ASTM D7418 Standard Practice for Set-Up and Operation of Fourier Transform Infrared (FT-IR) Spectrometers for In-Service Oil Condition Monitoring
9. Procedure for Collecting FT-IR Spectra
9.1 Background Collection - Collect a single-beam background spectrum at the beginning of each run and frequently enough thereafter such that changes in atmospheric water vapor levels and other changing ambient conditions do not significantly affect the sample results (for example, every 30 min). Four methods may be used to collect single-beam background spectra: (1) collecting an air (open-beam) background spectrum, (2) collecting a cell background spectrum, (3) collecting an air (open-beam) background spectrum and a cell reference spectrum, or (4) air background with corrected cell reference. The background spectrum shall be acquired using the operating parameters specified in 6.1.
NOTE 3 - It should be noted that changes in atmospheric conditions, such as humidity and temperature, can change the background spectrum. The frequency of background checks shall be determined by the individual laboratory.
9.1.1 Air Background - Collect a single-beam background spectrum with no cell in the sample compartment.
9.1.2 Cell Background - Collect a single-beam cell background with the clean empty cell in the sample compartment.
9.1.2.1 To use an empty cell background, either physical or computational fringe reduction methods (see Appendix X1) must be employed so as to reduce the superimposition of fringes from the spectrum of the empty cell onto the sample absorption spectrum.
9.1.2.2 To verify that the cell is empty and clean, an absorption spectrum of the empty cell should be collected using a previously collected or archived single-beam air (open-beam) spectrum as the background spectrum. Measure the maximum peak height between 3000 and 2800 cm(-1) relative to a baseline at 2700 cm(-1). If this value is <0.2 absorbance units, then the cell is adequately clean for recording an empty cell background. This spectrum may also be used to verify that the fringe reduction technique employed meets the criterion of sample spectral peak-to-peak noise (see Section 12).
9.1.3 Air Background with Cell Reference - Collect a single-beam background spectrum with no cell in the sample compartment. Obtain a cell reference spectrum by collecting a single-beam empty cell background spectrum, according to the procedure outlined in 9.1.2, and ratioing it against the newly acquired air background spectrum to give the absorption spectrum of the empty cell. This absorption spectrum is then subtracted in a 1:1 ratio from the absorption spectra of the samples collected using an air background.
9.1.4 Air Background with Corrected Cell Reference - Collect a cell reference absorbance spectrum according to the procedure outlined in 9.1.3. If any visible interference fringing is present after fringe-reduction methods have been employed, smooth the spectrum by convolution with a triangular function having full width at half-height (that is, FWHH) of 33 cm(-1) and unit area, or by an equivalent procedure. To account for the increase in reflection losses seen with an empty cell, subtract 0.067 (in absorbance) from the spectrum. The resultant spectrum is then subtracted in a 1:1 ratio from the absorption spectra of the samples collected using an air background. This procedure is appropriate only for cells held perpendicular to the optical axis and having two uncoated parallel-faced ZnSe windows. Wedging of the sample volume by a small angle (<<1°) does not affect the validity of the procedure.
9.2 Reference Collection (Differential Trend Analysis Only) - The reference oil sample should be shaken or agitated to ensure that a representative sample is taken from the bottle. Introduce the sample into the infrared transmission cell, either manually or using an automatic pumping system. Collect the absorption spectrum of the reference oil using the single-beam background spectrum collected as described in 9.1 and using the operating parameters specified in 6.1. If spectral normalization is being used (see 5.2.2), the absorption spectrum should subsequently be normalized (see 10.2) to a pathlength of 0.100 mm.
9.2.1 The reference oil sample may be diluted (see Appendix X3).
9.3 Sample Collection - The in-service oil samples to be analyzed should be shaken or agitated to ensure that a representative sample is taken from the bottle. Introduce each sample into the infrared transmission cell, either manually or using an automatic pumping system. Collect the absorption spectrum of each oil sample using the single-beam background spectrum collected as described in 9.1 and using the operating parameters specified in 6.1. If spectral normalization is being used (see 5.2.2), the absorption spectrum should subsequently be normalized (see 10.2) to a pathlength of 0.100 mm.
9.4 Cell Loading Check (optional) - A mechanism to verify that the cell is fully loaded with oil (no air bubbles) is useful to avoid misleading data. The integrated absorbance value calculated for the CH region between 2754 and 3039 cm(-1) relative to that in an archived absorption spectrum for a cell that has been ascertained to be fully loaded (by careful visual inspection) provides a simple means by which to check cell loading. The integrated absorbance values used to ascertain that the cell is fully loaded shall be determined by individual laboratories.
NOTE 4 - As a general guideline, the integrated absorbance after normalization to a pathlength of 0.100 mm (10.2) should be >2 for petroleum and ester-based oils.
