Nmr Hardware And Software

In this chapter, we will follow the process of acquiring and processing an NMR spectrum in chronological order: preparing the sample, inserting it in the spectrometer, locking, shimming, acquiring the free induction decay (FID) (with a detailed look at the hardware), and processing the data. The goal is to obtain a general understanding of how the spectrometer works and how the data are processed, independent of any specific spectrometer (Bruker, Varian, etc.) or NMR software package. As these steps are the same in all spectrometers and with all software, you can apply this knowledge to the specific instrumentation and programs available to you even though the terminology and specific software commands and parameters may be different.

At this point, we need to discuss briefly the instrument manufacturers ("vendors") and the models (generations) of NMR spectrometers in use today. There are three main vendors right now: Bruker (based in Germany and Switzerland), Varian (Palo Alto, CA), and JEOL (Japan). We will concentrate on Bruker and Varian due to lack of experience with JEOL. The earliest commercial instruments (continuous wave) are gone to the scrap heap. The first significant generation of Fourier-transform (FT) instruments was the Bruker WM and AM, and the Varian VXR and Gemini. These had built-in computers and two channels: a transmitter for XH, 13C, and other nuclei, and a decoupler devoted to XH only. The Varian Unity came along with an "industry standard" UNIX computer (made by Sun Microsystems) connected to the spectrometer using an SCSI interface, and two equivalent "broadband" channels (covering 1H and all other nuclei). Bruker introduced the AMX with a separate UNIX computer made by Silicon Graphics (SGI) interfaced to the spectrometer with an Ethernet link. The AMX had capabilities for shaped pulses and optional pulsed field gradients (Chapter 8), as well as three radio frequency (RF) channels. Varian introduced the Unity-Plus with these features as options. They were integrated into the design of the next generation: Bruker DRX and Varian Inova models, which also included oversampling and

NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology, by Neil E Jacobsen Copyright © 2007 John Wiley & Sons, Inc.

digital filtering (Section 3.8). These have a modular design that allows for any number of RF channels, so that the designations "transmitter" and "decoupler" are no longer relevant; for example, you might have XH, 13 C, 15N, and 2 H channels all working together in one experiment.

In the discussion of parameter names and software commands, upper case (SW, TO, etc.) will be used in describing the oldest generation (Bruker AM and Varian Gemini) and lower case (sw, tof, etc.) will be used for the newer (UNIX based) models. Software is constantly being upgraded, so that the reader will have to refer to the vendor's manuals for precise information; the parameter and command names used in this book are only illustrative.

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