There is rising attention in the world to food safety issues. The number of pesticides is growing and so does the list of pesticides that are regulated and monitored at various residual levels in food. The maximum residue levels for pesticides are getting lower following detection capability of modern analytical instrumentation and depend on type of food.
GC-MS analysis of pesticide residues in tea extracts is rather demanding due to complexity of the matrix – including presence of phenolic compounds, caffeine, flavones and flavonoles which interfere with some target compounds.
The goal of the work was to evaluate the novel Q-Tek GC-MS for quantitative analysis of several chlorinated pesticide residues at levels near those regulated by GB2367-2012 Standard in China. The green tea was used as sample matrix due to it’s complexity. Acetonitrile extraction by QUECHERS method was used to prepare samples prior to injection into GC-MSD.
The GC-MSD experimental workflow is shown in fig. 1, to get most of Q-Tek productivity tool Q-Tek SIM Wizard®. The method parameters are listed below:
Fig.1 The GC-MSD experimental workflow.
| GC method parameters: | |
| Column | RTX-5MS (30m x 0.25, 250)
Constant Flow mode, 1 ml/min flow |
| Inlet temperature: | 250°C |
| Inlet mode: | Splitless 1.2 min
Then, purge 40 mL/min |
| Oven program: | 60°C (1.25 min) |
| 20°C/min – 180°C (0.0 min), | |
| 5°C/min – 230°C/min (0.0 min); | |
| 25.0°C/min – 290°C (2.0 min); | |
| 30.0°C/min – 310°C (2.0 min) | |
| Transfer line temperature | 280 ° C |
| MSD data acquisition parameters: | |
| SCAN mode | Setting up SIM method |
| Mass Range: | 50 – 400 amu |
| SIM mode | Quantitation |
| Dwell time | automated by «SIM Wizard®» algorithm |
First, one must run a mix of standards containing all your target compounds at 50ppb each in full SCAN mode in order to optimize chromatographic separation parameters. From there Q-Tek SIM Wizard® is designed to lead you to optimal SIM method that would be then ideal for quantitation of all the targets.
Please, see more details in Technical note: Q-Tek SIM Wizard® productivity tool
TIC peak autointegration and simultaneous extraction of clean spectra for each compound is done initially. Autointegration algorithm confidently defines peaks and makes effective spectral library search even on co-eluting peaks.
As soon as clean spectra have been extracted from the target peaks the ion masses automatically selected by the algorithm as the best SIM-candidates are flagged on the compound spectrum for operator’s review and confirmation.
Then, Q-Tek SIM Wizard® will split the TIC into time segments to ensure required number of points per a mass peak during the SIM acquisition, thus, finalizing the SIM method parameters. Each time-segment will contain the least number of targets, as to increase method sensitivity resulting from longer dwell-time for each monitored compound in the given time-segment (see fig. 2)
Fig. 2 Extracted Ion Chromatogram of 16 chlorinated pesticide standards mix and automatic target compounds grouping into SIM-segments by iDwell® Time algorithm. 1 – α-HCH; 2- β-HCH; 3 – Lindane; 4 – δ-HCH; 5 – Heptachlor; 6 – Aldrin; 7 – Heptachlor exo-epoxide; 8 – α-Endosulfan; 9 – 4,4’-DDE; 10 – Dieldrin; 11 – Endrin; 12 – β-Endosulfan; 13 – 4,4-DDD; 14 – 4,4’-DDT; 15 – Endrin ketone; 16 – Methoxychlor.
At the end SIM Wizard populates SIM table with RT and selected ions to be monitored. The rest of data acquisition parameters are transferred from the original SCAN method. Thus, operator’s time to develop and optimize SIM method can be substantially reduced and free from human errors. Results are shown in Table 1.
Table 1. Retention times and monitored ions for all Chlorinated Pesticide targets in the mix.
The GC-MSD instrument was calibrated in range from 10 to 1000 ppb (ng/mL) for all 16 compounds of the mix. The linear calibrations coefficients are listed in the Table 2.
