IEC 62697-1 TEST METHODS FOR QUANTITATIVE DETERMINATION OF CORROSIVE SULFUR COMPOUNDS IN UNUSED AND USED INSULATING LIQUIDS - Part 1: Test method for quantitative determination of dibenzyldisulfide (DBDS)
4 Sampling
Samples shall be taken, following the procedure given in IEC 60475. A representative portion shall be taken after thorough mixing. The specific sampling technique can affect the accuracy of this test method.

Precautions should be taken to prevent cross-contamination during sampling.

5 Procedure
5.1 Principle
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), an atomic emission detector (AED), a sulfur chemiluminescence detector (SCD), a flame photometric detector (FPD), a mass spectrometer (MS) or a tandem mass spectrometer (MS/MS).

Separation of oil constituents, DBDS (if present) and DPDS is achieved with a suitable column such as a 30 m to 60 m x 0.25 mm (internal diameter) fused silica column with 5 % polyphenylsiloxane and 95 % methylpolysiloxane or other suitable stationary phase and helium or other suitable carrier gas. Separation is facilitated through temperature programming over a suitable temperature range. DBDS is monitored with the detector and quantified with the internal standard.

NOTE Other suitable detectors such as sulfur chemiluminisence detector or flame photometric detector can be used. However, these detectors were not used during the Round Robin Tests.

5.2 Significance and use
This test method describes the determination of DBDS in insulating liquids for analysis.

DBDS is an aromatic organosulfur compound, which may be present in insulating liquids and impart oxidation stability to the liquids. However, DBDS 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 62535).

DBDS has been found in insulating mineral oils at concentrations ranging between 5 mg kg(-1) and 600 mg kg(-1), but it may be present at levels outside this range, in oils that have been blended, or oils in which DBDS have been consumed through its reaction with the copper or other metals.

This method can be used for detecting and quantifying DBDS content in used and unused insulating liquids.

5.3 Interferences
5.3.1 Co-eluting compounds
Interferences experienced during quantitative determination of DBDS will vary with the detector used for quantification of DBDS 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 and disulfides. Presence of such compounds especially polychlorinated biphenyls (PCBs) in insulating liquids can cause interference. In such cases an alternate detector should be used.

5.3.3 Atomic emission detector (AED)
An AED responds to volatile and semi-volatile compounds separated with a gas chromatograph that contains carbon and selected heteroatoms, including sulfur, nitrogen, oxygen and halogens (fluorine, chlorine, bromine and iodine). AED can thus provide a carbon and heteroatom fingerprint of complex mixtures such as insulating liquids. It can be used for quantification of selected additives and their homologues with minimum interferences. It can also be used for determination of origin and formulation through pattern recognition. Interferences can arise from co-eluting sulfur compounds.

5.3.4 Mass spectrometer (MS)
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 give yield ions at m/z 246 or m/z 218 will cause interference if such compounds elute from the GC column with retention times similar to those of the DBDS and DPDS (IS).

5.3.5 MS/MS
MS/MS is a highly sensitive detector that can yield greater specificity for targeted volatile and semi-volatile compounds separated with a gas chromatograph. It minimizes background interferences arising from complex matrices and enhances certainty in quantitative determination of DBDS, other compounds, their isomers (compounds with the same elemental composition but different connectivity) and their homologues (compounds with the same functional group(s) but a different carbon chain) in insulating liquids. This detector provides a largely interference-free response.

5.3.6 Interference from the matrix
The insulating liquid matrix is comprised of hydrocarbons that do not respond well in the ECD; therefore, matrix interference should be low with GC-ECD.

AED response is selective for heteratoms present in an organic compound; therefore, matrix interference should not be encountered.

It is possible that certain insulating liquids can contain molecules that yield ions at m/z 246 and m/z 218. Such molecules can cause interferences with GC-MS.

MS/MS response is highly specific for target compound; therefore, matrix interference should not be present.

5.4 Apparatus
5.4.1 Balance
A balance with a capability for automatic tare, accuracy down to 0.001 g, and a maximum weight range of ≥ 100 g is required.

