Many TLC users prefer to use pre-coated
TLC plates, but others because of their special needs or because they
need special additives or a special thickness, coat their own TLC plates.
This paper is a quick review of what is needed for this process.
Initial Instructions for Plate Coating: If reproducible plates are needed, then
the user should invest in a coating device from Camag or Desaga (addresses
below). The devices consist of a tray that holds the glass plates and
a device that holds the slurry to be placed on the glass plates. A guide
allows the applicator (with an adjustable layer thickness gauge) to
be pulled along the glass plates as the slurry is dispensed. Some practice
is needed to get a continuous body motion pulling the applicator across
the plates to get an even layer.
The glass plates must be the same thickness
and extremely clean. Any oil or grease will result in a poor layer.
Handle the plates with rubber gloves when washing or rinsing. Care is
also needed if the edges of these plates are not polished or finished
off since a serious cut might result.
If only coating one or two plates, then
a glass plate can be built up on two opposite edges with masking tape
to an appropriate height to give the thickness needed. Slurry is poured
on to one end of the plate and a glass rod that extends passed the masking
taped edges can be used to pull the slurry evenly over the plate. There
will be considerable waste until the correct amount of slurry for the
thickness required is figured out from trial and error.
Smaller TLC plates can be coated by using
smaller glass plates (microscope slides) and placing two of them back
to back. The two slides are dipped into the silica (or other) slurry,
pulled out, separated and dried. If a small beaker is used and
methanol instead of water is used for the slurry, then the silica gel
slurry can be saved and used again.
Binders and TLC:
It is important that anyone making TLC
plates is aware of the binders required for silica gel TLC plates. The
fine silica will not stay on the plate after coating if some type of
binder is not included. It would literally fall or blow from the plate.
The classical binder used is gypsum (G), which is CaSO4ˇ˝
H2O, or Plaster of Paris. In most silica gels it is put in
at a level of 10-15% to give good binding to the glass plates. For coating
five 20x20 plates, about 30g TLC silica gel to 60ml water is needed.
Once made, the time available to put in the applicator and to pull across
the glass plates is limited, since the binder will hydrate and the slurry
will no longer be a flowing liquid.
The other formulation offered by Merck
KGaA, Darmstadt, Germany is the H binder, which is a silicic
acid. This also gives a soft layer plate but might be considered to
be pure silica gel. Both the G and H layers have to be handled and used
carefully so as not to disturb the layer. For a stronger layer, polyvinyl
alcohol or polyvinyl pyrollidone can be added to a TLC grade silica
gel (without G or H binders) in a level of 1-2% by weight as a polymeric
binder. Merck KGaA, Darmstadt, Germany, does not sell
these binders however. Heat drying after air-drying makes cross-linking
of this type of polymer binders.
Handling After the Coating Process:
After coating the plates, the plate holder is
lifted a couple of mm and dropped on the benchtop to help release air
bubbles that might be caught in the slurry. Then the plates in the
holder tray are allowed to remain flat while the gypsum (or other
binder) sets up or the excess moisture evaporates. The gypsum on
hydrating makes the layer turn from a translucent, shiny appearance to
a flat, matte finish.
Allow the plates to air-dry overnight.
This is particularly true if thicker preparative layers are being made.
Slower drying of these layers insures no cracking. The next day or before
use the plates can be activated, if desired, to remove more of the adsorbed
water. If not using immediately, then the water still remaining on an
unactivated plate will actually protect the surface more by not allowing
impurities to collect on the activated silanols. The plate can be stored
with this water and activated for 30 minutes at 110°C just before
use.
Other information concerning plate manufacturer
can be found in the Stahl book listed below. The manufacturers of the
coating equipment might also be helpful, so can be consulted.
Making TLC Plates with Additives
When making the TLC plates various additives
can be used. A buffered TLC plate might be desired. The slurry is made
up from the buffer instead of plain water. Likewise, if a plate is made
for the separation of double bonded species, then silver nitrate is
added to the slurry in about 5-10% w/v. These plates have to be dried
in the dark to prevent darkening of the TLC plate. An alternative to
using the additive to the slurry is to dip the plate in the buffer or
silver solution before using. Whether you use one technique or the other
depends on your time and number of plates being used at a time.
References:
1) E. Stahl, Thin Layer Chromatography,
Springer-Verlag, New York, 1969
2) Camag Scientific, Inc., 515 Cornelius Harnett
Drive, Wilmington, NC 28401, phone 800-334-3909, fax 919-343-1834
3) Desaga GmbH, P.O. Box 101969, D-69009
Heidelberg, Germany, phone 0-62-21-83590, fax 0-62-21-840887
