ARTICLE pubs.acs.org/est
New Sensitive and Quantitative Analysis Method for Organic Nitrogen Compounds in Urban Aerosol Samples ,† € Mustafa Z. Ozel,* Jacqueline F. Hamilton,† and Alastair C. Lewis‡ † ‡
Department of Chemistry, The University of York, Heslington YO10 5DD, York, U.K. National Centre for Atmospheric Science, Department of Chemistry, The University of York, Heslington, YO10 5DD York, U.K. ABSTRACT: Atmospheric aerosols contain a highly complex mixture of organic and inorganic compounds; however, as a chemical class relatively little is known about organic nitrogen (ON) content, with few satisfactory methods for speciated analysis. In this paper we report a sensitive and quantitative method for the speciation of ON within ambient atmospheric aerosol. Aerosol samples, collected on quartz microfiber filters, were extracted in water followed by solid phase extraction, elution, and concentration before analysis by comprehensive gas chromatography with a nitrogen chemiluminescence detection system (GCxGC-NCD). The NCD detection method was optimized using liquid standards. The GCxGC-NCD method showed high selectivity, sensitivity, and equimolarity in its response to individual organic compounds. Limits of detection (LOD) and limits of quantitation (LOQ) for four ON standards (1-nitropentane, o-toluidine, nonanenitrile, and quinoline) were determined to be in the range 0.16-0.27 pgN and 0.71-1.19 pgN, respectively. Between 21 and 57 different ON compounds were found in urban aerosol, (including 10 nitriles, 9 alkyl nitro compounds, 4 nitro-phenols, 4 amides, 3 nitrosamines, and 2 nitro-PAHs) on different dates from a city center location. Pyrrole (8.26-39.21 ngN m-3 air) and N-butyl-benzenesulfonamide (6.23-20.87 ngN m-3 air) were the most abundant ON compounds observed in all samples analyzed. The average mass loading of the total identified ON was 532.51 ngON m-3 air. The sensitivity, selectivity, and relative ease of quantitation of unknown ON components makes the technique a significant improvement over previous laboratory methods.
’ INTRODUCTION Organic nitrogen (ON) is ubiquitous in the environment and present in water,1 soil,2 air,3 and also in most petroleum products.4 In the environment ON plays a role in regulating nutrient supply yet in excess quantities can show toxicity. The atmospheric burden of ON is only poorly quantified; however, they have potential to be deposited to the terrestrial surface or undergo wash-out due to high solubilities (e.g for amines, nitrates, and some heterocyclic species such as pyridine, morpholine, and quinoline). Addition of nitrogen to an organic structure can greatly increase the potential carcinogenic and mutagenic effects of a compound,5 and this is of particular relevance to human health through inhalation and chemical transfer via the lungs. For example, the cancer Potential Equivalence Factor (PEF), a measure of carcinogenicity, of 6-nitrochrysene is 1000 times higher than chrysene.6 A number of the most important mutagens in air contain nitrogen.5 There are a number of ON species on the Environmental Protection Agency hazardous air pollutants list, including N-nitrosomorpholine, nitrobenzene, and nitropolycyclic aromatic hydrocarbons (nitro-PAH). In order to understand the toxicity and nutrient potential of atmospheric particles, a much better chemical knowledge of the ON composition is vital. While the sources, formation, and composition of oxygenated organics in the environment have been extensively studied in recent years, ON has received less attention, despite its environmental importance. This is in part due to a lack of satisfactory methods for analysis of trace amounts of ON compounds from difficult matrixes. r 2011 American Chemical Society
High performance liquid chromatography has been used to analyze amino acids and alkyl amines and ion chromatography has been used for urea.7 In a study carried out at Cape Grim, Australia, rain and bulk aerosol samples contained ON at concentrations representing, on average, between 19-25% of total nitrogen.8 Another study in California found a significant amount (∼20%) of water-soluble ON in an aerosol samples.7 The water-soluble organic nitrogen (WSON) content was determined to be around 20% of the total particulate nitrogen.7 The majority of studies of ON have concentrated on the WSON content, a proportion of which may be secondary in nature, formed by successive oxidation of the initial emitted ON species. However, in a study by Russell et al.9 aerosols collected at a coastal site indicated that approximately 10% of the total nitrogen was nonwater-soluble ON and that its concentration was considerably higher than the WSON. There is currently only limited information available on primary ON emissions (especially the nonpolar fraction) from anthropogenic sources, such as petrochemicals, biomass burning, and industrial processes. There has been some limited study of ON fractions of aerosols using gas chromatography (GC). Often, a small subset of organic nitrogen species are identified as part of a wider organic aerosol characterization. Hamilton et al.10 reported eight ON compounds, Schenelle-Kreis et al.11 identified a series of nitriles, Received: August 1, 2010 Accepted: December 20, 2010 Revised: December 10, 2010 Published: January 6, 2011 1497
