Direct analysis of dithiocarbamate fungicides in fruit by ambient mass spectrometry

Dithiocarbamates (DTCs) are fungicides that require a specific single-residue method for detection and verification of compliance with maximum residue limits (MRLs) as established for fruit and vegetables in the EU. In this study, the use of ambient mass spectrometry was investigated for specific determination of individual DTCs (thiram, ziram) in fruit. Two complementary approaches have been investigated for their rapid analysis: (i) direct analysis in real time (DART) combined with medium-high resolution/accurate mass time-of-flight mass spectrometry (TOFMS) and high-resolution/accurate mass Orbitrap MS, and (ii) desorption electrospray ionization (DESI) combined with tandem-in-time mass spectrometry (MS(2)). With both techniques, thiram deposited on a glass surface (DART) or Teflon (DESI) could be directly detected. With DART, this was also possible for ziram. Before the instrumental analysis of fruit matrix, an extract had to be prepared following a straightforward procedure. The raw extracts were deposited on a slide (DESI), or rods were dipped into the extracts (DART), after which thiram and ziram could be rapidly detected (typically 10 samples in a few minutes). In the case of thiram, the lowest calibration levels were 1 mg kg(-1) (DART-TOFMS, DESI-MS(2)) and 0.1 mg kg(-1) (DART-Orbitrap MS). For ziram, the achieved lowest calibration levels were 0.5 mg kg(-1) (DART-TOFMS) and 1 mg kg(-1) (DART-Orbitrap MS). In all cases, this was sufficiently low to test samples against EU-MRLs for a number of fruit crops. Using an internal standard, (semi)quantitative results could be obtained.