Development of a Fast and Cost-Effective Multi-Residue Method to Determine Pesticides in Tobacco by LC/MS/MS

Transcription

1 TECHNICAL NOTE Pesticides in Tobacco Development of a Fast and Cost-Effective Multi-Residue Method to Determine Pesticides in Tobacco by LC/MS/MS API 4000 LC/MS/MS System Overview This application note presents a fast and cost-effective LC/MS/MS method to detect 30 selected pesticides in tobacco samples. The high sensitivity and robustness of the API 4000 LC/MS/MS System with electrospray ionization (ESI) and selective detection in multiple reaction monitoring (MRM) mode allows fast, easy, and inexpensive sample preparation followed by a dilution step to minimize matrix effects. The method was validated for detection in tobacco matrix with maximum residue limits (MRL) between 0.1 and 2.0 μg/kg, depending on the pesticide. Introduction Tobacco is an important agricultural product processed from the leaves of plants in the genus Nicotiana. It is grown all over the world, with highest production in China, India, Brazil, and the US. Fresh leaves are harvested, dried, and cured prior to consumption, most commonly by smoking in the form of cigarettes, cigars, and pipes, though it is also chewed or sniffed. Most people are aware that tobacco consumption has significant effects on health, including an increased risk of lung cancer and cardiovascular disease. However, people may be less aware of problems

2 associated with the widespread application of pesticides used to grow tobacco. Tobacco ranks sixth among all agricultural commodities in the amount of pesticides used per acre. Thus, residues of pesticides in tobacco products, their metabolites, and their pyrolysis products generated while smoking are an additional cause of concern for human health. Although many countries have no maximum residue limits (MRL) for pesticides in tobacco like they do for many other food products, the tobacco-producing industry analyzes their products for possible pesticide residues. Due to the complexity of the matrix, powerful analytical techniques are required to detect pesticides in tobacco. Traditionally, gas chromatography (GC) with a variety of detectors and liquid chromatography (LC) with UV detection were used to test for a multitude of pesticides. However, complicated, time-consuming, laborintensive, and expensive sample preparation procedures were required to reach desired low detection limits and also to minimize matrix interference during analysis. 10 g tobacco sample Add 100 ml acetonitrile Ultra-turrax for 5 min Filter Take an aliquot of 5 ml Add 3 x 10 ml hexane/ethyl ether (9:1) Evaporate under nitrogen stream Add 2 ml water Extract using phenyl cartridge Wash 4x with 2 ml water Wash with 5 ml water/methanol (85:15) Extract with charcoal phenyl cartridge Wash with 5 ml water/methanol (65:35) Replace phenyl cartridge Elute with 8 ml of methanol/acetonitrile (80:20) Evaporate to a volume 0.5 ml Add 2.5 ml water Filter LC/UV analysis 10 g tobacco sample Add 100 ml acetone/water (2:1) Ultra-turrax for 2 min Figure 1. Comparison of tobacco matrix extraction procedures to analyze only Thiamethoxam and Clothianidin by LC/UV (left) with those to analyze 30 pesticides of different compound classes by LC/MS/MS (right). Filter Shake for 20 min Take an aliquot of 10 ml Centrifuge for 30 min at 3000 rpm and 5 C Dilute 50x with water/acetonitrile (1:1) LC/MS/MS analysis Recently, the combination of LC with tandem mass spectrometry (LC/MS/MS) has become available to laboratories performing pesticide residue analysis. Many publications highlight LC/MS/MS as a powerful tool that allows fast and cost-effective analysis of pesticides with high selectivity and superior sensitivity. Experimental Targeted Pesticides An LC/MS/MS method to detect the following 30 pesticides and pesticide metabolites in tobacco samples was developed: carbamates (Aldicarb, Aldicarb-sulfone, Aldicarb-sulfoxide, Carbaryl, Carbofuran, 3-Hydroxycarbofuran, Methiocarb, Methiocarb-sulfone, Methiocarb-sulfoxide, Methomyl, Oxamyl, Pirimicarb, and Propoxur); neonicotinoids (Imidacloprid, Clothianidin, and Thiamethoxam); organophosphates (Acephate, Demeton- S-methyl, Demeton-S-methyl-sulfone, Demeton-S-methyl-sulfoxide, Dimethoate, and Monocrotophos); and organonitrogens (Benalaxyl, Carbendazim, Clomazone, Dimethomorph, Diphenamid, Metalaxyl, Oxadixyl, and Pebulate). Sample Preparation Different sample preparation procedures are required depending on the selectivity and sensitivity of the detection method; traditional pesticide methods based on GC and LC/ UV require extensive multi-step cleanups. Very often these extraction procedures can be only used for a subset of pesticides or a single compound class. An example procedure (Figure 1) was developed to detect Thiamethoxam and Clothianidin by LC/UV. Compared to this procedure, the cleanup required prior to LC/MS/MS analysis is fast and easy, using a simple extraction with a mixture of acetone and water. After centrifugation, the extract was diluted to minimize possible matrix effects before injection into the LC/MS/MS system. Such a simple procedure can be applied to a large number of targeted pesticides of many compound classes. HPLC Separation An Agilent 1200 system was used. Separation was performed on an Agilent Eclipse C18 (150 x 4.6 mm, 5 μm) column with a mobile phase of (A) water/acetonitrile (90:10) + 10 mm ammonium acetate and 0.1% formic acid, and (B) water/acetonitrile (10:90) + 10 mm ammonium acetate and 0.1% formic acid. The following gradient profile was used at a flow rate of 800 μl/min (A/B): 0 min 70:30, 2.5 min 70:30, 5.5 min 0:100, 15 min 0:100, equilibration for 7 min. The injection volume was set to 20 μl. MS Detection The API 4000 LC/MS/MS System was operated with a Turbo V Source and electrospray ionization (ESI) probe in positive polarity. The following ion source parameters were used: IS 5500 V, GS1 50 psi, GS2 45 psi, and TEM 450 C. Multiple reaction monitoring (MRM) was used to detect all 30 pesticides for highest selectivity and sensitivity. In MRM

