Building on the File "Common Practices..." of the Chem 3012 directory, below are some additional instructions which should be referred to during the semester.
Running an IR Spectrum (This is a more general treatment than the "Check In…" file in the Chem 3012 directory.)
The Instrument is a Perkin Elmer Spectrum 1 fourier transform infrared instrument. State of the art!!! You should follow the on-screen instructions to collect a spectrum of your solid or liquid sample. However, the technique for preparing the sample will vary depending on whether it is a solid or a liquid. You will need to run a background spectrum (one without a sample but with your holder) that the instrument will automatically subtract out of your sample when you run it.
A liquid sample is mounted between two plates of NaCl. These plates need to be rinsed carefully with 100% ethanol and set to drain on a paper towel before use. Place one drop of your sample on one plate, cover it with the other, and place the "sandwich" in the holder carefully (these break easily), and run the spectrum.
Solid samples, however, will be analyzed using a diffuse reflectance apparatus on the instrument. For a solid sample, thoroughly grind approximately 100 mg of dry KBr in an agate mortar and pestle. Fill one of the "macrocups" with this ground KBr as a background. Add approximately 5 mg of sample to the remaining KBr and grind thoroughly. Add this mixture (sample + KBr) to the second "macrocup". Mount the background and sample macrocups onto the holder and carefully place it into the diffuse reflectance apparatus. If this is not installed in the instrument already, ask your instructor to do this for you.
Spectra with impurities: Whether the sample contains more than one substance should be ascertained. This problem is usually encountered when a mixture of compounds have been separated, and traces of one compound are registered in the spectrum of another. This is most easily determined by lining up the spectra of the two fractions and looking at both together through a window. Mark where large peaks from one spectrum are found in the spectrum of the second compound. If no peak appears for the second compound at this position, this is significant; it means that the second compound is not contaminated with the first. Now it should be determined if the first compound was contaminated with the second. Contamination peaks must be recorded.
Running NMR Spectra: The instrument is a JEOL 300
MHz Superconducting Magnet, Fourier Transform Nuclear Magnetic Resonance
Spectrometer. Wow, how cool!! This is state of the art! The instrument
is located in the small room adjacent to the organic laboratory. The instrument
contains an extremely powerful magnet which presents some dangers. People with
pacemakers or metal containing devices must stay well away from the magnet.
All metal objects must be kept outside the room (chairs, tools, keychains, etc.).
This magnet will also destroy metal (particularly analog) watches and credit card magnetic
strips so keep them well away. 5 gauss magnetic field lines are indicated on the
floor in the instrument room to show the extent of the field from the magnet.
Everyone should stay outside these lines except to insert or remove a sample from the
instrument.
The samples will be calibrated and integrated by the instrument, and the spectra will be
printed on a laser printer for you. Include these printouts in your notebook.
Running the samples can be a slow process so be organized and prepared ahead of time so
you are ready to run when the instrument is free. Your instructor will provide
individual instructions for running your samples.
Sample Preparation: NMR samples must be rigorously free of extraneous solvent or large amounts of impurities in order to obtain quality spectroscopic data. Samples are prepared by placing at least 5-10 mg of analyte into a clean glass NMR tube, dissolving them in an appropriate DEUTERATED solvent, and capping the tube with the plastic cap provided. An "appropriate" solvent is one that will completely dissolve the analyte and is specially deuterated. The term "deuterated" means that the hydrogens (protons) on the solvent molecules are all replaced by a heavier isotope called deuterium (2H1). The instrument can "observe" signals from the deuterium nuclei and use these signals to calibrate the spectrum of your analyte. What do you think you would observe in the PMR spectrum of an analyte dissolved in H2O (instead of D2O)? Since the water makes up about 99% of the sample, you would get a beautiful spectrum of water. This might be interesting once, but incredibly annoying and unproductive if you wanted to see the analyte resonances. Bottom line: be sure to dry your samples adequately before trying to obtain an NMR spectrum of them. Additionally, it would be a worthwhile venture to check out the PMR spectrum of diethyl ether ("ether"), dichloromethane and acetone since these solvents are often observed as impurities from poorly "dried" products. Their chemical shifts will likely prove useful.
Revised by Shane Phillips on 9/2/99.