FTIR Testing of Residues of Curved Metal Surfaces

Determining whether the metals components are clean or are adequately lubricated with oil is often important in a manufacturing process. Infrared analysis of tiny, flat pieces can be readily performed by in-compartment grazing angle specular reflectance. However, performing in-compartment analysis is trickier when the components are large and irregularly shaped or curved.

This article discusses oil measurement on the curved surface of a large metal cylinder outside of the sampling compartment of a spectrometer using a specular reflectance fiber optic probe.


Harrick Omni-Spec fiber optic specular reflectance probe with two PIR fibers, coupled to a commercial FTIR spectrometer via the Harrick FiberMate2 coupler, was used to perform the measurements (Figure 1). Spectra were acquired at a resolution of 8 cm-1 and an MCT-A detector was used to average the signal over 32 scans.

A 4.5” high, 6” diameter stainless steel cylinder with a machine finish was used as the sample. Sample cleaning was carried for the background measurement, followed by the application of a thin coating of Mazola oil to the same area before taking the sample measurement. The sample and background measurements were taken by holding the OmniSpec so that its sampling surface was tangential to the sample surface.

FTIR Testing

Figure 1. Omni-Spec specular reflection fiber optic probe.

Results and Discussion

The oil coated metal is shown in Figure 2. As expected, CH stretching bands are clearly observed in the 2800 - 2900 cm- 1 region, as well as bands at 1462 cm-1 caused by C-H bends, 1724 cm-1 caused by a C=0 stretch, and 1160 cm-1 from a C-0 stretch.

Increased noise is observed at around 3000 cm-1 due to the optical cut-off of the PIR fiber. The spectrum’s baseline is also strongly sloped because of the machine finish on the sample, which forms rough and shiny areas on the surface. The specular reflectance, as well as the some fraction of the diffused specular reflectance, is collected by the Omni-Spec. This combination leads to a sloping baseline.

FTIR Testing

Figure 2. Specular reflectance spectrum of oil on surface of machined steel cylinder.


Coatings and residues on large, curved metal surfaces can be effectively examined using specular reflection fiber optic probes such as the Omni-Spec by carefully positioning the probe corresponding to the sampling surface. The Omni-Spec fiber optic probe is a useful tool for investigating large samples as it can be operated outside of the spectrometer sample compartment.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.


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