Analysis by ultra-high pressure liquid chromatography of passivators and furan compounds in an insulating liquid

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1,2,3-benzotriazole (BTA), N-bis tolutriazole derivative[2-ethylhexyl] aminomethyltolutriazol (TTAA) and furan compounds in insulating oil are indicators of electrical equipment degradation. They are checked periodically in accordance with standard procedures described in BS148:2009 and ASTM D5837-15. This article presents the optimization of the two separate standard methods, offering simultaneous analysis in a single trial. Additional time was saved by moving the 30 minute high performance liquid chromatography (HPLC) gradient method to a 5 minute analysis using ultra high pressure liquid chromatography (UHPLC) conditions.

In transformers and capacitors, insulating oil and paper are used to insulate conductors and cool the interior of electrical devices. Long-term operation or exposure to heat can cause these insulators to degrade, which can ultimately lead to device failure. Therefore, periodic inspection to monitor
for any signs of degradation should be standard practice. The insulating paper is attached to the device and difficult to remove, but the insulating oil is easily collected and analyzed to check the condition of the device.

1,2,3-benzotriazole (BTA) and N-bis tolutriazole derivative[2-ethylhexyl] aminomethyltolutriazol (TTAA) are additives in insulating oil and act as passivators (metal deactivators). As they affect the risk of sulfurization, quantification of corrosion of BTA and TTAA is required as specified in British Standards (BS) BS148:2009 (1), which describes a high performance liquid chromatography (HPLC) method at this end. The insulating paper used as a coating for the windings of transformers and condensers is composed of cellulose, which decomposes at high temperatures and in contact with water or oxygen. It is then dissolved as furan compounds in the insulating oil. Thus, the concentration of furan compounds is an indicator of degradation of electrical equipment. ASTM D5837‑15 (2) specifies an HPLC method for this analysis. This article presents the optimization of the methods described by ASTM and BS, to offer simultaneous analysis of passivators and furan compounds in a single assay (3). As both methods were designed for a conventional HPLC system, the run time is 30 min. By simple transfer to an ultra-high pressure liquid chromatography (UHPLC) system, the analysis time was reduced to approximately 5 min (4).

Method

Analysis of the standard solution:Figure 1 shows the chemical structures of BTA, TTAA and five furan compounds. Standard solutions were prepared according to ASTM D5837-15. Furan, BTA and TTAA compounds were weighed individually, dissolved in acetonitrile, and then diluted with water. As there is no commercially available TTAA analytical standard, Irgamet 39 (manufactured by BASF) was used instead, as stated in BS148:2009.

Figure 2 shows the HPLC chromatogram of the standard solution. Since Irgamet 39 is a mixture of two TTAA isomers, it eluted as two partially overlapping peaks. In this analysis, in accordance with BS148:2009, the combined value of the peak areas of these two isomers was used for quantification.

Analytical conditions for HPLC analysis of BTA, TTAA and furan compounds: System: i-Series (LC-2050) (Shimadzu); column: Shim-pack VP-ODS (250 mm × 4.6 mm, 5 µm) (Shimadzu); flow rate: 1.0mL/min; mobile phase: A) water, B) acetonitrile; hourly program: 15% B (0 min) ” 45% B (10 min) ” 100% B (20 min) ” 15% B (30 min); column temperature: 40˚C; injection volume: 15 µL; detection (PDA): λ = 220, 260 and 280 nm.

After demonstrating that passivators and furan compounds in insulating oil can be analyzed simultaneously by optimizing the test method for furan compounds specified in ASTM D5837-15, an additional time saving was investigated by transferring method under UHPLC conditions.

The standard chromatograms of the UHPLC analysis can be seen in Figure 3. The baseline separation of the seven compounds of interest was carried out in 1 min in a 5 min gradient. The analytical conditions were adjusted so that the TTAA isomers elute in one peak.

