Analyzing Residues on Curved Metal Surfaces Outside of the Sampling Compartment Using Sampling FTIR

It is highly critical to establish whether metal components are well lubricated with oil or clean during the manufacturing process. In-compartment grazing angle specular reflectance enables performing infrared analysis of small, flat pieces. In many cases, however, components are mostly large and irregularly curved or shaped, thereby making in-compartment analysis difficult.

This article discusses the application of a specular reflectance fiber optic probe to determine oil on the curved surface of a large metal cylinder placed on the outer side of the spectrometer sampling compartment.

Experiment

Harrick Omni-Spec fiber optic specular reflectance probe equipped with two PIR fibers (Figure 1), interfaced to a commercial FTIR spectrometer via the Harrick FiberMate2 coupler, was used to carry out the measurements.

Signal averaged over 32 scans and spectra were acquired at an 8cm-1 resolution with an MCT-A detector.

A stainless steel cylinder with a machined finish and dimensions of 4.5” in height and 6” in diameter was chosen as the sample to be analyzed. First, the sample was cleaned and then coated with a thin layer of azola oil before performing the sample measurement. The sampling surface of the Omni-Spec must be positioned to be tangential to the sample’s surface for both the background and sample measurements.

Omni-Spec specular reflection fiber optic probe.

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

Results and Discussion

Figure 2 illustrates the metal well coated with oil. As anticipated, there are C-H stretching bands more pronounced in the 2800-2900cm-1 region, in addition to bands at 1724cm-1 because of a C=0 stretch, 1462cm-1 owing to C-H bends, and 1160cm-1 due to a C-O stretch.

It is to be noted that there is an increase in noise of around 3000cm-1 because of the optical cut-off of the PIR fiber. The spectrum’s baseline is also highly sloped because of the machine finish on the sample, which produces rough and shiny areas on the surface. The Omni-Spec acquires the specular reflectance along with a certain amount of the diffused specular reflectance, resulting in a sloping baseline.

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

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

Conclusion

Specular reflection fiber optic probes, such as the Omni-Spec, are very effective in analyzing residues and coatings on large, curved metal surfaces. However, care must be taken to ensure that the probe is positioned relative to the sampling surface. As the Omni-Spec fiber optic probe operates on the outer side of the spectrometer sample compartment, it becomes a useful instrument for large sample analysis.

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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