Pressurized Fluid Extraction Research Articles

新型持久性有机污染物分析方法研究进展

Polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs), perfluorooctanoic acid (PFOA), perfluorooctyl sulfonates (PFOS), chlordecone and polybrominated dibenzo- p -dioxin and dibenzofurans (PBDD/Fs), as the new listed POPs in Stockholm Convention and emerging POPs, were increasingly concerned by environmental scientists. The research work about the residue’s characteristic, source analysis, trends evolution, transport, bioaccumulation, and toxic effects of these emerging chemicals depended on the development of the analytical methods. Analytical procedures of PBDEs and PBBs were similar to those of organochlorine pesticides (OCPs) and poly chlorinated biphenyls (PCBs), due to their similar physical and chemicalcharacteristics with “Dirty Dozen”. Many extraction methods, such as Soxhlet extraction, automatic Soxhlet extraction, supersonic extraction, microwave assisted extraction, pressurized fluid extraction, were applied for water, sediment, soil and particulate matter. Common clean-up steps were sulfuric acid silica gel cartridge, multiple layer chromatography column and GPC. Although HRGC/HRMS and GC/MS/MS were effective for environmental background samples, GC/MS-EI and GC/MS-NCI were most popular for determinations of the PBDEs and PBBs indicators or markers. HPLC/MS/MS with solid phase extraction was developed for the perfluorinated alkyl acids, sulfonates and their salts. HLB solid phase column was efficient for PFOA/PFOSs recoveries from water samples, as the weak anion exchange column could raise the recoveries of perfluorinated alkyl acids and sulfonates with the carbon chain ranging from 4 to 14. Acetone was necessary in the extraction because of the polarity of clordecone. The GC injector and capillary column needed cleaning carefully when clordecone was analyzed by GC/ECD and GC/MS, otherwise the strong tailing of chromatographic peak would make the exact qualitative and quantitative impossible. The analysis procedures for polybrominateddibenzo-p-dioxin and dibenzofurans (PBDD/Fs) were similar to those of PCDD/Fs. Florisil or active carbon-impregnated silica gel column were packed to separate PBDEs and PBDD/Fs. The instrumental conditions were optimized to prevent from the thermal degradation of higher brominated compounds. The application of the method was also depended on the improvement of PBDD/Fs reference materials.

Pressurized fluid extraction of polyunsaturated fatty acids from the microalga Nannochloropsis oculata

Microalgae are the primary producers of economically valuable polyunsaturated fatty acids (PUFAs) and can be used as a resource for biorefining. In this respect, these high-value products may support the economical viable energy recovery from microalgae. The extraction of PUFA-containing lipids from Nannochloropsis oculata, a marine species rich in eicosapentaenoic acid (EPA), was tested with a commercially available pressurized fluid extraction technique (PFE, traded as Accelerated Solvent Extraction (ASE®)) in our study. Solvents, which are suitable for an application in the food and pharmaceuticals industry, were used (n-hexane, n-hexane/propan-2-ol (2:1 vol.%), ethanol 96 vol.%) to test the quantitative effect of solvent polarity on the gravimetric extraction yield, total fatty acid and EPA yield. The highest extraction yield resulted from ethanol extraction (36 ± 4 mass%), compared to low yields from n-hexane extraction (6.1 ± 0.3 mass%). A maximum fatty acid yield of 16.7 ± 0.6 mass% was determined for the biomass extracted with the green solvent ethanol. The EPA yield of 3.7 ± 0.1 mass% with the use of ethanol indicates that EPA production from N. oculata is economically beneficial, referring to prognosis from literature. The remaining biomass may eventually be used for energy recovery and other applications within a biorefinery concept.