Analytical conditions for UHPLC analysis of BTA, TTAA and furan compounds: System: Nexera X3 UHPLC (Shimadzu); column: Shim-pack XR-ODS III (75 mm × 2.0 mm, 1.6 µm) (Shimadzu); flow rate: 0.7mL/min; mobile phase: A) water, B) acetonitrile; time program: 20% B (0–0.3 min) ” 90% B (1 min) ” 100% B (3 min) ” 20% B (5 min); column temperature: 50 ˚C; volume d injection: 5 µL detection (PDA): λ = 220, 260 and 280 nm.

Results and discussion

Linearity and repeatability: Calibration curves were prepared from standard solutions in the concentration range of 0.2, 1, 5, 10 and 20 mg/L for BTA and TTAA, and in the range of 0.01, 0 .05, 0.25, 0.50 and 1 mg/L for furan compounds. Repeatability was determined at the highest concentration for each analyte. With r2 > 0.9999 and %RSD ≤ 0.25% for all compounds studied, good linearity and repeatability were proven. The detailed results are listed in Table 1.

Sample pretreatment and recovery determination: All samples were pretreated according to ASTM D5837-15 as shown in Figure 4. To ensure good recovery using this method, standards for BTA, TTAA, and furan compounds dissolved in toluene were were added to white oil and extracted using the proposed protocol. The diluted supernatant was analyzed by UHPLC. As shown in Table 2, high recovery (≥86% for passivators and ≥97% for furan compounds) and good reproducibility (%RSD ≤1.4) were achieved using the described pretreatment method.

Conclusion

A fast and simple UHPLC method for the simultaneous analysis of passivators and furan compounds in insulating oil has been developed by optimizing the test method for furan compounds specified in ASTM D5837-15. Although ASTM D5837‑15 and BS148:2009 specify separate analytical methods for the quantification of BTA, TTAA and furan compounds, the separation of the seven analytes from each other and from impurities in the insulating oil could be easily performed (1). Transferring the method to UHPLC conditions saved additional time. Quantification of all analytes of interest could be performed in a 5 min gradient assay using the proposed conditions.

References

  1. British Standards Institute, BS 148:2009: Reclaimed mineral insulating oil for transformers and switchgear. Specification (now superseded by: BS 148:2020: Recycled mineral insulating oil for transformers and switchgear. Specification), https://www.en-standard.eu/bs-148-2020-recycled-mineral-insulating-oil-for-transformers-and-switchgear-specification/
  2. ASTM International, ASTM D5837-15: Standard Test Method for Furan Compounds in Electrical Insulating Liquids by High Performance Liquid Chromatography (HPLC)https://www.astm.org/d5837-15.html
  3. M. Hayashida, Shimadzu Corporation Application News No. L576 (Shimadzu Corporation, November 2020).
  4. T. Yoshioka, Shimadzu Corporation Application News No. L587 (Shimadzu Corporation, December 2021).

Gesa Johanna Schad obtained a degree in chemical engineering from the NTA Technical University of Isny, Germany in 2004 and an MSc in pharmaceutical analysis from the University of Strathclyde in Glasgow, United Kingdom in 2005. She has worked until 2006 as a consultant in HPLC method development and validation in an analytical laboratory of the FAO/IAEA in Vienna, Austria. She obtained her research doctorate in pharmaceutical sciences from the University of Strathclyde in 2010 and was employed as an HPLC specialist in the R&D department of Hichrom Ltd in Reading, UK from 2009. Since 2013 she has worked as HPLC Product Specialist and since 2015 as HPLC Product Manager in the Analytical Business Unit of Shimadzu Europa in Duisburg, Germany.

Takuya Yoshioka graduated with a degree in applied biology from Osaka University in Osaka, Japan in 2016 and a master’s degree in biological engineering from the Graduate School of Engineering of Osaka University in Osaka in 2018. He worked as HPLC product specialist at Shimadzu Corporation in Kyoto, Japan since 2018.

Momoka Hayashida graduated with a degree in biology from Ochanomizu University, Tokyo, Japan in 2017 and a master’s degree in life sciences from the Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo in 2019. She worked as a specialist HPLC products at Shimadzu Corporation’s Solution Center of Excellence in Kyoto since 2019.

E-mail: [email protected]
Website: www.shimadzu.eu

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