Molecular Isomers Research Articles

Hydrogen-bonded Two-fold Interpenetrated Diamondoid Networks for Solid-State [2 + 2] Polymerizations of Criss-crossed Olefins: Molecular Connections vs Supramolecular Connections

The molecular isomers of a diene-containing amide and pyridine moieties revealed that they exhibit different crystal structures and reactivities. One of the isomer forms a two-fold interpenetrated diamonded network via N–H···N hydrogen bonds and undergoes a [2 + 2] photopolymerization in the solid-state despite having unfavorable criss-cross aligned double bonds.

Radiation-induced transformations of isolated CH3CN molecules in noble gas matrices

The transformations of isolated CH3CN molecules in various solid noble-gas matrices (Ne, Ar, Kr, and Xe) under the action of X-ray irradiation at 5K were investigated by FTIR spectroscopy. The main products are CH3NC, CH2CNH and CH2NCH molecular isomers as well as CH2CN and CH2NC radicals. The matrix has a strong effect on the distribution of reaction channels. In particular, the highest relative yield of keteneimine (CH2CNH) was found in Ne matrix, whereas the formation of CH3NC predominates in xenon. It was explained by differences in the matrix ionization energy (IE) resulting in different distributions of hot ionic reactions. The reactions of neutral excited states are mainly involved in Xe matrix with low IE, while the isomerization of the primary acetonitrile positive ions may be quite effective in Ne and Ar. Annealing of the irradiated samples results in mobilization of trapped hydrogen atoms followed by their reactions with radicals to yield parent molecule and its isomers. The scheme of the radiation-induced processes and its implications for the acetonitrile chemistry in cosmic ices are discussed.

Source-oriented risk assessment of inhalation exposure to ambient polycyclic aromatic hydrocarbons and contributions of non-priority isomers in urban Nanjing, a megacity located in Yangtze River Delta, China

Sixteen U.S. EPA priority polycyclic aromatic hydrocarbons (PAHs) and eleven non-priority isomers including some dibenzopyrenes were analyzed to evaluate health risk attributable to inhalation exposure to ambient PAHs and contributions of the non-priority PAHs in a megacity Nanjing, east China. The annual average mass concentration of the total 16 EPA priority PAHs in air was 51.1 ± 29.8 ng/m3, comprising up to 93% of the mass concentration of all 27 PAHs, however, the estimated Incremental Lifetime Cancer Risk (ILCR) due to inhalation exposure would be underestimated by 63% on average if only accounting the 16 EPA priority PAHs. The risk would be underestimated by 13% if only particulate PAHs were considered, though gaseous PAHs made up to about 70% of the total mass concentration. During the last fifteen years, ambient Benzo[a]pyrene decreased significantly in the city which was consistent with the declining trend of PAHs emissions. Source contributions to the estimated ILCR were much different from the contributions for the total mass concentration, calling for the introduce of important source-oriented risk assessments. Emissions from gasoline vehicles contributed to 12% of the total mass concentration of 27 PAHs analyzed, but regarding relative contributions to the overall health risk, gasoline vehicle emissions contributed 45% of the calculated ILCR. Dibenzopyrenes were a group of non-priority isomers largely contributing to the calculated ILCR, and vehicle emissions were probably important sources of these high molecular weight isomers. Ambient dibenzo[a,l]pyrene positively correlated with the priority PAH Benzo[g,h,i]perylene. The study indicates that inclusion of non-priority PAHs could be valuable for both PAH source apportionment and health risk assessment.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group.

