Techniques precursor

Fixed-product ion scans (sector instruments). High-voltage scan or linked scan at constant B /E. Both techniques give a spectrum of all precursor (parent) ions that fragment to yield a preselected product (daughter) ion. 

Figure 1 shows the decomposition sequence for several hydrous precursors and indicates approximate temperatures at which the activated forms occur (1). As activation temperature is increased, the crystal stmctures become more ordered as can be seen by the x-ray diffraction patterns of Figure 2 (2). The similarity of these patterns combined with subtie effects of precursor crystal size, trace impurities, and details of sample preparation have led to some confusion in the Hterature (3). The crystal stmctures of the activated aluminas have, however, been well-documented by x-ray diffraction (4) and by nmr techniques (5).

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

Many materials have been deposited by PECVD. Typically, the use of a plasma allows equivalent-quaUty films to be deposited at temperatures several hundred degrees centigrade lower than those needed for thermal CVD techniques. Often, the plasma-enhanced techniques give amorphous films and films containing incompletely decomposed precursor species such as amorphous siUcon (i -Si H) and amorphous boron (i -B H). 

M. C. Dodge, Combined Use of Modeling Techniques and Smog Chamber Data to Derive O ne-Precursor Relationships,Repott No. EPA-600/3-77-001a, U.S. Environmental Protection Agency, Research Triangle Park, N.C., 1977. 

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