5.4.2 Gas chromatography system
5.4.2.1 General
Gas chromatograph equipped with:
- a split/splitless injector with temperature stability of better than 0.5 °C and maximum operating temperature above 300 °C;
- an injection device suitable for introducing 1 µl - 10 µl liquids into the column (an automated sampling injection device is preferred);
- a 30 m à 60 m x 0.25 mm (internal diameter) fused silica capillary column with 5 % phenyl polysiloxane and 95 % methylpolysiloxane or other suitable stationary phase;
- a column oven capable of operation over the 30 °C - 300 °C range with ramp rates of up to 20 °C min(-1).

5.4.2.2 ECD
ECD with a
63Ni foil detector capable of operating at temperature ~ 300 °C with temperature stability of ≤ 0.5 °C.

5.4.2.3 Atomic emission detector(AED)
AED capable of detecting the sulfur emission line at 181 nm (or other suitable sulfur emission line).

5.4.2.4 Mass spectrometer (MS)
- quadrupole or other suitable MS with an electron ionization (EI) source, operated in positive ion selected ion monitoring (SIM) mode;
- electron energy 70 eV;
- GC-MS interface temperature 270 °C with temperature stability of ≤ 0.5 °C;
- source temperature 200 °C or as recommended by the manufacturer.

5.4.2.5 MS/MS
- triple quadrupole or other suitable MS with an (EI) source, operated in positive ion SIM mode;
- electron energy 70 eV;
- GC-MS interface temperature 270 °C with temperature stability of ≤ 0.5 °C;
- source temperature 200 °C or as recommended by the manufacturer;
- system shall permit selection of precursor ions, dissociation of precursor ion into characteristic fragment ions and quantification of the fragment ions.

5.4.3 Data system
For control, monitoring, acquisition and storage of analytical data.

5.5 Reagents and materials
5.5.1 Purity of reagents
Analytical reagent grade chemicals shall be used in all analysis performed with this method.

5.5.2 Gases
The carrier gas (He or other suitable gases) shall have purity equal to or better than 99.999 % (grade 5). Refer to the specifications provided by the manufacturer of the GC system to verify the purity requirements.

Make up gas for the ECD shall be nitrogen or other gas specified by the instrument manufacturer.

Collision gas for the MS/MS system shall be argon with purity equal to or better than 99.999 %.

5.5.3 Solvents
Toluene may be used for the preparation of the stock solution.

Iso-octane or other suitable solvents should be used for dilution.

Low-boiling solvents such as hexane should not be used because their volatility can cause problems during weighing.

5.6 Standard materials
5.6.1 Dibenzyl disulfide (DBDS)
DBDS is solid at ambient temperature (melting point 71 °C - 72 °C); its purity shall be ≥ 97 %.

Store DBDS in an amber glass bottle with screw cap in a secure place. Keep the bottle away from a source of heat.

5.6.2 Diphenyl disulfide (DPDS)
DPDS is solid at ambient temperature (melting point 61 °C - 62 °C); its purity shall be ≥ 97 %.

Store DPDS in an amber glass bottle with screw cap in a secure place. Keep the bottle away from any heat source.

5.6.3 Blank oil
Insulating liquid that is free from DBDS and DPDS is used for preparation of standard solutions and blank samples.

NOTE White mineral oil with viscosity in the same range as the insulating mineral oil samples is suitable for this purpose.

5.7 Standard solutions
5.7.1 Stock solution
Prepare a solution of DBDS in toluene with known concentration. It is recommended that a fresh stock solution should be prepared every 3 months. The stock solution should be stored in amber glass bottles with polytetrafluoroethylene (PTFE) lined screw caps in refrigerator at ~4 °C. The solution shall be brought to room temperature (~25 °C) prior to its use.

1000 mg kg(-1) stock solutions have been found to be stable for at least 3 months. Stability of stock solution should be checked with a fresh standard solution for periods longer than three months.

5.7.2 Internal standard (IS) solution
Diphenyl disulfide (DPDS) is recommended as the internal standard. A stock solution of DPDS should be prepared in toluene at 500 mg kg(-1) concentration. It is recommended that a fresh IS stock solution should be prepared every 3 months. The stock solution should be stored in amber glass bottles with PTFE lined screw caps in a refrigerator at ~4 °C. The solution shall be brought to room temperature (~25 °C) prior to its use.