Solid-phase extraction (SPE) is a fast, cost-effective sample preparation technique for purifying complex samples before their
analysis by gas chromatography (GC) or liquid chromatography (LC). The technique removes undesirable matrix compounds that
can interfere with the analysis. In many cases, SPE has replaced liquid–liquid extraction (solvent extraction), which often
uses copious amounts of organic solvent and is labor-intensive and difficult to automate, especially in its classical operation.
In SPE, analytes are partitioned between a solid phase and a liquid phase and the stationary phase usually is chosen to have
a greater affinity for the analytes than for the sample matrix. The classic SPE stationary phases range from bare silica and
other solid sorbents such as alumina, florisil, and carbon to chemically bonded silica with phases similar to those used in
high performance liquid chromatography (HPLC). Interestingly, as can be seen in Figure 1 (1,2), the usage of bonded sorbents
in SPE almost mirrors the usage of the same phases in HPLC. Silica, ion-exchange phases, and affinity phases show slightly
greater percentages of use in SPE. Included in Figure 1 are Florisil and alumina, non-silica-based but popular SPE packings
that see applications in SPE only.
Bonded silicas are still the most widely used SPE packings, with several decades of applications having been developed. Although
it has been debated as to who commercialized the first SPE cartridges (3), for many years before that date, researchers were
using adsorbents and polymeric polystyrene–divinylbenzene (PS-DVB) ion-exchange resins to do a rough cleanup of samples before
GC and LC. Only after these adsorbents were provided in convenient-to-use cartridge format did the SPE technique take off
and become a popular sample preparation technique. In recent years, there has been a renewed growth in SPE, primarily driven
by the introduction of newer packing materials based upon specialized polymer technology that overcomes some of the disadvantages
of silica-based materials. This installment of "Sample Prep Perspectives" will explore these new polymer-based packings and
compare and contrast them with popular silica-based packings. In addition, the use of polymers in other sample preparation
techniques is explored. In these applications, polymers are coated on fused-silica fibers, stir bars, micropipettes, vials,
and 96-well plates
Silica-Based Packings in SPE
Figure 1: Comparison of phase usage in SPE and HPLC.
For many years, silica-based sorbents have been the main types used in SPE. A wide variety of bonded phases (Figure 1) can
be applied to just about any SPE problem encountered. However, bonded silica sorbents in SPE have some of the same disadvantages
that they display in HPLC such as limited pH range (pH 2–9), and the presence of residual silanol groups. Under the appropriate
pH conditions (for example, above their pKa values around 4.5–4.7), these silanol groups become ionized and are negatively charged. Positively charged compounds such
as protonated amines can interact electrostatically with these ionized silanols. Such strong ionic interactions might be difficult
to overcome with organic solvent elution, and analyte recovery for these polar compounds is affected.
In the classic SPE experiment there are four main steps: conditioning the sorbent to solvate (or "wet") the stationary phase;
loading the sample; rinsing away interferences; and eluting the analytes of interest. For silica gel-based sorbents and chemically
bonded silica gel-based sorbents, it is fairly critical to ensure that the packing material does not dry out between steps,
particularly after the conditioning step. Drying of the sorbent often "dewets" or deactivates the stationary phase and changes
its properties so that analyte recovery is affected and reproducibility jeopardized. When using vacuum and positive-pressure
manifolds, it is sometimes easy to allow SPE phases to dry out by inadvertently forgetting to turn off the vacuum or pressure
during the critical steps.
Follow Us
RSS
Bookstore
