Quantitation and Confirmation of the Pesticide Malathion in Fruit Samples using Multiple ReactionMonitoring and MS-MS-MS

LC–MS-MS instruments operating in Multiple Reaction Monitoring (MRM) are widely used for targeted quantitation on triple quadrupole and hybrid triple quadrupole linear ion trap (QTRAP® ) systems because of their well known selectivity and sensitivity. In MRM mode, the first quadrupole (Q1) filters a specific precursor ion; the collision cell (Q2) generates fragments (product ions) which are filtered in the third quadrupole (Q3). Although this double mass filtering greatly reduces noise there is always a chance that elevated background levels or matrix signals interfere with the targeted analyte.

One possibility of improving quantitative results is using a more selective detection mode, such as MRM³ . When a QTRAP® System is operated in MRM³ mode first Q1 filters the first precursor ion; than Q2 generates product ions which are trapped in Q3 operating as a linear ion trap (LIT). Afterwards the LIT isolates the second precursor ion and generates the second generation of product ions which are scanned out towards the detector. In comparison to MRM mode, MRM³ provides higher selectivity due to one additional fragmentation step. Both modes of operation are illustrated in Figure 1.

Figure 1

The novel AB SCIEX QTRAP® 5500 system uses the new and patented Linear Accelerator™ Trap designed to reduce the fragmentation time and to increase the MRM³ excitation efficiency delivering a whole new level of MRM³ performance. This, together with the high sensitivity of the mass spectrometer and faster scanning of up to 20000Da/s, allows performing MRM³ with lower detection limits and with shorter cycle times than previous generations of QTRAP® systems.

Method Details

• Ultra High Pressure Liquid Chromatography using a Shimadzu UFLC XR system with a Phenomenex Synergi Fusion-RP (2.5μm) column and a fast gradient of water and methanol with ammonium formate buffer

• AB SCIEX QTRAP® 5500 System with Turbo V™ Source and ESI probe

• Experiment 1: detection of two MRM transitions with 100ms dwell time

• Experiment 2: detection of two MRM³ scans at 20000Da/s with 20ms fill time and 25ms excitation time, total cycle time of only 0.33s

The detected masses in MRM and MRM³ mode with compound dependent parameters are shown in Figure 2.

Figure 2

Results

Selectivity

A homogenized apple was spiked with 10ppb Malathion, extracted using a QuEChERS procedure, diluted 50 times to minimize matrix effects, and analyzed by LC–MS-MS. The resulting chromatograms using two MRM transitions and two MRM³ experiments are shown in Figure 3. The MRM transition 331/127 showed expected selectivity, while the second transition 331/99 had an elevated background level and also matrix interference. In contrast both MRM³ experiments showed superior selectivity for reliable quantitation.

Figure 3

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