In situ analysis of agrochemical residues on fruit using ambient ionization on a handheld mass spectrometer

We describe a rapid in situ method for detecting agrochemicals on the surface or in the tissue of fruit using a portable mass spectrometer equipped with an ambient ionization source. Two such ionization methods, low temperature plasma (LTP) and paper spray (PS), were employed in experiments performed at a local grocery store. LTP was used to detect diphenylamine (DPA) directly from the skin of apples in the store and those treated after harvest with DPA were recognized by MS and MS/MS. These data therefore allowed ready distinction between organic and non-organic apples. DPA was also found within the internal tissue of purchased apples and its distribution was mapped using LTP. Similarly, thiabendazole residues were detected on the skin of treated oranges in a grocery store experiment in which paper spray was performed by wiping the orange surface with a moist commercial lens wipe and then applying a high voltage to ionize the chemicals directly from the wipe. The handheld mass spectrometer used in these measurements is capable of performing several stages of tandem mass spectrometry (up to MS5); the compounds on the fruit were identified by their MS/MS fragmentation patterns. Protonated DPA (m/z 170) produced a characteristic MS2 fragment ion at m/z 92, while thiabendazole was identified by MS3 using precursor to fragment ion transitions m/z 202 → m/z 175 →m/z 131. These particular examples exemplify the power of in situ analysis of complex samples using ambient ionization and handheld mass spectrometers.