Experimental design and sampling

TD-NMR sampling is decidedly more difficult than HR-NMR because on-line applications of low-resolution NMR have tended to focus on solid materials such as tablets, polymers, toothpaste, cellulose, foodstuffs. 

In the case of HR-NMR the main stipulation is that all samples introduced to the analyzer must be in an entirely liquid form in order for the protons in the entire molecular distribution to be observed. Solids will not provide observable H signals and their presence will therefore reduce the efficacy of the on-line predictions or the model development. A sampling tap from the process lines must deliver separate streams (up to five) to the sample system at a flow rate of 260-340 liters per hour. The sample system must be designed to provide each sample stream to the NMR probe with a temperature variation of less than 3°C. Each sample system is custom designed to meet the above criteria by working in close collaboration with process engineers on the temperature, flows, pressures, viscosities, and solids content found in the various process lines. 

Excessive water in samples should be removed if possible, but there are processing methodologies to remove the effects of water contamination on the chemometric models used for predictions. Water appears in a specific area of the spectrum and can be digitally removed from the spectrum during the processing stages. 

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