Sample preparation has often been viewed as the bottleneck in analytical procedures. Surveys have shown that time is typically
the most frequent problem area for sample preparation procedures and that analysts can easily spend a majority of the total
analysis time on sample preparation. While newly developed extraction techniques address time, modern chromatography advances
are also moving toward faster separations. Based on these considerations, what is high-throughput sample preparation? Do modern
extraction methods adequately address the issue of time? How can we address the analytical need for speed?
The self-proclaimed world's greatest rock and roll band famously sang, "You can't always get what you want." It's easy to
apply the Rolling Stones admonition to analytical sample preparation because surveys (1–3) have consistently shown that time,
along with cost and solvent use, is among the most significant desires of analysts. Traditional sample preparation methods
are often the rate-limiting step in the overall sample analysis process. We can envision three approaches for addressing the
desire for high-throughput sample preparation: parallel sample processing, automation, and improvements in the process kinetics.
Evolution of High-Throughput Analysis from Microplates
Over the past several years, we've heard increasing calls for high-throughput sample preparation. But what is high-throughput
sample preparation? It many cases, high-throughput stems from the high-throughput screening approach to combinatorial chemistry.
This approach is the most established, based on the 96-well plate format. These microplates have been established for two
to three decades. Key considerations for the acceptance of this approach are standardization of microplate dimensions and
attributes by the Society for Biomolecular Screening and development of ancillary devices like repeating pipettes, vacuum
manifolds, and more. Countless vendors are involved. Sample preparation and analysis using microplates includes liquid–liquid
extraction (LLE), protein precipitation, solid-phase extraction (SPE), fluorescence, matrix-assisted laser desorption–ionization
mass spectrometry (MALDI-MS), and separations.
Wells (4) presented a list of some of the uses of filtration microplates in pharmaceutical development, including
- clarification of acid, base, or organic digests of plant materials in medicinal chemistry,
- SPE of natural products,
- solution-phase synthesis in combinational chemistry using resin scavenger media,
- dye terminator removal,
- plasmid DNA binding,
- lysate clarification,
- membrane-based proteolytic digestion before MALDI-MS,
- filtration of precipitated proteins,
- filtration of plasma or serum samples in combination with direct injection techniques,
- filtration of reconstituted extracts, and
- solid-supported LLE, SPE, or exclusion chromatography.
As we can see, the development of 96-well plates and the major applications of this approach are in bioanalysis of liquid
samples. This is because the transfer of liquid samples from analytical operation to another is somewhat straightforward.
Recently, Scoffin (5) reviewed liquid transfer in high-throughput systems. These liquid-handling systems operate either by
air displacement or positive displacement and can reliably work with microliter volumes of volatile or viscous liquids. Other
combinations of liquid handling and sorbent-based extraction, including solid-phase microextraction (SPME), automated disposable
pipette extraction, and microextraction by packed sorbent have also been used in high-throughput approaches (6,7). By combining
these solvent-free, sorbent-based extractions with syringe-needle or pipette configurations, direct extraction and liquid
transfer is accommodated.