FAQ
What Are the Advantages of Alumina over Silica Gel?
Dynamic Adsorbents is the industry
leader in the use of alumina for adsorption chromatography. While also
selling a comprehensive line of silica gel products that are the highest
quality in the industry and unique in providing customers with a
detailed analysis from each lot of silica gel provided the customer, we
recommend the use of since according to "August 2003 New Horizons and
Challenges in Fundamental Analysis and Monitoring" (see bottom of page),
Alumina is the most effective adsorbent used for this purpose.
Silica, along with 3 metal oxides
(alumina, titanium and zirconia), are all used in the stationary phase
of adsorption chromatography.
Attributes of silica include the
following:
- High sample loading
- Fewer unwanted reactions during
separation
- Wide range of chromatographic
forms of silica – wide range of surface areas and average pore
diameters
- The hydroxyl group in silica is
attached to silicon – hydroxyl groups are either free or hydrogen
bonded
- The silicas used are amorphous –
terms gels
While silica has a higher sample load,
there are several limitations in the use of silica:
- The silica backbone Si – 0 – Si
hydrolyzes at pH . 8
- Silica hydrolyzes at temperatures
greater than 40 degrees C
- Silica hydrolyzes in the presence
of phosphate and carbonate ions
- Silica is soluble in
aqueous/organic media in even slightly alkaline media at even
moderately elevated temperatures
- The siloxane bond is unstable at
pH < 4.5 and becomes even more unstable the lower the pH
The Alumina advantage
Alumina has several advantages over
silica gel and the other metal oxides.
Chromatographic alumina is gamma
alumina with 2 specific adsorption sites: acidic, basic, and electron
acceptor sites. The adsorption sites are exposed aluminum atoms,
strained Ai-O bonds or cationic sites. They are in a crystalline form –
either alpha, gamma states or in an amorphous state, has a pH stability
ranging from pH 3 to pH 13, with great stability across the entire pH
range higher thermal stability than silica.
Alumina advantages include the
following.
- Due to its low pHpzc, silica
allows only for a cation exchange while metal oxides such as alumina
behave as amphoroteric ion exchangers: they can be cation or anion
exchanges depending on pH. Of all the metal oxides, alumina performs
the best in this role as an amphoroteric ion exchanger.
- Alumina has Lewis acid sites that
are responsible for ligand exchange ability
- Alumina offers much higher values
of pHpzc: i.e. at neutral pH the surface of alumina does not have a
negative charge and this will not interact with charged bases via
electrostatic interactions
- There is a strong affinity of
alumina for phosphates and carboxylates
- Alumina has a strong affinity for
halides – for example, it is able to elute excess concentrations of
fluoride from drinking water, leading to enhanced column packing
stability.
For more information, download the
following article from Center of Excellence Environmental
Analysis and Monitoring, August 2003 New Horizons and Challenges in
Fundamental Analysis and Monitoring.
