Synthesis of isotope-labelled methoxypyrazine compounds as internal standards and quantitative determination of aroma methoxypyrazines in water and wines by solid-phase extraction with isotope dilution-GC-MS /

An efficient way of synthesizing the deuterium labelled analogues of three methoxypyrazine compounds: 2-d3-methoxy-3-isopropylpyrazine, 2-d3-methoxy-3- isobutylpyrazine, and 2-d3-methoxy-3-secbutylpyrazine, has been developed. To confirm that the deuterium labels had been incorporated into the expected positions in the molecules synthesized, the relevant characterization by NMR, HRMS and GC/MS analysis was conducted. Another part of this work involved quantitative determination of methoxypyrazines in water and wines. Solid-phase extraction (SPE) proved to be a suitable means for the sample separation and concentration prior to GC/MS analysis.Such factors as the presence of ethanol, salt, and acid have been investigated which can influence the recovery by SPE for the pyrazines from the water matrix. Significantly, in this work comparatively simple fractional distillation was attempted to replace the conventional steam distillation for pre-concentrating a sample with a relatively large volume prior to SPE. Finally, a real wine sample spiked with the relevant isotope-labelled methoxypyrazines was quantitatively analyzed, revealing that the wine with 10 beetles per litre contained 138 ppt of 2-methoxy-3-isopropylpyrazine. Interestingly, we have also found that 2-methoxy-3-secbutylpyrazine exhibits an extremely low detection limit in GC/MS analysis compared with the detection limit of the other two methoxypyrazines: 2- methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine.

Polytypism and Silicon carbide : a solid state nuclear magnetic resonance study

A survey of predominantly industrial silicon carbide has been carried out using Magic Angle Spinning nuclear magnetic resonance (MAS nmr); a solid state technique. Three silicon carbide polytypes were studied; 3C, 6H, and 15R. The 13C and 29 Si MAS nmr spectra of the bulk SiC sample was identified on the basis of silicon (carbon) site type in the d iff ere n t pol Y t Y pes • Out to 5.00 A fro mac en t r a lsi 1 i con (0 r carbon) atom four types of sites were characterized using symmetry based calculations. This method of polytype analysis was also considered, in the prelminary stages, for applications with other polytypic material; CdBr 2 , CdI 2 , and PbI 2 " In an attempt to understand the minor components of silicon carbide, such as its surface, some samples were hydrofluoric acid washed and heated to extreme temperatures. Basically, an HF removable species which absorbs at -110 ppm (Si0 2 ) in the 29 Si MAS nmr spectrum is found in silicon carbide after heating. Other unidentified peaks observed at short recycle delays in some 29 Si MAS nmr spectra are considered to be impurities that may be within the lattice. These components comprise less than 5% of the observable silicon. A Tl study was carried out for 29 Si nuclei in a 3C ii polytype sample, using the Driven Equilibrium Single-Pulse Observation of T1 (DESPOT) technique. It appears as though there are a number of nuclei that have the same chemical shift but different T1 relaxation times. The T1 values range from 30 seconds to 11 minutes. Caution has to be kept when interpreting these results because this is the first time that DESPOT has been used for solid samples and it is not likely in full working order. MAS nmr indicates that the 13C and 29 Si ~sotropic chemical shifts of silicon carbide appear to have a reciprocal type of relationship_ Single crystal nmr analysis of a 6H sample is accordance with this finding when only the resultant isotropic shift is considered. However, single crystal nmr also shows that the actual response of the silicon and carbon nuclear environment to the applied magnetic field at various angles is not at all reciprocal. Such results show that much more single crystal nmr work is required to determine the actual behavior of the local magnetic environment of the SiC nuclei.