MLD
packings (MLD
= monolayer dense)
are very stable solide supports, known in the literature as "mixed
self-assembled monolayers
(SAM)”.
They can be used both for basic and acidic compounds in a relatively
wide range of pH and with high selectivity. The buffer concentration can be held
less. With organic buffers one can successfully use these columns between pH
2 – 11.
The
application MLD1 demonstrates a separation on UltraSep
ES MLD RP18, which was performed already on UltraSep ES PHARM RP8E with 15 mM
phosphate buffer as eluent (Application PHARM3).
With
UltraSep ES MLD RP18 already £ 5 mM
phosphoric acid is sufficient for delivering the same chromatogram quality (Application
MLD2). Furthermore it is important that for obtaining
comparable short retention times of basic pharmaceuticals one can work with
50% acetonitrile (instead of 30%).
The
application MLD3 represents a chromatogram of basic pharmaceuticals at pH 11,
which was obtained after flushing the column with 6000 ml eluent, pH 11,
corresponding to a continuous use of approximately 4 weeks. Remarkable is the
still very good peak shape especially of perazine dimalonate.
For working with basic
eluents the usage of organic buffers and the addition of organic solvents is
recommended.
MLD
packing are, as already mentioned, suitable for the separation of organic acids
too (Application MLD4).
Using
MLD packings it is possible to obtain chromatograms with good peak shapes of
very different classes of compounds (Application MLD5).