6 Dec 2021

A better design of cell for FTIR spectroscopy of viscous oils

Infrared spectroscopy is routinely used for the analysis of lubricating oils, motor oils, greases, and fuel blends. Use of the technique is sufficiently well recognised for it to be included in several standard methods published by authorities such as ASTM, ISO, and British Standards.

Standard methods for in-service oil condition monitoring such as D7412 (Phosphates), D7414 (Oxidation), D7415 (Sulfates), and D7624 (Nitration) typically call for a transmission measurement. This is because when the contaminants are present in low concentrations (such as when the oil is nearly new) the signals associated with them are too weak for detection by ATR methods.

ASTM D7418 sets out the criteria for sample cells used for infrared spectroscopy for in-service oil monitoring. The cells should be capable of taking measurements in the 4000-550 cm-1 spectral range and have a pathlength of 0.100 mm (100 µm). ZnSe is usually the preferred material for these analyses due to its optical and physical characteristics. Salt windows, such as KBr, can also access the required spectral range but they are prone to damage by moisture in the samples and may also react with nitration products [1].

When oil samples are significantly aged or pick up contaminants due to machine breakages, they often have a higher viscosity than fresh oil. For fresh oil samples, the simplest experimental set-up is a sealed sample cell that can be cleaned by simply flushing through with suitable solvents after each measurement. For samples that have been aged, however, the increased thickness and viscosity begins to impede the flow of oil through the cell. At this point, a demountable cell that can be disassembled and cleaned after every measurement will be needed. This significantly increases the workload and also introduces issues with pathlength variability after each reassembly of the cleaned cell.

A better solution 

Specac’s Pearl™ liquid analyser contains our patented Oyster™ cells, which are designed to be simple and easy to open, clean, and reassemble while maintaining the same pathlength to within 1 µm [2]. The cell has two circular windows which are permanently mounted in stainless-steel rings. Three ball bearings maintain the separation of the rings at a fixed and repeatable distance from one another. The cell is used in a horizontal plane, eliminating leaks and holding  the top window in place using only gravity; the companion Pearl™ cell holder provides the necessary optical path for this arrangement.

You can see the working of the Oyster™ cell in this short video: 


Elimination of fringing 

Another advantage of the Oyster™ cell is the ability to reduce or eliminate interference fringing in the spectrum. When two parallel windows are used in close proximity, internal reflections within the window material superimpose an interference fringing pattern on the baseline of the spectrum. The frequency of this fringing pattern can be used to accurately measure the pathlength of the cell – a requirement of several measurement norms.

In cases where it’s desirable to minimise the interference, however, one window can be put fractionally out of parallel by introducing a “wedge” angle. Depending on the pathlength of the cell and the window material chosen, the small angle will either reduce or completely eliminate the fringing pattern. The design of the Oyster™ cell makes it possible to build a permanent wedge angle into the cell. The Oyster™ cell is also available in CaF2, which reduces interference fringing further due to its lower refractive index.


Further information 

The Specac Pearl™ is ideal for analyses of oils, lubricants, and greases. We have a selection of application notes giving further details below. Please contact sales@specac.com for any further questions or to request a quote. 



  1. ASTM D7418, Standard Practice for Set-Up and Operation of Fourier Transform Infrared (FT-IR) Spectrometers for In-Service Oil Condition Monitoring, ASTM International, last updated December 17 2020. 

  1. Specac Technical Note: Pathlength Repeatability for the Pearl™