Raman spectroscopy is an excellent technique for the identification and characterization of fuels. With no requirement for sample preparation and the power to identify and quantify materials using robust, handheld, portable instrumentation, it is no surprise that Raman has found so many uses in a range of industries. In the case of fuels, Raman spectra contain a wealth of spectral features due to the presence of different types of hydrocarbons resulting in a unique Raman fingerprint based on chemical composition.
Chemometrics models can also be developed from Raman spectral data to more fully characterize fuels, enabling the determination of a range of critical parameters such as octane and density. Raman spectroscopy can also play a major role in the detection of counterfeit fuels through the use of Raman spectral markers added to the fuel to enable rapid field confirmation of parameters such as fuel composition, quality and source.
Biodiesel is a non-petroleum-based diesel fuel made from vegetable oil or animal fats. It is produced using a process called transesterification where the oil or fat is reacted with an alcohol to remove glycerin (used in soaps and other products) and produce biodiesel in the form of fatty acid methyl esters. Biodiesel can be used in most diesel engines with little to no engine modification required. It can also be blended with petroleum diesel fuel to provide a cleaner burning, lower emission diesel fuel. With the move toward renewable energy sources with lower emissions and less toxicity than petroleum-based fuel, biodiesel production is on the rise. As this production increases, there are many opportunities to use Raman spectroscopy during the biodiesel production process to assess the incoming raw materials, monitor the production process and confirm the quality of the final product.
With the rise in Raman spectroscopy, there are a range of options available for Raman measurements including fully integrated systems like the PeakSeeker and portable PinPointer Raman systems as well as a large toolkit of modular components including spectrometers, lasers, probes and sampling accessories. One of the newest additions to the Raman modular toolkit is the Maya2000 Pro-NIR spectrometer. The Maya2000 Pro-NIR is a new addition to the high sensitivity, low noise Maya2000 Pro product family with unprecedented sensitivity in the NIR region. The Maya2000 Pro-NIR uses a new back thinned detector with peak quantum efficiency at 700 nm and greater than 40% quantum efficiency out to 1000 nm making it an excellent choice for Raman measurements in the NIR.
The Raman measurements described below were made with a Maya2000 Pro-NIR spectrometer, Laser-785-LAB 785 nm Raman laser, RIP-RPB-785 785 nm Raman probe and OOA-HOLDER-RFA probe holder. The 785 nm laser was chosen for Raman excitation to avoid the fluorescence background often seen with shorter wavelength laser excitation. Acquisition parameters were a 500 millisecond integration time with no scans averaged and no boxcar smoothing. Samples of corn oil (sometimes used as a diesel alternative) and petroleum-based diesel were placed in small glass vials for analysis to illustrate the power of Raman to distinguish diesel fuels and to characterize biodiesel raw materials.
The Raman spectra for corn oil and diesel are shown in Figure 1 below. While these spectra share some common features due to the hydrocarbon content of the samples, there are also a number of spectral differences observed for these samples in the fingerprint region from 500–2000 cm-1. Even though both samples are suitable for use as fuel in diesel engines, they have distinct Raman spectra that distinguish the corn oil/biodiesel fuel from the petroleum-based diesel fuel. Note that in addition to identifying the fuel type based on its Raman fingerprint, more quantitative information – including assessment of critical fuel parameters – could be obtained from these spectra using more extensive analysis and an appropriate chemometrics model.
One difference that stands out in particular for the spectrum of corn oil is the presence of stearate (a form of the fatty acid found in animal and vegetable fats and oils) in the region between 1600 and 1800 cm-1. As expected, these peaks do not occur in the Raman spectrum for petroleum-based diesel fuel. While these peaks along with the other spectral differences allow for easy discrimination of these fuels from one another, it is also possible to achieve even higher spectral resolution in the Raman fingerprint region with a custom configured Maya2000 Pro-NIR using a narrower slit to provide a higher Raman shift resolution over a narrower spectral range.
With their unique hydrocarbon compositions, fuels are well suited for identification and characterization using Raman analysis. The wealth of spectral features in the Raman spectra for fuels can be used in a range of applications including determination of critical fuel parameters, fuel classification and detection of counterfeit fuels. With its unprecedented sensitivity in the NIR region, the Maya2000 Pro-NIR is an excellent choice for Raman measurements using laser excitation in the NIR region.
While the setup described here is one possible set of tools for Raman measurements, there are a number of other possibilities, both integrated and modular, to enable a range of measurements for different sample types and conditions.
In addition to the Maya2000 Pro-NIR with exceptional performance for Raman measurements in the NIR region, the new QE65 Pro high sensitivity spectrometer is also available with a cooled back thinned detector to keep dark noise from interfering with the measurements when long integration times (several seconds) are required to detect low intensity Raman scattering. Preconfigured versions of the Maya2000 Pro and QE65 Pro spectrometers are available to provide optimized Raman measurements with 532 nm and 785 nm laser excitation as well as custom configured options with wavelength range and resolution tailored to specific measurement needs.
There are also a number of Raman excitation laser options available including 532 nm and 785 nm Raman lasers. Excitation lasers and spectrometers can be coupled to a range of Raman probes with built in laser line filtering and shutters with probes designed specifically for hostile process environments, immersion into samples and various focal lengths for laboratory measurements. Custom flow cells are also an option for monitoring Raman in a flowing sample stream, if desired.
With all the choices available, the modular approach to Raman measurements provides an endless choice of setups with the ability to change or add components to meet your changing measurement needs.