Qualitative and Quantitative Analysis

The data produced by the migration curves resulting from monitoring the conductivity, potential gradient thermometric etc. provides the general characteristics of the isotachophoretic separation and from these curves both qualitative and quantitative information can be obtained.

The information obtained from isotachophoretic migration curves, although appearing similar to those obtained from chromatographic separations, are completely different, in fact, interpretation is almost completely opposite to that of a chromatogram, integral, differential or other wise. In an integral chromatogram the step height is a measure of the quantity of the material present and the step width  (or the distance of the step from the injection point) gives an indication as to the nature of the eluting material. In the isotachopherogram the converse applies; the height of the step provides information that helps in identifying the materials migrating whereas the width of the step is proportional to the mass of material migrating.

An example of an isotachopherogram depicting the separation of five components is shown in figure 17.

Figure 17. An Isotachopherogram Depicting the Separation of Five Components.

The curves in figure 17 were obtained monitoring the migration of the ions by, resistance (inverse of conductivity) and UV absorption; the diagram also includes the differential of resistance with respect to time, which produces a series of sharp peaks at the junction between each migrating ion.  These peaks provide an accurate means of identifying the position where two substances meet. The precise location of this point is important in measuring the step width, which as already stated, will be proportional to the amount of material present in the step. The UV absorption step is normally used solely as a means of identifying those UV absorbing substance amid the total group of migrating ions but can also be used for quantitative work if necessary.