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Modifying QuEChERS for complicated matrices- High Sugar & Starch Samples

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  • Nancy Schwartz
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  • #Method Development
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  • #Food & Agriculture
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In previous blog posts for this series on QuEChERS, we discussed QuECHERS dSPE Selection, Modifying QuECHERS..-Dry Samples and Modifying QuEChERS..-High Fat Samples

Once again, I suggest to use the following 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

References may be made to information from a table that 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.

High Sugar and Starch Samples

Samples containing high amounts of carbohydrates may present unique challenges for an analysis. Molecular size of these interfering molecules can vary considerably, ranging from small monosaccharides like glucose to very large polysaccharides like starches. Although size and some of their properties vary, they are all composed of repeating units of one or more monosaccharides. Medium size polysaccharides (3-10 sugar units), also called oligosaccharides, can exist as glycans, where they are linked to a lipid, amino acid, or protein. The largest polysaccharides (>10 sugar units), include starch and cellulose, which more closely resemble a polymer because they are very long chains of repeating monosaccharide units. All of these molecules have the potential to wreak havoc with GC and LC analyses for various reasons. They may produce visible chromatographic interference and often they can also inflict damage to LC and GC systems by accumulating residue in injection ports, valves, tubing and sometimes in columns. To reduce the potential of instrument downtime and to reduce the cost of column replacement, it is always best to eliminate these types of materials from sample matrices as much as possible. Here are some techniques that have been used with Quechers to accomplish this.

dSPE with PSA/MgSO4 and possibly C18-EC – This is a demonstration of the classic QuEChERS technique, where the analyst would choose MgSO4 and PSA to remove the simple sugar molecules. PSA is indeed powerful in its interaction with small sugar molecules due to its ion exchange properties. However, it is limited in its interaction with larger and more complex molecules containing sugar, since they are less reactive, less polar and sometimes have little or no solubility in water. In the table shown above, the interaction with longer chain carbohydrate molecules and starches is noted as a secondary action for the C18-EC. Consequently, when more complex sugar molecules and starches are present, C18-EC sorbent can be the perfect complement to PSA to remove this matrix. For some sugary matrices that are dry, such as honey, adjustments to water content made in Stage 1 extraction also help to make the dSPE step go more smoothly. Here are some examples of successful use of these techniques. (Please note that some of these also use GCB, but its target for removal is pigment. Also, a quick reminder to use caution with GCB for samples containing planar compounds.)

 

Cooling/Freezing the QuEChERS extract- Similar to what we discussed in the previous blog post about high fat samples, the same approach mentioned on the QuEChERS website (https://www.QuEChERS.com/pdf/cleanup.pdf) can sometimes be used to help remove sugar-related or  starchy sample matrix.  Unfortunately, I was unable to find any examples where this was done successfully, other than the Quechers site itself.  I would be very interested to hear some readers share their experiences in this regard.

SPE cartridge (cSPE) cleanup- In some cases, the amount of sugars or carbohydrates is too high to efficiently remove with the above mentioned techniques.  Using cartridge SPE gives the analyst an option to use more sorbent to more effectively remove this interference. Sometimes alternate sorbents such as silica or florisil may be used as well.

Here are some examples of this technique.

 

Matrix matched standards- Sometimes despite extensive measures to remove matrix from a sample, there may be enough residue remaining to affect the analysis. This is often the case with more sensitive analytes, such daminozide. In order to mimic the same effect for calibration standards, they can be prepared to contain the same sample matrix.

Here are some examples of this technique.

 

Additional Resources:

 

Thanks for reading our discussion of high sugar and starch sample matrices. Please feel free to add your comments and share your experiences. We look forward to further discussion.  Also, please look for the next post in this series on soil samples.