Application of GC-Triple Quadrupole MS/MS for Multi-Residue Analysis of Pesticides in Complex Matrices

In this application note, gas chromatography (GC) with tandem quadrupole mass spectrometric detection (GC-MS/MS) was used to achieve the required selectivity.

Benefits

• The Waters Micromass Quattro micro GC can be used in routine analysis at 10 ms dwell times
  
• Good sensitivity for the majority of pesticide residues

There are over 800 registered pesticides in use worldwide, and an ever-increasing requirement to monitor the levels of these compounds in foodstuffs. Under EU legislation, maximum residue limits (MRLs) are specified for pesticides considered a potential health risk; guidelines for developing analytical methods must be followed.

The analytical challenge is to maximize the number of pesticides, minimize the variety of methods, keep run times short and achieve limits of detection (LODs) at or below the reporting level. Multi-residue methods are efficient for analysis of pesticide residues. For methods with a very wide scope, generic sample preparation procedures are employed. Inherent to this approach is that clean up of extracts is only possible to a limited extent. When applying such methods to complex matrices like baby food, herbs, spices and tobacco, enhanced selectivity in detection is required to make up for the low selectivity in sample preparation.

Whereas single quadrupole and ion trap MS instruments are suitable for simple matrices where LODs of >0.01 mg/kg are required, these detection systems provide insufficient selectivity for complex food matrices, such as baby food, garlic, ginger, herbs, and spices. In this work, gas chromatography (GC) with tandem quadrupole mass spectrometric detection (GC-MS/MS) was used to achieve the required selectivity.

Requirements of the analytical method:

• A simple and generic, rapid extraction method, allowing recovery of multiple classes of pesticides (OP, OC, pyrethroids, etc.)

• Analysis of 100 or more pesticides in 1 run

• LODs at or lower than the reporting level

• Need for targeting multiple compounds in a variety of produce/matrices

• Efficient use of time/instrumentation/personnel (Multiresidue methods)

• Selective MS detection method (required to compensate for less selective sample prep)

• Adequate sensitivity to keep amount of (dirty) matrix introduced into the GC system as low as possible

Method development

In order to maximize the response of the instrument for each residue the choice of precursor ion, product ion, and collision energy were optimized. Initially, the pesticide mixture was analysed in full scan mode (Figure 3).

The precursor ion was selected from the full scan spectra based on its relative abundance, e.g. Endosulfan-beta (MW = 404; Figure 4).

The collision energy for the product ion was optimised using a range of collision energies between 5–30 eV (Figure 5).

Method Performance

For the majority of pesticides, LoDs were below 10 pg on column (based on S/N >3:1; Table 1). The ability to quantify closely eluting peaks is a prerequisite of multiresidue methods. Therefore, the dwell time allocated for each transition must be sufficient to ensure that at least 10 data points are acquired for accurate quantification. To assess the effect of short dwell times on data quality a standard solution of hexachlorobenzene was acquired using a range of dwell times. 

Figure 6 illustrates that the signal intensity is unaffected by the shorter dwell time. With a 10 ms dwell time, the S/N measured is sufficient for quantification of the target compounds.

To assess instrument robustness, hexachlorobenzene (20 pg on-column) was analyzed repeatedly (n = 10) at a range of different dwell times, from 10–50 ms. The %RSD shown in Table 2 illustrates the repeatability of injection was less than 5%.

For 100 pesticide residues, the optimum experimental setup utilized 84 MRM transitions in 14 MRM function windows, with 4 to 8 transitions in each window (Figure 7). With this setup, at any point in time, the maximum number of transitions acquired was 15.

The linearity of the system is illustrated in Figure 8.

The analytical method was applied to the analysis of a range of complex food extracts, including fresh produce, baby food (matrix equivalent = 0.5 g/mL), dried herbs, spices, tobacco, ginkgo, and cannabis (matrix equivalent = 0.1 g/mL).

Figure 9 compares chromatograms from complex and non-complex matrices, showing the importance of high selectivity for the analysis of these types of sample. Figure 10 shows the GC-MS/MS chromatograms of selected pesticide residues in herbal tea.

In all cases (Table 3), a high percentage of the spiked pesticides were detected and measured, despite the highly complex matrices, generic sample preparation and limited clean up employed.

• A multiresidue method has been developed for the surveillance monitoring of 100 pesticide residues in a range of food matrices
• Sufficient selectivity has been achieved to allow generic sample clean up, even for very complex food matrices
• The Waters Micromass Quattro micro GC can be used in routine analysis at 10 ms dwell times. Subsequent expansion of this method to include an increased number of MRM transitions would be possible without a loss in sensitivity
• The system provided good sensitivity (1–10 pg on column) for the majority of pesticide residues, allowing the amount of matrix introduced into the GC system to be minimized