Table 2. Linear calibration coefficients (R2) for chlorinated pesticide mix
| Compound | R2 | Compound | R2 | |
| Aldrin | 0.999 | Dieldrin | 0.999 | |
| α-BHC (α-HCH) | 0.999 | α-Endosulfan | 0.999 | |
| β-BHC (β-HCH) | 0.999 | β-Endosulfan | 0.999 | |
| Lindane | 0.999 | Endrin | 0.999 | |
| δ-BHC (δ-HCH) | 0.998 | Endrin ketone | 0.998 | |
| 4,4-DDD | 0.998 | Heptachlor | 0.998 | |
| 4,4’-DDE | 0.999 | Heptachlor exo-epoxide | 0.999 | |
| 4,4’-DDT | 0.996 | Methoxychlor | 0.998 |
Green tea extract provided high background level which affected detection limits for few compounds. As an example, Dieldrin suffered from high matrix background at current separation conditions and, therefore, could only be detected at level of 50 ppb. Of course, it’s detection limit will be further improved having separation conditions optimized.
Table 3. Limits of Detection and Recovery values for 50 ppb spike of pesticide mix into green tea (* – Report of the working group on maximum residue levels (MRLS) and MRLS in the brew [1] ** – Commission regulation (EU) 2017/978 of 9 June 2017 [2] *** – Japanese Food Chemical Research Foundation [3])
| № | Compound | LOD, ppb | Recovery,
% |
Maximum Residue Levels, ppb | ||
| EU** | China* | Japan*** | ||||
| 1 | Aldrin | ≤ 10 ppb | 97% | 20 ppb | n/a | n/a |
| 2 | α-BHC (α-HCH) | ≤ 10 ppb | 75.2 % | 10 ppb** | 200 ppb* | n/a |
| 3 | β-BHC (β-HCH) | ≤ 10 ppb | 74% | 10 ppb** | 200 ppb* | 50 ppb*** |
| 4 | Lindane | ≤ 10 ppb | 82% | 10 ppb** | n/a | n/a |
| 5 | δ-BHC (δ-HCH) | ≤ 10 ppb | 64% | 10 ppb** | 200 ppb* | n/a |
| 6 | 4,4-DDD | ≤ 10 ppb | 90.5% | ND | 200 ppb* | n/a |
| 7 | 4,4’-DDE | ≤ 10 ppb | 84.5% | ND | 200 ppb* | n/a |
| 8 | 4,4’-DDT | ≤ 10 ppb | 86% | ND | 200 ppb* | n/a |
| 9 | Dieldrin | ≤ 50 ppb | 120% | n/a | n/a | n/a |
| 10 | α-Endosulfan | ≤ 50ppb | 112.4% | 10000 ppb* | 10000 ppb* | 30000 ppb* |
| 11 | β-Endosulfan | ≤ 100 ppb | 93.05% | 10000 ppb* | 10000 ppb* | 30000 ppb* |
| 12 | Endrin | ≤ 10 ppb | 112% | n/a | n/a | n/a |
| 13 | Endrin ketone | ≤ 100ppb | 107.5% | n/a | n/a | n/a |
| 14 | Heptachlor | ≤ 50 ppb | 54.4% | n/a | n/a | n/a |
| 15 | Heptachlor
exo-epoxide |
≤ 10 ppb | 99.8% | n/a | n/a | n/a |
| 16 | Methoxychlor | ≤ 10ppb | 60.0% | n/a | n/a | n/a |
The experiment demonstrated that, 11 of 16 chlorinated pesticides could be detected at below 10 ppb level in green tea matrix. The analytes recovery with QUECHERS sample preparation was from 70 to 110% .
The experimental results demonstrated capability of novel GC-MS to effectively detect and quantitate pesticides in complex extracts such as green tea, well below regulated levels.
The iDwellTime productivity tool provides a lot of value to a GC-MS operator to speed up method development for compound quantitation in complex matrices
References:
- Report of the working group on maximum residue levels (MRLS) and MRLS in the brew (based on feedback from India, UK, US, China and Canada)// CCP: TE 16/CRS15
- COMMISSION REGULATION (EU) 2017/978 of 9 June 2017
- Japanese Food Chemical Research Foundation
