ASTM D7171 Standard Test Method for Hydrogen Content of Middle Distillate Petroleum Products by Low-Resolution Pulsed Nuclear Magnetic Resonance Spectroscopy
6. Apparatus
6.1 Nuclear Magnetic Resonance Spectrometer:
6.1.1 This test method requires a low-resolution pulsed instrument capable of measuring a nuclear magnetic resonance signal due to hydrogen atoms in the sample in a linear fashion over the filling volume of interest. The instrument includes the following:
6.1.1.1 Permanent magnet to provide the necessary static magnetic field for the NMR test,

6.1.1.2 Sample compartment containing a radio frequency (RF) coil for excitation and detection, and

6.1.1.3 Electronic unit to control and monitor the resonance condition involving magnet temperature control and field offset coils.

6.1.2 The test method also requires that the instrument have the ability to equilibrate samples within the probe at a constant temperature (that is, 35 °C or 40 °C).

6.2 Conditioning Block - Block of aluminum alloy drilled with holes of sufficient size to accommodate the nominal 18 mm diameter test cells to a depth of at least 42 mm and with a centrally positioned well to house a temperature-sensing device, such as a thermometer or thermocouple.

6.3 Conditioning Apparatus - Bath or other temperature control device (into which the conditioning block is inserted) for controlling block temperature to 35 °C +/- 0.2 °C or 40 °C +/- 0.2 °C.

6.4 Test Cell - Glass tube (with a flat or round bottom) with an outside diameter of 17.6 mm to 18.1 mm and an inside diameter of 15.2 mm to 16.4 mm. Any tube length that permits easy insertion into and removal from the NMR sample chamber may be used.

6.5 Polytetrafluorethylene (PTFE) Plug - Device made of PTFE used to tightly fit and close the test cells. An example of one type of PTFE plug design is shown in Fig. 1.

6.6 Insertion Rod - Straight, rigid rod with a threaded end (to screw into the PTFE plug) for inserting and removing the PTFE plugs from the test cells. Any diameter and length rod that facilitates easy plug insertion and removal may be used.

6.7 Analytical Balance - Top loading pan-type balance, capable of weighing the test cells in an upright position to an accuracy of 0.001 g or better.

6.8 Volume Transferring Device - Capable of accurately and repeatedly delivering a fixed volume of material within +/- 1 % or better, for use in preparing test specimens and standards for analysis. A 10 mL capacity serological pipet with 0.1 mL marked subdivisions has been found suitable to use.

7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.

7.2 List of Available Calibration, Reference, or Check Standards (see Table 1). (Warning - Irritant. Combustible. Avoid breathing vapor. Avoid contact with eyes, skin, and clothing.) applies to all of the following:
7.2.1 3-cyclohexanepropionic acid, 99 % minimum purity.

7.2.2 cyclohexyl acetate, 99 % minimum purity.

7.2.3 diethyl malonate, 99 % minimum purity.

7.2.4 dodecane, 99 % minimum purity.

7.2.5 ethyl caprate, 99 % minimum purity.

7.2.6 ethyl heptanoate, 99 % minimum purity.

7.2.7 2-nonanone, 99 % minimum purity.

7.2.8 octyl acetate, 99 % minimum purity.

7.2.9 pentadecane, 99 % minimum purity.

7.2.10 2-phenylethyl acetate, 99 % minimum purity.

7.3 Quality Control (QC) Samples, preferably are portions of one or more petroleum distillate materials that are stable and representative of the samples of interest. These QC sample can be used to monitor the precision and stability of the testing process as described in Section 15.

8. Hazards
8.1 Wear appropriate personal protective equipment when working with the materials in Section 7. Transfer all reagents in a fume hood and immediately seal containers tightly. Avoid prolonged or repeated exposure to materials.

9. Sampling
9.1 Take a homogeneous sample in accordance with Practice D4057. Mix the sample prior to taking a representative aliquot as the test specimen.

10. Preparation of Test Specimens and Standards
10.1 Fig. 1 is an example of a PTFE plug. Dimensions may be varied to best seal the chosen test cell. Drill and thread the plug hole to mate the insertion rod thread. All test cells and PTFE plugs used shall be well cleaned and dry. Weigh a clean empty test cell and PTFE plug (W1) on an analytical balance to the nearest 0.001 g. Use of a jig for supporting the test cell on the balance pan is recommended for flat bottom test cells and required for round bottom test cells. For round bottom test cells, a paper clip dispenser (with hole in top) works well for this function. Transfer a fixed volume (in accordance with the tolerances specified in 6.8) of a reference standard or test specimen into the test cell to a fill height of nominally 32 mm (representing a nominal 6 mL volume). The choice of fill height is not critical, but fill height shall be the same for all standards and samples to ensure good results. Use the same test cell source (that is, manufacturer and part number) for all standards and test specimens. This will ensure uniformity of fluid height in the NMR sample compartment, an important parameter in the test method. Take care to not introduce the fluid down the side of the test cell. Seal the reference standard container immediately after the material transfer to minimize moisture pickup. Using the insertion rod with a PTFE plug attached, push the PTFE plug into the test cell until it is just above the liquid surface (that is, nominal 1 cm), keeping the tube upright. Gently twisting 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. Unscrew the insertion rod carefully without disturbing the plug. Weigh the filled, sealed test cell (W2) on the analytical balance to the nearest 0.001 g. Determine the sample mass from the difference (W2 - W1) of the two weighings.

10.2 Repeat 10.1 for all standards and test specimens.

11. Preparation of Apparatus
11.1 Place the conditioning block in the conditioning apparatus. Select a testing temperature of 35 °C or 40 °C based on the NMR instrument manufacturer's recommendation for the sample type to be tested. Adjust the conditioning apparatus setting to the selected 35 °C +/- 0.2 °C or 40 °C +/- 0.2 °C reading on a calibrated thermocouple or thermometer placed in a test cell filled with a test fluid and placed in the conditioning block/apparatus. Allow at least 30 min for temperature equilibrium after a setting adjustment. After adjustment, measure the aluminum block temperature at the thermometer/thermocouple well in the block, and control the conditioning apparatus at this block temperature.

11.2 Set the NMR instrument recycle delay to 20 s. Set the test specimen chamber temperature to 35 °C +/- 0.1 °C or 40 °C +/- 0.1 °C (consistent with selected conditioning temperature). Set all other NMR instrument parameters in accordance with the manufacturer's instructions. Ensure that the NMR spectrometer has been on for the minimum warm-up time recommended by the manufacturer and that the sample chamber temperature has equilibrated to 35 °C +/- 0.1 °C or 40 °C +/- 0.1 °C. Keep the room temperature below 28 °C to permit good thermal equilibrium of equipment.

11.3 The magnetic environment around the NMR spectrometer shall be kept constant during the calibration and testing. The position of substantial ferromagnetic metal objects, such as gas cylinders or metal chairs, cabinets, and doors, within 1 m of the spectrometer shall not be changed. Metal cabinet doors and drawers near (under) the spectrometer shall not be opened or closed during the analysis.