Two iron(ii) molecular isomers, namely face-Λ-[Fe(L1)3](ClO4)2 (1a) (L1 = R-1-phenyl-N-((1-hexyl-5-methyl-1H-imidazol-2-ylmethylene)ethanamine)) and face-Λ-[Fe(L2)3](ClO4)2 (1b) (L2 = R-1-phenyl-N-(1-hexyl-4-methyl-1H-imidazol-2-ylmethylene)ethanamine), are obtained via a well-designed strategy based on the tautomerization of the 4(5)-methylimidazole group. Structural investigations reveal that the two isomers are extremely similar with only differences in the methyl group position of the ligands. The iron(ii) center is surrounded by three bidentate imidazole Schiff-base ligands in the face-Λ conformation, which affords a distorted [FeN6] octahedral coordination sphere. The average Fe-N bond length of 1a (1.971 Å) is shorter than that of 1b (2.185 Å). Spectroscopy analyses, X-ray crystal structures and magnetic investigations show that 1a is in the low-spin state at room temperature due to the strong ligand field imparted by the electron-donating methyl groups at the 5-position on the imidazole moieties and undergoes a partial spin transition with an estimated T1/2 = 390 K. In contrast, 1b is stabilized in the high-spin state because of the strong steric hindrance when the three methyl "legs" are changed from the 5-position to the 4-position on the imidazole moieties. In addition, a new complex, 2, without methyl groups attached to the imidazole rings is introduced and characterized to further corroborate the steric influence on the spin state. Complex 2 exhibits a gradual spin-crossover behaviour with T1/2 = 258 K. Moreover, the diverse spin states of these complexes are computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that the competition of the electronic effect and steric crowding influence the spin states.

Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging.

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C2H2Br2). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

Versatile and Resilient Hydrogen-Bonded Host Frameworks.

Low-density molecular host frameworks, whether equipped with persistent molecular-scale pores or virtual pores that are sustainable only when occupied by guest molecules, have emerged as a promising class of materials owing to the ability to tailor the size, geometry, and chemical character of their free space through the versatility of organic synthesis. As such, molecular frameworks are promising candidates for storage, separations of commodity and fine chemicals, heterogeneous catalysis, and optical and electronic materials. Frameworks assembled through hydrogen bonds, though generally not stable toward collapse in the absence of guests, promise significant chemical and structural diversity, with pores that can be tailored for a wide range of guest molecules. The utility of these frameworks, however, depends on the resilience of n-dimensional hydrogen-bonded motifs that serve as reliable building blocks so that the molecular constituents can be manipulated without disruption of the anticipated global solid-state architecture. Though many hydrogen-bonded frameworks have been reported, few exist that are amenable to systematic modification, thus limiting the design of functional materials. This Account reviews discoveries in our laboratory during the past decade related to a series of host frameworks based on guanidinium cations and interchangeable organosulfonate anions, in which the 3-fold symmetry and hydrogen-bonding complementarity of these ions prompt the formation of a two-dimensional (2-D) quasi-hexagonal hydrogen-bonding network that has proven to be remarkably resilient toward the introduction of a wide range of organic pendant groups attached to the sulfonate. Since an earlier report in this journal that focused primarily on organodisulfonate host frameworks with lamellar architectures, this unusually persistent network has afforded an unparalleled range of framework architectures and hundreds of new crystalline materials with predictable solid-state architectures. These range from the surprising discovery of inclusion compounds in organomonosulfonates (GMS), as well as organodisulfonates (GDS), structural isomerism reminiscent of microstructures observed in soft matter, a retrosynthetic approach to the synthesis of polar crystals, separation of molecular isomers, storage of unstable molecules, formation of a zeolite-like hydrogen-bonded framework, and postsynthetic pathways to inclusion compounds through reversible guest swapping in flexible GS frameworks. Collectively, the examples described in this Account illustrate the potential for hydrogen-bonded frameworks in the design of molecular materials, the prediction of solid-state architecture from simple empirical parameters, and the importance of design principles based on robust high dimensional networks. These and other emerging hydrogen-bonded frameworks promise new advanced materials that capitalize fully on the ability of materials chemists to manipulate solid-state structure through molecular design.

Construction of Supramolecular Assemblies from Self-Organization of Amphiphilic Molecular Isomers.

