1. Solid-Phase Extraction:
What is it and what it does?
Solid-phase extraction (SPE) is one of various techniques available to an analyst to bridge gap between sample collection and analysis.
Filtration
homogenization
precipitation
chemical reaction
solvent exchange
concentration
matrix removal
solubilization

2. These are just a few of the available tools that may be used individually or in combination to get the sample into a form compatible with the analytical instrument required for analysis.
Solid-phase extraction is seldom used without other sample preparation steps, such as dilution or pH adjustment.
However, the action of performing SPE often simultaneously completes several other preparation goals.
Moreover, SPE has been coupled with an analytical technique or another preparation method to enhance the benefits of each separate technique.

3. BASIC STEPS OF SOLID-PHASE EXTRACTION
Liquid-liquid extraction (LLE) is excellent starting-point for interpretation of SPE.
In LLE ,sample is agitated in presence of an extracting solvent (immiscible).
When sample-solvent mixture settles, one layer should contain most analyte.
Sample passes into extracting solvent; equilibrium is established between two layers.
Equilibrium is described by partition coefficient for analyte, which is ratio of concentrations for analyte in the two liquids.

4. Very high partition coefficient means all compound of interest migrates into extractant
Low coefficient means very little of compound of interest moves into extractant.
For most liquid-liquid extractions, properly chosen conditions will result in most of analyte found in extracting solvent, implying that partition coefficient has been maximized.
This occurs when analyte interacts better with extracting solvent than with sample matrix.
In other words, the extracting solvent provides a better environment for the analyte.

5. Stages of SPE
1. Retention
An extraction solvent is substituted by a solid surface.
Then analyte distributes between liquid phase and solid surface,
Either by
simple adsorption to solid surface
or by penetration of molecules into outer layer on that surface,
Equilibrium is set up similar to LLE.
Can be defined that distribution by a coefficient, KD which indicates what fraction of analyte remains in solution and what fraction is adsorbed on or entered the solid phase.
This distribution coefficient should be defined in terms of activities of the analyte in either phase. However, convenience dictates that concentrations are used.

6. In a column packed with sorbent, it is no longer a "batch" partition like LLE.
Instead the process more closely parallels distillation and Equation 1 converts to
Where
V0 and VR are void volume and retention volume respectively.
If analyte is entirely trapped on solid surface, distribution coefficient will be very large.

7. Chromatographic extraction
in contrast to elution chromatography, is best described as a pseudo-equilibrium process whereby the analyte is adsorbed completely on solid surface.
Most Solid-Phase Extractions simply require a liquid sample to be passed through a bed containing sorbent particles onto which analyte will be retained.
It should be possible to find a technique for identifying & quantifying compound of interest while it is retained on the solid surface,

8. 2. Rinsing or Washing
During retention step, many compounds in complex sample may have been retained on solid surface at same time as compound of interest.
Likewise, at elution it is likely that some of these co-retained compounds will be eluted with compound of interest.
To minimize interferences of these undesirable compounds created during analysis stage, we may add one or more wash steps between retention and elution, to attempt to remove or rinse them out.
Each wash step involves another distribution between analyte and the co-retained species, solid surface and liquid that is passing over it.
Control of each step is accomplished by careful selection of wash, elution, and sample loading conditions.

9. 3. Elution
It is necessary to find a way to remove and collect analyte.
Some advanced & unusual techniques for removal from sorbent & collection of analytes could be applied, such as using a stream of supercritical gas to desorb the analytes.
But in a typical solid-phase extraction a simple liquid does the job very well.
When a liquid provides a more desirable environment for analyte than solid phase, then compound of interest is desorbed & collected in liquid in a process called elution.
k' between the concentration of analyte on solid surface and in eluting liquid that is very small.
One way to view solid phase is as an intermediary between sample matrix and elution solvent.
This highlights a very important difference between LLE and SPE.

10. 4. Complete Solid-Phase Extraction
The potential complexity but also the power of SPE is highly appreciable because each step performed can be controlled.
One can select the sorbent type (the solid phase that does the extraction)
One can manipulate the sample to enhance retention of one chemical species over another
One can select an elution liquid that has properties desirable to analyte, convenient for employed method of analysis, or for subsequent sample handling
In between one may use any number of wash steps to ensure that final extract is of desired purity.

11. An elution solvent may be used which is miscible with sample in a solid-phase extraction, because elution solvent and sample never come into direct contact.
Thus, sample may be aqueous but SPE elution solvent may be methanol, which is miscible in all proportions with water.
Such a scheme, impossible in a LLE, is not only possible with SPE - it accounts for majority of all solid-phase extractions!

