Technically Speaking: IR vs. FTIR

By Brett Winberg & Dave Wooton of Wooton Consulting

LubeTalk issue 139 was written all about FT-IR so we created an article on the different methods of IR (Infrared) testing when using this method for Used Oil Analysis.

The infrared spectroscopy of used lubricants is not a simple subject, and there are often many exceptions and potential for interference in samples to make a meaningful single practice. Infrared spectroscopy is a powerful tool, and if care is taken to accommodate the different scenarios encountered in used oils it is possible to obtain meaningful data, both on isolated and trended samples (the latter being preferred). The draft of the standard practice goes part of the way, but it does not adequately handle the differences between gasoline and diesel samples, and it is somewhat idealistic in its approach. As noted, this is only in draft form, and it could be a while before it becomes an accepted method, based on many of its shortcomings.

Infrared analyses are based on the absorption of different wavelengths of light, within the infrared spectral region, by the different chemistries of the sample. All infrared measuring instruments have three common components that are the bases of the measurement. These components include the infrared source, the infrared detector and the monochromator.

The infrared source is a device that will emit infrared radiation when heated by an electric current. There are several types of these devices. For example, the Nernst Glower is a source composition of mainly oxides of zirconium, yttrium and thorium, which the Globar is a silicon carbide rod. All of these devices produce nominally the same think – light within the infrared spectral region.

The infrared detector is the device that measures the amount of light energy from the source that has passed through the instrument. These devices change radiation energy into electrical energy for processing by the instrument. This transformation is based on two basic types: thermal – which measures the heating effects with response that is equal for all wavelengths, and selective detectors – whose response is markedly dependent on the wavelength. Most instruments utilize a thermal detector allowing the user the ability to measure full spectra.

Setting between the source and the detector is a device to analyze the radiation with respect to its individual wavelength elements. There are basically three types of monochromators: monochromoters - used in dispersive instruments, filter systems – used in filter wheel instruments, and interferometers – used in Fourier transform instruments.

The monochromoters is a prism or grating that will separate the elements of polychromatic light radiation by bending or reflecting the radiation different amounts for each wavelength. When the infrared light radiation enters the monochromoter through a slit, which focuses it by a mirror onto an exit slit. This process allows one wavelength only to focus onto the detector. Then by shifting either the mirror or the exit slit one wavelength at a time, the whole spectrum can be measured. The wavelength precision of this instrument becomes dependent upon the monochromoter/slit system’s ability to separate discrete wavelengths. This technique was the standard of most instruments sold prior to about 1980. Although a very workable technique, it did suffer from several disadvantages. These disadvantages were centered on the time to acquire a spectrum being long, the sensitivity for low-level concentration was poor and the resolution was often limited by the monochromoter’s construction ability.

Because of the limitations of monochromoters systems, newer types of techniques have been developed. One of these is the interferometers was developed around the 1980’s as the standard for infrared analyses. The interferometer is based on having a fixed and a moving set of mirrors and a beam splitter. Based on the movement of the interferometer, a phase change in the beam is generated. This will generate a fringe interference pattern called an interferogram. Unlike monochromoter, the time domain spectrum that is generated needs to be transformed into the spectrum domain to generate a wavelength-orientated spectrum. A Fourier Transform algorithm performs this transformation. Thus one generated what is called a Fourier Transform infrared spectrum (FTIR). The resolution of the spectrum is a constant over the whole spectrum and can be increased by simply increasing the length of travel of the moving mirror. The advantage of this technique is that the entire spectrum is generated simultaneously and not one wavelength at a time. This technique also allows the signal averaging of repetitive scans to produce the interferogram. This signal averaging overcomes low spectral energies by averaging out the noise, recording very weak bands.

The third type of monochromators is the filter system. In this type of instrument the infrared radiation is separated into its individual components, or wavelengths, by the use of filters. Instead of measuring the full infrared spectrum and then mathematically separating only the spectral wavelength region of interest, the filter system uses an optical filter that will only allow one wavelength, or a narrow range of wavelengths, to pass through to the detector. By placing a series of different filters in a wheel, one can rotate the wheel as desired to allow the different wavelength radiation to strike the detector. The detector is thus measuring only one wavelength, similar to the monochromoter/slit system, but controlled by the speed of the wheel’s rotation and the wavelengths of the filters. With this system, improved sensitive of weak bands and poor signal/noise problems can be easily corrected by slowing down the wheel’s rotational speed.

These instruments typically exhibit very stable measurement repeatability. The number of filters in the wheel is determined by the application. The typical analysis entails the studying of only a discrete number of infrared wavelength bands. The analyst typically ignores the remainders. With this type of monochromators one only puts a filter for the wavelengths one has interest. If one desires to measure oil oxidation, then one only uses the wavelength for this chemistry and not the entire spectral region. This will improve spectral acquisition time, lower signal/noise and improve selectivity of the analyses.

Special thanks to Dave Wooton for his input on both of these subjects.

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