Determination of phthalic acid diesters in human milk at low ppb levels

Phthalic acid diesters (PAE) are omnipresent in the human environment and food is a major contributor to the overall human exposure towards these chemicals. Due to developmental effects, PAE infants’ exposure via human milk has been subjected to a number of analytical studies. These previous studies, however, revealed that normal laboratory blank values are in the range of or even higher than human milk levels due to the presence of PAE in laboratory environments. In order to provide more reliable data on PAE exposure via human milk, the aim of this study was to develop and validate a robust and sensitive analytical method. This should be capable of removing matrix components efficiently and guarantee limits of quantification in the low ppb range. The method development took into account liquid–liquid extraction and selective pressurised fluid extraction (sPFE) as well as chromatography-based clean-up steps. The final method consisted of a liquid–liquid extraction followed by an automated chromatographic clean-up by an sPFE device. After volume reduction the cleaned extracts were analysed by quadrupole GC/MS. Quantification was based on internal standards. An extensive quality assurance and method test programme demonstrated conservatively determined limits of detection and quantification from 0.3 to 10 ng g−1 in human milk, with recoveries of internal standards from 50% to 101%. Thus, the method allowed the quality-assured detection of di-isobutyl phthalate (DiBP), di-n-butyl phthalate (DBP), di-2-ethylhexyl phthalate (DEHP), di-allyl phthalate (DAP), benzylbutyl phthalate (BBP) and di-cyclohexyl phthalate (DcHP) in 30 human milk samples provided by 30 volunteers from southern Germany. DiBP, DBP and DEHP were the most commonly detected PAE, with median levels of 1.0, 0.6 and 2.3 ng g−1, respectively.

TiO2 powders synthesized by pressurized fluid extraction and supercritical drying: Effect of water and methanol on structural properties and purity

Pressurized water extraction followed by sub/supercritical methanol drying was used for synthesis of TiO2 from gels prepared by sol–gel technique using Triton X-114 like template. It was proved that pressurized water causes titania crystallization. Subsequent sub/supercritical methanol drying is responsible for total elimination of the rest of organic phase originating from the template. The impact of temperature and pressure modifications on titania purity was more essential for pressurized water extraction. The specific surface area significantly increases with removal of the residual organic phase. Anatase was a dominant crystalline phase. The fraction of minor brookite was in the range 20–30wt.%. In situ XRD showed stability of titania crystallites size up to 300°C and the presence of brookite crystallites even up to 600°C. The correlation between Raman FWHM and XRD crystallite size was not influenced by the presence of organic phase and was more sensitive for anatase Eg band 647cm−1.

Pressurized fluid extraction of essential oil from Lavandula hybrida using a modified supercritical fluid extractor and a central composite design for optimization

Essential oil components were extracted from lavandin (Lavandula hybrida) flowers using pressurized fluid extraction. A central composite design was used to optimize the effective extraction variables. The chemical composition of extracted samples was analyzed by a gas chromatograph-flame ionization detector column. For achieving 100% extraction yield, the temperature, pressure, extraction time, and the solvent flow rate were adjusted at 90.6°C, 63 bar, 30.4 min, and 0.2 mL/min, respectively. The results showed that pressurized fluid extraction is a practical technique for separation of constituents such as 1,8-cineole (8.1%), linalool (34.1%), linalyl acetate (30.5%), and camphor (7.3%) from lavandin to be applied in the food, fragrance, pharmaceutical, and natural biocides industries.

Pressurized fluid extraction of polycyclic aromatic hydrocarbons using silanized extraction vessels

We have investigated the extraction efficiency of a pressurized fluid extraction system using an Ottawa sand matrix, soils and a certified reference material (HS-6) spiked with the 16 polycyclic aromatic hydrocarbon (PAHs) associated with method EPA 8100. Acceptable recoveries were achieved for all PAHs using a nominal sand concentration of 2.0 μg.g-1. However, similar experiments that were conducted at a concentration of 0.20 μg.g-1 afforded poor recoveries and poor reproducibility for the six-ring PAHs indeno(1,2,3-cd)pyrene, dibenz(a)anthracene, and benzo(ghi)perylene. These were not adequately addressed by the use of indeno(1,2,3-cd)pyrene-d12 and benzo(g,h,i)perylene-d12 surrogates. Silanization of vessels using dichlorodimethylsilane adequately passivates the system but is only required for the high surface area retention frits. Replicate analyses at concentrations of 200 and 20 ng.g-1 demonstrated that detection limits in the low ppb range (ng.g-1) are achieved for Ottawa sand, dry soil and soil with moisture contents up to a mass fraction of 30 %. Such performance is consistent with the analytical requirements of the Canadian Sediment Quality Guidelines. Improved analyte recoveries were also observed for the certified reference material HS-6 which were further improved by increasing extraction temperatures from 100 to 150 °C.

