ASTM D2163 Standard Test Method for Determination of Hydrocarbons in Liquefied Petroleum (LP) Gases and Propane/Propene Mixtures by Gas Chromatography
10. Procedure
10.1 Sampling - Sampling at the sample source and at the chromatograph shall always be done in a manner that ensures that a representative sample is being analyzed. Lack of precision and accuracy in using this test method can most often be attributed to improper sampling procedures. (See Practice D3700 and Practice D1265.)
10.2 Liquid Sample Valve Injection - For propene concentrates, butane samples, or other LPG samples, the sample may be introduced as a liquid by means of a liquid sample valve. It is strongly suggested that the use of a floating piston type sample cylinder be used and that the sample be pressurized to 1380 kPa (200 psi) above the vapor pressure of the sample prior to sampling.
10.2.1 In a hood, prior to connecting the cylinder, invert the cylinder and purge a small aliquot of the sample through the valve on the sample cylinder to remove any moisture or particulate matter which might be present.
10.2.2 Connect the pressurized liquid standard to the "sample in" port of the liquid sampling valve and close the waste vent shut-off valve. Open the outlet valve on the standard cylinder and open the waste shut-off valve for 10 to 15 s to allow sample to flow through the sampling valve. Flushing the valve several times prior to injection provides some local cooling, and it provides for more repeatable liquid injections. When liquid is flowing through the valve, quickly close the waste shut-off valve, then rotate the liquid sampling valve to inject the sample.
10.2.3 If the back flush option is being used, switch the back flush valve at the pre-determined time to elute the C5 =/C6+ composite to the detector.
10.3 Gas Sample Valve Injection (optional) - Vaporize the liquid sample according to the procedure given in 10.3.1 through 10.3.5, or using an on-line heated vaporizing device that is heat-traced to the gas sampling valve, as described in 10.3.6. Flush a gas sample loop with 5 to 10 mL of sample, close cylinder valve, and allow the sample pressure to equilibrate to atmospheric pressure (stopped flow) before introducing the sample into the carrier gas stream.
10.3.1 In a hood, prior to connecting the cylinder, invert the cylinder and purge a small aliquot of the sample through the valve on the sample cylinder to remove any moisture or particulate matter which might be present.
10.3.2 Attach a secondary sampling vessel, consisting of two ball valves joined together and having an internal volume of approximately one mL to the liquid outlet on the sample vessel.
10.3.3 Evacuate the secondary vessel to approximately 0.13 kPa (1 mm Hg), including the connection to the liquid outlet of the sample vessel. Close all valves.
10.3.4 Slowly open the sample outlet valve of the sample cylinder to fill the connection with liquid. Open the inlet ball valve of the secondary vessel and fill the vessel with liquid. Holding the liquid sample vessel vertically with the secondary vessel on the bottom, open the outlet ball valve and allow a portion of the liquid to purge through the secondary vessel. Shut the outlet ball valve, followed by the inlet ball valve and the sample cylinder outlet valve, in that order. Disconnect the secondary vessel.
10.3.5 Connect the secondary vessel to a container with an approximate volume of 100 mL which is fitted with needle valves or shut-off valves. Open the container valves and evacuate the container and connecting pipe work. Close the container outlet valve and slowly open the secondary vessel outlet valve to allow the liquid sample to vaporize into the evacuated vessel. Close all valves. The 100 mL container will contain a vapor that is representative of the liquid sample and have a gauge pressure of 69 to 138 kPa (10 to 20 psi). This gas may be used to purge the sample loop of the gas sampling valve as described in 11.1.
10.3.6 Alternatively, an on-line heated vaporizing device, which is heat-traced to the gas sampling valve, may be used. The device should consist of a volume of tubing of approximately 10 mL that is encased in a heated block (the block should be a high-mass block heated to approximately 60ºC). The outlet of the tubing should be heat-traced and connected to the gas sampling valve. Connect the liquid sample cylinder to the inlet of the heated tubing. Using the sample cylinder outlet valve, pulse several small aliquots of the liquid sample through the tubing successively. Allow the sample loop of the gas sampling valve to equilibrate to ambient pressure, and then rotate the gas sampling valve to inject the vaporized sample.
11. Calculation
11.1 External Standard Calibration Calculation (recommended) - Calculate the concentration of each component according to Eq 2. Determine the total amount of hydrocarbons by summing the component concentrations. If the sample is known to contain only hydrocarbons, then the results shall be normalized to 100.00%. Occasionally, normalized results will not equal precisely 100.00% due to rounding. In this case, small differences are typically added to the largest component. As stated in 1.2, this test method does not fully determine non-hydrocarbon materials and normalization could cause skewed data.
SCi = RFi x SAi
where:
SCi = concentration of component i in the sample,
RFi = response factor for component i, and
SAi = integrated area for peak i.
11.2 Theoretical Relative Response Calibration
Calculation - If a FID is being employed for the determination of those components, then theoretical response factors, as listed in Table 3, may be applied in place of RFi. The results shall then be normalized to 100%. Use of these response factors will produce results in mass percent units, which may be converted to other units (liquid volume percent or mole percent) by the user as needed. Alternately, the theoretical response factors may be converted to other units prior to quantitation. Quantitation using theoretical response factors does not account for the presence of non-hydrocarbon components. Example unit conversion calculations are found in Practice D2421. If non-hydrocarbon components are present, the results using this calculation method will not be representative or valid.
