Extraction of Planar Pesticides from Spinach and Fresh Oregano Using QuEChERS Methodology With a Novel Carbon Adsorbent

By: Michael Ye, and Jennifer Claus, Reporter US Volume 34.2

Introduction

Although vital to plant life, the presence of chlorophyll in GC/MS analysis is highly problematic. Chlorophyll, a pigment of high molecular weight, is known to quickly cause fouling of injection liners and GC columns due to buildup at the injection port and GC column head. To preserve the lifetime of the column, it is imperative to remove chlorophyll prior to analysis of desired analytes by GC/MS.

Graphitized carbon black, or GCB, has been historically beneficial for removing chlorophyll pigment interferences. Traditional GCB possesses a planar, graphitic surface, allowing for removal of nearly 100% of chlorophyll from green samples. Unfortunately, when analyzing planar aromatic compounds such as hexachlorobenzene and chlorothalonil in green samples, poor analyte recovery is observed. The interaction between the planar structure of the GCB and these planar pesticides causes unwanted retention on the carbon sorbent.

Recently a “Quick, Easy, Cheap, Effective, Rugged, and Safe” (QuEChERS) cleanup method has been developed with the use of Supel™ QuE Verde, a sorbent combination containing Z-Sep+, primary-secondary amine (PSA), and an improved GCB. This sorbent combination has been shown to provide improved recovery of planar pesticides over current QuEChERS sorbents while maintaining sufficient color removal in high cholorphyll matrices. The GC/MS analysis of select planar pesticides from spinach and oregano extracts is described herein.

Experimental

QuEChERS extraction and cleanup was performed in a similar fashion to that outlined in the AOAC International Official Method 2007.01.1 Organic spinach and freshly picked oregano were extracted with acetonitrile. Figure 1 describes the extraction and cleanup procedure for use with the 2 mL tubes. When using the 15 mL tubes, 8 mL of acetonitrile extract was transferred to the cleanup tubes. The acetonitrile extract was spiked with a mixture composed predominantly of planar pesticides in acetonitrile to a concentration of 50 ng/mL. Samples were spiked after extraction to study the performance of the cleanup sorbents only in relation to the planar pesticides. Extractions were performed in triplicate. Analysis of the final extracts was done by GC/MS using the GC conditions listed in Figure 1.

Planar Pesticides in (a) Spinach and (b) Fresh Oregano After Cleanup with Supel QuE Verde

Figure 1. Planar Pesticides in (a) Spinach and (b) Fresh Oregano After Cleanup with Supel QuE Verde

Results and Discussion

Background was evaluated by analysis of the final extracts by GC/MS in selected ion mode (SIM). The chromatograms of the spiked extracts are illustrated in Figure 1. In the analysis of the planar pesticides in selected ion mode (SIM), the Supel QuE Verde cleanup removed enough matrix interferences to easily identify and quantitate all 12 residues in a mixture containing planar pesticides (Figure 1).

Figure 2 shows the spinach extract before cleanup, as well as the spinach and oregano extracts after cleanup with Supel QuE Verde. Removal of the majority of the green pigment is observed. Total chlorophyll removal was evaluated using a spectrophotometer by measuring absorbance at 664 nm, 647 nm, and 630 nm, and comparing the processed extracts to the initial acetonitrile extract of each plant material (Figure 3). In all cases, chlorophyll removal was 95% or greater.

Average analyte recoveries obtained from 50 ng/mL spiked spinach and oregano extracts using 2 mL Supel QuE Verde cleanup tubes are summarized in Figure 4. Recoveries were in the range of 70% to 120% for all of the planar pesticides tested in both matrices. Reproducibility, determined as % RSD for n=3 spiked replicates, was very good for both matrices. Each pesticide exhibited a %RSD value less than 5% with the exception of chlorothalonil, which had a %RSD value of 20%.

QuEChERS Extracts

Figure 2. QuEChERS Extracts of (a) Spinach Extract Before Cleanup, (b) Spinach Extract After Cleanup with Supel QuE Verde, and (c) Oregano Extract After Cleanup with Supel QuE Verde

 

Percent Total Chlorophyll Removal by Supel QuE Verde From Spinach and Oregano Extracts

Figure 3. Percent Total Chlorophyll Removal by Supel QuE Verde From Spinach and Oregano Extracts

 

Average Percent Recoveries From Spinach and Oregano Extracts

Figure 4. Average Percent Recoveries From Spinach and Oregano Extracts Spiked at 50 ng/mL After Cleanup with the 2 mL Supel QuE Verde Tube (n=3). Note: Recoveries of planar pesticides with traditional carbon sorbents are typically below 70%

Conclusion

The general consensus among food analysts regarding analytical method performance requires pesticide recovery to be between 70% and 120%, and chlorophyll removal to be 95% or greater. Because of the strong interaction between planar pesticides and the planar surfaces of GCB, there remains a compromise between color removal and analyte recovery, especially regarding the most planar pesticides such as hexachlorobenzene and chlorothalonil. Supel QuE Verde combines a novel carbon with zirconia coated silica (Z-Sep+) to provide an optimum balance between planar pesticide recovery and color removal. As shown in this application, QuEChERS cleanup using Supel QuE Verde, may be used prior to GC/MS analysis to provide sufficient chlorophyll removal while maintaining excellent recovery of planar pesticides from spinach and oregano matrices.

Materials

     

 References

  1. AOAC Official Method 2007.01, Pesticide Residues in Foods by Acetonitrile Extraction and Partitioning with Magnesium Sulfate

 

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