Amphiphilic coil-rod-coil molecules, incorporating flexible and rigid blocks, have a strong affinity to self-organize into various supramolecular aggregates in bulk and in aqueous solutions. In this paper, we report the self-assembling behavior of amphiphilic coil-rod-coil molecular isomers. These molecules consist of biphenyl and phenyl units connected by ether bonds as the rod segment, and poly(ethylene oxide) (PEO) with a degree of polymerization of 7 and 12 as the flexible chains. Their aggregation behavior was investigated by differential scanning calorimetry, thermal optical polarized microscopy, small-angle X-ray scattering spectroscopy, and transmission electron microscopy. The results imply that the molecular structure of the rod building block and the length of the PEO chains dramatically influence the creation of supramolecular aggregates in bulk and in aqueous solutions. In the bulk state, these molecules self-organize into a hexagonal perforated lamellar and an oblique columnar structure, respectively, depending on the sequence of the rod building block. In aqueous solution, the molecule with a linear rod segment self-assembles into sheet-like nanoribbons. In contrast, its isomer, with a rod building block substituted at the meta-position of the aryl group, self-organizes into nanofibers. This is achieved through the control of the non-covalent interactions of the rod building blocks.

Computational design of multi-states monomolecular device using molecular hydrogen and C20 isomers

We perform detailed calculations for the interaction of molecular hydrogen with C20isomers in the framework of density functional theory method. The adsorption of H2 outside the C20-e isomer with parallel orientation with respect to the plane of the hexagon is found to be the most stable adsorption configuration. Thus this might have potential for the hydrogen storing. We have also investigated the number and the position of adsorption sites in the pentagon for the parallel configurations of the H2/C20 systems. We find two stable configurations of the molecule for the C20-bowl isomer that have a small difference in energy. Thus, surprisingly, despite their apparent simplicity these H2/C20-bowl systems are shown to exhibit the flip-flop motion by a small current pulse. Hence, it might be a candidate for multi-states monomolecular device. Convenient experimental techniques such as field emission microscopy are proposed to test these predictions.

Increasing Polyaromatic Hydrocarbon (PAH) Molecular Coverage during Fossil Oil Analysis by Combining Gas Chromatography and Atmospheric-Pressure Laser Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS).

Thousands of chemically distinct compounds are encountered in fossil oil samples that require rapid screening and accurate identification. In the present paper, we show for the first time, the advantages of gas chromatography (GC) separation in combination with atmospheric-pressure laser ionization (APLI) and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) for the screening of polyaromatic hydrocarbons (PAHs) in fossil oils. In particular, reference standards of organics in shale oil, petroleum crude oil, and heavy sweet crude oil were characterized by GC-APLI-FT-ICR MS and APLI-FT-ICR MS. Results showed that, while APLI increases the ionization efficiency of PAHs, when compared to other ionization sources, the complexity of the fossil oils reduces the probability of ionizing lower-concentration compounds during direct infusion. When gas chromatography precedes APLI-FT-ICR MS, an increase (more than 2-fold) in the ionization efficiency and an increase in the signal-to-noise ratio of lower-concentration fractions are observed, giving better molecular coverage in the m/z 100-450 range. That is, the use of GC prior to APLI-FT-ICR MS resulted in higher molecular coverage, higher sensitivity, and the ability to separate and characterize molecular isomers, while maintaining the ultrahigh resolution and mass accuracy of the FT-ICR MS separation.

Determination of high-molecular weight polycyclic aromatic hydrocarbons in high performance liquid chromatography fractions of coal tar standard reference material 1597a via solid-phase nanoextraction and laser-excited time-resolved Shpol'skii spectroscopy

This article presents an alternative approach for the analysis of high molecular weight – polycyclic aromatic hydrocarbons (HMW-PAHs) with molecular mass 302Da in complex environmental samples. This is not a trivial task due to the large number of molecular mass 302Da isomers with very similar chromatographic elution times and similar, possibly even virtually identical, mass fragmentation patterns. The method presented here is based on 4.2K laser-excited time-resolved Shpol'skii spectroscopy, a high resolution spectroscopic technique with the appropriate selectivity for the unambiguous determination of PAHs with the same molecular mass. The potential of this approach is demonstrated here with the analysis of a coal tar standard reference material (SRM) 1597a. Liquid chromatography fractions were submitted to the spectroscopic analysis of five targeted isomers, namely dibenzo[a,l]pyrene, dibenzo[a,e]pyrene, dibenzo[a,i]pyrene, naphtho[2,3-a]pyrene and dibenzo[a,h]pyrene. Prior to analyte determination, the liquid chromatographic fractions were pre-concentrated with gold nanoparticles. Complete analysis was possible with microliters of chromatographic fractions and organic solvents. The limits of detection varied from 0.05 (dibenzo[a,l]pyrene) to 0.24µgL−1 (dibenzo[a,e]pyrene). The excellent analytical figures of merit associated to its non-destructive nature, which provides ample opportunity for further analysis with other instrumental methods, makes this approach an attractive alternative for the determination of PAH isomers in complex environmental samples.

