Quantifying 250 Pesticides Found in Fruit Juices Using Single Positive-Negative Switching LC-MS/MS

The role of crop yield enhancing agrochemicals is vital in devising effective food production strategies in order to meet the demands of ever-increasing world population.

The increasing use of many different pesticides for crop yield improvement increases the demand for more stringent pesticide monitoring to ensure the safety for consumers. QC labs worldwide are looking for monitoring methods capable of shortening analytical cycle time with improved sensitivity, consistency and robustness to address increasing demands and productivity targets.

Drawbacks of Traditional LC-MS/MS Systems

Simultaneous monitoring of both positive and negative ions is not possible with traditional LC-MS/MS systems. As a consequence, MRM monitoring needs to be run in both positive and negative modes to make up for co-eluting compounds in order to obtain complete analytical confidence. This means the requirement for roughly double the sample volume and analysis time.

Bruker’s EVOQ Elite (LC-MS/MS) System

This limitation can be addressed with improved efficiency using the EVOQ Elite (LC-MS/MS) system from Bruker. Both positively and negatively charged species can be simultaneously detected using the smart Compound Based Scanning (CBS) software. In a multiple residue assay, a scan time is assigned by CBS for each pesticide, enabling simplified method development with accelerated analysis times by automatically switching between positive and negative modes.

The EVOQ Elite provides highly sensitive and effective pesticide quantification with advanced developments such as IQ Dual Ion Funnel and Active Exhaust, which maintain superior robustness and high signal to noise ratios even for trace amounts of pesticide compounds. As a consequence, downtime involved in routine instrument maintenance is reduced, which is of high importance for high throughput QC labs. In addition, simplified ‘dilute-and-shoot’ sample preparation eliminates sample enrichment, thus minimizing the effort and time required for the analysis.

This article discusses the application of the EVOQ Elite triple quadrupole mass spectrometer in the quantification of 250 pesticides in vegetable juice, orange, cranberry, and apple, demonstrating the sensitivity and speed of this technique for pesticide analysis in food. Only one method with positive/negative switching was employed for more than 500 MRM transitions.

Experimental Procedure

The sample preparation involved dilution of 50µL samples of vegetable juice, grape, orange, cranberry, and apple with 450µL of solvent of MeOH/water in 10:90 (v/v) in a filter vial by means of a press filter plunger (0.2µm PVDF) to filter. Bruker’s Ultra High Performance Liquid Chromatography (UHPLC) system was used to separate the samples, which were then analyzed on the EVOQ Elite (LC-MS/MS) system using a CTC Autosampler for automated sample loading. The instrument conditions are listed as follows:

LC Parameters

Column: YMC-Pack ODS-AQ 3µm, 150mm x 3mm (I.D.)
Column Temperature: 40°C
Injection Volume: 30µL
Mobile Phase A: 5mM ammonium fluoride in water
Mobile Phase B: Methanol

Gradient:

Time (s) %A %B Flow(µL/min)
0.00 90 10 400
0.20 90 10 400
0.20 30 70 400
10.0 0 100 400
15.0 0 100 400
15.1 90 10 500
17.5 90 10 500
18.0 90 10 400

MS Parameters

Source: HESI
Spray Voltage (Positive): 4000V
Spray Voltage (Negative): 4000V
Cone Gas Flow: 20-unit
Cone Temperature: 250°C
Heated Probe Gas Flow: 40 – unit
Heated Probe Temperature: 400°C
Nebulizer Gas Flow: 60 – unit
Exhaust Gas: On

Dwell times are automatically assigned by the CBS software for MRM determination based on the data points and peak width required, enabling polarity switching of 250 positive and negative compounds for more than 500 MRM transitions with reduced manual interference. The calibration curve for negative pesticide Fipronil and positive pesticide Cyazofamid and their corresponding co-eluting chromatographic plots are shown in Figure 1.

Calibration curve of negative pesticide Fipronil and positive pesticide Cyazofamid and their co-eluting chromatograms.

Figure 1. Calibration curve of negative pesticide Fipronil and positive pesticide Cyazofamid and their co-eluting chromatograms.

Outstanding linearity is demonstrated during the calibration of triplicate injections, between 0.01-10ppb, and the r2 value is 0.999 for both pesticides. The response factor RSD over three orders of range is below 5%, revealing the good reproducibility of the method. In total, 12 pesticides were identified in the samples (Table 1).

Table 1. Twelve pesticides were detected in orange, cranberry, apple or vegetable juice. No pesticides were detected in white grape juice.

Fruit Juice=> Apple Juice Orange Juice Cranberry Juice White Grape Juice Vegetable Juice
Pesticide µg/L (ppb)
Azoxystrobin ND ND 0.32 ND 0.48
Boscalid ND ND 0.16 ND ND
Carbaryl ND 0.39 1.47 ND ND
Carbofuran ND 0.14 ND ND ND
Dimethoate ND 0.30 ND ND ND
Imidacloprid ND ND 0.60 ND 0.20
Mandipropamid ND ND 0.59 ND ND
Metalaxyl ND ND 0.21 ND ND
Methoxyfenozide ND ND ND ND 0.84
Tebuconazole ND ND 0.32 ND ND
Thiabendazole 1.8 ND ND ND ND

This technique took only 18 minutes for the entire analysis through multiple injections or extraction/enrichment based sample preparation, thus saving considerable time over traditional LC-MS/MS techniques. With remarkable sensitivity and linearity, the ‘dilute-and-shoot’ sample preparation method is effective even for trace amounts of pesticide residues.

Conclusion

Sophisticated LC-MS/MS methods enable rapid detection of hundreds of pesticides using a single positive-negative switching technique. Moreover, straightforward sample preparation techniques support food analyzers to implement regulatory requirements and satisfy productivity targets, even for trace amounts of compounds in complex matrices.

This information has been sourced, reviewed and adapted from materials provided by Bruker - Chemical And Applied Markets.

For more information on this source, please visit Bruker Daltonics.

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