A filter membrane could have charged sites on it or a small amount of extractables.

Most manufacturers, including MicroSolv, produce filters with the lowest extractables possible. If these extractables are soluble in the solvent you are using, you should see them only in the first few milliliters. This aliquot should be discarded.

If your filter has charges on it and it adsorbs your analyte, once all the charged sites are taken up (normally in the first few milliliters), no more adsorption should take place and the concentrations of your filtrate will remain constant. This is called “the steady state”.

Please check with your SOP if discarding filtrate is acceptable practice as this can vary from lab to lab.

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