For the most efficient separation, the LC separation modes in the first and second dimension should be different. In practice, however, it is not convenient or even possible to combine certain LC modes with eachother.
Typical problems include:

• mobile phase immiscibility,
• precipitation of buffer salts
• incompatibility of the mobile phase used in the first dimension and the stationary phase in the second dimension.

In addition, the solvent strength is an essential parameter in the transfer, because the volume of the fractions is typically several hundreds of microlitres. In LC-LC coupling, the eluent in the first dimension should preferably be a weak eluent in the second dimension. In this way, larger fractions can be transferred into the second column without peak deformation. If the fraction is in a solvent that is a strong solvent in the second separation, the analytes start migration already during the transfer, causing serious peak broadening. For example, because aqueous buffers used in I(E)C are weak solvent in RPLC, these two techniques can be easily coupled together, and during the transfer, efficient reconcentration takes place. An example of opposite situation is the combination of NPLC and RPLC which are difficult to couple because the organic solvents in NPLC are strong eluents in RPLC.

- difficult to combine; + possible, but not convenient; ++ relatively easy, +++ easy.

Column dimensions.
Another important parameter that contributes directly to the volume of the fraction is the diameter of the column. The  smaller the column i.d. is, the smaller is the volume of the fraction. For example, if the column diameter is decreased from 4.6 mm to 2.1 mm, the size of the fraction is decreased from ca. 1 ml to 0.2 ml. However, the sample capacity also decreases if the i.d. decreases. A good compromise of reasonable sample capacity and small fraction size is to have column with i.d. of 2-3 mm for the first dimension. The dimensions of the second column are not as critical, but it should not have i.d. smaller than the column used for the first dimension.

Combining different LC modes in LCxLC
In principle, the same parameters that are described for heart cut LC also are valid in the selection of LC modes for LCxLC. However, because the transferred fractions in LCxLC have typically much smaller volumes (15-100 μl), the eluent miscibility or eluent strength are not as critical in LCxLC. For example, it is possible to combine NPLC with RPLC, although careful selection of conditions is required to avoid band broadening