Explore this issueApril/May 2012
Organophosphorus pesticides are widely used in the agricultural industry for crop protection. Human toxicities for this class of molecules have shown acute and chronic effects from pesticide poisoning. OP pesticides affect the nervous system of insects and mammals by inhibiting an enzyme, acetylcholinesterase, that is important in helping regulate nerve impulses.1
Children are considered more susceptible to organophosphate toxicity because their pesticide dose per body weight is larger compared to that of adults.2 Children also have lower levels of detoxifying enzymes that deactivate OP pesticides, contributing to their vulnerability to pesticide exposure.3,4 Recent studies have shown a correlation between OP pesticide exposure and an increased risk for attention deficit hyperactivity disorder and other neurodevelopmental deficits in children.5-7 Because the main source of exposure for children is through consumption of food containing OP pesticide residues, analytical testing capable of determining residual pesticides in food samples is critical.8
The multiresidue determination of pesticides in fruits and vegetables usually involves an organic extraction of the pesticides from the plant matrix, followed by a cleanup procedure to remove co-extractives and other interferences. Anastassiades and colleagues developed the QuEChERS [quick, easy, cheap, effective, rugged, and safe] method for the analysis of pesticide residues in produce.9 This approach simplifies the traditional, labor-intensive extraction and cleanup procedure, while providing a fast, robust, and cost-effective method suitable for extracting pesticide residues.
Chromatographically active compounds such as OP pesticides can adsorb onto active sites in the sample flow path, particularly at trace levels, compromising an analyte’s response. These pesticides tend to show peak tailing through interaction with active sites in a chromatographic system. This makes analysis challenging, particularly in difficult sample matrices. Minimizing activity in the gas chromatography column is essential to ensure accurate quantitation. Agilent’s J&W DB-35ms Ultra Inert column minimizes column activity so difficult and active analytes can be consistently analyzed at trace levels. The use of the mid-polarity DB-35ms UI phase also offers additional selectivity over a non-polar phase, which can assist in resolving potentially co-eluting peaks, or shift a peak of interest away from matrix interferences.
A gas chromatographic system capable of multisignal detection can provide complementary data for identification, confirmation, and quantitation of target analytes from a single injection. This method provides simultaneous detection of OP pesticides by gas chromatography/ mass spectrometry/selected ion monitoring and flame photometric detector in phosphorus mode by splitting the column effluent between the mass selective detector and FPD. The approach chosen here uses a GC/MSD/FPD system to identify and confirm the order of elution for peaks of interest. Once the elution order is established, the chromatographic parameters can easily be transferred to a GC/FPD system. The use of FPD detection without flow splitting is expected to increase sensitivity threefold, further improving lower level detection.
The GC/MS system was also equipped with backflush capability, which shortens instrument cycle time by backflushing late-eluting matrix components through the inlet purge valve. Long bakeout times between injections are avoided by using this technique. Backflushing has the additional benefit of increasing the time intervals for source cleaning by effectively clearing deleterious matrix components from the system.10
An Agilent 7890A GC, combined with an Agilent 5975C GC/MSD equipped with a flame photometric detector and an Agilent 7683B automatic liquid sampler, were used for this series of experiments. A purged two-way capillary flow technology device was used to split the effluent 3:1 to the MSD:FPD. The CFT device also allowed for post-column backflush. Table 1 lists the chromatographic conditions used for these analyses. Table 2 lists the flow path consumable supplies used in these experiments.
Reagents and Chemicals
All reagents and solvents were high-performance liquid chromatography or Ultra Resi grade. Acetonitrile from Honeywell (Muskegon, Mich.), toluene from Honeywell Burdick & Jackson, and acetone from JT Baker were purchased through VWR International (Radnor, Pa.). The 12-component custom pesticide standard was prepared by Ultra Scientific (N. Kingstown, R.I.).