The Use of QuEChERS

QuEChERS is a method now commonly used for pesticide residue analysis. It is an acronym for:-

• Quick
• Easy
• Cheap
• Effective
• Rugged
• Safe

Traditional Multiresidue methods are considered inefficient. The main disadvantages are they need large sample sizes which implies a ‘macro-approach’, there are often too many analytical steps involved, some of which are highly complicated. The consequence of this is they are wasteful of sample used and consume a lot of solvents and materials in the process of analysis. The other consequence is the length of time required for analysis and they are troublesome. the end result is that such methods used routinely are environmentally critical, expensive and with the added complexity, prone to error.

The QuEChERS-method was developed by an analytical scientist, Michelangelo in 2011 who was analysing for veterinary drugs in animal tissues. Having established it as a method that was ideal for extracting polar compounds and basic compounds especially, he then applied it to pesticide residue analysis in various plant materials. The QuEChERS method is regarded as a multiclass and multiresidue method (MRM) which is ideal for pesticide analysis especially in matrices with a high water content – between 80 and 95%.

The method is now formally adopted as AOAC Method 2007.01 – Determination of pesticide Residues in Foods by Acetonitrile Extraction and Partitioning with Magnesium Sulfate.

The method is validated and its veracity endorsed for the measurement of pesticides in dry foods especially such as dried fruits, cereals, fatty foods such as meat and fish, and in tea.

Acetonitrile is the solvent of choice because it can extract a very broad range of organic compounds without co-extracting lipophilic and fatty materials although there is a small amount.

The method involved the following:-

1. Sample Preparation and Extraction: all materials to be tested are ground to a uniform size, suited to extraction with a defined quantity of acetonitrile. The extract is centrifuged.
2. Sample Extract & Clean-Up: The extract is cleaned up using DSPE. These are bulk drying salts and dSPE (dispersive solid phase extraction) sorbent packings. These materials removed any excess water and unwanted contaminants. The extract is then shaken to ensure uniform dispersion before centrifugation. This is usually through vortex mixing of the sample in an Eppendorf tube for 30 seconds before centrifuging at 3000 rpm for 5 min.
3. In some cases, the clean-up stage can vary by using freezing out. This is where the acetonitrile phase is transferred to a centrifuge tube and stored for a minimum of 2 hours in the freezer. This causes any fats and lipids, wax, low-molecular weight sugars and extraction materials which have low solubility in acetonitrile. The dSPE stage then follows on.

The dSPE stage used drying agents such as PSA (primary secondary amine) with MgSO4 which is ideal for the majority of samples. The acetonitrile extract is transferred into a polypropylene centrifuge tube which contains PSA and magnesium sulphate where it is agitated for full dispersion. The dSPE approach has the advantage over SPE or solid phase extraction because it is completed in a few minutes as opposed to being ready in 1 hour.

There are d-SPE materials which use PSA, MgSO4 and ODS too for removing fats and lipids. This type of clean-up material is recommended for extracts containing more than 50mg of lipids. If GCB is used instead of ODS, then it removes carotenoids and chlorophyll. The approach is ideal for tomatoes, peppers, green leafy foods etc.

4. Extract concentration and Make-Up: The extract is concentrated in a suitable evaporator for samples between 2 and 5 ml. It is made up to the required volume in a suitable solvent. If gas chromatography is considered, n-hexane is used and for liquid chromatography it is methanol. The analysis is either by GC/MS (mass spectroscopy) or by LC/MS.