Articles, Tribology

Using FTIR to Determine Soot Concentrations

A procedure used to determine the amount of soot in lubricants is called Fourier Transform Infrared (FTIR) spectroscopy.  In this process, an infrared beam (laser) is passed through a sample.  Some of the infrared energy will be absorbed by the sample and some will pass through, depending on what chemical structures (molecules) are present.  Different molecules will have different spectral “fingerprints” or absorption characteristics.  This means that, if a specific molecule is present within the sample, a peak will be observed at a constant absorption wavelength.  For example, molecules that contribute to oxidation will generate a peak in the 1800 – 1670 cm-1 range and those that contribute to nitration are in the 1650 to 1600 cm-1 range.  The peak will depend on the amount of molecules present within the sample (concentration).e

Unlike other molecules, soot has no specific wavelength of absorption, because soot particles will block the laser beam.  This means that infrared energy is not absorbed and does not pass through to the detector.  Soot particles like to clump together.  As the concentration of soot within the sample increases, clumping increases, and less of the laser beam will reach the detector.  This correlates to an increase in absorbance value that is directly related to the percent of soot in the sample.  Visually, this is observed as a shift and tilt in the baseline of the spectral fingerprint, as shown in Figure 1.  Eventually, the laser beam is blocked to the extent that there is too much noise, and too much shift in the baseline, to reliably interpret what molecules are present.  The increased noise is demonstrated in Figure 1 when the spectral fingerprint is no longer a smooth line, but appears scribbled (3% to 7.5%).

ALS determined that the shift and tilt in the baseline is directly proportional to the soot concentration in samples containing between 0% to 5% soot.  This differs from the ASTM D7844 method, which indicates the relationship is linear up to 3% soot concentration, and should be the upper reporting limit for this method.  ALS is able to provide better service to clients by extending the upper reporting limit to 5% soot.  At concentrations >5%, the soot particles within the sample cause too much interference to produce reliable data.  This is visually observed by the change in slope (moving average line), as shown in Figure 2, for soot concentration >5%, and the increased noise (scribbling) observed in the spectral fingerprints in Figure 1.  Samples having soot concentration >5% will be reported as “>5%” on client reports.

At soot concentrations >3%, the tilt observed in the baseline begins to interfere with other FTIR parameters such as oxidation and nitration.  These parameters are determined by measuring the peak at defined wavelengths.  As soot concentration increases and the baseline shift is more pronounced, the peak for other FTIR parameters is falsely increased.  When soot concentrations above 3% are indicated, our internal reporting system will automatically report “–“ for any FTIR parameters (i.e. oxidation, nitration, etc.), which means that the actual concentration could not be determined due to interference from soot.  This is just another example of how ALS is improving to provide more accurate and reliable services to our clients.

Example of the FTIR spectral fingerprints for samples with various soot concentrations (green 0%, purple 1%, black 3%, red 5%, gray 7.5%). By ASTM D7844, the extent of the baseline shift is measured at 2000 cm-1 to determine the concentration of soot within the sample.

Demonstration of how the relationship between absorbance and soot concentration changes with increasing soot concentration.

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