Universal Structure Conversion Method for Organic Molecules: From Atomic Connectivity to Three‐Dimensional Geometry

We present a powerful method for the conversion of molecular structures from atomic connectivity to bond orders to three‐dimensional (3D) geometries. There are a number of bond orders and 3D geometries corresponding to a given atomic connectivity. To uniquely determine an energetically more favorable one among them, we use general chemical rules without invoking any empirical parameter, which makes our method valid for any organic molecule. Specifically, we first assign a proper bond order to each atomic pair in the atomic connectivity so as to maximize their sum and the result is converted to a SMILES notation using graph theory. The corresponding 3D geometry is then obtained using force field or ab initio calculations. This method successfully reproduced the bond order matrices and 3D geometries of 10 000 molecules randomly sampled from the PubChem database with high success rates of near 100% except a few exceptional cases. As an application, we demonstrate that it can be used to search for molecular isomers efficiently.

High-Performance Liquid Chromatographic Analysis of Sarcosine as a Fluorescent Levofloxacin Derivative.

Sarcosine, N-methyl glycine, could be used as a biomarker for the diagnosis of prostate cancer. It exists in biosamples at low levels; therefore, sensitive methods are necessary for its detection. In this study, we developed a sensitive and selective method for the analysis of sarcosine, based on derivatizing sarcosine with a fluorescent reagent levofloxacin acyl chloride. The resulting derivative is highly responsive to a fluorimetric detector (λex = 290 nm, λem = 460 nm). The sarcosine derivative can be separated from its molecular isomers (α-l-alanine, α-d-alanine and β-alanine) on a hexyl-phenyl column by gradient elution using sodium acetate buffer (pH 3.8; 50 mM) and tetrahydrofuran as the mobile phase. The method showed a determination range of sarcosine in water over 44.5-1780.0 ng/mL (0.5-20.0 µM) and the limit of detection at 8.9 ng/mL (0.1 µM) (S/N = 3 with 20 µL injected). Intra- and inter-day precision (as % relative standard deviation) and accuracy (as % relative error) were all below 4.8%. Application of the method to the analysis of sarcosine in human urine proved feasible.

INVESTIGATING THE MINIMUM ENERGY PRINCIPLE IN SEARCHES FOR NEW MOLECULAR SPECIES—THE CASE OF H2C3O ISOMERS

Recently, Lattelais et al. (2009) have interpreted aggregated observations of molecular isomers to suggest that there exists a "minimum energy principle'', such that molecular formation will favor more stable molecular isomers for thermodynamic reasons. To test the predictive power of this principle, we have fully characterized the spectra of the three isomers of C$_{3}$H$_{2}$O toward the well known molecular region Sgr B2(N). Evidence for the detection of the isomers cyclopropenone (c-C$_{3}$H$_{2}$O) and propynal (HCCCHO) is presented, along with evidence for the non-detection of the lowest zero-point energy isomer, propadienone (CH$_2$CCO). We interpret these observations as evidence that chemical formation pathways, which may be under kinetic control, have a more pronounced effect on final isomer abundances than thermodynamic effects such as the minimum energy principle.

Photoisomerization action spectroscopy: flicking the protonated merocyanine-spiropyran switch in the gas phase.