dx.doi.org/10.1021/es102528g | Environ. Sci. Technol. 2011, 45, 1497–1505
Environmental Science & Technology and Ochiai et al.12 found seven ON compounds in aerosol using comprehensive two-dimensional gas chromatography (GCXGC). Ma and Hays13 used thermal extraction two-dimensional gas chromatography (TE-GC-GC-MS) to identify 14 nitrogen containing heterocyclic compounds from biomass aerosol. We recently reported ON in urban aerosol samples3 analyzed using a direct thermal desorption technique (DTD) together with comprehensive two-dimensional gas chromatography-time-offlight mass spectrometry (GCxGC-TOF/MS). Between 17 and 57 ON compounds were found in individual samples. Nearly one-third of the ON compounds were nitriles, 16 were amides, 8 were amines, and 7 were nitro compounds. Quantification of these species in aerosol was exceptionally difficult, however, since individual calibration curves were required for each component, yet in many cases pure components were not commercially available. The use of element specific detectors for chromatography is a key approach in separation science. Resolution of target classes of analytes in highly complex samples can often be most easily achieved with such detectors. Atomic emission detection (AED),14 nitrogen phosphorus detection (NPD),1 and nitrogen chemiluminescent detection (NCD)1,3 have all been used for detecting ON compounds in combination with GC. Nitrogen detection by AED is subject to some interference from hydrocarbons within the sample matrix and cannot be used for the study of ON compounds at very low nitrogen levels, unless prior separation is carried out.14 Sensitivity for ON compounds can also be low using GC-NPD, because of significant matrix interferences.1 Conventional universal detectors such as the mass spectrometer (MS)15 and the flame ionization detector (FID)16 do not always provide sufficient discrimination of ON when present with a higher background of hydrocarbons and other organics compounds. Chemiluminescent detectors which are highly specific for nitrogen and sulfur are however especially attractive for ON. In a NCD all ON compounds are combusted in a high temperature furnace to form NO. This is then reacted with ozone to form excited NO2*. Relaxation to the ground state results in emission of light, which is measured by a spectrometer. GC-NCD is not a new technique, but it is becoming more popular with improved instrumentation and has recently been used to resolve ON within diesel fuel4 and wastewater.1 One advantage of NCD is that it should theoretically have an equimolar response for all organic nitrogen species (with the exception of azo-compounds, which form N2 rather than NO during combustion), unlike NPD where each compounds needs to be calibrated individually. Comprehensive two-dimensional gas chromatography (GCxGC) is a multidimensional GC technique that has an increased separation power often coupled to reduced analysis time. GCxGC has advantages over single column GC and when compared under optimized conditions, sensitivity and resolution are generally much better with GCxGC. The limit of detection (LOD) is typically three to five times lower, and the number of satisfactory resolved peaks is up to 10 times greater with GCxGC than with 1D-GC.15,17 The objective of this study was to couple GCxGC separation to a NCD detector as a means to achieve highly selective and sensitive detection of ON in atmospheric aerosols. Even with the potentially high analytical specificity of GCxGC-NCD, some form of sample clean up is required when ON is present in a high background matrix. Solid phase extraction (SPE) or solid phase microextraction (SPME) are the most
ARTICLE
common preparative techniques for ON in water matrices. Grebel et al.1 for example identified 8 nitrosamines from wastewater using SPME clean up. Here we couple a straightforward SPE cleanup with GCxGC-NCD to resolve a range of ON found in urban aerosol samples.
’ EXPERIMENTAL SECTION Chemicals and Samples. Pentane, toluene, chloroform, dichloromethane (DCM), methanol, water (HPLC grade), and the remainder of the chemical standards were supplied by SigmaAldrich (Dorset, UK). Stock standard solutions were prepared in DCM and stored at -4 °C. On various dates ranging from July 2007 to April 2008, samples of airborne fine particulate matter (aerodynamic diameter
