Analyzing pesticides in herbal tea using QuEChERS and GCxGC-TOFMS of course!

Herbal tea, a non-caffeinated drink made from plants, herbs, or spices has been used throughout history for its potential medicinal benefit.   Various herbal material is mixed depending on the desired medicinal or flavor properties for the tea.   As with any plant based commodity, there is the potential for pesticide residues to remain in the final product.   Herbal tea also falls into a gray area of pesticide regulation because it can also be viewed as a dietary supplement.

It can be very challenging to detect trace levels of pesticide residues in dried plant material found in herbal tea.  The extract, even after cleanup can contain a large amount of coextractive material that can completely overwhelm the target pesticides, making trace detection very difficult. Furthermore, nonvolatile material not removed during extract cleanup deposit onto the inlet and column requiring more frequent maintenance to be performed (Figure 1). Luckily we have been down this road before and came out the other side successfully (See Dietary Supplements and Tobacco application notes).  When encountered with trace level pesticides and difficult food based commodities we employ the QuEChERS methodology and evaluate and quantitate the samples using GCxGC-TOFMS.

The tea samples were first ground to a powder and then we weighed 1 g of tea and added 10 mL of water to hydrate the sample.  After letting that sit for 30 min, we went through the EN QuEChERS extraction procedure.  The extracts were cleaned up using dSPE tubes containing 150 mg MgSO4, 50 mg PSA, 50 mg C18, 7.5 mg GCB.

The primary column was a 1 m x 0.25 mm Rxi Guard column connected to a 30m x 0.25mm x 0.25µm Rxi-5ms.  The guard column (aka retention gap) allowed for better solvent focusing and improved the peak shape of the early eluting pesticides while protecting the analytical column (Figure 2). The secondary column was a 1 m x 0.25 mm x 0.25 µm Rtx-200.  All of the columns were connected using the SGE SilTite µ-Union.  For quantification, I used our new QuEChERS Performance mixes which cover a range of pesticide volatility and overlapped with many of the GC amenable pesticides commonly found in tea or the other ingredients included in the herbal teas.  The herbal tea “Wildberry Zinger” had the most incurred pesticides of the teas that we evaluated (Figure 3 and Table I).

After 28 injections of tea samples, nonvolatile residue was present on the liner.  After changing the liner GC performance was restored.

After 28 injections of tea samples, nonvolatile residue was present on the liner. After changing the liner, GC performance was restored.

The use of the 1 m retention gap allows for better solvent focusing and improves the peak shape of the early eluting pesticides while protecting the analytical column.

The use of the 1 m retention gap allows for better solvent focusing and improves the peak shape of the early eluting pesticides while protecting the analytical column.

Although the herbal tea is very complex, the GCxGC-TOFMS helps to separate the matrix from the pesticides of interest.

Although the herbal tea is very complex, the GCxGC-TOFMS helps to separate the matrix from the pesticides of interest.

Percent recoveries for a subset of the pesticides evaluated.  We achieved good recoveries at both spike levels and found a few incurred pesticides too.

Percent recoveries for a subset of the pesticides evaluated. We achieved good recoveries at both spike levels and found a few incurred pesticides too.

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