Liquid Chromatography - The Thermogravimetric Analysis of Silica Gel 1
Probably the most informative experimental procedure to help in the elucidation of the structure of the silica surface is thermogravimetric analysis (TGA). Formally, the silica was heated for known times at known temperatures and the loss in weight noted (21). With the advent of the programmed TGA apparatus this procedure has been simplified a great deal and the basic thermogram can now provide considerable information on the nature of the surface hydroxyl groups and adsorbed water (22).
The Thermogravimetric Analysis of Silica Gel
The sample is suspended from the arm of a continuously recording micro balance in a temperature controlled furnace and is heated from a defined starting temperature to a specified final temperature at a designated heating rate usually given as temperature change per unit time. The temperature and the sample weight are continuously digitized and the data stored. The results can then be printed out or presented as an appropriate graph relating sample mass to temperature. To help identify the desorption of different species, derivative curves can also be produced. The results obtained from a sample of Matrex 20m LC silica gel taken directly from the Perkin Elmer TGA instrument is shown in figure 33.
The derivative curve is seen to have three distinctly different desorption processes. The first takes place from about 30oC to 130oC; the second between about 200oC and 450oC and the third between about 400oC and 900oC. The three different desorption processes are distinct and unambiguous and are similar to those previously identified by Scott and Traiman (22). The total loss from the sample was about 5%w/w but it would appear from the TGA curve that the desorption may not be entirely complete at the temperature of 900oC. The curve relating mass of water lost (obtained by subtracting each data point from the initial total mass of sample) against temperature in a TGA analysis, was a type of desorption isotherm that could be described by assuming three distinct and separate desorbable species on the silica surface, each evolving water over a specific temperature range.