Measurements of chain length distributions in calcium phosphate glasses using 2D 31P double quantum NMR

31P double quantum (DQ) NMR is used for improved studies of the structure of phosphate glasses. The common Qn notation of the network forming tetrahedral phosphate units with n bridging oxygen atoms is extended to Qn,jkl indicating the Q type of the bonded adjacent units by the additional superscripts j,k and l. It will be shown that: (i) Q1 units have different isotropic chemical shifts depending on whether they form diphosphate anions in the amorphous state (Q1,1) or end groups of chains (Q1,2) and (ii) that even Q2 units possess different isotropic chemical shifts such that in the DQ dimension Q2,11 can be distinguished from Q2,12 and middle groups of longer chains or rings (Q2,22). Based on this result the opportunity for measuring chain length distributions in the amorphous state is discussed and first results for binary calcium phosphate glasses are presented. The advantages and also the limitations of this novel approach will be discussed.