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"I have observed that in one of your answers for a previous helpdesk blog, ‘Retention time shifts in NP-HPLC’, you used the term ‘dry’ solvents. What is the difference between dry solvents and ordinary HPLC solvents, and what is the use of anhydrous sodium sulfite in mobile phase?"
"Anhydrous or dry solvents have been treated to remove trace amounts of water that may be present. It is possible to buy anhydrous solvents from your normal solvent supplier or you can use a variety of methods to dry solvents in your laboratory. The most suitable method depends on the actual solvent in question but typical methods include addition of anhydrous salts, distillation and use of a molecular sieve (usually 0.4nm). The suggested use of solvents which are half saturated with water (in the previous blog post) is a method by which retention times can be kept reasonably constant by controlling the amount of water present in the mobile phase, since the amount of residual water present in mobile phase can affect retention times considerably.
Another source of trace amounts of water is the sample which is injected onto your HPLC column, thus the suggestion to consider drying this sample by the addition of anhydrous sodium sulphate. This salt is widely used as an inert drying agent for removing traces of water from organic solutions in the laboratory. I would not recommend adding it to the mobile phase. The aim in NP-HPLC is to achieve a fairly consistent amount of water in the mobile phase rather than using completely dry solvents. Once opened dry solvents will absorb water over time and this change will affect retention times in NP-HPLC."