Ronald E. Majors, Agilent Technologies, Wilmington, Delaware, USA.
Over the years, "Sample Prep Perspectives" columns by myself and invited guest authors have covered various aspects of solid-phase
extraction (SPE) including the basics,1–7 applications and method development,8–12 automation,13–16 dedicated symposia17,18 and special supplements.19 Today, SPE remains one of the more important sample preparation techniques for chromatography and has branched out into
other areas by employing new formats, new chemistries via novel sorption and partition mechanisms, multidimensional techniques
and miniaturized systems. This instalment of "Sample Preparation Perspectives" will review some of these advances in newer
phase chemistries that might be directly applicable to solving your everyday sample preparation problems, especially when
dealing with complex mixtures. I will divide my coverage into different categories that employ related concepts.
Quick Review of the Basics of SPE
SPE is a sample preparation technique based upon principles similar to those of high performance liquid chromatography (HPLC).
It is used for the selective sorption of analytes or interferences from simple to complex matrices. It is used for sample
clean-up and analyte concentration preceding HPLC, gas chromatography (GC), ion chromatography and other separation techniques.
It has replaced many of the classic liquid–liquid extraction (LLE) techniques. Relative to LLE, SPE uses much less solvent,
improves sample throughput, provides more tunable selectivity by appropriate choice of stationary phase, is more readily automated
and avoids the formation of emulsions. In many instances, SPE provides cleaner extracts and provides higher and more reproducible
recoveries. 
Figure 1
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In its simplest form, SPE uses a packing material such as bonded silica (40–50 μm) or polymeric media packed into a plastic,
medical-grade syringe barrel. Similar to an HPLC column, porous polymeric or metallic frits contain the packing in the cartridge
format. Other popular SPE formats are disks, pipette tips and 96-well plates. The stationary phases typically used in SPE
cartridges are reversed-phase (C8, C18), ion-exchange (strong anion and strong cation exchange), or normal-phase (silica,
cyano, amino) packings. Typically, there are four steps in SPE operation: conditioning, sample addition, washing and elution
(Figure 1). The conditioning step solvates the bonded phase so that it can readily accept the liquid sample load, the washing
step removes interferences and the elution step involves the use of a strong solvent to elute the analyte of interest in a
small volume for direct injection into chromatographic column. Sometimes, the eluent is blown down by solvent evaporation
to further concentrate the analyte or to allow redissolution of the analyte in a solvent more compatible with the subsequent
chromatographic technique.
More and more applications are going to on-line or automated SPE. Compared with manual methods, automated SPE is less labour
intensive, relieves the tedium of manual operation, requires less sample handling providing better recovery, is more reproducible,
is performed in a closed system (less chance of sample oxidation or solvent evaporation) and allows smaller samples to be
transferred and accommodated more easily. There are several approaches used for automation. The SPE phase can be packed into
a stainless steel column and used repeatedly via column switching. Some SPE phases are packed into special high-pressure cartridges
and used in an automated sampling device. Other forms of automation involve SPE cartridges, 96-well SPE plates, disks or pipette
tips that are handled by robotic or xyz liquid-handling devices. In these approaches, the sample is sometimes exposed to the atmosphere during handling.
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