Insulating oils are often considered the life blood of electrical devices such as transformers. Proper functioning of these devices is essential for uninterrupted electrical power over long periods, often stretching into decades. To ensure longevity of insulating oils and improve certain characteristics additives are often added to the oils. On the other hand, the presence of contaminants can deteriorate the performance of the oils and can cause extensive damage to the electrical devices. To ensure optimal operation of electrical devices, additive and contaminant levels should be periodically monitored. Because of the complex chemical composition of insulating mineral oils, the determination of additives and contaminants is a difficult and often laborious task. However, the task can be made manageable with the use of current state-of-the art analytical instrumentation. This article demonstrates the use of electrospray ionization–mass spectrometry for rapid and specific determination of widely used metal deactivators Irgamet 30 and Irgamet 39. The article also presents specific quantitative determination of a highly corrosive sulfur compound dibenzyl disulfide and its principle nonsulfur by-product bibenzyl with gas chromatography and tandem mass spectrometry (GC–MS-MS).
Corrosion of metals, such as copper, resulting from chemical reactions with sulfur and other chemical species in insulating liquids has been a matter of concern for decades. The presence of corrosive species in general and in particular corrosive sulfur has been linked to failures of electrical equipment used in generation, transmission, and distribution of electrical energy. For this reason, the International Electrotechnical Commission (IEC) standard for mineral insulating oils states that corrosive sulfur compounds shall not be present in unused and used insulating liquids (IEC 60296 clause 6.10) (1).
It is generally accepted that the presence of corrosive sulfur species in mineral oil leads to cuprous sulfide (Cu2S) deposits on the surface of copper conductors. Cuprous sulfide is a semiconductor, and its buildup and subsequent migration into insulating paper over time disrupts the integrity of the insulating paper, which leads to short-circuit faults, sometimes accompanied by windings deformation (2–4). Efforts have been made to assess Cu2S formation resulting from the presence of corrosive sulfur compounds in mineral insulating oils for more than 60 years. Clark and Raab (5) developed a qualitative method for detecting corrosive sulfur in mineral insulating oil during the 1940s. Their approach is still being used with some modifications as standard test methods; for examples, see ASTM D-130 (6), ASTM D-1275 06 (7), DIN 51353 (8), and IEC 62535 (9). However, the standard test methods yield only qualitative results for whether oil contains corrosive sulfur compounds or not under test conditions. Furthermore, the IEC standard test method has been shown to yield false positive results with aged insulating oils and false negative results with oils containing metal deactivators or passivators (10). To overcome the limitations of the current standard test methods for corrosive and potentially corrosive sulfur species, the IEC initiated the development of a quantitative method for a highly corrosive sulfur compound dibenzyl disulfide (DBDS) as well as the development of methods for the quantitative assessment of total corrosive sulfur and specific corrosive species in insulating liquids (11).A part of this article deals with the specific method for the determination of DBDS and its principle nonsulfur by-product bibenzyl in mineral insulating oils.
This article also deals with the determination of metal deactivators in insulating mineral oils. Metal deactivators or passivators are corrosion-inhibiting chemicals (additives) that interact with metal surfaces and form 0.5–1 nm thick monolayers that limit the access of the corrosive chemical species to the metal surface (12). Common metal passivators added to insulating mineral oils are triazole and benzotriazole derivatives; the two most commonly used are Irgamet 39 and Irgamet 30, which are produced and marketed by BASF (13). Irgamet 39 is a mixture of two isomers, N,N-bis(2-ethylhexyl) 4-methyl-1-benzotriazole-1-methyl amine and N,N-bis(2-ethylhexyl) 5-methyl-1-benzotriazole-1-methyl amine. Irgamet 30 consists of N,N-bis(2-ethylhexyl)-1-triazole-1-methyl amine. Both metal deactivators contain two 2-ethylhexyl chains attached to the triazole or the tolyltriazole moieties. The presence of the 2-ethylhexyl chains in the structure enhances solubility of these compounds in mineral oils. However, the addition also makes the molecules highly unstable and extremely difficult to detect and quantify with methodologies that involve the use of gas chromatography (GC) or liquid chromatography (LC). In fact a well-accepted reversed-phase LC–UV absorption-based method for determination of Irgamet 39 in insulating mineral oil, quantifies only toluyl triazole and not the intact Irgamet 39 (14). However, reversed-phase LC coupled with electrospray ionization (ESI) and tandem mass spectrometry (MS-MS) has been used for quantitative determination of triazole and benzotriazole derivatives in aqueous environmental samples at very low concentrations (15). An LC–ESI-MS-MS method for determination of Irgamet 39 and Irgamet 30 has been reported (16). The method makes use of fragment ions resulting from precursor ion m/z 242 for quantification of both Irgamet 39 and Irgamet 30. The authors assumed that intact Irgamet 30 and Irgamet 39 molecules are separated with reversed-phase LC and the ion m/z 242 results from fragmentation of the parent ion in the ESI source. The ion m/z 242 results from protonation of di 2-ethylhexyl amine, which is present in both metal deactivators. Results obtained in our laboratory indicate that di 2-ethylhexyl amine is released from metal deactivators before their introduction into the LC column. Thus, the approach as described is unlikely to yield results that can be used for specific quantification of Irgamet 30 and Irgamet 39. Therefore, it is apparent that current methods for the detection of these widely used metal deactivators or passivators in mineral insulating oils are unsatisfactory. This article reports a rapid and specific method for the determination of Irgamet 39 and Irgamet 30 in mineral insulating oils with direct infusion ESI-MS, detection limits of the method for both metal deactivators were lower than 1 mg/kg.