Evaluation of Instrument and Associated Software

As is true of most modern instruments, the biggest Alearning curve@ is in learning the software commands for the instrument. The Anasazi Instruments modification uses two different sets of software. The operating software is Anasazi Instruments= PNMR instrument operating system. This software was updated twice after instrument installation (which slowed down the Alearning curve@). The latest version is Windows8-based, which is a big improvement over the DOS-based initial versions. After acquisition, the FID data is transformed and displayed using the NUTS8 software developed by Acorn NMR. This allows the data to be processed Aoff instrument@ on another computer, if desired, but adds to the number of commands that must be memorized or looked up. Despite this inconvenience, several students and faculty have become proficient in software use.

Despite the electronic upgrade to Fourier Transform technology, the instrument remains a low-field instrument. The energy level population difference (Dn0), which impacts both the resolution and the sensitivity of the instrument, is a function of magnet field strength (Bo) the number of nuclei in the sample (N):

Dn0 = N g S Bo / 2kT

Therefore, to make up for the lower field strength, the concentration must be enhanced. This is certainly the case for carbon-13 spectroscopy. Even with FT signal-averaging technology, a 60 MHz magnet (Bo = 1.41 T) will have far less sensitivity than a cryogenic 300 MHz instrument (Bo = 7.05 T). Natural abundance spectra of carbon-13 on this instrument have been found to require solutions in the 1-2 Molar range.

Two other important Ahigh frequency@ instrument features that this conversion lacks (and which were lacking on the original instrument) are lack of temperature control and lack of a deuterium lock. The first can result in frequency shifts over a long acquisition time, such as carbon-13 or multidimensional spectra, with the lack of the latter giving no method for automatic correction. We have observed a limit to the number of signal-averaging pulses that may be effectively employed on this instrument as a result. This further limits carbon-13 and 2-D spectra, as well as not allowing a planned in-depth investigation of nuclei relaxation times as per G. D. Bailey and W. R. Carper (J. Inorg. Biochem. 1993, 52, 99).

With these limitations in mind, the Anasazi Instruments modification has still resulted in a vast improvement in NMR technology at SAU. We now have the capability for signal-to-noise enhancement as a result of computer signal averaging, carbon-13 capability, two-dimensional methods (COSY, HETCOR, etc.), computer processing and analysis of the free induction decay (FID) signal, and even automatic computer-controlled shimming (allowing even our students to tune the instrument!). These features have been productively employed to enhance student understanding of chemical phenomena, as seen below.