Automobile Tires—A Potential Source of Highly Carcinogenic Dibenzopyrenes to the Environment

Eight tires were analyzed for 15 high molecular weight (HMW) polycyclic aromatic hydrocarbons (PAH), using pressurized fluid extraction. The variability of the PAH concentrations determined between different tires was large; a factor of 22.6 between the lowest and the highest. The relative abundance of the analytes was quite similar regardless of tire. Almost all (92.3%) of the total extractable PAH content was attributed to five PAHs: benzo[ghi]perylene, coronene, indeno[1,2,3-cd]pyrene, benzo[e]pyrene, and benzo[a]pyrene. The difference in the measured PAH content between summer and winter tires varied substantially across manufacturers, making estimates of total vehicle fleet emissions very uncertain. However, when comparing different types of tires from the same manufacturer they had significantly (p = 0.05) different PAH content. Previously, there have been no data available for carcinogenic dibenzopyrene isomers in automobile tires. In this study, the four dibenzopyrene isomers dibenzo[a,l]pyrene, dibenzo[a,e]pyrene, dibenzo[a,i]pyrene, and dibenzo[a,h]pyrene constituted <2% of the sum of the 15 analyzed HMW PAHs. These findings show that automobile tires may be a potential previously unknown source of carcinogenic dibenzopyrenes to the environment.

Strategies for selecting optimal sampling and work-up procedures for analyzing alkylphenol polyethoxylates in effluents from non-activated sludge biofilm reactors

Trace-level analysis of alkylphenol polyethoxylates (APEOs) in wastewater containing sludge requires the prior removal of contaminants and preconcentration. In this study, the effects of the types of alkylphenols present, their degree of ethoxylation, the biofilm-wastewater treatment and sample matrix were investigated on optimal work-up procedures for these purposes. The spot for sampling APEO-containing specimens from an industrial wastewater treatment plant was optimized, and boxed round the tubing carrying the wastewater to prevent sedimented sludge contaminating the collected samples. Following these changes, the sampling precision (in terms of dry matter content) at a point just under the tubing leading from the biofilm reactors was 0.7 % RSD. The findings were applied to develop a work-up procedure for use prior to a high-performance liquid chromatography-fluorescence detection analysis method capable of quantifying nonylphenol polyethoxylates (NPEOs) and poorly investigated dinonylphenol polyethoxylates (DNPEOs) at low μg L−1 concentrations in effluents from non-activated sludge biofilm reactors. The selected multistep work-up procedure includes lyophilization and pressurized fluid extraction (PFE) followed by strong ion exchange solid phase extraction (SPE). The yields of the combined procedure, according to tests with NP10EO-spiked WWTP-derived wastewaters, were in the 62–78 % range.

Antioxidant activity of grape skin aqueous extracts from pressurized hot water extraction combined with electron paramagnetic resonance spectroscopy

Pressurized hot water extraction (PHWE) was employed to prepare extracts from dried grape skin of two wine grape varieties (St. Laurent and Alibernet) at various temperatures (from 40 up to 120°C) and amounts of sample (0.5, 1.0 and 1.5g). To assess the antioxidant activity of the extracts, electron paramagnetic resonance (EPR) spectroscopy was applied involving DPPH and ABTS+ assays. Other extract characteristics including HPLC profile of anthocyanins and total phenolic compound content were obtained as well. PHWE has also been compared with earlier results of extractions of the same grape skin samples with compressed methanol and compressed ethanol under the conditions of pressurized fluid extraction (PFE). From this comparison, PHWE emerges as the more benign and efficient extraction method to recover valuable phenolic antioxidants from grape skins for the prospective use in functional food supplements.

Comparison of two extraction procedures for the assessment of sediment genotoxicity: Implication of polar organic compounds