3 mode, the first quadrupole (Q1) filters the precursor ion of the targeted analyte, and the collision cell (Q2) is optimized to produce characteristic fragments (product ions), which are filtered in the third quadrupole (Q3). Two MRM transitions were detected for each pesticide to allow quantitation and identification in a single injection (Table 1). All MRM transitions were automatically optimized using the compound optimization tool of Analyst software. Figure 2 illustrates MRM and shows example Q1 and product ion spectra of Benalaxyl generated automatically during optimization. Results and Discussion The combination of simple cleanup, extract dilution, LC separation, and selective detection in MRM allowed the analysis of 30 targeted pesticides in tobacco samples with Maximum Residue Limits (MRL) between 0.1 and 2 μg/kg (Table 1). An example chromatogram of a spiked tobacco sample is shown in Figure 3. The developed method was validated for tobacco; recoveries at the MRL were determined between 87% and 106%, with a relative standard deviation (RSD) of less than 15%. The validation data are summarized in Table 2. Table 1. MRM transitions of detected pesticides. Pesticide MRM Pesticide MRM Methomyl 163/58 163/88 Acephate 184/ /143 Carbendazim 192/ /160 Carbaryl 202/ /145 Pebulate 204/57 204/128 Aldicarb-sulfoxide 207/ Aldicarb 208/89 208/116 Propoxur 210/ /168 Oxamyl 220/ /163 Carbofuran 222/ /165 Aldicarb-sulfone 223/ /166 Monocrotophos 224/98 224/127 Methiocarb 226/ /169 OH-Carbofuran 238/ /163 Pirimicarb 239/72 239/182 Clomazone 240/89 240/125 Diphenamid 240/ /167 Methiocarb-sulfoxide 242/ /185 Demeton-Smethyl-sulfoxide 247/ /153 Clothianidin 250/ /163 Imidacloprid 256/ /209 Methiocarb-sulfone 258/ /209 Demeton-Smethyl-sulfone 263/ /169 Oxadixyl 279/ /219 Metalaxyl 280/ /192 Thiamethoxam 292/ /211 Dimethoate 230/ /199 Demeton-S-methyl 231/61 231/89 Benalaxyl 326/ /294 Dimethomorph 388/ /301

4 Figure 3. Total ion chromatogram (TIC) and extracted ion chromatograms (XIC) of both MRM transitions of detected pesticides at a concentration of 0.5 ng/ml.

