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Modifying QuEChERS for complicated matrices- Soil and Sediment

7 Dec 2020

In previous blog posts for this series on QuEChERS, we discussed QuECHERS dSPE Selection, Modifying QuECHERS..-Dry Samples, Modifying QuEChERS..-High Fat Samples, and Modifying QuEChERS..-High Sugar & Starch.

Once again, I suggest using the following information from the official QuEChERS website (QuEChERS.eu), maintained by CVUA Stuttgart, specifically QuEChERS: About the method.

References may be made to information from a table that indicates indicates dSPE Primary and Secondary Actions, which originated in previous posts from this series. You may find it useful as you read the discussion below.

 

Soil and Sediment Samples

Soil and sediment samples present challenges for an analysis in many different ways. They contain a mixture of inorganic and organic ingredients and can be highly variable, depending on their source. The organic components usually contain a certain amount of lignins and complex carbohydrates, lipids, waxes and resins, flavonoids and other pigments, as well as carboxylic acids to name a few of the chemical classes represented. Of course, we all are familiar with “dirt”, so the potential to leave residue in analytical equipment is fairly well understood. Failure to properly prepare samples can result in considerable downtime for cleaning and maintenance procedures. Although QuEChERs was not originally designed for use with these matrices, the technique has been used with some success for pesticide analysis and other environmental pollutants. At this time, there are really not any well-established methods and virtually no standardization for QuEChERs with this matrix, so the literature reflects many different versions of modifications that could be made to use the QuEChERs technique. Although the reported methods in literature vary widely, I will point out and discuss some common themes.

Stage 1- Extraction into organic solvent

Extraction of pesticides and organic pollutants from soil, sediment and related matrices can be difficult because these analytes can become chemically bound to the matrix, making it difficult to move them into an organic solvent layer.  For this reason, traditional extraction methods that are fairly exhaustive, such as Soxhlet extraction or Pressurized Fluid extraction (see EPA SW-846 Methods 3540 and 3545) are still very commonly used. Even with these challenges, the QuEChERS technique has been successful in many studies and offered as an acceptable alternative to the more traditional methods. An article by Vera et al. (https://www.degruyter.com/view/journals/sampre/1/2013/article-p54.xml) gives a nice overview of various QuEChERS modifications that have been successfully used for a variety of analytes with soil or sediment matrices. For best results, I recommend that a method be developed or optimized for each specific combination of analytes and soil type. The following are common trends observed with respect to QuEChERS modification for extraction of organic analytes from these matrices.

 

Stage 2- Cleanup of Extracts

As mentioned previously, soil and sediment contain a tremendous variety of materials as sample matrix. Composition also varies according to the source of sampling, so it is not surprising that a great assortment of cleanup techniques can be found in current literature on the topic. Some analysts do choose to forego a cleanup step for this type of sample if interfering molecules are not visible in their analytical results. However, this results in the need to frequently trim and/or replace LC and GC columns, as well as associated consumables and tubing. It can also create the need to frequently clean instrument injection ports, valves, and detectors, which can be costly in terms of downtime, part replacement and repair expenses. This is particularly true when using mass spectrometry.

Traditional approaches for cleanup of soil extracts represented in EPA SW-846 methods involve a combination of techniques, possibly including acid-base partitioning (Method 3650) as well as GPC (Method 3640) or adsorption column cleanup (Florisil, alumina or silica/ Methods 3630, 3620 and 3630). A QuEChERS dSPE procedure could possibly take the place of some of these procedures, depending on the analytes and desired sensitivity.

dSPE with MgSO4  and PSA, often with C18-EC – Representing the classic QuEChERS approach, several scenarios are seen throughout the literature, but PSA seems essential in almost all cases. A combination of MgSO4 and PSA (without C18 or GCB) has sometimes been successful, but a combination of MgSO4, PSA and C18 seems to be very common and is the predominant product sold by chromatography vendors for this purpose. It is likely this combination works well due to the known interactions with humic acids, proteins, lipid- related and carbohydrate-related molecules known to be present in the organic matter. Referring to our table of dSPE Primary and Secondary Actions, it has been shown that C18-EC has an affinity for lipid related molecules and a secondary affinity for starch and proteins, while PSA has affinity for humic acids and many of the carbohydrate-related molecules. In most cases, it seems that these sorbents are sufficient also to remove pigments, although GCB could be considered in some cases if needed, and if planar analytes are not an issue. Some analysts have successfully used PSA with no C18 or GCB and a smaller handful have used C18 alone. Some of these variations were usually due to certain target analytes that were acid sensitive and/or planar molecules, although the reasoning was not always clear.

Here are some examples where QuEChERS dSPE technique was successfully used in this manner.

 

Cooling/Freezing the QuEChERS extract- Similar to what we discussed in previous blog posts for other sample types, cooling or freezing soil extract may also be helpful. Here is an example.

 

SPE cartridge (cSPE) or online cleanup- In some cases, more traditional methods using cartridge SPE may provide more removal of the interferences due to the option to use more sorbent and/or the ability to better control the exposure time and surface area.  The best choice in sorbents depends on the nature of the analytes and desired sensitivity. Variation in type of soil or sediment can also be a factor. Online SPE could also be coupled with LC analysis for ease of automation.

Here's an example of this technique.

Cartridge SPE:

 

On-line SPE (for NSAIDS):

 

Additional Resources:

Thanks for reading our discussion of soil and sediment matrices. Please feel free to add your comments and share your experiences. We look forward to further discussion.