2.3. Sample collection
The sampling site selected was the CIEMAT research centre (Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas), 7 Km northwest of Madrid’s city centre. Fine particulate (PM10) air pollution samples were collected on previously heat–treated (400 °C for 24 hours) Whatman glass fiber filters using a high volume MCV CAV–A/mb sampler. After collection, the filters were wrapped separately in aluminum foil and stored in a freezer at –20 °C until analysis. The volume of ambient air drawn through the filter over 24 hours of sampling was about 550 m3. From February 2008 to February 2009, we collected 55 samples of 550 m3 (one sample per week).
2.4. Optimizing conditions of extraction, clean–up and chromatography
Initial experiments were designed to establish the best extraction procedure. To improve efficiency and reduce the amount of solvent and time needed for extraction, the effects of several factors including mobile phase composition and analysis time were explored as indicated below. Each experiment was run at least in triplicate.