Encyclopedia of Analytical Chemistry

Multidimensional (MD) separation is a combination of two or more separation techniques that increases peak capacity. There are two requirements or criteria in MD as suggested by Giddings. First is orthogonality, which means that the separation mechanism in each step must be based on different molecular properties such as size, charge, hydrophobicity, or chirality. Second is that the separation obtained in the first dimension must be preserved in the subsequent dimensions. There are two types of MD separation, comprehensive and heart cutting. In the first type, all compounds present in the mixture are separated and detected, while in the second, only selected fractions in the mixture are analyzed. The connection between the dimensions that must be tight and dead-volume free is the most important part of an MD separation system. The most common connection was an interface that joins two or more chromatographic columns and/or electrophoretic capillaries. In the case of capillary electrophoresis (CE), there is an alternative strategy to obtain MD separations without the use of an interface. This is the so-called heart-cutting MD-CE in a single capillary or interface-free MD-CE. In interface-free MD-CE, mobilization of fractions between dimensions is achieved by manipulation of voltage and/or pressure. Although MD-CE has been developed for more than 15 years, applications to real samples were not as prevalent as those reported for MD gas and liquid chromatography. MD-CE research reports were on tighter interfaces development as well as sensitivity and performance enhancements in two-dimension format (2D-CE). This article is devoted to the wide range of interfaces and interface/interface-free methods developed for 2D-CE. CE separation modes used in 2D-CE were also briefly discussed for readers not familiar with CE.