Susan Berets, Jeffrey Christenson, Harrick Scientific Products, Inc.

	In the manufacturing process, it frequently becomes important to determine if metal parts are clean or are sufficiently lubricated with oil. Infrared analysis of small, flat pieces can be readily carried out by in-compartment grazing angle specular reflectance. However, parts are frequently large and irregularly shaped or curved, which makes in-compartment analysis trickier. This note explores the use of a specular reflectance fiber optic probe to measure oil on the curved surface of a large metal cylinder outside of the spectrometer sampling compartment.

	The measurements were carried out using Harrick Omni-Spec fiber optic specular reflectance probe with two PIR fibers (Figure 1), interfaced to a commercial FT-IR spectrometer through the Harrick FiberMate2 coupler. Spectra were collected at an 8 cm

 resolution and signal averaged over 32 scans with an MCT-A detector. The sample investigated was a stainless steel cylinder, 6 in. in diameter and 4.5 in. high, with a machined finish. The sample was cleaned for the background measurement and a thin coating of Mazola oil was applied to the same area prior to the sample measurement. For both the sample and background measurements, the Omni-Spec was held so its sampling surface was tangential to the surface of the sample. 

	Figure 2 shows the oil coated metal. As expected, there are C-H stretching bands prominent in the 2800–2900 cm

 from a C-O stretch. Note that there is increased noise about 3000 cm

. This is due to the optical cut-off of the PIR fiber. The baseline of the spectrum is also strongly sloped. This is due the machine finish on the sample which creates shiny and rough areas on the surface. The Omni-Spec collects the specular reflectance in addition to some fraction of the diffused specular reflectance. This combination results in a sloping baseline. 

	Specular reflection fiber optic probes such as the Omni-Spec can be used effectively to examine coatings and residues on large, curved metal surfaces, provided that care is taken in positioning the probe relative to the sampling surface. The Omni-Spec fiber optic probe operates outside of the spectrometer sample compartment, making it a useful tool for examining large samples.

Articles

FT-IR Sampling of Residues on Curved Metal Surfaces Outside of the Sampling Compartment

	ATR is the most widely used FT-IR sampling technique, and is frequently used for quantitative, qualitative, and kinetic studies. New high-throughput accessories are expanding the scope of experiments by providing improved signal-to-noise performance. This application note demonstrates the potential of using a new high-throughput single-reflection diamond ATR accessory to study reaction kinetics by examining the heating of an aqueous sugar slurry.

	Infrared spectra were collected on an FT-IR spectrometer equipped with the Harrick DiaMaxATR™ single-reflection diamond ATR accessory with its Temperature Controlled Liquid Cell (Figure 1), Temperature Controller, and TempLink Software. The system was purged to remove water vapor and CO

 resolution and signal-averaged over 32 scans. A background spectrum was collected at ambient temperature. Confectioner's sugar was mixed with a small amount of water to generate a slurry and injected into the liquid cell. A sample spectrum was collected at ambient temperature. Then the cell was heated to 100 °C and additional sample spectra were collected at 2 min intervals.


	Figure 1: The DiaMaxATR™ with its Temperature Controlled Liquid Cell and associated Temperature Controller.

	Figure 2 shows the resulting spectra. Initially, there is a broad band centered around 3276 cm

, both of which are attributable primarily to water with a smaller component from the O-H stretches in confectioner's sugar, which is mainly sucrose. There are also bands in the 900–1150 cm

 region from the C-O and C-C stretches and bands in the 1400–1200 cm

 region due to the O-C-H, C-C-H, and C-O-H bends in the sugars, as expected.


	Figure 2: ATR spectra of sugar slurry over time at 100 °C, including plot of peak area versus time at 1519 cm

	Upon heating, the bands attributable to the O-H stretch and H-O-H bend both change shape, as the water evaporates and the primary contribution comes from the bound hydroxyl groups. The bands in the 900–1200 cm

 region also change, as the sucrose breaks down into glucose and fructose which in turn react to become small aromatic compounds and a brown-colored polymer.

 is attributed to aromatic compounds and grows as a function of time. The rate of formation of the aromatic compounds can be determined by plotting the peak area over time. Fitting the curve gives a linear relationship, where the peak area = 0.0026 x (time in minutes) with an R

 of 0.9981. Given the complexity of the reaction, it is somewhat surprising that the data could be fit easily with a linear curve but this may be due to the time scale and temperature used for the measurements.

	As seen herein, high-throughput, single-reflection ATR is valuable for examining thermally induced changes in materials. In the case of the heating an aqueous sugar slurry, it can be used to extract information about the kinetics of the reaction. Further analysis is possible to obtain more rigorous identification of the heat-induced structural changes.


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Susan Berets, Jeffrey Christenson, Harrick Scientific Products, Inc.

Articles

FT-IR Sampling of Residues on Curved Metal Surfaces Outside of the Sampling Compartment

A Heated ATR Single Reflection Diamond ATR Study of an Aqueous Sugar Slurry