In all Forms the Matrix

Today, Solid Phase Extraction (SPE) has become a widely used sample preparation tool for analytical chemists. This technique was developed as an alternative to extracting liquid (LLE). Historical evidence and some scholars claimed that the first reference literature to the SPE was found in the Bible. But this technique was applied in a late experimental manner, developments that led to widespread use and adaptation to current analytical methods began in the 1970

Initially, SPE was used to concentrate small amounts of organic pollutants present in wastewater samples, but their use has now extended to a wide range of matrices including oil, serum, milk, blood, urine, plant and animal tissues and pharmaceuticals. Simplify the complex sample matrix combined with compound purification, low ion suppression or enhancement in mass spectrometry applications, the ability to segment the sample matrix to analyze the compounds by category, and enrich trace level trace compounds are the main advantages of the SPE.

Mechanism of SPE

To determine the accuracy of the desired analytical concentration in a sample, the quantitative extraction of analyzes from other components in the sample matrix (which may interfere during analysis) is necessary. During the SPE, analyzes in the solution are enriched, separated (or purified) by retaining the absorbent material (solid phase), followed by suction with a suitable rinse. To successfully conduct a SPE process, the mechanical understanding of the interaction between the absorbent and analytical substances of interest is essential. It is also necessary to evaluate their properties based on frantic, polar or inorganic characteristics. The most common retention mechanisms depend on van der Waals forces (non-polar interactions), hydrogen bonding, bipolar forces (polar interactions) and cationic-anionic reactions (ionic reactions). These mechanisms include reverse phase SPE, SPE normal phase, ion exchange-SPE, and polymer-based pipettes. In the reverse phase (SPE), the mobile phase (sample matrix) is polar or moderate phase and stable non-polarity. Interest analysis is usually moderate to non-polar. The normal SPE phase involves a polar analysis, a moderate to non-polarity matrix (mobile phase) and a polar constant (pipette) stage. In the ion exchange technique (SPE), the mechanism of retention depends on the electrostatic attraction between charged functional groups of the analytical and static phase. A polymer-based pipette is used to retain water-loving compounds that contain some water-loving functions, especially aromatics.

SPE Process

Solid phase extraction uses the difference of affinity between an analyte and interferents, present in a liquid matrix, for a solid phase (sorbent). This affinity allows the separation of the target analyte from the interferents.

A typical solid phase extraction involves four steps:

1. First, the cartridge is equilibrated or conditioned with a solvent to wet the sorbent.
2. Then the loading solution containing the analyte is percolated through the solid phase. Ideally, the analyte and some impurities are retained on the sorbent.
3. The sorbent is then washed to remove impurities.
4. The analyte is collected during this elution step.

The method of SPE

The chromatographic bed can be used to separate the different compounds in a sample, to make subsequent analytical testing more successful. For example, SPE is often used for the selective removal of interferences.

The technically correct name for this technology is “Liquid-Solid Phase Extraction,” since the chromatographic particles are solid and the sample is in the liquid state. The same basic chromatographic principles of liquid chromatography that are used in HPLC are also used here, but in a different format and for a different reason. Here, chromatography is used to better prepare a sample before it is submitted for analytical testing.

Application of SPE

SPE technique is a useful tool for many purposes through its versatility. Isolation, concentration, purification and clean-up are the main approaches in the practices of this method. Food structures represent a complicated matrix and can be formed into different physical stages, such as solid, viscous or liquid.

Simplification of Complex Sample Matrix along with Compound Purification

One of the most difficult problems for an analytical chemist is when compounds of interest are contained in a complex sample matrix, such as mycotoxins in grains, antibiotic residues in shrimp, or drug metabolites in plasma, serum, or urine. The large number of interfering constituents or substances in the sample matrix along with the compounds of interest makes analysis extremely difficult.

The first problem to solve is the resulting complexity of the analysis itself due to the presence of so many entities which must be separated in order to identify and quantitate the compound[s] of interest. See Figure 2.

Reduce Ion Suppression or Enhancement in MS Applications

The second problem with complex sample matrices can be seen when we look at mass spectrometer output [LC/MS or LC/MS/MS]. For proper MS signal response [sensitivity], the compound ion must be allowed to form properly. In cases where the formation of the compound ion is suppressed by interferences in the sample matrix, the signal strength is greatly diminished.

We can see this effect in Figure 6. The upper output is the signal for our compounds of interest when injected in a saline solution. The lower trace shows significant reduction in response [> 90% suppression] of these same compounds when they were analyzed in human plasma. For the lower trace, only a common protein precipitation step was performed. This technique does not clean up the matrix interferences that cause the ion suppression, resulting in poor signal response.

Common SPE Applications:

• Pharmaceutical compounds and metabolites in biological fluids
• Drugs of abuse in biological fluids
• Environmental pollutants in drinking and wastewater
• Pesticides, antibiotics or mycotoxins in food/agricultural matrices
• Desalting of proteins and peptides
• Fractionation of lipids
• Water and fat soluble vitamins

Conclusion

The SPE system developed with low-cost biomass has no matrix effects during analysis. The minimum pre-treatment of samples without loss of target solution provided by low-cost additives is an important feature of this technique as the added steps during Sample preparation will increase the time of analysis, solvent consumption, change, and large losses of target and sensitivity. In general, the results obtained during this study enable a new analytical methodology for the routine analysis of methylene blue in wastewater samples. The SPE system developed during this study can be a good alternative to commercially available SPE systems. 

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