Laser-Desorption Time-of-Flight Mass-Spectrometry Analysis of Soot from Various Hydrocarbon Fuels

Laser-desorption time-of-flight mass spectrometry (LDMS) was used to analyze soot collected from seven different hydrocarbon fuels (methane, n-hexane, n-heptane, n-octane, n-decane, benzene and toluene) burning with air under normal atmospheric pressure. To evaluate the technique, samples of two different types of furnace blacks, two types of graphite, a synthetic diamond and a C 70 fullerene also were analyzed. With proper experimental design, the C 70 produced a single peak of mass 840 Da, thereby showing the ability of LDMS to remove and singly ionize large intact molecular species from the sample for the purpose of chemical analysis. The LDMS soot analysis showed that each mature soot has a unique chemical fingerprint. A family of fuels (e.g., normal alkanes) produces soot that is similar in appearance when mature and that therefore might be expected to yield similar mass spectra, but it is found instead that each spectrum has unique features that are different for each parent fuel. These initially surprising results point to the need for further study of fuel pyrolysis in flames.