Four sediment samples (Vaïne Airport VA, Vaïne Center VC, Vaïne North VN and Reference North RN) were collected in the Berre lagoon (France). Sediments were analyzed for polycyclic aromatic hydrocarbons (PAHs) by use of pressurized fluid extraction with a mixture of hexane/dichloromethane followed by HPLC with fluorescence detection analysis. Organic pollutants were also extracted with two solvents for subsequent evaluation of their genotoxicity: a hexane/dichloromethane mixture intended to select non-polar compounds such as PAHs, and 2-propanol intended to select polar contaminants. Sediment extracts were assessed by the Salmonella/microsome mutagenicity test with Salmonella typhimurium TA98+S9 mix and YG1041±S9 mix. Extracts were also assessed for their DNA-damaging activity and their clastogenic/aneugenic properties by the comet assay and the micronucleus test with Chinese Hamster ovary (CHO) cells. The PAH concentrations were 611ngg−1dw, 1341ngg−1 dw, 613ngg−1dw and 482ngg−1dw for VA, VC, VN and RN, respectively. Two genotoxic profiles were observed, depending on the extraction procedure. All the non-polar extracts were mutagenic for TA98+S9 mix, and VA, VC, VN sediment samples exerted a significant DNA-damaging and clastogenic activity in the presence of S9 mix. All the polar extracts appeared mutagenic for TA98+S9 mix and YG104±S9 mix, and VA, VC, VN were genotoxic and clastogenic both with and without S9 mix. These results indicate that the genotoxic and mutagenic activities mainly originated from PAHs in the non-polar extracts, while these activities came from other genotoxic contaminants, such as aromatic amines and nitroarenes, in the polar extracts. This study focused on the important role of uncharacterized polar contaminants such as nitro-PAHs or aromatic amines in the global mutagenicity of sediments. The necessity to use appropriate extraction solvents to accurately evaluate the genotoxic hazard of aquatic sediments is also highlighted.

An evaluation of analytical quality for selected PAH measurements in a fuel-contaminated soil

The measurements of polycyclic aromatic hydrocarbons (PAHs) in soil require optimized analytical methods that assess reliable mass fraction results. This is particularly important for analysing very complex matrix such as contaminated soils with crude fuels. The main objectives of this work were focused to minimize analytical effort and assess result reliability in analysis of PAH by high-performance liquid chromatography with fluorescence detection (HPLC/FD). First, analysis of soil samples with/without fuel contamination was well established by sonication (US) and pressurized fluid extraction (ASE) using minimal amount of sample and minimal treatment of sample. On the other hand, an extensive study with spiked and field soil samples was performed by checking proportional and constant bias for analytical validation. The major components for estimating uncertainty contributions were evaluated on the basis of intermediate precision with two fuel matrix, PAH mass fraction and dates of analyses.

Extraction of Antioxidants from Spruce (Picea abies) Bark using Eco‐friendly Solvents

Antioxidants are known to avert oxidation processes and they are found in trees and other plant materials. Tree bark is a major waste product from paper pulp industries; hence it is worthwhile to develop an extraction technique to extract the antioxidants. To develop a fast and environmentally sustainable extraction technique for the extraction of antioxidants from bark of spruce (Picea abies) and also to identify the extracted antioxidants that are abundant in spruce bark. A screening experiment that involved three different techniques was conducted to determine the best technique to extract antioxidants. The antioxidant capacity of the extracts was determined with DPPH (2,2-diphenyl-1-picrylhydrazyl) assay. Pressurised fluid extraction (PFE) turned out to be the best technique and a response surface design was therefore utilised to optimise PFE. Furthermore, NMR and HPLC-DAD-MS/MS were applied to identify the extracted antioxidants. PFE using water and ethanol as solvent at 160 and 180°C, respectively, gave extracts of the highest antioxidant capacity. Stilbene glucosides such as isorhapontin, piceid and astringin were identified in the extracts. The study has shown that PFE is a fast and environmentally sustainable technique, using water and ethanol as solvent for the extraction of antioxidants from spruce bark.

Determination of polycyclic aromatic hydrocarbons in urban street dust: Implications for human health

