Atomic Force Microscopy Identifying Fuel Pyrolysis Products and Directing the Synthesis of Analytical Standards.

Here we present a new method that integrates atomic force microscopy (AFM) with analytical tools such as high-performance liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) absorbance, and mass spectrometry (MS) along with synthetic chemistry. This allows the detection, identification, and quantification of novel polycyclic aromatic hydrocarbons (PAH) in complex molecular mixtures. This multidisciplinary methodology is employed to characterize the supercritical pyrolysis products of n-decane, a model fuel. The pyrolysis experiments result in a complex mixture of both unsubstituted as well as highly methylated PAH. We demonstrate the AFM-driven discovery of a novel compound, benz[ l]indeno[1,2,3- cd]pyrene, with the chemical structure assignment serving as input for the chemical synthesis of such molecule. The synthesis is verified by AFM, and the synthesized compound is used as a reference standard in analytical measurements, establishing the first-ever unequivocal identification and quantification of this PAH as a fuel product. Moreover, the high-resolution AFM analysis detected several five- to eight-ring PAH, which represents novel fuel pyrolysis and/or combustion products. This work provides a route to develop new analytical standards by symbiotically using AFM, chemical synthesis, and modern analytical tools.

Processing of atmospheric polycyclic aromatic hydrocarbons by fog in an urban environment.

Polycyclic aromatic hydrocarbons (PAH) are ubiquitous pollutants in the atmosphere, predominantly known for their toxicity. Although there has been substantial work on the atmospheric degradation of PAH, little is known about how the presence of atmospheric droplets (e.g., a fog cloud) affects the fate of PAH. In order to assess the processing of PAH and their corresponding oxidation products during a fog event, two field-sampling campaigns in Fresno, CA and Davis, CA were conducted. The simultaneous evaluation of concentrations of the PAH and oxygenated polycyclic aromatic compounds (OPAC) in the gas phase, particulate matter and fog water droplets before, during and after fog allows for the characterization of transformative and transport processes in a fog cloud. By tracking the ratio of OPAC to PAH in the individual atmospheric phases, two major polycyclic aromatic compounds-processing pathways can be identified: (i) the dissolution of OPAC from particulate matter and (ii) the uptake and oxidation of PAH in the fog water droplets. Wet deposition steadily decreases the pollutant concentration in the fog cloud droplets during a fog event; however, uptake and concentration via evaporative water loss upon the dissipation of a fog cloud cause an increase in the atmospheric pollutant concentration.

First identification of benzo[cd]phenanthro[1,2,3-lm]perylene by high-pressure liquid chromatography with ultraviolet-visible spectroscopy and mass spectrometry.

Benzo[cd]phenanthro[1,2,3-lm]perylene has been identified as a product of supercritical 1-methylnaphthalene pyrolysis from an experiment performed at 585 degrees C, 110atm, and 140s in a supercritical fluid flow reactor. The identification of benzo[cd]phenanthro[1,2,3-lm]perylene is based on the product's mass spectrum, HPLC elution time, and UV absorbance spectrum. The mass spectrum of the identified 1-methylnaphthalene pyrolysis product, called component I here, reveals a molecular weight of 426, corresponding to a C(34)H(18) polycyclic aromatic hydrocarbon (PAH). The extremely long HPLC elution time, 20-48min longer than those of the four other C(34)H(18) PAH components in this product mixture, indicates that component I has a planar structure with a high length-to-breadth ratio. Annellation theory is used to interpret and compare the UV spectrum of component I with those of the C(30)H(16) benzo[cd]naphtho[1,2,3-lm]perylene and the C(36)H(18) teropyrene, structures with one ring less and one ring more, respectively, than that of component I. This analysis of component I's UV spectrum, in conjunction with its mass spectrum and HPLC elution behavior, all lead to the identification of component I as the nine-ring PAH benzo[cd]phenanthro[1,2,3-lm] perylene, a molecule whose existence has never before been documented.

Design and evaluation of a dopant-delivery system for an orthogonal atmospheric-pressure photoionization source and its performance in the analysis of polycyclic aromatic hydrocarbons.

Atmospheric-pressure photoionization (APPI) mass spectrometry benefits from the addition of an ionization-enhancing dopant such as benzene. A passive dopant-delivery system has therefore been designed for use with the orthogonal APPI source within a commercial liquid chromatographic instrument with mass spectrometric detector. By providing the dopant in the gas phase, the newly designed equipment avoids mixing problems and other difficulties associated with liquid dopant addition. The system is a simple and durable design that can reliably deliver virtually any dopant with sufficient vapor pressure in the temperature range of 20 to 120 degrees C. At the optimum dopant flow rate (10% of the mobile phase flow rate) for high-performance liquid chromatography with narrow-bore (2.1 mm) columns, the system allows for uninterrupted routine analysis for up to two weeks. The performance of the device has been evaluated with benzene as dopant and with a test mixture consisting of four polycyclic aromatic hydrocarbons (PAH): naphthalene, 9H-fluorene, anthracene, and phenanthrene. All four PAH can be detected with an excellent signal-to-noise ratio in the scanning mode and a limit of detection down to 0.42 ng on column (51 pg in single-ion monitoring mode). The concentration calibration curves are linear over a range of three orders of magnitude, with correlation coefficients greater than 0.99. The utilization of benzene as dopant not only increases the sensitivity significantly - 20-fold, compared with dopant-free operation - but the low m/z values of the background ions observed also allow for the effective quantitative and qualitative analysis of PAH.

