Variation associated with pI is, however, not corrected at all.
This was not a major problem in earlier experiments, because
commercially available pI-strips showed high quality [2,4].
Already 20 years ago, substituted aminomethylphenols
have been proposed as low-molecular markers for the electrophoretic and chromatographic focusing of ampholytes[6],
but never really gained much attention among GE users. With
the expanding field of proteomics this line of research has
attracted new interest. As detailed in Ref.[7],pI-markers
should meet a number of criteria such as good focussing ability
and hydrophilicity, chemical stability and purity. A pI colour
code in the visibly range would also be useful. Theˇ Slais group
has been active in this research area for many years synthesizing marker ampholytes derived from nitrophenol and
methyl red[6–8]. Coloured azo pIstandards were made available which covered the entire pI-range[7,8]. The majority of
those compounds could serve as primary standards, because
their pI values were determined with methods independent
of isoelectric focussing[7].