The determination of sixteen polycyclic aromatic hydrocarbons in urban street dust has been done. Samples were collected from 12 sampling locations in a city centre location (Newcastle upon Tyne, north east England) and extracted using in situ pressurised fluid extraction followed by gas chromatography mass spectrometry. From the results it was possible to identify three groups, with respect to PAH concentration, with PAH contents ranging between 0.6–2.3 mg kg −1, 15.6–22.5 mg kg −1 and 36.1–46.0 mg kg −1. The total PAH content of samples from these sampling sites has been compared to 22 urban locations around the world; comparable levels were found in these samples compared to the other cities around the world. The potential source of PAHs has been investigated by investigating the proportion of pyrogenic and petrogenic material in urban street dust using specific individual PAH ratios. The results indicate that the PAH content of urban street dust from the chosen sites are more likely to be due to pyrogenic sources i.e. vehicle exhaust emissions. The particle size fractions (<63 μm; 63–125 μm; 125–250 μm; 250–500 μm; 500–1000 μm; and 1000–2000 μm) of individual PAHs in three selected sampling sites was investigated. In two of the selected sites the PAH content was independent of particle size whereas in sampling site 10 elevated PAH levels are noted in the <63 μm size fraction. Sampling site 10 is located at the junction of three road tributaries which are used as major access points to the east of the city centre. Finally, the potential health risk for unintentional consumption of PAHs was assessed in terms of a mean daily intake (based on an ingestion rate of 100 mg d −1). It was found that all 4–6 membered ring PAHs had concentrations in excess of the mean daily intake thereby reflecting a potential health risk, particularly in the smallest size particle fractions.

Temporal trend (1988–2008) of hexabromocyclododecane enantiomers in herring gull eggs from the german coastal region

Levels of α-, β-, and γ-hexabromocyclododecane (HBCD) were determined in pooled eggs from herring gulls ( Larus argentatus) sampled on three bird sanctuaries near the German North Sea coast between 1988 and 2008 (Mellum and Trischen) and the German Baltic Sea coast between 1998 and 2008 (Heuwiese) and archived by the German Environmental Specimen Bank. Pressurized fluid extraction, gel permeation chromatography, and LC–MS/MS using 13C 12-labelled isotope standards and a chiral column were applied. α-HBCD was the dominating diastereomer and ranged between 3.7 and 107 ng g −1 lw while β- and γ-HBCD were throughout close to LOQ. The highest α-HBCD concentration was found in eggs from Mellum sampled in the year 2000. Interestingly, HBCD in eggs from the three islands displayed similar time courses with levels increasing to a peak contamination around 2000 and decreasing levels ever since. Chiral signatures of α-HBCD in eggs differed among the islands but indicated a preferential enrichment of the first eluting enantiomer (−)-α-HBCD.

Analysis of phthalic, isophthalic and long-chain (C4–C12) dicarboxylic acids in atmospheric aerosols by UPLC/ESI/ToF-MS

A new analytical method for the simultaneous determination of phthalic, isophthalic and 14 long-chain dicarboxylic acids in ambient aerosols by ultra-performance liquid chromatography/time-of-flight mass spectrometry with negative-ion electrospray ionization was developed and validated. A fast and quantitative pressurized fluid extraction procedure with water as solvent was optimized using sampling filters spiked with a standard solution and experimental design to determine the influential factors and interactions. The extracts were analyzed without any further clean-up step. Chromatographic separation was achieved with a mobile phase consisting of 20 mM formic acid in water and methanol/acetonitrile (60 : 40, v/v). Excellent precision and accuracies in the MS mass measurements were observed. Matrix effects were investigated by spiking actual samples with a solution containing standards. The method was applied to air particulate reference materials and atmospheric samples collected by a low-volume sampler at a French urban background site.

Offline combination of pressurized fluid extraction and electron paramagnetic resonance spectroscopy for antioxidant activity of grape skin extracts assessment

A comprehensive characterization of grape skin methanolic and ethanolic extracts prepared by pressurized fluid extraction (PFE) at various temperatures within 40 to 120 °C from two wine grape varieties, St. Laurent and Alibernet was performed. For the first time, an offline combination of PFE and electron paramagnetic resonance (EPR) spectroscopy together with other experimental methods was employed to assess the effect of extraction conditions on numerous extract characteristics including antioxidant or radical-scavenging ability, HPLC profile of anthocyanins, total phenolic compounds content (TPC), tristimulus color values (CIE Lab), and pH values. The properties of extracts depend on the solvent used, the mass of grape skins as well as on the extraction conditions among which the temperature plays a crucial role. In spite of wide interval of extraction temperatures, all extracts still retain their antioxidant and/or radical-scavenging properties, indicating that the extracts prepared by PFE can serve as potential source of functional food supplements or color enhancers.

