4.3.1.1.3 Field Amplified Sample Stacking (FASS) and Field AmplifiedSample Injection (FASI)

Samples zones are sharpened when the conductivity of the sample matrix is substantially lower than that of the buffer. Essentially, this is the reverse of electro-dispersion, described previously. Thus, samples with low salt content can be actively sharpened and/or large quantities of them can be loaded onto a capillary before zone-broadening effects begin to occur. When this phenomenon is utilized to allow larger sample lengths to be loaded hydrodynamically, the concentration is called field amplified sample stacking (FASS). When the injection is performed electrokinetically, it is called field amplified sample injection (FASI).

In the case of FASI, if the conductivity of the sample varies or is unstable, results can be unreliable. In such a case, FASS can be used instead and, while peak resolution may be affected by variability in sample conductivity, the peak areas are still reliable.

Often 100 times more sample can be loaded by FASS before separation is greatly affected. The actual amount of sample that can be loaded by FASS is regulated by the sample's conductivity and factors such as Joule heating. As the conductivity of the sample must necessarily be much lower than that for the running buffer, the applied electric field can induce a large degree of heating within the sample plug, which can change the viscosity of the sample and result in degassing and/or boiling.

Additionally, due to the difference in ionic strength between the sample plug and the running buffer, the electro-osmotic mobility will vary between the sample plug and the buffer. This can result in a parabolic flow profile between the plug and the running buffer that may cause zone broadening. Similarly, a substantially different pH between the sample plug and the buffer can also result in differences in analyte mobility, so a buffer with a similar pH to the sample solution should be chosen.