A variety of samples is analyzed at PPD® Laboratories’ GMP lab in Middleton, Wisconsin, using methodologies that a scientist would consider to range from being pretty elementary — such as the measurement of pH or light absorbance — to highly complex — such as chromatography in tandem with high-resolution mass spectrometry. In a perfect world, these samples would be directly compatible with these instruments. However, in reality, the samples may need to be processed prior to analysis for a number of reasons, such as:
- Remove extraneous substances that would interfere with the analysis
- Enrich the concentration of target compounds to allow for trace level detection
- Transfer the target compounds from the sample matrix into one that is compatible with the instrument
Techniques commonly employed for these purposes include liquid/liquid extraction (LLE), evaporation/reconstitution, static headspace extraction and filtration/centrifugation.
Another technique that can perform these tasks, but is not used as extensively, is solid phase extraction (SPE). What’s interesting to note is that depending on the sample matrix and the task to be performed, SPE can provide several advantages over the other techniques, most notably LLE, to which SPE is most comparable.
SPE offers several advantages over LLE due to major differences in their respective operating mechanisms.
First, SPE does not have the same concerns with emulsion formation that can hinder LLE. This is a result of SPE being performed not with a liquid sample phase but a solid extraction phase, which eliminates the presence of incompatible solvents (as are used in LLE).
Second, SPE is less labor intensive than LLE, which can be attributed to the mechanism of the technique that allows the compounds of interest to be extracted from the sample in a single loading step.
Furthermore, SPE is more easily automated, which increases the potential to nearly eliminate manual labor. In contrast, LLE involves a series of lengthy manual shaking and collection steps, as well as pooling and evaporating of the extracting phases. Finally, SPE is better suited to enriching the concentration of the target compounds in a given sample.
Despite the advantages that are possible using SPE, some potential limitations do exist and must be properly planned for.
First, compounds that can be processed by SPE must be compatible with the mode of chromatography employed. Considering the most commonly used mode—reverse phase—relatively non-polar compounds are isolated from a polar (usually aqueous) sample. This makes this particular mode less ideal for the extraction of highly polar compounds. This issue can be remedied by switching the mode of SPE to normal phase or ion exchange, or by selecting a different methodology altogether, which actually is a better choice for the polar compound of interest and/or sample matrix.
The second potential limitation of SPE is the loss of a compound of interest during the loading of the sample onto the sorbent, more commonly known as breakthrough. Because breakthrough is directly related to retention, it is most commonly encountered with compounds least suited to the chosen mode. Breakthrough can be remedied by optimizing the procedure to reduce the amount of sample loaded onto the sorbent, and/or by increasing the mass of sorbent, which in turn increases the capacity factor.
In summary, SPE provides a straightforward and robust means for processing samples that are too dilute, insufficiently pure and/or in an incompatible matrix for analysis by various analytical techniques. From our perspective, even though SPE and LLE can be used for the same purposes, SPE is less laborious, not susceptible to problematic emulsions and better suited to enriching the concentration of target compounds for trace level analysis.
Steve Zdravkovic is a research scientist in the PPD® Laboratories GMP lab.