Alkaline Fraction
Definition
The organic phase recovered when the aqueous is basified to pH 9 to 10 with ammonium hydroxide or sodium hydroxide and shaken with chloroform-isopropanol or hexane-ethyl acetate. Contains the basic drugs with pKa above 7.5: opioids, tricyclics, SSRIs, antipsychotics, amphetamines, cocaine, LSD, atropine, nicotine and quinine.
- pH condition
- Basified to pH 9 to 10
- Basifying agents
- Ammonium hydroxide or sodium hydroxide
- Extraction solvents
- Chloroform-isopropanol or hexane-ethyl acetate
- Drug pKa range captured
- Above 7.5
- Example drugs
- Opioids, tricyclics, SSRIs, amphetamines, cocaine
Common questions
Why does raising the pH pull these particular drugs into the organic solvent?+
Basic drugs are protonated and water-soluble at low pH but become neutral and lipophilic once the solution pH exceeds their pKa, so basifying past pH 9 converts them to their uncharged form and allows them to partition into the organic layer.
What is left behind if the alkaline fraction is removed for separate analysis?+
Acidic and neutral drugs that were not protonated by the alkaline shift remain in the aqueous phase or require a separate acidic extraction step, since a single basification does not efficiently recover the full range of drug classes present in a sample.
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