Analysis of Dithiocarbamate Pesticides

Keywords food safety dithiocarbamate fungicides DTCs hydrolysis Thiram GC-MS SIM 

Dithiocarbamate fungicides (DTCs) are widely used in agriculture. They are non-systemic and typically remain at the site of application. DTCs are characterized by a broad spectrum of activity against various plant pathogens, low acute mammal toxicity, and low production costs (Crnogorac and Schwack, 2009). 

DTCs are not stable and cannot be extracted or analyzed directly. Contact with acidic plant juices degrades DTCs rapidly and they decompose into carbon disulfide (CS2) and the respective amine (Crnogorac and Schwack, 2009). DTCs cannot be extracted by organic solvents from homogenize plant samples, as it is the QuEChERS standard procedure in pesticide-residue analyses. The described method is a non-specific DTC sum method that does not distinguish between the different species of DTCs in the sample. Interferences are known from natural precursors, for example from crops or brassica, that can produce CS2 as well during hydrolysis (Reynolds, 2006 Crnogorac and Schwack, 2009). 

The earlier published SnCl2/HCl acid-hydrolysis method was employed for sample preparation (CRT, 2005). The described method follows the established methods applied in the EU reference laboratories and European commercial testing laboratories for CS2 analysis. From the homogenized sample 25 g are taken in a 250 mL glass bottle, 75 mL of the reaction mixture is added, followed by 25 mL iso-octane. The bottle is closed gastight immediately and placed in a water bath at 80 C for 1 h with intermittent shaking and inverting the bottle after every 20 min. 

After cooling the bottle to 20 C by ice water, a 1-2 mL aliquot of the upper iso-octane layer is transferred into a micro centrifuge tube, and centrifuged at 5000 rpm for 5 min at 10 °C. The supernatant is then transferred into GC vials and the residues of DTCs are estimated by determining the CS2 concentration by GC-MS. The sample preparation procedure depending on the type of food used takes l-2h. 

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Dithiocarbamates have been covered in CCC (1987),378 in a comprehensive early review (together with xanthates)379 and in a book.380 They continue to be extensively used and much interesting novel chemistry has been reported since. Several reviews on dithiocarbamates cover the electro-chemistry, photoelectron spectroscopy, analytical applications of dithiocarbamates (e.g., for the determination of metals in foodstuff, water and environmental samples and the analysis of dithiocarbamate pesticides),383-386 their use as NO trapping agents,387-389 or in the heavy-metal removal from wastewaters.390... 

Dasgupta, S., Mujawar, S. et al. (2013) Analysis of Dithiocarbamate Pesticides by GC-MS, Thermo Fisher Scientific Application Note AN10333. 

CRL (2005) Analysis of Dithiocarbamate Residues in Foods of Plant Origin involving Cleavage into Carbon Disulfide, Partitioning into Isooctane and Determinative Analysis by GC-ECD, http //www.crl-pesticides.eu/library/docs/srm/ meth DithiocarhamatesCs2 EurlSrm.PDF (accessed 2 November 2014). 

Dithiocarbamates can be quantitatively converted to carbon disulfide by reaction with tin(II)chloride in aqueous HCl (1 1) in a closed bottle at 80 C. The CS2 gas produced is absorbed into iso-octane and measured by GC-MS. The analysis of DTCs for this application follows the acid-hydrolysis method using SnCl2/HCl (Reynolds, 2006). For method validation of the DTC pesticides, Thiram (99.5% purity) was used as representative DTC compound considering its simple structure (1 mol of Thiram = 2 mol of CS2 = >1 mg of Thiram theoretically generates 0.6333 mg CS2, 1 mL of 100 ppm Thiram in 25 g of grapes = 2.5 ppm of CS2), see Figure 4.38. The residues were estimated by analysis of CS2 as the DTC hydrolysis products by GC-MS. 

From these early studies, dithiocarbamates and their transition metal complexes soon found a host of applications. For example, as a result of their insoluble nature they are widely used in inorganic analysis (3,4). They can also be used to separate different metal ions by high-performance liquid chromatography (HPLC) (5-10) and capillary gas chromatography (GC) (11, 12), and find use as rubber vulcanization accelerators (13), fungicides (14), and pesticides (15). Concomitant with the development of these applications came a burgeoning interest in their general transition metal chemistry and the characteristics and properties of the complexes formed. 

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