due to auto oxidation of the samples during incubation see fig2 and 3 subsequently the nps were sedimented by centrifugation e g zro 2 u2014 nps or by using a magnet e g fe 3 o 4 u2014 nps and the supernatant containing unbound material was removed finally the remaining nps were washed by re suspension in 500 u03bcl meoh sedimented again and the supernatant discarded for the detection of oxpcs in positive ionisation mode maldi the nps were mixed with 2 u03bcl att matrix 10 mg ml dissolved in etoh h 2 o 90 10 v v containing 2 mm gua and 5 mm dahc 7 for detection of oxpcs in the negative ionisation mode maldi a 9aa matrix 10 mg ml dissolved in iso acn 60 40 containing 5 mm gua was used 22 in either case 1 u03bcl of the np matrix suspension was deposited on fleximass u2122 ds flds sample plates shimadzu biotech manchester uk and inserted into the maldi instrument using a specific adaptor carrying up to four sample plates axima precision u2122 shimadzu manchester uk mass spectra were obtained using an axima cfrplus shimadzu biotech manchester uk curved field reflectron time of flight rtof mass spectrometer equipped with a 337 nm pulsed nitrogen laser 3 ns pulse width measurements were performed either in positive or negative mode using delayed ion extraction for optimized mass resolution the ion acceleration voltage was set to 20 kv and the reflectron analyzer was operated at 25 kv the laser energy was adjusted to 5 10 above threshold irradiation an integrated video imaging system u02dc 25 u00d7 magnification allows direct observation of the sample spots under investigation a hybrid maldi quadrupole.