## 2 Quantitative Test Methods for Different Suphides in Transformer Oil
The sulfur element in the transformer oil mainly exists in the form of mercaptan, bisulphide, thioether, and thiophene, and its content is closely related to the producing area of crude oil. In order to further understand the distribution and content of different sulfur components in oil samples, a quantitative method for the determination of different sulfur compounds in transformer oil was proposed. The test content is total sulfur content, elemental sulfur content, hydrogen sulfide content, mercaptan sulfur content, DBDS content, thiophene sulfur content, and thioether sulfur content. The above operations referring to the following standards and equipment is shown in Table 2.  
In the above operations, as the determination of thiophene and thioether sulfur is the most difficult, the whole can be divided into the following three steps. 1. Methylation  
The thiophene and thiophene compounds in crude oil are methylated to form the corresponding sulfur salts. 2. Reduction of thiophene  
The thiophene compounds in the methyl thiosulfate/methylated sulfur salt are reduced by 7-azindole, and the amount of thiophene compounds is obtained after separation. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Test Items** & **Reference Standard** & **Equipments** \\ \hline \multirow{2}{*}{Determination of total sulfur} & ASTM D5453-12 & Jena-MultiEA3100 \\  & Standard Test Method for Determination of Total & Sulfur nitrogen analyzer \\  & Sulfur in Light Hy (detection limit is 5 mg/L) & \\ \hline \multirow{2}{*}{Determination of elemental sulfur} & Laboratory method & \\  & Content of elemental sulfur in the sample should & \\  & be determined by Oscillographic polarography & \\  & (detection limit is 0.1 mg/L) & \\ \hline \multirow{2}{*}{Determination of hydrogen sulfide} & GB/T 26983-2011 & \\  & determination of sulfuretted hydrogen, methyl & Agilent-6890N gas chromatograph \\  & mercaptan and ethyl mercaptan in crude oil & Gas chromatography PFPD detector \\  & (detection limit is 2 mg/L) & \\ \hline \multirow{2}{*}{Determination of mercaptan sulfur} & GB 1792-1988 & Kyoto Electronics \\  & Determination of mercaptan sulphur (Potenio \\  & metric titration method)(detection limit is 3 mg/L) & AT-710M Automatic potentiometric \\  & & itrant \\ \hline \multirow{4}{*}{Determination of DBDS} & IEC 62697-1-2012 & \\  & Test methods for quantitative determination of & \\  & corrosive sulfur compounds in unused and used & Gas chromatography-mass \\  & insulating liquids-Part 1: Test method for & spectrometry(GC-MS) \\  & quantitative determination of dibenyl disulphide & \\  & (DBDS) & FT-ICR MS \\ \hline \multirow{2}{*}{Determination of thiophene sulfur and thioether sulfur} & Laboratory method & (Bruker Apex Ultra 9.4T AS) \\  & The distribution characteristics of thiophene sulfur & Agilent 3555 CD \\ \multirow{2}{*}{and thioether sulfur} & and thioether sulfur should be analyzed in depth & Sulfur Chemiluminescence Detector \\  & Agilent/890A-5975C Gas \\ \multirow{2}{*}{chromatography-mass spectrometry} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reference standards and equipment for the quantitative determination of different sulfur compounds in transformer oil. 3. Reduction of thioethers The 4-dimethylaminopyridine is added to the sample which has been separated from thiophene. The thioether compounds are reduced and the amount can be obtained after separation. The specific steps are shown in Figure 1.  
In order to further analyze the distribution characteristics of sulfur in thiophene and thioether, a combination of GC-SCD (gas chromatography-sulfur chemiluminescence detection), GC-MS, and FT-ICR MS (Fourier transform ion cyclotron resonance mass spectrometry) was adopted. The distribution spectra of sulfur in thiophene and thioether, with different carbon number and different unsaturation, can be generated by FT-ICR MS, respectively. The molecular formula of sulfur compounds with higher content in oil samples could then be obtained. Then, the molecular structure could be speculated on.