Sample matrix
There are as many sample preparation options available as there are matrices. Some are more regulated than others, but it all starts by looking at the matrix under consideration.
A highly water-based matrix is a different challenge than a fat-rich or protein-containing matrix. The job of sample preparation is to separate the analytes from the matrix and then concentrate the analytes to make them analytically compatible so the required detection limits can be reached.
Think about the nastiest matrix you can. For some, this is a meat sample containing maggots. For others, it might be rotten fish. However, this is not what makes it complicated. The matrix itself, as well as the analytes and their chemical properties, are the base for the right sample preparation.
A lot of errors occur during sample collection, storage, and transportation. There are a lot of possibilities where things can go wrong and where contamination can occur.
Most samples, if not all, must be homogenized so that a representative sample can be taken for analysis. If this is not done in the right environment, for example, using cooled ball mills to prevent heat generation and thus degradation, that’s where the problem starts. You do not want to lose analytes during the whole process. That is critical for the result. For that reason, there are a lot of parameters that need to be taken into consideration depending on what you are looking for.
Sample preparation
If you are looking into solids or oil, liquid-liquid extraction (LLE) is the best first route. You need to get the particles (e.g., matrix) removed and extract the analytes into a more analytically compatible solution. Now, this is universal enough, but is it clean enough?
The analytical system can be easily contaminated and requires a lot of maintenance afterward, thus cleaning becomes necessary. That costs time and resources, not what’s needed in a lab that needs to produce results.
That’s where special cleanups such as solid-phase extraction (SPE) come into place. Sure, this is more costly and labor-intensive, however, it gives you more specific and analytically compatible results. The more specific you can get, the better the detection limits will be. There are a lot of different options and it all depends on the chemistry and properties of your analytes. There are solutions, proven and widely used.
Fig. 1 shows the objectives of sample preparation to clean, concentrate, and the analytical combability.
Fig. 1 Objectives of sample preparation.
Another route might be QuEChERS, which is a solid-liquid extraction (SLE) that may also require an SPE step afterward.
If you have a liquid sample you might dilute, derivatize, and inject, how sure are you that the matrix doesn’t contaminate your analytical system? The better the cleanup is, the better the results.
If the focus is on volatile substances, there are special methods to treat the sample as well. That might be static headspace (HSS), solid-phase micro-extraction (SPME), or, in some cases, purge and trap (P+T).
An additional option is accelerated-solvent extraction (ASE). This technology is especially useful for difficult matrices such as honey or Dioxin-containing samples.
Sometimes it can be necessary to adopt methods, existing or new, to a group or one specific new analyte that was not on the radar screen two years ago. The same strategy applies — matrix and analyte properties determine the sample preparation.
It is always the goal to find a fast, cheap, and reliable method. However, sometimes a little bit more effort is recommended to really accomplish the necessary detection limits, and infant food is a completely different story when it comes to detection limits and sample prep.
Conclusion
Always know what you want to get rid of and what you need to detect, and how low you can go. That is the bottom line for food analysis and sample preparation. It is a good starting point for our experts to find the optimal solution for your analytical challenge.
Helpful links
QuEChERS and ASE:
https://analyteguru.com/webinars/part-1-of-4-tips-and-tricks-for-sample-preparation-using-quechers-a…
QuEChERS:
https://analyteguru.com/resources/the-quechers-three-minutes-experience-screening-targeted-data-anal…
QuEChERS product information: https://www.thermofisher.com/search/browse/featured/de/de/80013164/Quechers%20Dispersive%20Spe
SPE extraction guide:
https://www.thermofisher.com/de/de/home/industrial/chromatography/chromatography-sample-preparation/…
SPE consumables:
https://www.thermofisher.com/de/de/home/industrial/chromatography/chromatography-sample-preparation/…
Chromatography consumables now available online:
https://www.thermofisher.com/de/de/home/products-and-services/promotions/industrial/chromatography-c…