Identification of a new C28H14 polycyclic aromatic hydrocarbon as a product of supercritical fuel pyrolysis: Tribenzo[cd,ghi,lm]perylene.

Tribenzo[cd,ghi,lm]perylene has been identified as a product of the supercritical pyrolysis of both toluene and Fischer-Tropsch synthetic jet fuel. This identification is based on HPLC/UV/MS data, which show that compound I, eluting immediately after five other C28H14 isomers, is also a C28H14 PAH. The UV spectrum of compound I has features of a benzenoid PAH, of which there are only eight C28H14 isomers. Four of these isomers--benzo[a]coronene, phenanthro[5,4,3,2-efghi]perylene, benzo[cd]naphtho[3,2,1,8-pqra]perylene, and benzo[pqr]naphtho[8,1,2-bcd]perylene--have already been identified as supercritical pyrolysis products by matching their UV spectra with those of respective reference standards. A fifth C28H14 PAH--benzo[ghi]naphtho[8,1,2-bcd]perylene, which does not have a reference standard--has also been recently identified through MS and UV data, use of annellation theory to predict UV spectral characteristics, and length-to-breadth ratio/retention time data. Of the remaining three isomers, bisanthene (IUPAC name phenanthro[1,10,9,8-opqra]perylene) has been determined not to be present in our product mixture, as its UV spectrum does not match that of any of our product PAH. Using annellation theory, we predict the UV spectral characteristics of the two remaining C28H14 benzenoid isomers, for which there are no reference standards (tribenzo[cd,ghi,lm]perylene and naphthaceno[3,4,5,6,7-defghij]naphthacene). Results from this analysis show that the predicted UV spectral features of tribenzo[cd,ghi,lm]perylene match those of compound I--and that those of naphthaceno[3,4,5,6,7-defghij]naphthacene are inconsistent with those of compound I. The length-to-breadth ratio of tribenzo[cd,ghi,lm]perylene also agrees with compound I's HPLC elution behavior. This is the first time that tribenzo[cd,ghi,lm]perylene (IUPAC name phenanthro[2,1,10,9,8,7-pqrstuv]pentaphene) has been identified as a product of fuel pyrolysis or combustion.

First identification of benzo[ghi]naphtho[8,1,2-bcd]perylene as a product of fuel pyrolysis, using high-performance liquid chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometry.

We present HPLC/UV/MS evidence to support the identification of benzo[ghi]naphtho[8,1,2-bcd]perylene as a product of supercritical toluene pyrolysis. Mass spectral data confirm that compound I-eluting in between co-eluting benzo[a]coronene/phenanthro[5,4,3,2-efghi]perylene and benzo[pqr]naphtho[8,1,2-bcd]perylene, all three of which have been unequivocally identified as C(28)H(14) products of toluene pyrolysis-is also a C(28)H(14) product component. The UV spectrum of compound I is presented, and indicates that it is a benzenoid polycyclic aromatic hydrocarbon (PAH). Five of the eight benzenoid C(28)H(14) PAH isomers have published UV spectra, and characteristics of the remaining three are deduced from annelation theory. Only one of these compounds, benzo[ghi]naphtho[8,1,2-bcd]perylene, is predicted to have a UV spectrum with characteristics that we find in the spectrum of compound I. In addition, benzo[ghi]naphtho[8,1,2-bcd]perylene is the only benzenoid C(28)H(14) isomer whose length-to-breadth ratio is consistent with the HPLC retention time of compound I. The reaction mechanism through which benzo[ghi]naphtho[8,1,2-bcd]perylene is formed in this environment is shown, and is consistent with reaction pathways of other large PAH found in this product mixture.

Uptake and UV-photooxidation of gas-phase PAHs on the surface of atmospheric water films. 1. Naphthalene.

The adsorption and photochemical reaction of naphthalene vapor at the air-water interface of water films (22 microm and 450 microm) were studied in a horizontal flow reactor. Experiments were conducted in the regime where gas-phase mass transfer resistance did not limit the uptake. The equilibrium uptake was dependent on water film thickness only below 1 microm. Bulk water-air and air-to-interface partition constants were estimated from the experiments. The equilibrium partition constant between the water film and air decreased with increasing temperature. Photochemical reaction products were isolated in the water film after exposure to UV light. Four main oxygenated products were identified (1,3-indandione, 1(3H)-isobenzofuranone (phthalide), 2H-1-benzopyran-2-one (coumarin), and 1-naphthol). The initial rates of product formation were 46 to 154% larger for the thin film (22 microm) compared to both a thick film (450 microm) and bulk aqueous phase photooxidation. The atmospheric implications of reactions in water films are discussed.