The determination of the phospholipids concentration in liposomal suspensions is a key step in phospholipids science. Standard phosphate tests date back several decades and require extended hands-on time. The procedures consist in a chemical digestion step where the phosphodiester moiety of the phospholipid is oxidized to an orthophosphate. This step is often carried out by heating the phospholipid suspension in the presence of a strong acid or oxidizing agent in a open vessel. Evaporation of solutes calls for an extra, time consuming volumetric step. The orthophosphate is the detected colorimetrically as a metallo complex. Many coloring agents can be used mainly molybdenum compound such as vanadomolybdophosphoric acid, molybdenum blue or phophomolybdenum-malachite green complex[1].
The use of microwave assisted chemical digestion allows skip the volumetric step as no solvent is evaporated during the oxidation of the phospholipids. Vanadomolybdophosphoric acid has a maximum absorption wavelength (402 nm) close to the standard 405 nm filter found on almost every plate reader. The plate reader allows to analyze several samples in parallel with a high number of replicates. The new phosphate test method has proven to be efficient even for low concentration suspensions (2 µM to 200 µM of phosphate).
Etienne Stalder, Andreas Zumbuehl, CHIMIA International Journal for Chemistry, 67, (2013), 819-82.
etienne.stalder@unifr.ch
