LC-MS/MS and GC-MS/MS Multi Residue Pesticide Analysis in Fruit and Vegetable Extracts on a Single Tandem Quadrupole Mass Spectrometer

Hundreds of pesticides are commercially available and are approved for use on various fruit and vegetable plants to prevent pest infestation and improve shelf-life of fresh produce. Maximum Residue Levels (MRLs) are set at the highest level of pesticide that the relevant regulatory body would expect to find in that crop when it has been treated in accordance with good agricultural practice. In the EU, if a pesticide is not explicitly mentioned in the MRL legislation, a default MRL is used for enforcement. This default value is set to be equal to the limit of quantification (LOQ) achievable with the analytical methods used for analysis. National authorities control and enforce MRLs by testing samples for pesticide residue levels using analytical surveillance programs. These programs check for compliance with MRLs, assess dietary exposure, and check for use of unauthorised pesticides. The food industry also carries out its own due diligence analyses. 

Mass spectrometry (MS) coupled with both gas chromatography (GC) and liquid chromatography (LC) is needed to provide comprehensive analysis of a wide range of pesticide residues with sufficient sensitivity to meet global MRL regulations. The use of Quick, Easy, Cheap, Efficient, Rugged and Safe (QuEChERS) sample extraction and clean up has streamlined analytical efficiencies for multi residue analyses [1]. The advantage of ultra high performance liquid chromatography (UHPLC) coupled with tandem quadrupole mass spectrometry (MS/MS) for multi residue pesticide analysis is widely reported [2]. More recently the use of GC-MS/MS utilising atmospheric pressure ionisation (APGC) has been shown to offer significant improvements in performance over EI for challenging pesticides, in terms of selectivity, specificity, and speed of analysis [3,4]. 

The APGC source ionises compounds using a corona discharge at atmospheric pressure in an APCI-like manner. Therefore, this ionisation mechanism is a much softer technique than classic electron impact (EI) ionisation and produces larger amounts of intact parent ions, especially in the case of fragile or easily fragmented compounds. APGC ionisation can occur using two mechanisms; proton transfer (wet source) or charge transfer (dry source). In proton transfer ionisation, [M+H]+ ions are formed, whereas in charge transfer ionisation, M+· ions are formed.

In this work, a single workflow for the multi residue analysis of pesticides is demonstrated on a variety of fruit and vegetable samples. Utilising the universal source of Waters Xevo^® TQ-S micro mass spectrometer allows for LC (electrospray ionisation) and GC (atmospheric pressure ionisation) analyses to be completed on the same tandem quadrupole MS instrument, with less than 30 minutes needed to switch between chromatographic inlets. The performance of the method will be highlighted in terms of sensitivity, repeatability, and linearity for both LC and GC in compliance with the SANTE guidelines (11945/2015) for pesticide analysis [5].

Methods

The LC and GC suites of pesticides analysed in this study (listed in appendix tables) were chosen to cover a wide range of different pesticide classes and chemistries. The multi residue MS/MS methods were generated using the Quanpedia™ database, with separate databases utilised for generation of the LC and GC methods. Each database contains MRMs and retention time information for each compound. When the MS method is generated the MRM function windows are automatically set for each compound. For the LC method, a window of 1 minute was placed around each compound’s expected retention time. For the GC method, a window of 30 seconds was used due to the narrower peak widths exhibited in GC analysis. In addition to the MS methods, the TargetLynx™ software data processing methods and LC inlet method were also generated through the Quanpedia database.

Sample Extraction and Cleanup

Celery, lemon, corn, and kale samples were purchased at a local grocery store. Samples were chosen to be representative of different types of matrix complexity from different commodity groups, including high water content (celery and kale), high acid content (lemon), and high starch/protein with low water content (corn). Samples were immediately homogenised in a food processer and frozen until sample preparation was performed. QuEChERS extraction was performed according to the official AOAC method 2007.01 using Waters DisQuE™ Dispersive Solid Phase Extraction (d-SPE) product [6]. Figure 1 highlights the sample extraction. 

Experimental

LC-MS/MS Conditions

LC system:             ACQUITY UPLC H-Class
Column:             ACQUITY UPLC BEH C18
            1.7 µm, 2.1 x 100 mm
Column temp.:         45°C
Injection volume:     5 µl
Flow rate:            0.45 mL/min
Mobile phase A:    Water + 10 mM
            ammonium acetate
Mobile phase B:     Methanol + 10 mM
            ammonium acetate
Gradient: 

MS System:     Xevo TQ-S micro
Ionisation mode:     ESI+
Capillary voltage:     1 kV
Desolvation temp.:    500°C
Desolvation
gas flow:         1000 L/hr
Source temp.:    150°C

GC-MS/MS Conditions

GC System:    7890A
Autosampler:    CTC PAL RTC
Column:         30 m x 0.25 mm x
        0.25 µm Rxi-5MS
Carrier gas:     Helium
Flow rate:     2.0 mL/min

Injection:         Splitless
Injector temp:     280°C
Injection volume:    1 µl
Makeup gas:     Nitrogen at 250 mL/min
Transfer line temp.:     320°C
Oven program:
MS system:     Xevo TQ-S micro
Ionisation mode:     APGC+
Ionisation
mechanism:     Proton transfer
        (3 vials of uncapped
        water in source)
Corona current:    20 µA for first 3.5 min
        3.0 µA for rest of run
Cone gas flow:    0 L/hr
Auxiliary gas flow:    250 L/hr
Source temp.:    150°C

Results and Discussion

Method Management Using the Quanpedia Database

Working with methods involving large numbers of compounds can be time consuming when done manually and is prone to errors when setting up time segmented acquisition. Quanpedia is a compound centric database typically used for method generation, but it can also function as a method management tool. Initial methods for this analysis were generated using existing LC and APGC databases (Figure 2). Retention time changes resulting from further method development or method changes were updated in the database. This allowed for immediate and automatic updates to be made in the MS processing methods by re-generating the methods with three simple clicks.