Laser spectroscopy and ion mobility spectrometry are combined to provide structural and photochemical information on photoisomerizing molecules in the gas phase. The strategy exploits the fact that an ion packet propelled through buffer gas by an electric field separates spatially and temporally into its constituent isomers because of small differences in their collision cross sections. Isomers selected by an electrostatic ion gate are exposed to wavelength tunable radiation, promoting formation of photoisomers that are separated in a second ion mobility stage. The approach is demonstrated for protonated merocyanine and spiropyran isomers formed through electrospray ionization. Four isomers are observed whose relative abundances depend on pretreatment of the electrosprayed solution with either ultraviolet or visible light, and on collisional excitation before the ions are launched into the drift tube. The observations are interpreted in the light of accurate double-hybrid density functional theory calculations for the protonated spiropyran and merocyanine isomers that are used to predict structures, relative energies, isomerization barriers, collision cross sections and electronic absorption spectra. The two most abundant isomers, are merocyanine forms, in which the proton resides on the quinone oxygen atom, with either a trans or cis central bond in the linking polymethine chain. These two mero forms can be interconverted through photoexcitation, with different wavelength dependences for the forward and reverse photoisomerization processes. Protonated spiropyran is formed from protonated merocyanine isomers through collisional activation, but in only minor amounts through their photo-excitation over the 300-700 nm range.

Importance of Molecular Structure on the Thermophoresis of Binary Mixtures.

Using thermal lens spectroscopy, we study the role of molecular structural isomers of butanol on the thermophoresis (or Soret effect) of binary mixtures of methanol in butanol. In this study, we show that the thermal lens signal due to the Soret effect changes its sign for all the different concentrations of binary mixtures of butanol with methanol except for the one containing tertiary-butanol. The magnitude and sign of the Soret coefficients strongly depend on the molecular structure of the isomers of butanol in the binary mixture with methanol. This isomerization dependence is in stark contrast to the expected mass dependence of the Soret effect.

Effect of Concentration on Isomerization of Rhodanine Derivatives of Merocyanine Dyes in Polar Solvents

Rhodanine derivatives of merocyanine dyes with residues of 1,3,3-trimethyl-3H-indole and 3-ethylbenzothiazoline have been found to possess two molecular forms in diluted solutions of polar solvents such as dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidinone. The first molecular form was observed to prevail at low concentrations of the dyes, normally up to 10−5 M. The second one prevails at higher concentrations and is displayed through a new band in the electronic absorption spectrum, which is red-shifted with respect to the absorption band of the first form. No similar effect was found for these dyes by use of nonpolar solvents or upon alkyl-substitution of the molecules at nitrogen atom in the rhodanine moiety. We assign the above two forms to different molecular isomers and the analogous spectral changes were shown to take place by light or heat influence which correspond to a typical isomerization effect for the related merocyanine dyes. It is discussed that the isomer transformation is facilitated by the increased mobility of the proton bonded to the nitrogen atom of the rhodanine moiety in the polar environment and the increased amount of dye-dye collisions.

Enhanced lipid isomer separation in human plasma using reversed-phase UPLC with ion-mobility/high-resolution MS detection

An ultraperformance LC (UPLC) method for the separation of different lipid molecular species and lipid isomers using a stationary phase incorporating charged surface hybrid (CSH) technology is described. The resulting enhanced separation possibilities of the method are demonstrated using standards and human plasma extracts. Lipids were extracted from human plasma samples with the Bligh and Dyer method. Separation of lipids was achieved on a 100 × 2.1 mm inner diameter CSH C18 column using gradient elution with aqueous-acetonitrile-isopropanol mobile phases containing 10 mM ammonium formate/0.1% formic acid buffers at a flow rate of 0.4 ml/min. A UPLC run time of 20 min was routinely used, and a shorter method with a 10 min run time is also described. The method shows extremely stable retention times when human plasma extracts and a variety of biofluids or tissues are analyzed [intra-assay relative standard deviation (RSD) <0.385% and <0.451% for 20 and 10 min gradients, respectively (n = 5); interassay RSD <0.673% and <0.763% for 20 and 10 min gradients, respectively (n = 30)]. The UPLC system was coupled to a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometer, equipped with a traveling wave ion-mobility cell. Besides demonstrating the separation for different lipids using the chromatographic method, we demonstrate the use of the ion-mobility MS platform for the structural elucidation of lipids. The method can now be used to elucidate structures of a wide variety of lipids in biological samples of different matrices.

