Mobility of Peaks/Bands
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The speed and, thus, the temporal appearance of DNA fragments (electrophoresis peaks/bands), i.e. their mobility, depends on many parameters and varies between different sample runs.

Table 1 lists some factors that influence the fragment mobility.

Tab. 1.
Electrophoresis buffer
The older (depletion of ions) the slower the run.
Viscosity of the gel matrix
The lower the faster the run.
The higher the faster the run.
Sample DNA concentration
The higher the faster the run.

Different electrophoresis parameters can cause a 500 base pair fragment to pass the detection unit after 20 minutes in one case and after 25 minutes in a second case. This explains why the same fragment has different data point values (time values) but identical base size labels. Therefore, a size standard is run in an own color channel or gel lane in each run.

Figure 1 shows the same sample (here: a size standard) run under different electrophoresis parameters. The data is scaled by data points, i.e. the curve points are drawn according to their temporal appearance.
Fig. 1
The same size standard run under different conditions. The pattern is very similar but in the lower run, the sample peaks appear earlier and in a shorter time interval. Therefore, the whole sample appears 'compressed' if compared with the upper one.

Also see
> Sizing