ASTM D2268 Test Method for Analysis of High-Purity n-Heptane and Isooctane by Capillary Gas Chromatography
6. Preparation of Resolving Column
NOTE 5 - There are many different procedures for coating capillary columns. A suitable procedure is given in 6.1 through 6.3. Other columns may be used provided they meet resolution and repeatability requirements of the method.
6.1 Connect a 229 mm (9-in.) section of stainless steel tubing 6.4 mm (1/4-in.) outside diameter, total volume of approximately 5 mL) to a high-pressure cylinder of argon, helium, or nitrogen through a pressure regulator. Connect at least 61 m (200 ft) of Type 316, 321, or 347 stainless steel tubing (0.25-mm (0.010-in.) inside diameter) to the 229-mm section of 64 mm tubing which is to be used as a reservoir for the coating solution. The capillary column is generally coiled on a suitable mandrel before coating. To the other end of the capillary column, connect an additional 30 to 9 to 12 m (40 ft) of capillary tubing through a 1.6 mm (1/16-in.) Swagelok union.
6.2 Clean the tubing by passing 25 to 30 mL (5 to 6 reservoir volumes) of n-pentane through the tubing with about 1.7 to 2.1 MPa ( 250 to 300 psig gage) of inert gas. After the column has been cleaned, disconnect the up-stream end of the reservoir tube and allow the pressure in the tubing to return to atmospheric.
6.3 Prepare a solution containing 6 volume percent of squalane in n-pentane. Fill the reservoir tube with the coating solution and promptly connect to the gas cylinder. Pass the coating solution through the column at 500 psig (3.5 MPa gage) until the solution begins issuing from the end of the capillary tubing; gradually reduce the inlet pressure in order to keep the flow of the solution at a relatively even rate of 40 to 60 drops/min. When the coating solution has been expelled from the column, reduce the inlet pressure to 345 kPa (50 psig gage) and allow gas to pass through the column for 1 to 2 h. Disconnect the 9 to 12-m (30 to 40-ft) tail section and then mount the column in the chromatograph.
6.4 To test column resolution use Fig. 1 and calculate R, from the distance between the cyclohexane and n-heptane peaks at the peak maxima, d, and the widths of the peaks at the baseline, Y1 and Y2.
R = 2(d1 - d2)/(Y1 + Y2)
Resolution (R), using the above equation, must exceed a value of 10.
7. Sample Preparation
7.1 Place 20 to 30 mL of the reference fuel (n-heptane or isooctane) into a 100-mL volumetric flask which has been previously weighed.
7.2 Weigh the sample. Using a 0.10-mL volumetric pipet, add 0.10 mL of the internal standard cyclohexane (99 mol %, min) and reweigh. Dilute to the mark with the n-heptane or isooctane sample and weigh. Use a 200-g analytical balance accurate to +/- 0.0002 g. From these weights (masses) and the relative density (specific gravities) of cyclohexane and n-heptane or isooctane, calculate the volume percent of the cyclohexane internal standard to the nearest 0.001 volume percent. (Relative density (specific gravity) of cyclohexane at 20°C = 0.7786; n-heptane = 0.6838, and 2,2,4-trimethylpentane = 0.6919.)
Cyclohexane, volume % = wt cyclohexane/rel dens cyclohexane / (wt reference fuel/rel dens reference fuel) x 100
