A method and an arrangement for determining the concentration of an oxygen-containing compound in a test gas in an external volume are presented with the external volume containing also additional oxygen. The inlet brings mist-laden air through an airflow straightening area into the cyclone body in a tangential manner, creating a spiral flow path within the cyclone body. A circuit chip is provided with a contact pad above the carrier substrate. The releasable plug preferably has an MRI detectable element. The second server may match the content item with relevant advertisements based on keywords associated with the content item and the advertisements.