Instrument probe tuning

An automatic probe tuning and matching (ATM) accessory allows one to automatically tune the NMR probe to the desired nuclei s resonant frequency and match the resistance of the probe circuit to 50 Q [7]. Traditional NMR instruments are designed so that one must perform these adjustments manually prior to data acquisition on a new sample. The advent of the ATM accessory allows the sampling of many different NMR samples without the need for human intervention. The ATM in conjunction with a sample changer enables NMR experiments to be conducted under complete automation. The sample changers are designed so that once the samples are prepared, they are placed into the instrument s sample holders. Data are then acquired under software control of both the mechanical sample delivery system as well as the electronics of the spectrometer. 

Robustness Even if the NMR instrument is not properly calibrated (for example, the probe tuning and pulse length calibration are not optimized), as... 

Modern instruments usually offer an on-board choice between a quadrature phase detector and some kind of diode or square detector. The latter, however is mostly used just for instrument setup (probe tuning, etc.) while signal acquisition is done almost exclusively by the phase detector. 

Probe tuning. The adjustment of the complex impedance of the probe to maximize the delivery of RF power to the sample (forward power), to minimize reflected RF power, and to maximize the sensitivity of the instrument receiver to the NMR signal emanating from the sample following the application of the pulse sequence. 

Many NMR probes now available are designed so that probe tuning response is relatively insensitive to sample changes. Consequently, we do not need to tune our probe or calibrate our RF pulses before carrying out various 1-D and 2-D NMR experiments. A probe of this nature is an essential feature of a high-throughput instrument. 

Probe tuning is not the final topic in the delivery of RF to our sample. Highpass, lowpass, bandstop, and bandpass analog filters are used to select which portion(s) of the RF frequency spectmm will get from one side of the filter to the other. Different filters are used, depending on need, and there are many variations from instrument to instrument. Nonetheless, many NMR instruments have similar filtering and RF configurations. Many NMR instruments normally operate with three active RF channels. 

Of all the inlets, the GC is the most foolproof, since operation of high vacuum valving, the chief danger with probe and batch inlets, is not necessary. Thus, once the instrument is properly tuned and operating, even novices may run their own GC/MS with minimum supervision. 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

With practice, we will be able to tune two RE channels of an NMR probe in under a minute, but we must take care not to damage the relatively delicate glass capacitors found in NMR probes (some newer NMR probes no longer contain these electrical elements—a point for rejoicing for those charged with maintaining NMR instrumentation in the multiuser environment). 

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