Identification of fatty acid steryl esters in margarine and corn using direct flow injection ESI-MSn ion trap-mass spectrometry

An approach for identification of steryl esters using flow injection ESI-MS2 and ESI-MS3 ion trap mass spectrometry has been developed. Sterols and other lipids extracted from samples of margarine and corn using hexane were subjected to solid phase extraction. The steryl ester fraction was eluted with hexane:diethyl ether (98:2, v/v). In ESI-MS experiments fatty acid steryl esters were easily detected as ammoniated adducts [M+NH4]+ using ammonium acetate as dopant. Steryl esters including molecular isomers could be identified using ESI-QIT MS2 by the facile ester cleavage whereby the charge resides with the steryl moiety. For positive confirmation of the sterol group, ESI-QIT MS3 was carried out on the intact steryl fragmentation cation. The resulting CID spectra of the steryl cation were found to be unique and similar to those from APCI-QIT MS2 CID of free sterol standards. All major steryl esters were identified in both margarine (12 esters) and corn (7 esters) extracts. Based on the ion intensity of the ESI-MS spectra the major steryl esters in margarine were: β-sitosteryl stearate, β-sitosteryl oleate, β-sitosteryl linoleate, campesteryl stearate and campesteryl linoleate. In corn they were: β-sitosteryl oleate, campesteryl stearate, campesteryl oleate and stigmasteryl linoleate. This method can be very useful for rapid and complete identification of fatty acid steryl esters extracted from other biological samples.

Ternary Hybrid Nanoparticle Isomers: Directing the Nucleation of Ag on Pt–Fe3O4 Using a Solid-State Protecting Group

Colloidal hybrid nanoparticles are an important class of materials that incorporate multiple nanoparticles into a single system through solid-state interfaces, which can result in multifunctionality and the emergence of synergistic properties not found in the individual components. These hybrid structures are typically produced using seeded-growth methods, where preformed nanoparticles serve as seeds onto which additional domains are added through subsequent reactions. For hybrid nanoparticles that contain more than two domains, multiple configurations with distinct connectivities and functionalities are possible, and these can be considered as nanoparticle analogues of molecular isomers. However, accessing one isomer relative to others in the same hybrid nanoparticle system is challenging, particularly when the formation of a target isomer is disfavored relative to more stable or synthetically accessible configurations. Here, we show that an iron oxide shell installed onto the Pt domain of Pt-Fe3O4 hybrid nanoparticles serves as a solid-state protecting group that can direct the nucleation of a third domain to an otherwise disfavored site. Under traditional conditions, Ag nucleates exclusively onto the Pt domain of Pt-Fe3O4 heterodimers, resulting in the formation of the Ag-Pt-Fe3O4 heterotrimer isomer. When the Pt surface is covered with an iron oxide protecting group, the nucleation of Ag is redirected onto the Fe3O4 domain, producing the distinct and otherwise inaccessible Pt-Fe3O4-Ag isomer. Similar results are obtained for the Au-Pt-Fe3O4 system, where formation of the favored Au-Pt-Fe3O4 configuration is blocked by the iron oxide protecting group. The thickness of the iron oxide shell that protects the Pt domain can be systematically tuned by adjusting the ratio of oleic acid to iron pentacarbonyl during the synthesis of the Pt-Fe3O4 heterodimers, and this insight is important for controllably implementing the protecting group chemistry.

Emission Characteristics for Polycyclic Aromatic Hydrocarbons from Solid Fuels Burned in Domestic Stoves in Rural China

Emission characterization of polycyclic aromatic hydrocarbons (PAHs) from residential combustion of crop residues, woody material, coal, and biomass pellets in domestic stoves in rural China are compared in term of emission factors (EFs), influencing factors, composition profiles, isomer ratios and phase distributions. The EFs of PAHs vary by 2 orders of magnitude among fuel types suggesting that a detailed fuel categorization is useful in the development of an emission inventory and potential in emission abatement of PAHs by replacing dirty fuels with relatively cleaner ones. The influence of fuel moisture in biomass burning is nonlinear. Biofuels with very low moisture display relatively high emissions as do fuels with very high moisture. Bituminous coals and brushwood yield relatively large fractions of high molecular PAHs. The emission factor of benzo(a)pyrene equivalent quantity for raw bituminous coal is as high as 52 mg/kg, which is 1-2 orders of magnitude higher than the other fuels. For source diagnosis, high molecular weight isomers are more informative than low molecular weight ones and multiple ratios could be used together whenever possible.