Surfaces were swabbed using a dry cotton swab over a ~20 x 20 cm area (400 cm 2 ) for ~1 min (to ensure full loading of the swab). The head of the swab was then immersed in 500 uL of 10 mM sodium phosphate solution at pH 7 in a Eppendorf tube, closed and sealed with parafilm and stored prior to measurement. To undertake analysis, the Eppendorf tube was placed on a shaker for few seconds, then an aliquot of 500 µL was removed and pipetted directly onto a chip mounted in the chip holder and analyzed by CV.