The structure of synthetic aluminosilicates prepared at pH 6 has been investigated by 29Si and 27A1 high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy. Fourier transform infrared (FTIR) spectroscopy and electron microscopy have also been used to characterize the products. The amount of Si and A1 in protoimogolite, disordered allophane and other structures has been measured. There is a fair correlation between the intensity of the 349 cm -1 band in the FTIR spectra and the proportion by weight of protoimogolite Si measured by NMR. It is shown that disordered allophanes have similar structures to those proposed by van Reeuwijk and de Villiers (Soil Sci. Soc. Am. Proc. 32 (1968) 238-240), i.e. octahedral A1 surrounding a tetrahedral core. Moreover, it is clear that at high AI:Si ratios (~>1:1), protoimogolite can compete with disordered allophane precursors for aluminum. The driving forces for formation of protoimogolite rather than allophane appear to be long range AI-A1 repulsive forces through oxygen.
2733on the tandem quadrupole-Fourier transform instrument for melittin at the 100 pmol level is displayed in Figure 5A. The SIN ratio achieved in this experiment is clearly substantially less than that observed for the other peptides discussed above. This spectrum is highly reproducible, however, and melittin at the 100-pmol level is often employed to test the tuning of the instrument. Glucagon and the insulin B chqin, peptides having molecular weights in the range 3300-3500 daltons, afford signals for (M + H)+ ions that are about the same as that observed for melittin. Insulins, in the molecular weight region 5600-5800 daltons, afford signals for (M + H)' ions that are barely visible above background on the present instrument. In the case of solid cesium iodide, Figure 5B, ions are readily observed out to mlz 9746, a limit imposed by our electronics and the software package on our present data system, but the SIN and resolution both deteriorate rapidly above mlz 3510.To make the tandem quadrupole-Fourier transform mass spectrometer perform routinely in the mass range above mlz 3000 on biological samples at the subnanomole level will require improvements in both the electronics and data system. Efforts are presently under way to modify both the excitation and detection circuitry, to increase the computing power of the data system beyond 16K transforms, and to alter the software controlling the instrument to permit selective accumulation of high mass ions in the cell prior to mass analysis. ACKNOWLEDGMENTThe authors are indebted to 0. P. Tanner and Gary Mappas of the Monsanto Co. and Robert Finnigan of Finnigan-MAT Corp. for their encouragement and support during this research effort.Registry No. Renin substrate tetradecapeptide, 98064-97-2; vitamin BIZ, 68-19-9; bradykinin, 58-82-2; gramicidin S, 113-73-5; cesium iodide, 7789-17-5; melittin, 37231-28-0. LITERATURE CITEDJohlman, C. L.; White, R. L.; Wilklns, C. L. Mass Spectrom. Rev.
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