Check if there is a sample in the magnet. Look at the analog meter for spin and lock signal lights. The monitor receives the direct signal from the magnet. The signal splits and goes to the computer. If you do not have a log in name use the temporary generic class login and password. After log in and entering a password the Solaris desktop environment is displayed. Select the VNMR icon to start the systems software --VNMR. Do not double click on the icon. Multiple copies of the software will operating problems.
The proton homonuclear decoupling procedure and meaning of the function keys are detailed. The first line is the permanent VNMR menu. It is always available on the upper row of the menu system. To make a choice, move the cursor over the button label with the action desired and click (press down and release) the left mouse button. Unlike all other menus, the function keys on the keyboard can not be used to make selections on this menu. Most of the terms are easy to understand. Flip alternately uncovers or conceals the text window. Flip is used when the graphics window covers the text window. Glide is an automated version of VNMR.
The second line off buttons is the Main Menu, this is the most important menu because it is a focal point from which all other menus can be reached. Think of the menu system as a tree, the Main menu is the trunk from which all the other menus grow. Since the Main menu can be reached at all times by using the Main Menu button in the Permanent menu, you can vary quickly go from any place in the menu system to any other place.
The acquisition status window shows if the instrument is idle or acquiring, who the user is, the number of experiments queued, etc. The first row of function keys are useful VNMR shortcut keys. The command line allow the user to enter commands two ways. Either type the short letter commands or click on the buttons. The most important VNMR paramaters for acquisition and decoupling follow.
Data is displayed in window #4.
Remember the following dps = display pulse sequence, d1 = relaxation delay, p1 = pulse, d2 = relaxation delay, pw - pulse, acquire (in units of time). Example: d1 and p1 = 0, pulse 0.5 u sec/acquire 1 second; repeat pulse/acquire 1000x. Also shows decoupler settings. Three channels are connected to the probe:
1. For proton NMR, power is sent to the decouple channel (outer coil). 2. For other nuclei, power goes to the observe channel (inner coil), and we much specify the target nucleus. 3. The lock channel is completely separate. The deuterium signal is used as a reference for all frequencies to keep the field optimized, and it does not interfere with acquiring spectra. Last is acqi -- window #5. The lock signal window does not display the peak but the integral of the peak. If baseline of the peak is flat and the peak is completely symmetric (phased correctly), a narrow peak gives an integral that looks like a long-division sign rather than a wavey-sine line. Base frequencies must be set so the peak is at the right position. The computer integrates over a certain range, so if the peak is out of range, the signal will be a flat or wavy integral. The lock is a function of: concentration of deuterium in the sample, the power set for the lock channel, and the volume (gain). The power is how hard we hit the sample (e. g. chimes) and the volume is the lock gain or how loudly we play back the signal. We reach saturation when the first hit is so hard that subsequent hits show a decrease in signal. At saturation, we will not be able to see small changes. To use the lock signal correctly, we need to decrease the power and increase the gain, which is analogous to hitting chimes more softly and turning up the microphone to hear them better, but that can produce to a lot of noise. So for each sample, the lock signal needs to be adjusted with respect to both power and gain. If everything is the same: the same nucleus, solvent, volume, probe, and constant amount of deuterated solvent, just recall the prior settings. The base frequency is Zo. With the lock "off", if the lock signal is flat or wavy, click with the left and right mouse buttons to change Zo and bring peak into area of integration. As you get closer, you will see less waves. If you notice saturation, turn down the power again until the signal stops bouncing up and down. When Zo is set, turn the lock on and then adjust the field (i.e. shim).
Shimming -- The window shows intensity of the peak. (This same level is displayed on the analog meter.) Change the current in 13 different shim coils, starting with the course adjustment of Z1. Keep changing until it does not get any better and shift to another (i.e. go back and forth between Z1 and Z2, then between Z3 and Z4) and repeat the process. It is easier with practice and generally only the Z1C shim coil needs adjustment for the same type of sample. Once the sample is shimmed type "go". Check lock signal, spin, and acquisition status. Note: ADC board is off during pulse and opens immediately afterward. Transmitter is on when the receiver is off. When the computer has acquired data, look at the signal, type `df' to display the FID. View the ppm scale by typing `dscale'. Expand part of the signal to see special details about the shape of the FID. Look at sinusoidal wave. Example if 11 cycles = 10 milliseconds, then the main signal is coming in at ~1100 Hz. Recall that we are digitizing the beat patterns (destructive/constructive interference) in the audio range, not radio frequency. We are measuring the difference between audio and radio; therefore, a 1 millisecond time range contains 8 data points per cycle (you can expand the scale to see each individual point). This is sufficient to get a good digital signal. Look again when more data is acquired. Note that there is a higher signal-to-noise ratio after averaging. Next, obtain a weighted-Fourier transform by typing `wft'. You should see your reference compound as well as signal(s) of interest. A signal out of range is a glitch (a problem with the electronics), mirror image of the glitch at an equal distance away from the true signal is a ghost, and there may also be a small amount of contaminating protons. A peak which is not a perfect Lorentzian shape means imperfect shimming. The presence of spinning side bands, small peaks at equal distance away from the major peaks, indicates inhomogeneity of the field in the xy plane. As the sample spins, most of the sample is experiencing the same field, but part is experiencing a different field. A slightly different field strength gives a slightly different energy gap.
Stepwise procedure for the determination of protons with the Unity 400 or a stepwise procedure for the determination of carbon with the Unity 400.
