12. Procedure
12.1 Acquire a single beam background spectrum. This background spectrum may be used in the conversion of all subsequent spectra for at least one day.
12.2 With a syringe or other injection device, fill the cell with the fresh oil, and record its single beam sample spectrum. Convert this spectrum to a transmittance spectrum by dividing it by the single beam background spectrum and to a fresh oil absorbance spectrum by taking the negative logarithm (base 10) of the transmittance spectrum. Accumulate an adequate number of scans for a satisfactory noise level of < 2 mAbs @2000 cm(-1).
NOTE 3 - Assuming there are no absorbance peaks in the range from 2050 to 1950 cm(-1) for the sample, the noise level may be estimated as the standard deviation of the absorbance data over this spectral range.
12.3 Empty and clean the cell. Heptane may be used. Fill the cell with the aged oil, and record its single beam sample spectrum. Convert this spectrum to a transmittance spectrum by dividing by the single beam background spectrum, and to an aged oil absorbance spectrum by taking the negative logarithm (base 10) of the transmittance spectrum.
NOTE 4 - It may happen that the aged oil is too viscous to fill the cell. Then it is possible to proceed to a dilution as described in 12.4.1.
12.4 Generate a differential spectrum by subtracting the fresh oil absorbance spectrum from the aged oil absorbance spectrum (see Fig. 2). Locate and zoom on the carbonyl region centered at 1720 cm(-1). Processing may continue if the maximum absorbance of this carbonyl region is lower than 1.5.
NOTE 5 - Since the carbonyl region absorption minima (close to 1820 cm(-1) and 1650 cm(-1)) can vary with the type of oil sample being tested, it was decided not to use fixed baseline limits for calculating the area A.
NOTE 6 - The carbonyl band may consist of more than one peak maxima.
NOTE 7 - Do not calculate the differential peak area by difference of the peak area of the aged oil with the peak area of the fresh oil.
12.4.1 If the maximum absorbance of the carbonyl region of the differential spectrum is higher than 1.5: dilute with 1 % accuracy by weight both fresh and aged oils with the same dilution factor, D (PAO 4 is recommended as dilution oil). For example, D = 2 for a 50 % (1:1) wt/wt dilution. Record the two spectra, convert them to absorbance and subtract them. If the maximum absorbance of the carbonyl region is still higher than 1.5, then use a higher dilution factor. This occurrence could happen in the case of ester or soot-containing oils.
NOTE 8 - The cell pathlength may be changed to 0.05 mm or 0.025 mm if absorbance in the assessment area is greater than 1.5.
NOTE 9 - Dilution factors are commonly chosen between 2 and 10.
12.4.2 If the maximum absorbance of the carbonyl region of the differential spectrum is lower than 1.5: draw a base line connecting the absorption minima located at each side of this region as shown on the spectrum in Fig. 2. These minima are usually close to 1820 cm(-1) and 1650 cm(-1) within +/- 20 cm(-1). Calculate and record the differential peak area as area A. (This may be done automatically with the spectrometer software.)
13. Calculation of Results
13.1 The results are reported as PAI (peak area increase): carbonyl region area, A multiplied by the dilution factor, D and divided by the cell pathlength, e in mm:
13.1.1 If no dilution was needed, the dilution factor, D is 1.
14. Procedures for Interferences
14.1 The results of this test method may be affected by the presence of other components with an absorbance band in the zone of 1600–1800 cm(-1). Low PAI values may be difficult to determine in those cases. The following procedures may be used if interferences are present.
14.2 Soot-Containing Oils - The presence of soot degrades the spectra by decreasing the transmittance level. This case may require a dilution as described in 12.4 in order to obtain an absorbance lower than 1.5.
14.3 Ester-Containing Oils - The ester functions contained in some lubricants, especially those formulated with ester base oil, interfere with the oxidation peak. Dilution may be needed with these types of lubricants and it is recommended to use a cell with a small pathlength (0.05 mm maximum). Check the shape of the spectrum before interpreting it. The residual positive or negative peaks at 1740 cm(-1) showing the presence of ester function may make it difficult to correctly perform the subtraction operation between the aged oil spectrum and the fresh oil spectrum. The different examples below show the different cases that could be encountered and describe the baselines settings needed to eliminate these ester residual interfering peaks.
14.3.1 Example 1 (see Fig. 3) - This differential spectrum is representative of a lubricant containing no ester base oil or containing ester but showing no interference. In this case, draw the baseline between the absorption minima located on either side of this region as shown on the spectrum in Fig. 2. These minima are usually close to 1620 cm(-1) and 1850 cm(-1) within 20 cm(-1).
14.3.2 Example 2 (see Fig. 4) - There is a small residual negative peak at 1740 cm(-1). This negative peak does not cross the baseline between 1650 and 1820 cm(-1). Draw a first baseline close to 1650 and 1820 cm(-1) as described in 12.4. This baseline creates the area A1. Draw a second baseline above the residual peak creating the area A2, representative of the ester interference. This second baseline has to be set in order to obtain a peak shape similar to a peak showing no interference as shown in Example 1, that is, a peak at approximately 1730 cm(-1) and a smaller peak at approximately 1780 cm(-1). The PAI is calculated from the area A defined here by:
Area A = A1 + A2
14.3.3 Example 3 (see Fig. 5) - There is a tall residual negative peak at 1740 cm(-1) crossing the baseline between 1650 and 1820 cm(-1). Draw a first baseline close to 1650 and 1820 cm(-1) as described in 12.4. This baseline creates the areas A1 + A2 - A3. Draw a second baseline above the residual peak creating the areas A3 + A4, representative of the ester interference. This second baseline has to be set in order to obtain a peak shape similar to a peak showing no interference as shown in Example 1, that is, a peak at approximately 1730 cm(-1) and a smaller peak at approximately 1780 cm(-1). The PAI is calculated from the area A defined here by:
Area A = (A1 + A2 - A3 ) + (A3 + A4 ) = A1 + A2 + A4
14.3.4 Example 4 (see Fig. 6) - There is a residual positive peak at 1740 cm(-1). Draw a first baseline close to 1650 and 1820 cm(-1) as described in 12.4. This baseline creates the areas A1 + A2. Draw a second baseline under the residual peak creating the area A2, representative of the ester interference. This second baseline has to be set in order to obtain a peak shape similar to a peak showing no interference as shown in Example 1, that is, a peak at approximately 1730 cm(-1) and a smaller peak at approximately 1780 cm(-1). The PAI is calculated from the area A defined here by:
Area A = (A1 + A2) - A2 = A1
14.3.5 Example 5 (see Fig. 7) - The differential spectrum is a negative interference peak at 1740 cm(-1). No PAI value can be determined. This occurrence could happen with ester-containing oils with very low oxidation level.
15. Quality Control
15.1 Confirm the performance of the test procedure by analyzing a quality control (QC) sample that is, if possible, representative of the samples typically analyzed.
15.2 Prior to each series of used lubricant measurements, a measurement shall be conducted on the QC sample using the procedure described above.
15.3 The results for the QC samples shall be analyzed as described in Practice D 6299 or by another similar procedure to ensure that the measurement system is in control prior to use. Minimally, an I-Chart and MR-Chart shall be used. If the I-Chart or MR-Chart analysis indicates an out-of-control situation, the cause of the out -of-control performance shall be diagnosed and corrected before the testing of used lubricants continues.