13. Goal,
Not to discard compounds of interest but to collect them
Concentrate analytes from a sample and removing compounds need not be analyzed.
Strip the sample away from the analyte (compound to be analyzed) and to put that analyte into a small volume of a different liquid.
In summary, achieve any or all of three things:
1) Concentration
2) Removal of unwanted molecules from the sample (clean-up)
3) Removal of the sample matrix-solvent exchange

14. A. The Objectives of Solid-phase Extraction
1. Concentration
To measure the quantity of a compound accurately one needs to concentrate it as much as possible.
This will ensure the largest response from detection system and will minimize errors in precision caused by background noise.
As shown in scheme (Figure 2), one can identify three ways in which SPE can help us.
1) Pass a large volume of sample through smallest bed of sorbent that will completely retain all of compound of interest.
2) Elute compounds of interest in the smallest volume of solvent possible.
3) Elute compounds of interest in a solvent that permits easy concentration, such as a volatile organic solvent.

16. In order to optimize each of these processes, one needs to know more about
Capacity of a solid-phase sorbent and how that relates to sample type,
Sample pH,
Ionic strength,
How properties of different sorbent types affect retention & elution steps for compound of interest.
This helps identifying solvents that give excellent elution while permitting further concentration, if this is required!
Note that concentration requires analyte to be retained on sorbent bed.
Occasional methods utilizing SPE simply retain sample interferences on sorbent bed and do not retain analytes, thus precluding concentration during the SPE process.

17. 2 Clean-Up
Concentration of an analyte is pointless if it can not be measured in a final concentrated solution.
Most common reason for this is that extracted sample contains interfering compounds.
These are components of sample that mask analyte during analysis
for example, when two or more compounds co-elute in a chromatographic experiment like gas or liquid chromatography.
Sample preparation permits removal of such interferences before analytical separation step.

18. Sample preparation permits removal of such interferences before analytical separation step.
Figure 3 shows a hypothetical example of how a chromatogram would look before and after a SPE clean-up.
The cleaned-up extract gives clearly identifiable signals from extracted components in sample. Such a chromatogram is readily interpreted & the quantities of desired analytes present in that sample are easily measured.
Clean-up may be achieved either by retaining analyte on a solid phase sorbent or washing out interferences or by retaining interferences and washing out analyte.

20. Sample Matrix Removal/Solvent Exchange
Many analytical instruments (e.g., gas- or liquid-chromatographs, NMR-, or IR-spectrometers) require sample to be analyzed in a specific environment.
e.g., injection of aqueous sample onto GC would ruin delicate instrument.
One would also, coincidentally, be lucky to see any chromatogram at end of experiment.
Whether sample is drinking water or whole blood, urine or face cream, one needs to remove sample matrix, & convert sample into a form compatible with instrument to be used.
A significant advantage of SPE over LLE is that solvents that are miscible with sample matrix may be used to elute the analytes.

21. Thus, a human plasma sample containing compounds that are to be analyzed by reversed-phase high-performance liquid chromatography (i.e., using an organic-aqueous mobile phase) may be retained onto a SPE sorbent from the water-based sample, & can then be eluted with a water-organic mixture
e.g. H2O-Methanol eluent can be injected directly into a reversed-phase HPLC system.
corresponding LLE commonly leaves analytes in a water-immiscible solvent that must be dried down & the residue reconstituted in a suitable solvent before analysis may begin.

22. B. Constraints that operate during Solid-Phase Extraction
The three aspects of a sample preparation using SPE can be represented by a triangular diagram as shown in Figure 4. It may be possible to achieve all three goals:
Satisfactory clean-up
Sufficient concentration
Efficient matrix removal in one simple SPE extraction.
However, it is more common to have to compromise. To understand why?

23. Consider the following:
For a given sample matrix there is an optimum sorbent that will give excellent retention & excellent elution for one specific analyte.
If one tries to extract more than one analyte at a time (e.g, a screen of drinking water for a range of environmental pollutants, or of urine for a parent drug & various metabolites of that drug) it is unlikely that one sorbent will be the best choice for every one of these compounds.
The wider the range of analytes, the better the chance that several may retain or elute poorly from a given sorbent.
Another explanation can be found if we consider the process of retention and elution.

25. To achieve good concentration one wants to pass a large volume of sample through the extracting sorbent.
Must have strong retention under these conditions to ensure all analyte is retained, as no analyte "breaks through" the sorbent bed. Strong interaction implies, however, that elution will not be easy, so a larger-than desired volume of elution solvent may be required to fully desorb analyte.
To help analyst overcome such problems, manufacturers of SPE devices have developed an extensive range of sorbents, ones that utilize varying strengths of Van der Waals (non-polar), hydrogen bonding or dipolar polar) or coulombic (ion exchange) interactions. A select range is shown in Table 1.
The variety of sorbent types adds complexity to SPE but it also adds power..