ASTM D4864 Test Method for Determination of Traces of Methanol in Propylene Concentrates by Gas Chromatography
9. Sampling
9.1 The propylene concentrate sample shall be in the liquified state and be representative of the material in the storage tank or process line. Also, for purposes of this method as well as for safety considerations, there must be a vapor space of about 15 % in the sampling container. It is recommended that sampling cylinders of the type listed in Section 7 be used. They can be equipped with an outage tube to effect the 15 % vapor space requirement.
10. Preparation of Apparatus
10.1 Prepare a water injection device. A suitable device is shown in Fig. 3. However, any other device that will deliver from 8 g to 15 g of water may be used.
10.2 Prepare a 300 mL sample cylinder for use as a methanol cylinder, as shown in Fig. 4. (This cylinder must not contain an outage tube.) Drill a 3 mm to 4 mm (approximately 1/8 in.) hole in a 1/4 in. NPT brass cap, insert an 11 mm septum into it, and screw it onto the plug valve.
10.2.1 As an alternative, the cylinder may be equipped with 1/4 in. male NPT to a 6.35 mm (1/4 in.) outside diameter tubing plug valve. Then a 6.35 mm tube fitting nut can be used with the septum, thus avoiding the necessity of drilling a brass cap.
10.3 Set up the chromatograph in accordance with the manufacturer's recommendations. Install the analytical column and adjust the gas flows and temperatures so that methanol will elute at the desired time. Condition the column at operating conditions until a stable baseline is recorded at the required sensitivity.
11. Calibration
11.1 Determination of Methanol Response Factor - Prepare several aqueous solutions of methanol in the same concentration range as expected for samples to be analyzed.
NOTE 3 - This should be approximately 40 mg/kg to 400 mg/kg (ppm mass) on the basis of propylene concentrate sample sizes of 100 g to 120 g, water extract volumes of about 10 g, and methanol concentrations in the propylene concentrate of 4 mg/kg to 40 mg/kg.
11.1.1 Methanol Stock Solution - Weigh an empty volumetric flask of at least 25 mL capacity to the nearest 0.0001 g. Add 20 mL ofdeionized water to the flask and reweigh. Finally, add 2 mL of methanol and again reweigh. Stopper and mix thoroughly. This should contain approximately 73 000 mg/kg (ppm by mass) of methanol. Calculate the exact concentration from the actual mass used. (Warning - Methanol is toxic and flammable. Use with adequate ventilation and keep away from ignition sources.)
NOTE 4 - Refer to Practice D4307 for additional information in preparing this solution and the calibration solution in 11.1.2.
11.1.2 Methanol Calibration Solutions - In similar manner, make serial dilutions by mass until two different concentrations in the range from 40 mg/kg to 400 mg/kg are prepared.
11.1.3 With the GC at the proper operating conditions, inject an appropriate quantity of each calibration solution, in duplicate, and obtain the area of the methanol peak.
NOTE 5 - The quantity of solution to be injected will depend largely on the type of detector in use. It varies from about 3 µL for FIDs to 25 µL for TCDs.
11.1.4 For each solution, calculate the response factor for methanol as follows:
F = C/H
where:
F = methanol response factor,
C = concentration of methanol, mg/kg, in the blend, and
H = area of the methanol peak (average of duplicate injections).
11.1.5 When the response factors at the two concentrations agree within 5 %, average them for use in the final calculation given in Section 13.
11.2 Determination of Methanol Extraction Factor - Since the methanol is not extracted quantitatively due to solubility competition between the water and the propylene, the extraction efficiency must be determined experimentally as follows.
11.2.1 Collect 100 g to 120 g (190 mL to 230 mL) of methanol-free propylene in a tared 300 mL sample cylinder. Reweigh the cylinder to ensure that it contains the proper amount. (Warning - Liquified propylene concentrate is at high pressure, can cause frostbite, and is flammable. Use appropriate care in handling.)
11.2.2 As shown in Fig. 4, attach the opposite end of the septum-equipped methanol cylinder to a vacuum source. Be sure that this assembly is leak-free.
11.2.3 Open both valves and evacuate the cylinder up to the septum. Then close the plug valve (next to the septum) and continue the evacuation. Finally, close the other cylinder valve, disconnect the cylinder from the vacuum source, and weigh it to the nearest 0.1 g.
11.2.4 Flush a 10 µL or 25 µL syringe several times with methanol, then fill it to the desired volume (see Table 2), wipe off the tip, pull the plunger back about 1 µL, and weigh it to the nearest 0.0001 g.
11.2.5 Open the cylinder plug valve at the septum and immediately inject the methanol in the syringe through the septum into the cylinder. Close the valve and immediately reweigh the syringe to determine the amount of methanol injected. The difference between this mass and that obtained in 11.2.4 is the mass of methanol injected, W1.
11.2.6 Cool the evacuated cylinder to about 20 °C below the temperature of the propylene concentrate cylinder.
11.2.7 As shown in Fig. 5, connect the cylinder containing propylene to the evacuated cylinder containing methanol via a hex coupling, a short length of 6.35 mm (1/4 in.) outside diameter SS tubing, or any other suitable fitting. Before tightening, flush the connector with a small amount of propylene concentrate by briefly opening the lower valve on the propylene concentrate cylinder.
11.2.8 With both cylinders in a vertical position and the propylene concentrate cylinder on top, open the valves between them (propylene concentrate cylinder first) and allow the liquified propylene concentrate to flow into the evacuated cylinder.
11.2.9 Close the valves, disconnect the cylinders, and allow the lower cylinder to warm to room temperature. Wipe off any water condensation and allow to dry.
11.2.10 Weigh the cylinder containing the methanol and propylene concentrate blend to the nearest 0.1 g. The difference between this mass and that obtained in 11.2.3 is the mass of methanol and propylene concentrate, W2. Calculate the concentration of methanol as follows:
C = (W1 x 10(6))/W2
where:
C = concentration of methanol, mg/kg,
W1 = mass of methanol injected, g, and
W2 = mass of propylene concentrate plus methanol, g.
11.2.11 Shake the cylinder vigorously to mix the propylene concentrate and methanol. Then extract the methanol and analyze the extract as described in 12.2-12.13. Analyze the extract in duplicate and average the methanol peak areas.
11.2.12 Calculate the methanol content of the extract as described in 13.1, but exclude Fx, the methanol extraction factor.
11.2.13 Calculate the methanol extraction factor as follows:
Fx = C/D
where:
Fx = methanol extraction factor,
C = methanol concentration calculated in Eq 2, and
D = methanol concentration calculated in 11.2.10.
NOTE 6 - It is recommended that the extraction procedure be repeated at a different concentration to verify the accuracy of the factor. Extraction factors of 1 to 2 are typical.
