New sorbent materials for solid-phase extraction (SPE), dispersive solid-phase extraction (dSPE), solid-phase microextraction
(SPME), and other solid–liquid extraction methods continue to be developed both in academia and the commercial sector. Here,
we cover some areas of significant development, including molecularly imprinted polymers (MIPs), dSPE for QuEChERS, nanomaterials,
and mixed-mode and silicone monolith sorbents.
Solid–liquid extraction is probably the most popular method for sample preparation in the chromatography laboratory. Solid–liquid
extraction techniques such as solid-phase extraction (SPE), solid-phase microextraction (SPME), stir-bar sorbent extraction
(SBSE), and dispersive solid-phase extraction (dSPE) continue to see increased application in diverse markets such as environmental,
food and food safety, forensics, and the life sciences. As sample sizes are continually decreased, more selective sorbents
are required to eliminate matrix compounds that may cause interference in the analytical measurement. With the increased use
of liquid chromatography–tandem mass spectrometry (LC–MS–MS) and gas chromatography–tandem mass spectrometry (GC–MS–MS), the
potential for ion suppression and ion enhancement, which affects signal intensity, continues to draw attention to sample preparation
technologists. In this instalment, I cover some newer sorbent materials that have the potential to help in solving specific
laboratory problems. In next month's instalment, I will look at entirely new sample preparation technologies that may break
away and become the next technique to help in eliminating the laboratory sample preparation bottleneck.
Molecularly Imprinted Polymers
Molecularly imprinted polymers (MIPs) are among the most selective phases used in SPE. The technique is sometimes referred
to as molecularly imprinted solid-phase extraction (MIP-SPE). Molecular imprinting is a technique that has been used in areas
where selective recognition is required for complex separations or sample cleanup. An introductory article (1) outlines the
basics of MIP technology, and review articles (2–5) and several books (6–8) provide detailed information on the use and potential
of MIPs in SPE.
A MIP is a highly stable polymer that possesses recognition sites that are adapted to the three-dimensional shape and functionalities
of an analyte of interest. The most common approach through the use of noncovalent imprinting involves a print molecule (template)
that is chemically coupled with one of the building blocks of a polymer. After polymerization, the resulting bond must be
cleaved to obtain a free selective binding site (receptor). The synthesis process is shown schematically in Figure 1. The
selective interactions between the template and the monomers are based on hydrogen bonding and ionic or hydrophobic interactions.
The most often used monomers are based on methacrylic acid or methacrylates. The basic idea of a MIP is the "lock and key"
concept in which a selective receptor or cavity on the surface of a polymer perfectly fits the template analyte that was used
to prepare the MIP. The receptor site is complementary to the template in terms of its size, shape, and functionality. The
concept is similar to immunoaffinity SPE phases, but obtaining and linking a suitable antibody for these immunoaffinity sorbents
can be very time-consuming and expensive.
Figure 1: Synthesis of a molecularly imprinted polymer stationary phase.
The removal of the template from the polymeric MIP is important not only to make the interaction sites available for increased
sample capacity, but also to ensure that the analyte to be isolated can be measured quantitatively. The lack of removal of
the template molecules, even with exhaustive extraction, has been one of the main problems with the acceptance of MIPs. The
template molecules frequently bleed, sometimes give baseline drifts, and interfere with the quantitation of the desired analyte,
especially at low levels. One approach to overcome this limitation is to use a template that is similar to the analyte of
interest. An example would be to use a brominated analogue template rather than a chlorinated molecule of interest. If the
analogue can be separated from the analyte of interest, then the MIP will function as desired.
With aqueous mobile phases, MIPs can display reversed-phase and ion-exchange interaction because selective polar interactions
are impaired. The MIP phases show the greatest selectivity when the experimental conditions are chosen that generate the selective
interactions that are usually obtained in organic solvents used for the MIP synthesis. This approach allows the MIP to be
used for trapping analytes from aqueous solution by hydrophobic or ionic interactions, then washed with a solvent that breaks
selective binding of matrix components, and finally with an organic solvent that disrupts the strong bonds between the analyte
and the MIP polymer matrix.
As the SPE packing material is a polymer, depending on the degree of crosslinking, there may be some swelling or shrinkage
with a change in solvent. Such a physical change can modify the size of the receptor and change the selectivity of the MIP
for the target analyte. In this regard, perhaps the synthesis of molecularly imprinted organic–inorganic hybrid polymers (9)
may generate a more rigid substructure that does not swell and shrink.
A disadvantage of the MIP approach to SPE is the fact that each sorbent must be custom made. One determines the specificity
of the MIP by choosing the appropriate template molecule. The MIP can be synthesized in the laboratory using published procedures,
or the template molecule can be sent to a specialty laboratory that will make a custom MIP. Because of the relatively long
process involved in making a MIP for SPE, one can justify it only if the application will frequently be required or if there
is no other way to perform sample cleanup.
Off-the-shelf MIPs have been introduced. These standard MIP phases have been designed for specific analytes that are popularly
encountered in complex matrices. Among those currently available from Supelco and Biotage are sorbents optimized for:
- Clenbuterol in biological fluids.
- Beta agonists: Multiresidue extractions in urine and tissue samples.
- NNAL (4-methylnitrosamino-1-[3-pyridyl]-1-butanol): tobacco-specific nitrosamine in biological matrices.
- Riboflavin (vitamin B2) in aqueous samples.
- Triazines: Multiresidue extraction in water, soil, and food products.
- Chloramphenicol: Antibiotic in biological matrices.
- Beta blockers: Multiresidue extractions in water and biological samples.