An investigation into the occurrence and distribution of polycyclic aromatic hydrocarbons in two soil size fractions at a former industrial site in NE England, UK using in situ PFE–GC–MS

Polycyclic aromatic hydrocarbon (PAH) concentrations were determined in 16 topsoils (0-10cm) collected across the site of a former tar works in NE England. The soils were prepared in the laboratory to two different particle size fractions: <250μm (fractionA) and >250μm to <2mm (fractionB). Sixteen priority PAHs were analysed in the soils using insitu pressurised fluid extraction (PFE) followed by gas chromatography-mass spectrometry (GC-MS). The average total PAH concentration in the soils ranged from 9.0 to 1,404mg/kg (soil fractionA) and from 6.6 to 872mg/kg (soil fractionB). These concentrations are high compared with other industrially contaminated soils reported in the international literature, indicating that the tar works warrants further investigation/remediation. A predominance of higher-molecular-weight compounds was determined in the samples, suggesting that the PAHs were of pyrogenic (anthropogenic) origin. Statistical comparison (t-test) of the mean total PAH concentrations in soil fractionsA and B indicated that there was a significant difference (95% confidence interval) between the fractions in all but two of the soil samples. Additionally, comparisons of the distributions of individual PAHs (i.e. 16 PAHs×16 soil samples) in soil fractionsA and B demonstrated generally higher PAH concentrations in fractionA (i.e. 65.8% of all individual PAH concentrations were higher in soil fractionA). This is important because fractionA corresponds to the particle size thought to be most important in terms of human contact with soils and potential threats to human health.

Comprehensive Two-Dimensional GC of Supercritical Fluid and Pressurized Liquid Extracts of Tobaccos

The developments of an analytical strategy based on rapid extraction techniques coupled to comprehensive bi-dimensional gas chromatography (GCxGC) for the characterization of the volatile fraction of tobaccos are presented. The high-peak capacity of GCxGC allows considering global extraction techniques that do not focus on restricted chemical families of products. Thus, potential of pressurized fluid extraction and supercritical fluid extraction (SFE) to provide informative extracts is evaluated and compared. As expected, both techniques enable rapid extraction of compounds of interest. However, because of the easy removal of the extracting fluid, SFE provides more concentrated extracts, allowing a GCxGC direct injection without any concentration step. Moreover, the use of pure carbon dioxide with tuneable extraction power favors the extraction of the compounds of interest while reducing the extraction of interfering heavy compounds. GCxGC conditions, especially concerning the set of columns used, are selected thanks to the comparison of separations obtained on model compounds. The developed methodology is applied not only to compare the three main types of tobaccos but also to discriminate between different tobaccos of the same type.

Comparison of pressurised fluid and ultrasonic extraction methods for analysis of plant antioxidants and their antioxidant capacity

Abstract The analytical method based on the high-performance liquid chromatography coupled with UV detection (HPLC/UV) for determination of selected antioxidants (i.e., esculetin, scopoletin, 7-hydroxycoumarine, rutin, xanthotoxin, 5-methoxypsoralen and quercetin) in plant material was developed. Pressurised fluid extraction (PFE) and ultrasonic extraction (USE) methods for the isolation of these compounds from ten real plant samples were used. Both extraction methods were optimised and compared to each other. For the proposed HPLC/UV method the LOQ values (limit of quantification) in the range from 22.7 (xanthotoxin) to 97.2 ng mL−1 (rutin) were obtained. For all extracts the antioxidant capacity based on the reduction of free 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) was also determined. Results ranged from 82.04 to 94.43% of DPPH radical inhibition for PFE method and from 76.01 to 89.94% in the case of USE method.

HETEROGENEOUS REACTIONS OF OH RADICALS WITH PARTICULATE-PYRENE AND 1-NITROPYRENE OF ATMOSPHERIC INTEREST

The heterogeneous reactivity of OH radicals with pyrene and 1-nitropyrene adsorbed on model particles has been investigated using a discharge flow reactor, in the presence of a large excess of NO 2 . Graphite was chosen as a simple model of carbonaceous particles, whereas silica was chosen as a representative surrogate of mineral atmospheric aerosol. The reaction kinetics was investigated by measuring the remaining pyrene and 1-nitropyrene adsorbed on particles after pressurized fluid extraction and gas chromatography/mass spectrometry analysis. Pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate polycyclic compound concentrations versus reaction time. Rate constants were measured at room temperature for reactions of OH radicals with, respectively, pyrene, 1-nitropyrene, both adsorbed on silica, and 1-nitropyrene adsorbed on graphite, in the presence of NO 2 .