when verifying and troubleshooting these systems.
Many factors must be considered when deciding on the best sulphur selective detector to employ for a given application. Attributes such as selectivity, response factor behaviour, quenching, column compatibility, and sensitivity should be considered and matched to the application.
The first consideration: Sample Matrix
The sample, sample matrix and component of interest that have to be detected and quantified are very important for the choice of detector.
Gas samples show less quenching because of the low boiling point and less hydrocarbon present in the matrix. A column separation is enough to use a FPD or a PFPD detector. For liquid samples with higher boiling points a detector like SCD is more desirable.
Equimolar detectors with a big linear range like SCD and PFPD have the advantages over FPD and AED. Equimolarity is a big advantage when calibrating a system. Equimolarity is more important when the sulphur compound elutes together with the hydrocarbon matrix. That is the reason why SCD is a more desirable detector for more complex samples like gasoline and middle distillates. Calibrating an equimolar detector requires not more than one component; for identification individual components are required.
The second consideration: Passivation
Sulphur compounds are very active compounds that will react with every active site, resulting in the loss of these compounds before reaching the detector. Deactivation of every part that comes in contact with the sample is inevitable. That means sample canisters, transfer lines, injection port liner, connectors, valves, fittings, and every part of the GC system should be passivated. Besides the deactivation of the active sites, the prevention of active chemical compounds entering the system is as equally important, including moisture, air and other contaminants. Gas filters are highly recommended in combination with high carrier gas purity.
Any active site in the Gas Chromatograph can cause a headache that can only be solved by replacing that part. However there are users who do attempt to deactivate the active site in their gas chromatograph by purging the sample and flow paths with high concentrations of Hydrogen Sulphide. An active liner as shown below cannot be deactivated by purging high concentration of sulphur through the liner.
The Siltek® be passivated using Siltek®
deactivation contributed a lot here as all parts that are in contact with the sample can deactivation.
Hydrocarbons are non-reactive but sulphur compounds, especially hydrogen sulphide and methyl mercaptan, are easily adsorbed by undeactivated surfaces. Therefore, there are two areas of concern with micropacked or packed column sulphur analysis: one is the inertness and selectivity of the solid support, and the other is the inertness of the tubing walls. Packed and micropacked
Figure 2: tailing peaks as result of partly adsorption OCTOBER / NOVEMBER 2013 •
WWW.PETRO-ONLINE.COM Figure 1: effect of an active liner on sulphur adsorption
columns typically use metal tubing for ruggedness but the surface is very adsorptive for sulphur compounds. PTFE tubing is also an option, but it has a limited temperature range, is permeable, and will expand and contract during temperature changes. These characteristics will negatively affect column efficiency and stability.
Apart from totally losing the sulphur species in the system, the sulphur species can be partly absorbed and will show a tailing (Figure 2) indicating the presence of adsorption in the system.
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