What is the difference between an assay and an extemporaneous preparation?
An assay is an investigative (analytic) procedure for qualitatively
  assessing or quantitatively measuring the presence, amount, or
  functional activity of a target entity (the analyte).


E.g percentage purity of a given substance, viscosity, bulk density, total organic content etc.

The analyte can be a drug, a biochemical substance, or a cell in an organism or organic sample.

Extemporaneous preparation refers to the preparation (mixing, assembling, packaging and labelling) of a medicinal product by a pharmacist (or licensed practitioner) according to prescription requirements for a specific patient . Extemporaneous preparations  are made in  relatively small quantities  for short-term supply and use.

For example: Magesium trisilicate mixture can be extemporaneously prepared from the following:


Magnesium trisilicate
Light magnesium carbonate
Sodium Carbonate
Concentrated Peppermint emulsion
Double Strength Chloroform water


(Magnesium trisilicate mixture is an oral suspension containing 5% w/v of these excepients in suitable vehicle.)

Guidelines of extemporaneous preparations can be found in the Pharmacopoeia.

Where you want to determine the content of say one of the excepients , is assaying. For example you may wish to experimentally find the actual content of magnesium in the mixture, you may perform titration with a suitable standard solution, then you can actually be able to calculate its percentage weight in volume