IEC TR-1948-3-2018 pdf download.Test methods for quantitative determination of corrosive sulfur compounds in unused and used insulating liquids – Part 3: Test method for quantitative determination of elemental sulfur.
5 Procedure
5.1 Principle
5.1 .1 Determination with gas chromatography The oil sample is diluted approximately 1 :20 with a suitable solvent, fortified with a known amount of an internal standard (IS) such as DPDS, and injected into the split/splitless injector of a gas chromatograph equipped with a suitable detector including an electron capture detector (ECD), a sulfur chemiluminescence detector (SCD), a flame photometric detector (FPD), an atomic emission detector (AED) or a mass spectrometer (MS). Separation of oil constituents, elemental sulfur (if present) and IS is achieved with a suitable column such as a 1 5 m to 30 m × 0,25 mm (internal diameter) fused silica column with 5 % polyphenylsiloxane and 95 % methyl polysiloxane or other suitable stationary phase and helium or other suitable carrier gas. Separation is facilitated through temperature programming over a suitable temperature range. Elemental sulfur is monitored with the detector and quantified with the internal standard. NOTE During the Round Robin Tests on the ECD, an FPD and an MS were used. Other suitable detectors such as sulfur chemiluminisence and atomic emission detector were not used.
5.1 .2 Determination with differential pulse voltammetry The test procedure is based on two standard additions. 1 0 ml of the base electrolyte is positioned in the test cell with the electrodes. 0,5 ml of the oil to be tested is added. The current density is recorded.
5.2 Significance and use This test method describes the determination of elemental sulfur in insulating liquids for analysis. The most common form of elemental sulfur is the octacyclic form with formula S 8 . If present, elemental sulfur can react with copper and other metal conductors in transformers, reactors and other similar devices to form copper and other metal sulfides. Therefore, this compound is classified as potentially corrosive sulfur (see IEC 6 2535 ). Elemental sulfur is present in petroleum and may be present in insulating mineral oils at concentrations ranging between 1 mg kg –1 and 400 mg kg –1 , but it may be present at levels outside this range, in oils that have been blended, or oils in which elemental sulfur has been consumed through its reaction with the copper or other metals. This method can be used for detecting and quantifying elemental sulfur content in used and unused insulating liquids.
5.3 Interferences
5.3.1 Co-eluting compounds Interferences experienced during quantitative determination of elemental sulfur will vary with the detector used for quantification of elemental sulfur separated with the gas chromatographic column.
5.3.2 Electron capture detector (ECD) An ECD is a very sensitive and selective detector that responds to volatile/semi-volatile compounds with high electron affinity. It has gained wide acceptance and use due to its very high sensitivity and selectivity for certain classes of compounds, including halogenated hydrocarbons, organometallic compounds, nitriles, or nitro compounds, disulfides and elemental sulfur (S 6 , S 7 and S 8 ). Presence of compounds with high affinity for thermal electrons especially polychlorinated biphenyls (PCBs) in insulating liquids can cause interference. In such cases an alternate detector should be used.
5.3.3 Flame photometric detector (FPD) The FPD provides a selective signal for sulfur species including DBDS in the form of broad light emission centred around 393 nm. Emission is separated with an interference filter with a 20 nm band pass with peak transmission at 393 nm. The filter is mounted in front of a photomultiplier that provides electrical signal that is amplified and linearized. Sulfur containing organic compounds which elute with the same retention time can lead to interference; a column with different stationary phase can be used to eliminate or minimize interference. 5.3.4 Mass spectrometer (MS) An MS is a very sensitive and selective detector that responds to the volatile and semi-volatile compounds. It has gained wide acceptance and use due to its very high sensitivity and selectivity for a broad class of compounds. Compounds present in the GC effluent that yield ions at m/z 256 and ions at m/z 21 8 will cause interference if such compounds elute from the GC column with retention times similar to those of the S 8 and DPDS (IS).IEC TR-1948-3 pdf download.