8. Preparation of Apparatus
8.1 Read the following instructions in conjunction with the manufacturer's handbook. Preparation of the instrument is not critical but take care to prevent rapid temperature fluctuations of the instrument and the conditioning block, for example, avoid them from direct sunlight or from drafts.
8.2 The results obtained during the use of the equipment are susceptible to error arising from changes in the magnetic environment. Exercise care to ensure that there is a minimum of magnetic material in the immediate vicinity of the equipment and that this be kept constant throughout the course of a series of determinations.
8.3 Set the instrument controls to the following conditions:
NOTE 2 - On new NMR instruments with variable gates the gate should be set at 1.5 gauss to comply with nonvariable gate instruments.
8.4 Switch on the main supply to the spectrometer and allow it to warm up for at least 1 h.
8.5 Place a test cell containing sample in the coil and adjust the tuning of the instrument until the two resonance curves on the oscilloscope are coincident. This setting may need to be readjusted during determinations.
8.6 Remove the test cell from the coil and observe that the signal readout is now zero ±3 digits. This should be checked periodically during the series of tests to ensure that no contamination of the coil has occurred.
9. Preparation of Samples and Standard
9.1 Take a clean, dry test cell and PTFE plug and weigh them together to the nearest 0.01 g and record the mass. Add 30 ± 1 mL of the reference standard to the tube, taking extreme care to prevent splashing the liquid above the line inscribed on the tube. The use of a pipet is recommended for this operation.
9.2 Using the insertion rod, push the PTFE plug into the tube until it is just above the liquid-surface, keeping the tube upright. A gentle twisting of the plug as it is inserted will aid the escape of air from the test cell and normally ensure that the lip of the plug is turned up around the entire circumference. Take care to ensure that this is so, as a plug that is not properly inserted will allow rapid sample evaporation and give rise to change in the results obtained.
NOTE 3 - The insertion of the PTFE plug can be facilitated by inserting a length of thin (less than 0.2 mm diameter) copper wire down the inside surface of the disc until it is approximately 38 mm from the graduation mark and then pushing the PTFE plug down past the wire which is then removed.
9.3 The bottom rim of the plug should be at or slightly below the 51-mm mark on the test cell. Unscrew the insertion rod carefully without disturbing the plug.
9.4 Place the reference standard in the sample conditioning block.
9.5 Repeat the procedure outlined in 9.1-9.4 using the material to be tested.