The determination of phosphorus containing compounds in dental casting investment products by 31P solid-state MAS-NMR spectroscopy.
OBJECTIVE
To use (31)P solid-state MAS-NMR to determine the phosphorus compounds that occur in dental casting investment material: (a) as-received, (b) after setting and (c) after burn-out and discover whether such compounds are the same in each material across a product range.


METHODS
[(1)H] High powered decoupling (HPDC) and [(1)H] cross-polarization (CP) (31)P solid-state MAS-NMR spectroscopy at a resonance frequency of 121.4 MHz were used. Six commercial products were examined. Manufacturer's instructions were followed and a special liquid was used without dilution.


RESULTS
All products contain ammonium dihydrogen phosphate as the acid phosphate required for the setting reaction. All set by the formation of struvite and significant amounts of amorphous magnesium orthophosphate. In three products, lesser amounts of newberyite were present and in another the equivalent amorphous compound was formed. When burnt-out, magnesium metaphosphate or pyrophosphate was the dominant matrix compound. A higher burn-out temperature favoured pyrophosphate formation. Farringtonite was present to a lesser extent with the metaphosphate.


CONCLUSIONS
Compounds that were not detected in earlier X-ray powder diffraction spectroscopy studies were detected by NMR, notably amorphous and glassy compounds (magnesium orthophosphate in set investment and magnesium metaphosphate in burnt-out material). The variation between products was significant and far greater than expected from the published scientific literature. Since the formation of compounds is affected by technical procedure and ambient conditions, these findings could offer some insight into the cause of the unpredictability of expansion measurements between laboratories. Further research is being undertaken.
Scrimgeour
Sheelagh N
SN
Dental School, University of Dundee, Dundee DD1 4HN, Scotland, UK.
Chudek
John A
JA
Lloyd
Charles H
CH
D003734:Dental Casting Investment / Q000737:chemistry*; D011756:Diphosphates / Q000032:analysis; D017616:Magnesium Compounds / Q000032:analysis; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D010710:Phosphates / Q000032:analysis; D017553:Phosphorus Compounds / Q000032:analysis*; D010759:Phosphorus Isotopes; D000069877:Struvite
D003734:Dental Casting Investment; D011756:Diphosphates; D017616:Magnesium Compounds; D010710:Phosphates; D017553:Phosphorus Compounds; D010759:Phosphorus Isotopes; C024788:ammonium phosphate; C030781:magnesium phosphate; D000069877:Struvite