Analytical scientists now have a wide variety of sample preparation techniques to choose from: solid phase extraction (SPE), solid phase microextraction (SPME), microextraction in packed syringe (MEPS), liquid phase microextraction (LPME), single drop microextraction (SDME) dispersive liquid-liquid microextraction (DLLME), hollow fiber supported liquid-phase microextraction (HFLPME), three phase LPME, dispersive liquid-liquid microextraction (LLME), supercritical fluid extraction (SFE), accelerated solvent extraction (ASE), or pressurized liquid extraction (PLE), fabric phase sorptive extraction (FPSE), microwave assisted solvent extraction (MASE), stir bar sorptive extraction (SBSE), micro SPE (μSPE), quick, easy, cheap, effective, rugged and safe extraction (QuEChERS), dispersive solid phase extraction (dSPE), magnetic solid phase extraction (MSPE), matrix solid phase dispersion (MSPD), disposable pipette extraction (DPX)... the list is almost endless. When I go into class and introduce all these techniques to my students, I joke that the reason so many exist is to make their exams more difficult. I am not sure that they are convinced otherwise (that is, until they start working in the lab and encounter a tricky matrix). But why do so many exist?
Well, we all know that sample preparation is an essential step prior to any chemical analysis – and optimizing various aspects for different analytes and matrices has resulted in the plethora we see today. And though sample preparation usually consists of several sub-steps – dilution, filtration, deproteinization, centrifugation, purification, derivatization – the most important step is extraction. Even in cases where a ‘dilute and shoot’ approach is an option, extraction is still useful in terms of protecting instruments and prolonging the life of chromatographic columns. Extraction can be either sorbent- based or solvent-based and, as noted, has been applied in many formats and versions through the years, as advances in instrument technology have demanded more efficient sample pretreatment, from liquid-liquid extraction (LLE) and solid-liquid extraction (SLE), to modern microextraction techniques. SPE can be considered the descendant of LLE. The few hundred milliliters of solvents needed in LLE have been reduced to just a few milliliters. Reversed phase, normal phase, ion exchange, mixed mode and many more selective sorbents - for example, molecularly imprinted polymers (MIPs) - can cover all analytical needs.
The next optimization trend was to further reduce the volume of solvents –the birth of microextraction techniques – and it demanded the use of new advanced materials that were only made possible by the evolution of nanotechnology. Nanosorbents with higher surface area and sorption capacities improved the efficiency of dispersive μSPE. Meanwhile, the increased sensitivity of instrumentation was able to overcome the resulting drawback: lower overall sorbent capacity. Miniaturized techniques promise to improve sample handling and increase efficiency and accuracy by using low quantities of sorbent materials and requiring no or low solvent volumes together with low sample volumes. Another important factor behind the popularity of microextraction techniques in sample handling is that they comply with green analytical chemistry demands. Selectivity, sensitivity and lower quantification limits are among the impressive performance characteristics of these techniques, which are required to meet modern legislation criteria. The unique properties of microextraction techniques have also resulted in savings in both time and money.
So, when faced with our long list of sample preparation options, we have serious choices to make. And although the best sample preparation would be no sample preparation, the second best choice is ‘just enough’ sample preparation – a suitable technique that provides selective, fast, cheap extraction – all while following the twelve principles of green chemistry. Choosing a sample prep technique by considering their advantages and disadvantages for the purpose of analysis should be common sense for an experienced analytical chemist (but how often is it forgotten?). For young chemists? Well, they need to live it before they believe it... To answer my own question in the opening of this article: “yes, sample preparation is still the bottleneck in chemical analysis!” However, we now at least have many tools in our hands to make it easier and more efficient. Perhaps one day we will be able to dispense with sample prep altogether but, for now, Benjamin Franklin’s famous quote remains disturbingly true: “By failing to prepare, we are preparing to fail.”
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