Our FTIR Product manager chats about one of his favourite products in this video – the Specac Pearl, a revolutionary Liquid Transmission FTIR accessory utilising a defined pathlength spacerless Oyster cell.
The Magic of the Pearl
At the heart of the Pearl™ lies the Oyster™, a defined pathlength spacerless cell. The system has several advantages over the traditional liquid cell assembly: The Oyster cell is designed for rapid sample screening and easy cleaning ensuring a high sample throughput.
Traditional cell designs suffer from the possibility of leakage which is disastrous; once the cell leaks it must be dismantled, cleaned, and reassembled followed by laborious recalibration. With the horizontal configuration of the Oyster cell the possibility of cell leakage is greatly reduced and so downtime for recalibration is minimised.
Wedged or Parallel?
Oyster cells can we supplied with the windows either parallel to one another or with the bottom window wedged.
Parallel cells allow for the cell pathlength to be accurately determined, using the interference fringing method [2], however this can cause the appearance of a sinusoidal wave pattern in your data if the refractive index of the material between the windows is changed between background and sample. Therefore, they work best when analysing solutes, where the neat solvent can be used for the background.
Wedged cells supress the interference fringing pattern so are a great choice when you want to analyse something with air as the background, such as when analysing neat samples.
Unlike a traditional cell that cannot be disassembled without altering the pathlength (by as much as 10%), the Oyster is designed to consistently form the same pathlength, with a reproducibility of less than 1 micron every time the cell is assembled and disassembled [1].
This reproducibility opens the door to routine FTIR analysis of thick viscous samples. Previously it was impossible to obtain a reproducible pathlength, and hence quantitative data collection was out. With the Pearl & Oyster system however a uniform sample thickness is achieved, opening the door to the analysis of new sample types with FTIR, such as thick transformer oils.
Transformer Oil degradation analysis
One of our collaborators, Smith et al. investigated the effect of transformer oil aging, with FTIR analysis used to follow the breakdown over time [3,4]. Transformers use oil as a coolant to provide protection against overheating and also as an insulator to electrically isolate the transformer. Over time the oil breaks down and the insulation capacity of the oil decreases, requiring regularly scheduled replacement of the oil. A rapid field test of transformer oil using FTIR could enable oils to be replaced less frequently when they are beginning to breakdown, but before the chemical degradation reaches the point where the oil ceases to function, with huge cost benefits for the electrical companies and their customers.
To understand the impact of different environments on transformer oils Smith and co-workers investigated transformer oils in contact with common components of transformers: Kraft paper, copper and both materials. Samples were also tested under ambient conditions – where the oil did not fail the required specifications over the course of the study – and under a thermally stressed environment where samples began to fail the required electrical performance in just under a month. In the FTIR spectrum, recorded with a 100 μm CaF2 Oyster cell, the changes in electrical performance were linked to the growth of acidic peaks between 1800-1700 cm-1. These changes were linked to total acid number (TAN) using a standard analytical titration method. This standard method for determining the TAN content of the oils is too involved and time consuming to be done in the field, however, while FTIR is simple enough that once it is calibrated in a central lab, it could be deployed in the field by someone with relatively little training. The current study provided an excellent calibration plot with an R2 value of 0.999. Failure of the oil was shown to occur at TAN concentrations above 40 mg/g, with literature references indicating values above 0.16 indicate the oil is in terminal decline and requires replacement.
Hints and Tricks
A lesser known feature of the Pearl is the ability to secure the bottom Oyster window to the Pearl with an M2 screw. This makes separating windows when using sticky substances a breeze.
The highly stressed samples with the longest exposure to the thermal conditions of the transformer were particularly viscous. Conventional FTIR spectroscopic liquid transmission cells would prove useless for this kind of a calibration since it would be virtually impossible to inject the oil into a sealed cell assembly. Although you could use a demountable cell (disassembling the cell every time to load samples), the variability on the pathlength of a demountable cell would render any calibration plot of dubious value. The Pearl thus offers a unique advantage for this kind of analysis: the pathlength repeatability (for a single cell) is less than 1 µm every time it is assembled. For the cell used in this study that is at least a 10-fold improvement over the repeatability of a demountable cell that has been reassembled.
References
[1] Specac Application Note: Pathlength Repeatability
[3] G. M. Smith, A. S. Holmes-Smith & S. G. McMeekin, 2019 IEEE 20th International Conference on Dielectric Liquids (ICDL), 2019, pp. 1-4, DOI: 10.1109/ICDL.2019.8796742.
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