Dialysis
Definition
Separation of small molecules from large ones across a semipermeable cellulose membrane. In forensic toxicology, used for clean-up of blood and plasma before HPLC, LC-MS/MS or AAS.
- Mechanism
- Separation across a semipermeable membrane
- Membrane material
- Cellulose
- Used in
- Blood and plasma clean-up before instrumental analysis
- Downstream methods
- HPLC, LC-MS/MS, AAS
Common questions
Why does forensic toxicology use dialysis as a clean-up step?+
Blood and plasma contain large proteins and cellular debris that can foul chromatography columns or interfere with mass spectrometry ionisation, so dialysis removes small target molecules like drugs or metals into a clean dialysate while retaining the bulky matrix components on the other side of the membrane.
What limits the usefulness of dialysis compared to other extraction methods?+
Dialysis is comparatively slow and works on the principle of passive diffusion, so it is generally reserved for specific applications such as metal analysis or particular clean-up needs, rather than being the default extraction method for routine drug screening, where solid-phase or liquid-liquid extraction is faster.
Related terms
- Conway Microdiffusion Cell
- A two-compartment glass or ceramic dish with a central well and an outer ring, sealed at the rim. The poison is released...
- Equilibrium Dialysis
- Variant in which the drug is allowed to redistribute across the membrane until the unbound concentration on both sides is equal. The...
- MWCO
- Molecular weight cut-off, the nominal size above which the membrane retains solutes. Forensic dialysis tubing typically uses 12 to 14 kDa MWCO,...
- Simple Distillation
- Heating an aqueous sample in a round-bottom flask and collecting the condensed vapour. Used for volatiles that boil below 100 degrees C...
- Steam Distillation
- Passing live steam through a sample so that volatile organic compounds co-distil with water at a temperature below their own boiling point....