ASTM D2505 for Ethylene, Other Hydrocarbons, and Carbon Dioxide in High-Purity Ethylene by Gas Chromatography
8. Preparation of Apparatus
8.1 Silica Gel Column - Dry the silica gel in an oven at 204°C (400°F) for 3 h, cool in a desiccator, and store in screw-cap bottles. Pour the activated silica gel into a 0.9-m (3-ft) length of 6.4-mm (1/4-in.) outside diameter copper or aluminum tubing plugged with glass wool at one end. Tap or vibrate the tube while adding the silica gel to ensure uniform packing and plug the top end with glass wool. Shape the column to fit into the chromatograph.
8.2 Silver Nitrate - β,β'-Oxydipropionitrile - Activated Carbon Column - Weigh 10 g of β,β'-oxydipropionitrile into a brown 125-mL (4-oz) bottle. Add 5 g of silver nitrate (AgNO3) crystals. With occasional shaking, dissolve as much AgNO3 as possible, and allow the bottle to stand overnight to ensure saturation. Prepare this solution fresh, as required. Without disturbing the crystals at the bottom of the bottle, weigh 2.5 g of supernatant AgNO3 solution into a 250-mL beaker and add 50 mL of methanol. While stirring this mixture, slowly add 22.5 g of activated carbon. Place the beaker on a steam bath to evaporate the methanol. When the impregnated activated carbon appears to be dry, remove the beaker from the steam bath and finish drying in an oven at 100 to 110°C for 2 h. Plug one end of a 4-ft (1.2-m) length of 6.4-mm (1/4-in.) outside diameter aluminum or stainless steel tubing with glass wool. Hold the tubing vertically with the plugged end down and pour freshly dried column packing into it, vibrating the column during filling to ensure uniform packing. Plug the top end with glass wool and shape the tubing so that it may be mounted conveniently in the chromatograph.
8.3 Hexamethylphosphoramide Column (HMPA) - Dry the 35 to 80-mesh inert support at 204°C (400°F). Weigh 75 g into a wide-mouth 500-mL (16-oz) bottle. Add 15 g of HMPA to the inert support and shake and roll the mixture until the support appears to be uniformly wet with the HMPA. Pour the packing into a 6-m (20-ft) length of 6.4-mm (1/4-in.) outside diameter copper of aluminum tubing plugged at one end with glass wool. Vibrate the tubing while filling to ensure more uniform packing. Plug the top end of the column with glass wool and shape the column to fit into the chromatograph.
8.4 Hexadecane-Squalane Column - Dissolve 30 g of hexadecane into approximately 100 mL of acetone. Add 70 g of 80 to 100-mesh inert support. Mix thoroughly and pour the mixture into an open pan for drying. The slurry should be stirred during drying to ensure uniform distribution. When the acetone has evaporated, add a portion of the packing to a 7-m (25-ft) length of 3.2-mm (1/8-in.) outside diameter nylon tubing which has been plugged at one end with glass wool. Vibrate the column while filling to ensure more uniform packing. Fill the column with packing to only 4 m (15 ft) of the length of the column. Fill the remainder of the column with squalane packing prepared in the same manner as the hexadecane packing. Plug the open end of the tubing with glass wool and shape the column to fit into the chromatograph with the hexadecane portion of the column at the front end of the column. The column shall be purged under test conditions (no sample added) until a constant baseline is obtained.
NOTE 3 - Columns made with liquid phases listed above were used satisfactorily in cooperative work. Other columns may be used (see 5.3).
9. Calibration
9.1 Preparation of Standard Mixtures:
9.1.1 Preparation of Concentrate - Prepare a concentrate of the impurities expected to be encountered. A certified calibration blend containing the expected impurities can be obtained and used as the concentrate. An example of a satisfactory concentrate is given in Table 1. The concentrate can be prepared using the gas blending manifold as shown in Fig. 2 or using a similar apparatus as follows: Evacuate the apparatus and add the components in the order of increasing vapor pressure; that is, propylene, carbon dioxide, ethane and methane. Record the increase in pressure on the manometer as each component is added. Close the reservoir and evacuate the manometer after each addition.
9.1.2 Dilution of Concentrate - Dilute the concentrate with high-purity ethylene in a ratio of approximately 1:4000. This can be done by adding the calculated amount of the concentrate and high purity ethylene to an evacuated cyclinder using the gas-blending apparatus (Fig. 2). Use a source of high-pressure, high-purity ethylene equipped with a needle valve and a pressure gage capable of accurately measuring the pressure of the blend as the ethylene is added to the cylinder containing the concentrate. Add the calculated amount of ethylene; warm one end of the cylinder to ensure mixing of the blend. Allow the temperature to reach equilibrium before recording the final pressure on the cylinder. Prepare at least three calibration samples containing the compounds to be determined over the range of concentration desired in the products to be analyzed.
9.2 Calculation of Composition of Standard Mixtures - Calculate the exact ratio of the concentrate dilution with ethylene by correcting the pressure of the ethylene added for the compressibility of ethylene. Multiply the dilution ratio or factor by the percentage of each component present in the original concentrate. These calculations give the amount of each component that has been added to the high-purity ethylene blend stock. The actual composition of the final blend must be ascertained by making corrections for the impurities present in the high-purity ethylene used for the blend stock. The amount of correction is determined by making chromatograph runs on the high-purity ethylene and measuring the peak heights of the impurities. These peak heights will be used in adjusting the calibration factors described in 9.3. Since peak height is very sensitive to changes in conditions, it is extremely important in correlating peak heights obtained in making calibrations, calibration adjustments, and final impurity determinations that these values be obtained under the same GLC column operating conditions in all cases.
9.3 Determination of Calibration Factors - Chromatograph the standard blend and the high-purity ethylene blend stock by each of the procedures given in Section 10.
9.3.1 Calculate calibration factors for carbon dioxide, methane, ethane, propylene, and heavier hydrocarbons as follows:
F = C/(S - B)
where:
F = mol percent per unit of peak height,
C = concentration of component added to the high-purity ethylene blend stock, mol %,
S = mm peak height of component in standard mixture, and
B = mm peak height of component in high-purity ethylene blend stock, mm.
9.3.2 Calculate calibration factor for acetylene as follows:
Fa = C/(Sa - Ba)
where:
Fa = weight percent per unit area,
C = concentration of methane added to the high-purity ethylene blend stock in weight percent,
Sa = area of methane peak in standard mixture, and
Ba = area of methane peak in high-purity ethylene blend stock.
