Fraction a
Definition
The acidic ether extract obtained by partitioning the acidified aqueous tissue digest with diethyl ether at pH 2 to 3. Contains barbiturates, salicylates, paracetamol, glutethimide, NSAIDs and phenolic compounds. Sometimes called the acidic drug fraction.
- Extraction pH
- 2 to 3
- Solvent
- Diethyl ether
- Compound class
- Acidic and neutral drugs
- Examples
- Barbiturates, salicylates, paracetamol, NSAIDs
- Also called
- Acidic drug fraction
Common questions
Why does acidifying the sample to pH 2-3 make acidic drugs extractable into ether?+
Acidic drugs like barbiturates and salicylates carry an ionisable acidic group that stays largely un-ionised and thus more lipid-soluble at low pH, allowing them to partition into the nonpolar ether layer instead of remaining dissolved in the aqueous phase.
How does Fraction A relate to Fraction B in the overall Stas-Otto scheme?+
They are sequential, complementary extracts from the same digest: Fraction A is pulled out under acidic conditions to capture acidic and neutral drugs, and the remaining aqueous residue is then basified to pull the alkaline Fraction B, so together they partition the case sample by drug chemistry.
What happens to drugs that are neither strongly acidic nor strongly basic in this scheme?+
Amphoteric or weakly ionisable compounds can partly distribute into both fractions or be lost between them, which is a recognised limitation of the classical scheme compared to modern solid-phase extraction methods that target a broader pH range more precisely.
Related terms
- Ammonium Sulphate Modification
- The Otto variant in which saturated (NH4)2SO4 is added to a protein-rich homogenate (liver, kidney, blood) to salt out proteins before partitioning....
- Colour Spot Test
- A presumptive identification reaction in which a small amount of concentrated fraction is treated with a chromogenic reagent (Marquis, Mecke, Mandelin, Mayer,...
- Defatting
- A clean-up step before partitioning in which the acidified aqueous is washed with petroleum ether or n-hexane to remove fats, waxes and...
- Fraction B
- The alkaline chloroform extract obtained after basifying the residual aqueous to pH 9 to 10 with ammonium hydroxide and partitioning with chloroform....
- Stas-Otto Method
- Sequential acid-alkaline solvent partitioning that isolates organic poisons from biological matrix in two fractions: acidic ether (Fraction A) for acidic and neutral...