Modifying QuEChERS for complicated matrices- Dry Samples

Before getting in to this discussion, I recommend reading my previous blog post first regarding classical applications for methods based on Quechers. https://blog.restek.com/quechers-dspe-selection-which-one-is-best/

QuEChERS methods were originally written to analyze pesticides in fruit and vegetable matrices, most of which have high water content and low fat content. More recently, the technique has been used for a greater variety of food and agricultural products, as well as other environmental matrices. It has also been adapted for analytes other than pesticides in some cases. We will discuss some of the possible modifications you may need to make for complicated matrices. For all of the sample types we will discuss, the best general references I can give are from the official QuEChERS website (quechers.com), maintained by CVUA Stuttgart, these specific documents:

For Extraction (Stage 1): https://www.quechers.com/pdf/reality.pdf

For dSPE Cleanup (Stage 2): https://www.quechers.com/pdf/cleanup.pdf

For greater detail, I found this reference useful: https://www.degruyter.com/view/journals/chem/open-issue/article-10.1515-chem-2015-0109/article-10.1515-chem-2015-0109.xml

For many samples, particularly food products, you may find it helpful to use the USDA database at https://fdc.nal.usda.gov/ to obtain a listing of content for water (as well as protein, total lipids, fatty acids, carbohydrates, sugars, and cholesterol).  We will discuss techniques for dry sample matrices in this blog post. Other types of sample matrices will be discussed in subsequent posts.

For samples with little or no water content, water must be added before using Quechers extraction salts. For these samples, reduce the sample weight from 10 g to 5 g or less and add 10 mL of water prior to adding acetonitrile and QuEChERS extraction salts. (For those using the AOAC QuEChERS method, the sample size is reduced from 15 g to 7.5 g or less and 15 mL of water is added.)  The sample weight should be adjusted according to the amount of chromatographic interference anticipated (or how “dirty” the matrix is perceived to be). Here are several examples of this technique. In some cases, such as cannabis, you might see total water volumes less than or greater than 10 ml, but the sample weights are adjusted accordingly as well.

For samples that contain a little bit of water but less than 80%, the amount of water can be adjusted accordingly to estimate a combined water content of 10 mL.  Some good examples of this technique are shown below.

Additional Resources:

This concludes discussion of dry sample matrices. Please look for the next post on samples containing high amounts of lipids and waxes.

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