Laser Technique Facilitates Mass Spectrometry of Petroleum
For those feeling pain at the pump and hoping for less expensive fill-ups, the news from the US Department of Energy in December was not good. The department has projected oil prices to remain well above $50 a barrel for the years to come, signaling the end of cheap oil. But does it mean that the world faces a crash in production?
That’s where chemistry professor Hilkka I. Kenttämaa of Purdue University in West Lafayette, Ind., comes in. Working with other researchers from the university and from ExxonMobil Research and Engineering Co. in Annandale, N.J., she has been using laser-induced acoustic desorption to improve the characterization of petroleum feedstock.
Researchers are using laser-induced acoustic desorption to study petroleum samples. A laser fired at the foil produces a shock wave that travels through the titanium and desorbs sample molecules on the other side. Courtesy of Hilkka I. Kenttämaa, Purdue University.
Better oil analysis could boost production. For example, there may be more efficient ways to refine and use heavy petroleum, which has a different chemical composition from the light petroleum favored by the industry. Knowledge of the composition and, therefore, the quality, of a petroleum feedstock enables companies to decide whether or not they should pump at a particular location, Kenttämaa explained.
The problem is that exacting petroleum characterization is not easy, so most petroleum is not analyzed, she said. Most techniques provide information about bulk chemical composition, and some can characterize polar components at the molecular level, but about 70 percent of crude oil is nonpolar hydrocarbons.
Mass spectrometry produces detailed molecular information, but it requires desorbing the petroleum distillate and ionizing it quickly, efficiently and without fragmenting the long hydrocarbon chains. For that, the researchers turned to laser-induced acoustic desorption.
The technique has been investigated for many purposes. After years of effort, the Purdue group has developed an approach that is optimized for the desorption of neutral molecules, that needs no tuning before use and that can be introduced into the probe inlets of many mass spectrometers.
The investigators deposit a petroleum sample on one side of a 12.7-μm-thick titanium foil and then fire 50 to 200 laser shots at the other side in a circular pattern. The resulting ablation creates acoustic waves that travel through the foil, freeing the molecules on the other side without producing damaging heat, and at such slow speeds that collisions do not break the molecules apart.
For their initial demonstration, which they reported in the Dec. 15 issue of Analytical Chemistry, they used a Continuum Nd:YAG laser with a frequency-doubled beam at 532 nm and with a pulse width of 3 to 5 ns. The wavelength was not critical, but the power density of the focused spot on the foil was. It had to be great enough to generate the desired acoustic waves, which had to be fast, irreversible and adiabatically compressive.
The researchers then used chemical ionization to create the charged molecules needed for mass spectrometry. This enabled them to analyze the resulting molecular cloud, yielding a plot of relative abundance versus molecular weight for a given sample. They compared the spectra for samples provided by ExxonMobil with those obtained at the company’s laboratories using existing methods and concluded that the new approach generates intact neutral molecules, extending the characterization to the nonpolar hydrocarbons.
Laser-induced acoustic desorption has potential in other areas as well, including the characterization of coal and of biological molecules. It could be used in the field after being adapted to smaller mass spectrometers.
“I am sure that we have no idea yet of the full scope of this technique, but everything we’ve seen has been fantastic,” Kenttämaa said.
- The study and measurement of spectra and their components.
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