The Need for Proper Practice

Some analytical techniques are sample specific. The group frequency bands in an infrared spectrum are largely independent of the method used to obtain the spectrum, whether it is run as a solid KBr disc, a Nujol mull or a solution and whether it is obtained by a dispersive or a Fourier 

This is not always so in the case of thermal methods. The results obtained depend upon the conditions used to prepare the sample, the instrumental parameters selected for the run and the chemical reactions involved. That is not to say that results are not reproducible provided similar conditions are selected. For example, it is possible to compare samples of a polymer to see if their behaviour is good or bad according to their potential use, but the experimental parameters used for running each sample must be the same. 

The useful acronym SCRAM (sample-crucible-rate of heating-fltmosphere-mass) will enable the analyst to obtain good, reproducible results for most thermal methods provided that the following details are recorded for each run  

The sample A proper chemical description must be given together with the source and pre-treatments. The history of the sample, impurities and dilution with inert material can all affect results. 

The crucible The material and shape of the crucible or sample holder is important. Deep crucibles may restrict gas flow more than flat, wide ones, and platinum crucibles catalyse some reactions more than alumina ones. The type of holder or clamping used for thermomechanical methods is equally important. The make and type of instrument used should also be recorded. 

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