Abstract
New Developments in QuEChERS methodology
Since its introduction, the QuEChERS method has been readily accepted by many pesticide residue analysts. Some modifications to the original QuEChERS method had to be introduced to ensure efficient extraction of pH dependent compounds (e.g. phenoxyalcanoic acids), to minimize degradation of susceptible compounds (e.g. base and acid labile pesticides) and to expand the spectrum of matrices covered. Buffering with citrate salts has been introduced in the first extraction/partitioning step to adjust the pH to a compromise value of 5 to 5.5, where most acid and base labile pesticides are sufficiently stabilized. To improve stability of base-labile compounds in the sample extracts, a small amount of formic acid is added to the final extract after PSA cleanup, while acidic pesticides are directly analyzed from the raw extract before PSA cleanup. When dealing with matrices having a high load of chlorophylls, the cleanup procedure is improved by adding GCB to the PSA at amounts, where some residual chlorophyll remains in the final extract. Dry commodities such as cereals, dried fruits or teas require the addition of water prior to extraction to weaken interactions of pesticides with the matrix and to ensure adequate partitioning. Even commodities with a high lipid load, such as avocados or plant oils can be employed. However, due to a partition to the lipid phase, highly non-polar pesticides give relatively low but typically consistent recoveries (< 70 %). Co-extracted lipids in the extracts can be removed, to a high degree, by a freezing-out step or a C18 cleanup. Excellent cleanup results have also been achieved using a mixed mode sorbent containing C18 and amino functionalities. Two inter-laboratory ring tests were performed in 2005 using the modified method, leading to very good recoveries and repeatabilities for numerous pesticide/commodity combinations at different spiking levels using GC-MS or LC-MS/MS.