5 Table 2. Maximum residue limits (MRL) and summary of validation data. Recoveries (Rec.) at the MRL were determined between 87% and 106% with a relative standard deviation (RSD) of less than 15%, except for Demeton-S-methyl, which was affected by a closely eluting matrix peak. Pesticide MRL (μg/kg) 0.5x MRL MRL 2x MRL Rec. (%) RSD (%) Rec. (%) RSD (%) Rec. (%) RSD (%) Aldicarb Aldicarb-sulfone Aldicarb-sulfoxide Carbaryl Carbofuran Hydroxycarbofuran Methiocarb Methiocarb-sulfone Methiocarb-sulfoxide Methomyl Oxamyl Pirimicarb Propoxur Imidacloprid Clothianidin Thiamethoxam Acephate Demeton-S-methyl Demeton-S-methylsulfone Demeton-S-methylsulfoxide Dimethoate Monocrotophos Benalaxyl Carbendazim Clomazone Dimethomorph Diphenamid Metalaxyl Oxadixyl Pebulate

6 The API 4000 System is a high-sensitivity mass spectrometer offering limits of detection for pesticides much below the desired MRL of this study. The extra sensitivity was used to add a 50-fold dilution step to the sample preparation procedure. This dilution has two major advantages for the developed method: (1) Diluting the tobacco extracts minimizes or even eliminates possible matrix effects during ionization. This is crucial for the analysis of complex and concentrated matrices like tobacco, as well as other dried plant materials such as tea and herbs. (2) In addition, the injection of diluted samples greatly enhances the robustness of the LC and the MS/MS system due to less matrix being present to possibly clog the LC column or contaminate the MS/MS interface. A simple but powerful procedure to evaluate matrix effects is to infuse an internal standard, post-column, while injecting the matrix extract into the LC/MS/MS system (Figures 4 6). LC Column MS/MS Figure 4. Evaluation of matrix effects by post-column infusion of an internal standard and injection of matrix samples. Cost Efficiency of the Developed Method The presented LC/MS/MS method is routinely used in our laboratory to monitor pesticides in tobacco samples. A single operator can easily prepare, analyze, and process data from 30 samples per day. Before the introduction of this technology, GC/MS, GC/NPD, GC/FPD, and LC/UV had to be used in parallel to measure all pesticides. Different sample preparation procedures were required for each detection method because of limited selectivity and sensitivity. At least 5 operators were required to process the same number of tobacco samples and monitor a similar panel of pesticides. Figure 5. Evaluation of matrix effects caused by tobacco extract compared to a solvent injection using the described experiment.

7 Summary A method was developed and validated for the analysis of 30 targeted pesticides in tobacco samples. The high selectivity and sensitivity of LC/MS/MS using electrospray ionization (ESI) and multiple reaction monitoring (MRM) allowed simplification of the existing sample preparation procedure; an easy and fast liquid extraction followed by 50-fold dilution was sufficient to clean up tobacco samples. The validated method showed excellent detection limits, high robustness, reproducibility, and minimal matrix effects, even though crude extracts were injected. Pesticides were successfully quantitated and confirmed using MRM ratios in tobacco samples with maximum residue limits (MRL) of 0.1 to 2.0 μg/kg. Future Developments Future plans include the application of fast and high-resolution HPLC to further minimize potential matrix interference. Also, possibilities of sample preparation automation will be investigated to increase the throughput of tobacco analysis while maintaining the laboratory staff. Authors Jorge Ghelli (Souza Cruz, Tobacco Analysis Laboratory, Porto Alegre, Brazil), Helio A. Martins-Júnior (Applied Biosystems, São Paulo, Brazil) and André Schreiber (Applied Biosystems, Concord, Ontario, Canada) Acknowledgments The authors would like to thank Jose Roberto Silva, Marcos Vinicius Gama, and Cristina Maciel of the tobacco analysis laboratory. References 1. Guideline on pesticides residues in tobacco of CORESTA (Centre de Coopération pour les Recherches Scientifiques Relatives au Tabac). Figure 6. Visualization of matrix effects by plotting the quotient of matrix injection to solvent injection with % tolerance bars. Matrix effects can be minimized effectively by simple dilution of the extracted sample.

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