Capillary Electrophoresis: complex technology for complex science

Capillary electrophoresis (CE) is a technique that allows researchers to separate analytes based on their differential mobilities in aqueous media when an electric field is applied.

According to Agilent Technologies product specialist Brendon Greirson, it is used across a broad range of analytical chemistry and biochemistry applications, in the food and agricultural industries, the bioscience and pharmaceutical sector and even in the petrochemical industry.

But for many applications, CE has been overshadowed by liquid chromatography.

"In the early 90s, it was touted as becoming the replacement for liquid chromatography but it never took off," said Greirson.

Greirson explained that the complexity of the system and lack of understanding of the chemistry behind it discouraged users. But companies that persevered with the method reaped the benefits, with cheap operating costs, and reproducible assays.

"It promised a lot. If you persevered it delivered," he said.

Sensitivity can also be an issue, said Greirson. While it is not necessarily as sensitive as chromatography methods, it can be used for detection of samples in the parts per million range and sometimes below. It is also good for samples in a dirty matrix.

CE falls into two general categories. Traditional CE is used for a variety of different separations, from simple organic and inorganic anions and cations, to biological macromolecules including DNA, proteins, and carbohydrates.

While 10 years ago there were numerous manufacturers of CE instruments, these days the market is dominated by Agilent Technologies' CE System and Beckman's P/ACE MDQ series of instruments.

Agilent's CE System provides several different kinds of detection units, including UV, diode arrays, fluorescence and mass spectrometry, according to Greirson.

On the other hand, Beckman has taken a solution-based approach with its P/ACE series, with systems configured especially for particular uses, such as quality control, glycoproteins or DNA.

The second category of CE systems is based on capillary gel electrophoresis (CGE). These have become a dominant method for DNA sequencing and genotyping applications, particularly in high throughput, automated systems, such as those used for the Human Genome Project.

While Agilent and Beckman have the capabilities to perform CGE using a polyacrylamide gel matrix on their standard CE systems, several companies have come up with CGE systems. Typically, these systems are fully automated from preparation of the capillary gel and sample denaturation to data analysis.

Beckman has a dedicated system for CGE - the CEQ8000 Genetic Analysis System, which supports an array of eight capillaries.

The two big players in this end of the CE spectrum however, are Amersham Biosciences and Applied Biosystems, who both manufacture a range of instruments for DNA sequencing and genetic analysis.

Amersham has the MegaBACE range, which provides arrays of 16 capillaries up to 384 capillaries. Applied Biosystems instruments range from the single capillary capacity of the ABI Prism 310 Genetic Analyzer to the 96 capillary array capacities of the ABI Prism 3700 DNA Analyzer.

New applications of CE have slowly developed. Agilent has lead the field in integrated CE/MS, which Greirson claims is a popular system for analysing proteins and peptides, particularly in a dirty matrix. Another new method is capillary electrochromatography, which combines CE and liquid chromatography.

Agilent is also pioneering the "Lab-on-a-Chip" system, which uses the same principles of CE but in a microchannel on a 2cm-by-2cm-sized chip. Tiny electrodes at the ends of the channels generate the electric force.

"The lab-on-a-chip is all based on CE theory. The driving force within a capillary and in a chip is an electric field that produces an electro-osmotic flow," explained Greirson.

He said that while the idea has been around since the mid-90s, it is only now becoming big business, and has received a lot of attention from the computer industry.

The Agilent 2100 Bioanalyzer lab-on-a-chip system can be used for protein sizing and quantitation, RNA and DNA detection and quantitation, and apoptosis, among other applications.