Laser-induced fluorescence measurements of vapor-phase polycyclic aromatic hydrocarbons. I. Evaluation of one- and two-photon excitation processes for mixture analysis. II. Flame temperature measurements using the anomalous fluorescence of pyrene
Abstract
One-photon excited laser-induced fluorescence is shown to be a viable technique for the quantitation of polycyclic aromatic hydrocarbons (PAH) in the vapor phase. Mixtures of anthracene, fluoranthene, and phenanthrene were analyzed in a vapor cell at temperatures ranging from 50 to 90$\sp\circ$C. The individual concentrations of PAH's in these mixtures were calculated by a least-squares matrix technique and were accurate to within 5.2% for anthracene,.98% for fluoranthene, and 2.9% for phenanthrene. This one-photon excited technique was compared to the corresponding two-photon excited method. Even though the two-photon method is insensitive to background scatter, it was unable to quantitate the PAH's at the concentrations of interest because of the low two-photon absorption cross-sections of the PAH's. The first use of the anomalous fluorescence of pyrene to determine flame temperature is reported. Vapor-phase pyrene was injected into fuel-rich ethylene/oxygen flames at 80 torr and hydrogen/oxygen flames at 22 and 72 torr. The relative laser-induced (S$\sb2$ $\to$ S$\sb0$)/(S$\sb1$ $\to$ S$\sb0$) emission was monitored as a function of height above the burner and compared to the temperature profile as measured by thermocouples and sodium d-line reversal. The good agreement between the ratioing technique and the flame temperature profile demonstrates the potential of the method. The thermal behavior of pyrene is discussed in terms of a modified intermediate strong-coupling model. Because of efficient quenching of pyrene in the flame, the ratio of the S$\sb2$/S$\sb1$ yields can be predicted with a model which was proposed for isolated pyrene molecules. The theoretical calculation of the temperature dependence of S$\sb2$/S$\sb1$ agreed well with the experimental flame-temperature data. Based on this agreement, the use of pyrene as a flame temperature molecular probe is evaluated and other potential PAH probes are discussed.
Degree
Ph.D.
Advisors
Laurendeau, Purdue University.
Subject Area
Analytical chemistry
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