identification of volatile organic compounds by gas chromatography-mass spectrometry in aerosols...
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Identification of volatile organic compounds by gas chromatography-mass spectrometry in aerosols collected at T1 (Tecamac, Estado de México)
Sandra I. Ramírez1 and Telma Castro2
1Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos. Av. Universidad #1001 Col. Chamilpa 62209 Cuernavaca, Morelos MEXICO. [email protected]
2Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México. Circuito Exterior S/N Ciudad Universitaria, Del. Coyoacán 04510 D. F. MÉXICO. [email protected]
INTRODUCTION
The MILAGRO (Megacity Initiative: Local and Global Research Observation) campaign was an integrative and collaborative scientific program with the objective of observing and quantifying the fate of anthropogenic pollutants emitted from the world’s second largest city: Mexico City and its metropolitan area.
During March 2006, scientists from Mexico, the United States and some European countries performed ground-based and aircraft measurements. The ground-measurement were mainly performed at three supersites: T0 located at CENICA, T1 located at the Universidad Tecnológica de Tecamac, and T2 located at Rancho Bisnaga (Figure 1).
Among the specific scientific objectives of MILAGRO campaign, one was focused on the characterization of chemical composition and diurnal variation of volatile organic compounds (VOC’s) contained in particulate matter collected at boundary sites (Figure 2). In this work we present preliminary results of the identification of some volatile organic compounds identified by the gas chromatography-mass spectrometry coupled analytical techniques in samples collected on aluminum filters at 12-hour sampling intervals at Tecamac.
Figure 1. MILAGRO surface measuring supersites
METHODOLOGY AND INSTRUMENTATION
The analyzed samples were collected in diurnal and nocturnal twelve-hour periods on aluminum filters with a ten-stage MOUDI. The filters were cut in two parts, one was used for a gravimetric analysis (results presented in a different work) and the other half was used for the chemical identification (Figure 3), that was performed only on stages 7 and 8. A preliminary test demonstrated that the quantity of atmospheric aerosols collected in the rest of the stages was below the detection limit of the analytical system.
In this work we present an innovation on the sample’s pre-treatment. Prior to the chromatographic separation, each half-filter was placed into a special glass device designed to allow the desorption of the VOC’s, bounded to the particles, using a temperature treatment (200°C) in an oxygen-free environment during 60 minutes (Figure 4).
CONCLUDING REMARKS
The improvement on the sample handling, avoiding manipulation and solvent extraction, has probe to be a positive change in the treatment of atmospheric aerosol samples.
The identified compounds correspond to species derived from the use of carbon-based fuels, biomass burning, transportation, and other anthropogenic activities. It is important to mention the detection of propanone (acetone), a long-living intermediate marker.
It is necessary to finish the quantitative analysis of all the identified species, look for correlations of quantitative data sets, and perhaps use these information on modelings studies.
Figure 2. Panoramic view of Mexico City, an evident lost of long-range visibility, mainly due to suspended particles, is observed.
RESULTS
At the moment, 70% of the received filters have been analyzed. Figure 6 shows a SIM (Single Ion Monitoring) chromatogram where almost all the detected compounds are observed. The areas of each detected compound can be related with their content as shown in Table 1.
Figure 3. A: half-filter from stage 8. B: half-filter from stage 5.
A B
Figure 4. Pyrex glass device used for the temperature treatment of the aluminum filters.
Table 1. Volatile organic compounds identified by GC-MS in particulate material collected al Tecamac, Estado de México during the MILAGRO campaign.
Figure 6. SIM chromatogram (m/z = 40, 44, 57, 71) of VOC’s detected at filter form Tecamac reconstructed by mass spectra (EI at 70 ev)
Chromatographic conditions. Column: PoraPLOT Q fused-silica 30 m 0.32 mm I.D. 20 m thickness polystyrene-divinylbenzene. Carrier flow: He (UHP) 1.2 ml min-1. Program temperature: isothermal 150°C for 3 min, 10°C min-1 up to 250°C for 20 min. Injector temperature: 200°C.
This analytical methodology allowed us to identified three saturated hydrocarbons, one aromatic hydrocarbon, four aldehydes, three ketones, and one halogenated hydrocarbon, as shown in Table 1.
Figure 5. Instrumental facilities to handle gas-phase materials at CIQ-UAEM.
The VOC’s was then immediately introduced, via a heated stainless steel transfer line and a pneumatic automatic six-port valve, to the gas chromatograph (Trace GC ultra, Finnigan) coupled to an ionic trap mass spectrometer detector (Polaris Q, Finnigan; Figure 5). The chromatographic separation was based on the TO-15 EPA protocol while the chemical identity of each compound was done by digital and visual comparison of individual spectra with the NIST library (v. 2002).
The quantitative analysis of the detected VOC’s were performed using a certified gas mixture of C-2 and C-3 hydrocarbons (PRAXAIR, México).
Figure 5. Coupled gas chromatograph-mass spectrometer at CIq-UAEM.