Methods: 400 µL of 0.50 or 1.00% potassium dodecyl phosphate (PDP), sodium dodecyl phosphate (SDP), sodium dodecyl sulfate (SDS) or deionised water was added to 100 mg HA powder and agitated for 2 min. The HA slurry was centrifuged at 5000 rpm for 5 min and the supernatant removed. The HA was then rinsed three times in 900 µL deionised water for 2 min with vortexing; the HA slurry was centrifuged and the supernatant discarded between each rinse stage. The HA powder was then allowed to dry overnight under ambient conditions. TGA was used to measure mass loss of the dried powder by heating the HA to 800°C at a rate of 10°C.min-1. The mass loss between 200 and 600°C was calculated as the percentage mass loss relative to the initial mass.
Results: MAP as received has a purity of ca. 80% with ca. 20% dialkyl phosphate (DAP). It has previously been shown that DAP does not bind substantively to HA powder and is completely removed within three rinse stages. The deionised water treatment gave rise to a mass loss of 0.87%. Subtracting this from the total mass lost, and correcting for the 80% purity of the MAP, the relative mass loss for 1.00% PDP was 2.21%. The relative mass losses from HA treated with 0.50% PDP, SDP or SDS were 1.01%, 0.97% and 0.37% respectively.
Conclusion: TGA analysis allowed us to conclude that MAP binds substantively to HA, in contrast to the sulfate analogue which was statistically inferior (p<0.05).
Financial support received from GSK CH R&D.