ASTM D7171 Standard Test Method for Hydrogen Content of Middle Distillate Petroleum Products by Low-Resolution Pulsed Nuclear Magnetic Resonance Spectroscopy
12. Procedure
12.1 Place the filled, weighed test cells in the conditioning block/apparatus. The test cells shall remain in the conditioning block/apparatus for a minimum of 20 min to reach thermal equilibrium.
12.2 Reference standards which may be used for this test method and their theoretical % hydrogen concentrations are shown in Table 1. These reference standards may be used as either calibration standards or check standards. Select calibration standards from Table 1 consistent with the instrument manufacturer's instructions for the calibration line. Use at least two pure compounds (99 % purity or better) bracketing the region of interest for establishing the calibration line. If more than two reference standards are used for calibration, the regression coefficient shall be ≥ 0.98.
12.3 Select a conditioned test cell containing a reference standard. Quickly wipe its exterior surfaces with a lint-free cloth and carefully insert it into the NMR instrument sample chamber. The test cell should be positioned so that the material to be analyzed is in the optimum measurement region of the NMR sample chamber. Allow at least 10 s for the test cell assembly to reach thermal equilibrium in the instrument. Follow the instrument manufacturer's instructions for establishing the calibration line. Remove the test cell from the instrument. Similarly, run the other calibration standards to complete the calibration.
12.4 Insert a conditioned test cell (in the same manner described in 12.3) containing a check standard into the sample chamber. The check standard is a material of known % hydrogen, but not one of the materials used as a calibration standard. In Table 1, 2-nonanone is a good check standard since its hydrogen content is the closest in Table 1 to that of a number of middle distillate products. Allow at least 10 s for thermal equilibrium, and run the check standard in accordance with the instrument manufacturer's instructions for analyzing test specimens. The % hydrogen value determined shall agree within 1.5 % relative of the known value before proceeding with testing of petroleum distillate samples. If the check standard is not within 1.5 % relative of the known value, recalibration of the NMR instrument is required. The check standard shall be run prior to each new batch of distillate test specimens and at least once on each day of analysis.
12.5 Insert a conditioned test cell (as described in 12.3) containing a middle distillate test specimen into the sample chamber. Allow at least 10 s for the middle distillate test cell to reach thermal equilibrium. Follow the instrument manufacturer's instructions for analyzing the test specimen. Similarly, run other test specimens. Similarly, analyze the check standard after completing the batch of middle distillate test specimens. If the result is not within 1.5 % relative of the known concentration, recalibrate the instrument and reanalyze all test specimens run since the last acceptable check standard analysis.
13. Calculation
13.1 Pulsed NMR instruments have software routines that calculate the % hydrogen based on the calibration entries, the instrument response, and the sample mass entries as appropriate for the instrument manufacturer's programs.
