A great deal of progress has been made toward the development of the micro total analysis system (micro-TAS) since its inception in 1990. A wide variety of applications, including genomics, proteomics and drug discovery, have prompted the development of analytical methods capable of very high throughput while maintaining low cost. The micro-TAS concept addresses both of these requirements. Electrophoresis has been a key element in the development of the micro-TAS. Most chemical and biochemical assays utilize a separation component at some point during analysis. Genomics, in particular, depends almost exclusively on electrophoresis for size-based separations of DNA. This review examines sample introduction into microfabricated electrophoretic devices, or chips, primarily for DNA analysis. Sample introduction is an important component of these systems and is an essential process for making chip electrophoresis a widely applicable analytical technique. Specific issues, such as automation, the delivery of large numbers of samples to microfabricated devices and injection of picoliter-sized sample plugs into a separation lanes on chips, are presented.
With the release of the human genome sequence, there has been increasing attention given to other genetic analyses, including the detection of genetic variations and fast sequencing of multiple samples for pharmacogenomics studies. Rapid injections of samples in multiplexed separation channels by optically gated sample introduction are shown here for DNA separation. Serial separations of four amino acids are shown in less than four seconds on a microchip with four multiplexed channels. Five short oligonucleotides have also been rapidly separated in 2% LPA with four channels using this technique. In addition, multiple unique samples have been simultaneously separated and five-base resolution has been demonstrated.
Continuous analysis of a DNA restriction enzyme digest on a microfabricated device is demonstrated with minimal intervention and enhanced time resolution. A 62-base-pair fragment of dsDNA containing a KpnI site was used to demonstrate this process. A capillary was used to transfer sample from a single reaction mix to a microfabricated chip with parallel separation lanes. The 6-carboxyfluorescein-labeled DNA fragments were detected with a CCD camera as they separated in the lanes, which were filled with linear polyacrylamide. The products of the restriction enzyme digest were monitored for up to 60 min at an average sampling rate of 1 injection/46 s, with consecutive injections as short as 1 injection/14 s. The digest was injected directly into the chip, eliminating the need for any sample-handling steps after addition of the enzyme to the reaction mix. The effects of temperature and restriction enzyme concentration were briefly examined, as well. This work shows the potential of this method to provide valuable information about the process of restriction enzyme cleavage.
Kinetic analysis of RNA enzymes, or ribozymes, typically involves the tedious process of collecting and quenching reaction time points and then fractionating by polyacrylamide gel electrophoresis (PAGE). As a way to automate and simplify this process, continuous analysis of a ribozyme reaction is demonstrated here using completely automated capillary sample introduction onto a microfabricated device with laser-induced fluorescence detection. The method of injection is extremely reproducible thereby standardizing data analysis. A 30-nucleotide ribozyme model, the self-cleaving lead-dependent ribozyme, or "leadzyme", which cleaves into a 24-mer and a 6-mer in the presence of Pb(2+), was end-labeled with fluorescein (FAM) and used to demonstrate the potential of this technique. After manually initiating the cleavage reaction by Pb(2+) addition, reaction samples were automatically injected directly into the parallel separation lanes of the chip via a capillary at predetermined time intervals, thus eliminating the need for additional sample-handling steps. The FAM-labeled leadzyme starting material and products were monitored for 60 min in order to ascertain kinetic information. The effect of lead acetate concentration on cleavage rates was also studied, and the results are in agreement with rates determined by conventional hand-mixing/PAGE analysis. This work demonstrates, through the use of a simple ribozyme model, the potential of this method to provide valuable kinetic information for other, more complex, biologically relevant RNA and protein enzymes.
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