The strict detection limits that are required for the quantitation of pesticide residues in food and environmental samples requires the use of very sensitive, selective, and rugged LC/MS/MS technologies. In this application note the benefits of using LC/MS/MS for this type of analysis is described. In particular the role of the collision cell in the MS/MS experiment is addressed. A method for the simultaneous multiresidue screening of 55 pesticides in both soil and leek extracts in less than 15 minutes is described.
Materials and methods
The 55 pesticides are listed in Table 1. Standard solutions (0.1 250 g.L-1 ) are prepared in a mixture of 50/50 (v/v) Water/Acetonitrile. Leek samples preparation consists of adding 250 ml acetone to 25 gram of leek. 20 ml of this solution is cleaned up by using liquid/liquid extraction (2 x 50 ml dichloromethane). After evaporation to dryness the extract is reconstituted in 2 ml 50/50 (v/v) water/ACN and spiked with a solution containing 10 g.L-1 of each of the pesticides listed in Table 1. The soil samples are treated identically with the exception that the acetone extraction is performed in an acidic environment. HPLC is performed on a Perkin-Elmer Series 200 system equipped with an Altima column (C18, 5 m, (250 x 2.1 mm). Elution is performed in gradient mode (Solvent A: water + 10 mM ammoniumformiate + 0.1% formic acid; Solvent B: ACN + 0.1% formic acid). Liquid flow rate is 200 l.min.-1 Injection volume is 10 l. Mass spectrometry is performed on an API 2000TM LC/MS/MS System, using TurboIonSp ray ionization in positive mode.
Chromatographic considerations Operating the mass spectrometer in the selective and sensitive MRM mode allows fast elution of the compounds and avoids the necessity of full chromatographic separation of all compounds of interest. However, some important aspects have to be considered and in some cases chromatographic separation may still be required: Compounds measured at the same MRM (i.e. the same precursor/ product ion combination) (e.g. Terbutylazine/Sebutylazine) (Figure 2) The compounds produce similar (overlapping) product ions upon fragmentation (Figure 1) The compounds that are measured are using the same product ions (but with different precursor ions) (Table 2). Here the risk of cross talk phenomena can occur when conventional collision
cells are used. Because the API 2000 system is equipped with the LINACTM collision cell, cross talk effects cannot occur. This allows a much greater flexibility and ease of use during method development. Figure 3 confirms the absence of cross talk, even at low MRM dwell times (10 ms only).
Benefitsof the LINAC collision cell for multi-residue analysis
The use of the LINAC collision cell does not only prevent cross talk phenomena, it also allows the use of very short MRM dwell times without sensitivity loss. This in turn permits the simultaneous monitoring of multiple MRMs. Consequently highthroughput multi-component analysis is enabled. To prove this several experiments are performed (Figure 5). In each experiment the number of compounds measur ed simultaneously is increased. In fact this means that a different (lower) dwell time for each MRM transition is used. The total cycle time (the time needed to measure all MRMs one time) remains the same. This is important to preserve quantitative integrity by ensuring that enough datapoints across each chromatographic peak are obtained to govern reliable peak area integration. In the example given in Figure 5, a constant cycle time would require dwell times of 5, 30, or 100 ms when respectively 196, 56, or 19 compounds are measured simultaneously.
The effect of applying different dwell times is given in Figure 6 for hexazinon. Dropping the dwell time to 1 ms does not lead to signal (sensitivity) loss. Detection limits (Figure 7) are only affected below 10 ms dwell times. This is due to the fact that the noise level will slightly increase. A simple smoothing procedure during data processing would correct this easily. For this study a dwell time of 30 ms for each MRM was used.
Analysis of leek and soil extracts
Figure 4 demonstrates the analysis of the 55 pesticides of interest (spiked at 10 g.kg-1) in a leek extract. Sensitivity, selectivity and specificity are clearly demonstrated in this complex matrix. A similar analysis would not be possible in single MS mode (e.g. with a single quadrupole mass spectrometer) due to the many interfering compounds. For soil extracts similar results are obtained. This means that the method is suitable for analysis of residues in complex matrices. The detection limits and recoveries are very good (Table 1).
Simultaneous fast analysis of 55 pesticides in less than 15 minutes in complex matrices is demonstrated.