Robust and Rapid Data Acquisition

For the successful analysis of large
numbers of pesticides and their metabolites, it is important that the mass spectrometer can maintain sufficient sensitivity while acquiring MRM transitions with a fast scan speed in order to provide enough data points across each chromatographic peak (e.g. minimum of 12 points per peak).
The fast scanning speeds of the Xevo TQ-S micro provide this robust and rapid data acquisition while maintaining large retention time windows to accommodate any shift in retention time due to column maintenance (GC) or chromatography changes caused by the different matrices [6]. Figure 3 highlights one of the busiest sections of the APGC MS method. In this example, flutolanil is just one of approximately 30 pesticides (set across 30 channels, each acquiring at least two transitions per compound) eluting in a 1.5 minute time window.
The dwell time calculated for this compound using the autodwell function was 0.006 s.
The resulting chromatogram of three replicate injections of 0.010 mg/kg of flutolanil in a celery matrix can be seen in Figure 3. Even with the fast scanning speed, 19 points were collected across the peak and the RSD of three consecutive injections in matrix was 5.2%. The same is true for the LC method used for this analysis.

Pesticides in Matrix

Matrix matched standards were prepared in celery, lemon, corn, and kale over a range of 0.001 to 0.050 mg/kg, and replicate injections made using the LC and GC methods. A TIC overlay for a selection
of pesticides is shown in Figure 4, with
0.010 mg/kg in celery extract from both the A. APGC, and B. UHPLC analyses. The data were fitted with the best fit calibration: for the UHPLC data, the response was shown to be linear, whereas the APGC response over the range investigated was non-linear and so it was fitted with a quadratic calibration. A majority of the compounds in both analysis methods had correlation coefficient (R²) values of 0.995 or greater. Figure 5 shows the matrix matched calibration curves and the peak response at 0.001 mg/kg of a representative pesticide from each analysis method in the four matrices. Residuals from triplicate injections at each calibration point were within ±20%. Ion ratios were also shown to be within 30% tolerance of the reference values.

For convenience, all sample extracts were spiked at the default MRL of 0.01 mg/kg. Figure 6 demonstrates the number of pesticides in each method detected in
the spiked matrices at 0.01 mg/kg.
However many pesticides could also be detected at 0.001 mg/kg as demonstrated in Figure 5 which shows leptophos (APGC compound) and carbofuran (UHPLC compound) in the different matrices. The precision of the measurements was excellent with more than 90% of the detected pesticides exhibiting RSDs of peak area of <10% (n=3). The exception was the APGC analysis of the kale matrix, which had more than 80% of pesticides exhibiting RSDs of <10%
(Figure 7).

Conclusions

Complex multi residue pesticide analysis was demonstrated using both LC and GC analysis on the same tandem quadrupole instrument. Analysis methods were generated and maintained using Quanpedia databases, making method generation and maintenance fast and simple. Although the multi residue methods contained approximately 200 compounds each, the reliable scanning speed of the tandem quadrupole mass spectrometer employed  (Xevo TQ-S micro) produced accurate and precise measurements. The performance for the determination of pesticide residues analysed in four matrices of varying complexity complied with the SANTE guidelines for pesticide residue analysis. Detection at the EU default maximum residue limit of 0.01 mg/kg was easily achieved for >99% of the pesticides analysed with good precision (RSDs <10%) for most analytes in the food samples. Having the flexibility of the MS Universal Source architecture to provide access to both LC-MS/MS and GC-MS/MS on the same instrument, allows for an increase of laboratory efficiency, while maintaining required sensitivity and repeatability.

References

1. D Shah, E McCall, G Cleland. Single LC-MS/MS Method for Confirmation and Quantification of Over 400 Pesticides in a Complex Matrix Without Compromising Data Quality. Waters Application Note no. 720005559EN. January, 2016.

2. T Kovalczuk, M Jech, J Poustka, J Hajslova. UPLC-MS/MS: A novel challenge in multiresidue pesticide analysis in food, Analytica Chimica Acta, 577, 2006.

3. M Tienstra, T Portoles, F Hernandez, J G J Mol. Fast gas chromatographic residue analysis in animal feed using split injection and atmospheric pressure chemical ionisation tandem mass spectrometry. J. Chrom. A. 1422, October, 2015.

4. L Cherta, T Portoles, J Beltran, E Pitarch, J G Mol, F Hernandez. Application of gas chromatography-mass spectrometry with atmospheric pressure chemical ionisation for the determination of multiclass pesticides in fruits and vegetables. J. Chrom. A.  1314: 224-240, November, 2013.

5. European Commission. SANTE/11945/2015. Guidance document on analytical quality control and method validation procedures for pesticides residues analysis in food and feed. 2015, rev. 0.

6. AOAC Official Method 2007.01. Pesticide residues in foods by acetonitrile extraction and partitioning with magnesium sulfate. 2013.

Pesticides in GC Method

Pesticides in LC Method