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].
Nevertheless, the variation in the first electrophoretic dimension remained an uncontrolled factor in the experiment. The
scientist was dependent on the manufacturer delivering good
batch-to-batch strip reproducibility in terms of gradient linearity or non-linearity, respectively, depending on applications. In
order to resolve this unsatisfactory situation, we tried to find
suitable protein standards for that purpose[4]. However, both
the pI coverage and the purity of the commercially available
proteins were not convincing.