The easiest way to cut down on acetonitrile is to reduce the inner diameter of the column. The method can be retained. All that has to be done is to adapt or reduce the flow and the injection volume. By reducing the inner diameter of the column from 4.6 mm to 3 mm you can save as much as 57 percent of the solvent. If the particle size of the HPLC column packing is altered at the same time, you can also reduce the length of the column and thus save even more acetonitrile. If you switch for instance from a 150 mm long column with 5 µm particles to a 100 mm long column with 3 µm particles another 33 percent of solvent and thus acetonitrile can be saved. With simple means you can thus save up to 72 percent of acetonitrile.
There are however limits to the reasonable reduction of the inner diameter, the length of the column and the particle size. Experience shows that with all current HPLC devices it is possible to reduce the inner diameter to 3 mm and the length to 75 mm without any problem. All sizes below these dimensions can be used only with volume-optimized HPLC systems. Many users do not dare to modify their existing, usually validated methods, even though the US Pharmacopeia (USP) and also the European Pharmacopeia (EP) explicitly allow changes up to a certain degree. For example length and inner diameter of the separating column may deviate upward or downward by 25 percent from the column stipulated for the method and also the particle diameter may be reduced by up to 50 percent. The approved replacement of a 250 x 4 mm column with 5 µm particles by a 200 x 3 mm column with 3 µm particles is possible without revalidation and reduces the consumption of acetonitrile by 55 percent.
We have devised a column calculator (Column Econo Calculator) which figures out how much solvent can be saved by altering the column and whether you are still within the limits given by USP and EP. The calculator can be obtained from us free of charge. You are also welcome for individual advice.
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