Editorial
Sampling of Small Spaces
L
ast month, this column discussed integrated aspects of analytical sampling. In particular, sampling when the analyte or analytes are preconcentrated, sometimes selectively, from the original matrix into a smaller volume, which subsequently delivers—through a simple procedure—the analyte (s) to a measuring instrument. This month, I want to emphasize sampling associated with small spaces. Interestingly, small spaces almost invariably lie in heterogeneous matrices. The heterogeneity may be on length scales happily comparable with that which one can sample, but often they are smaller. The requirements for small space sampling are demanding and incredibly diverse. Although this area is receiving vigorous research attention, I foresee that it will be a longlived research frontier. One approach to small space sampling involves isolating it from its matrix and then applying a technique that can deal with small quantities but that is not intrinsically a focussing or imaging methodology. This is what is done, for example, in studies of a single biological cell, where, packaged in its natural membrane, it is placed into a capillary electrophoresis column or a mass spectrometer. In such procedures, the concept of sampling has its traditional meaning. Small space sampling is more often approached by miniaturizing the analytical probe. Sampling becomes synonymous with how miniaturization is accomplished. Although one can consider that the analysis is done "in-place", this is still sampling, and the importance of the sampling elements (representativeness, throughput, selectivity, etc.) remains. There is a wonderful and still-growing army of miniaturized measurement probes, including laser-ablation microprobes, near-field microscopy, and scanning electrochemical microscopy. Small spaces can also be isolated through timing, as in fast potential steps at microelectrodes. In area, the length scales involved are submicron down to a few nanometers, and the volume scales extend from nanoliters down to femtoliters. Some really hard nuts have yet to be cracked, however, such as polymer laminates with multiple and sometimes structurally similar polymer layers with irregular
interfaces. This topic cries for improvements in infrared imaging spectroscopy. Other examples include substructures of hard, but intrinsically heterogeneous, materials such as ceramics, hard metal alloys, and aerogels—topics in contemporary materials science that need shorter sampling length scales than now available. Single-molecule detection is the ultimate act of sampling. This new world of chemistry has witnessed exciting progress in the last several years. Numerous strategies for measurement have been identified and proven, at least in preliminary form. Force and tunneling microscopies have provided observational capabilities on molecular length scales, and tactics for chemical selectivity in the images are gradually emerging. Most single-molecule observations involve probes of much larger dimensions. These studies involve volumes and concentrations sufficiently small that the sampled space contains only a single molecule, eliciting repeated fluorescent emissions or electron transfers from the molecule in order to bring the measurement sensitivity within reach. This field is too young to know whaa its sapacity will be, even in a couple of years; but almost certainly, ,he energetics and lifetimes of single-molecule excited states will be a hotly researched topic. The field is allo too young, I suspect, for many in the analytical community to clearly connect single-molecule observations with traditional ideas so chemical analysisi "Concentration" loses its meaning at the single-molecule level, but for me, variations in excited state lifetimes, molecular orientations local solvent environment, and so on, remain sharp challenges in the general sense of analytical chemistry as the science of chemical measurements. Finally, the vast world of sampling, in large and small spaces, does not occupy as prominent a place in our teaching of analytical chemistry as it deserves. Its systematics are absent from most texts. After thinking through this area, I have resolved to rectify this in my own classroom. I hope that others will do the same.
Analytical Chemistry News & Features, June 1, 1997 3 2 7 A
