Compositional changes of a dicalcium phosphate dihydrate cement after implantation in sheep.

A hydraulic calcium phosphate cement having dicalcium phosphate dihydrate (DCPD) as end-product of the setting reaction was implanted in a cylindrical defect in the diaphysis of sheep for up to 6 months. The composition of the cement was investigated as a function of time. After setting, the cement composition consisted essentially of a mixture of DCPD and beta-tricalcium phosphate (beta-TCP). In the first few weeks of implantation, the edges of the cement samples became depleted in DCPD, suggesting a selective dissolution of DCPD, possibly due to low pH conditions. The cement resorption at this stage was high. After 8 weeks, the resorption rate slowed down. Simultaneously, a change of the color and density of the cement center was observed. These changes were due to the conversion of DCPD into a poorly crystalline apatite. Precipitation started after 6-8 weeks and progressed rapidly. At 9 weeks, the colored central zone reached its maximal size. The fraction of beta-TCP in the cement was constant at all time. Therefore, this study demonstrates that the resorption rate of DCPD cement is more pronounced as long as DCPD is not transformed in vivo.

Negative chemical ionization of alcohols.

Reactions of the oxygen radical anion at low pressure (ICR conditions) with aliphatic alcohols are seen to give a limited number of product anions which allows for unambiguous determination of the structure of the neutral. The reactions of OH- are also investigated and mechanisms of reactions are discussed. Analytical application of NCI is also considered. OH- negative chemical ionization mass spectra, negative metastable ion spectra, and negative collisional activation spectra (negative product from negative precursor ions) have been applied to stereochemical problems of cyclic diols. The spectra of the cis and trans isomers of 1,3- and 1,4-cyclohexanediols and 1,2- cyclopentanediols show substantial and characteristic differences. The (M-H)- alkoxide anions produced by OH- reagent ions are stabilized for the cis isomers by formation of an intramolecular hydrogen bridge involving both oxygen functions. However, trans geometry of the (M-H)- ions is indicated by abundant (M-H3)- and/or (M-H-H2O) fragment ions. The stereospecificity increases with ion internal energy (high temperature, collisional activation) or especially with ion life times (metastable ions). It is shown that in F- negative chemical ionization the relative intensities of the fluoride attachment ions MF- and of the proton abstraction ions (M-H)- reflect the stereochemistry of the cyclic diols. The results demonstrate the potential of negative ion mass spectrometry for stereochemical investigations.