ASTM D6550 Test Method for Determination of Olefin Content of Gasolines
ASTM D6550 Standard Test Method for Determination of Olefin Content of Gasolines by Supercritical-Fluid Chromatography
6. Apparatus
6.1 Supercritical-fluid Chromatograph (SFC) - Any SFC instrumentation can be used that has the following characteristics and meets the performance requirements specified in Section 8.

NOTE 2 - The SFC instruments suitable for Test Method D5186 are suitable for this test method, if equipped with two switching valves, as described under 6.1.7.

6.1.1 Pump - The SFC pump shall be able to operate at the required pressures (typically up to about 30 MPa) and deliver a sufficiently stable flow to meet the requirements of retention-time precision (better than 0.3 %) and detection background (see Section 8). The characteristics of the pump will largely determine the optimum column diameter. The use of 4.6-mm internal diameter (i.d.) columns requires a pump capacity of at least 1 mL/min of liquid CO2. Columns with an inside diameter of 2 and 1 mm require minimum pump capacities of 200 and 50 µL/min, respectively.

6.1.2 Detectors - A FID is required for quantitation. A flow restrictor shall be installed immediately before the FID. This restrictor serves to maintain the required pressure in the column, while allowing the pump and detector to perform as specified. A (diode-array or variable wavelength) UV detector for establishing optimum switching times (see Sections 8 and 9) is optional. Such a detector can be incorporated in two different manners.
6.1.2.1 A UV detector with a very small dead volume can be inserted between the column and the FID and operated in series.

6.1.2.2 A post-column splitting device, consisting of a T-junction with an appropriate flow restrictor to the FID, can be inserted between the column and the UV detector. Using the T-junction, the two detectors can be operated in parallel. The combination of restrictors (before the FID and after the UV detector) shall allow the pump and detector to perform as specified.

6.1.3 Sample-inlet System - A liquid-sample injection valve5 is required, capable of introducing (sub-)microlitre volume with a precision better than 0.5 %. A 1-µL injection volume was found to be adequate in combination with 4.6-mm inside diameter columns. Corresponding injection volumes are 200 and 50 nL for columns with inside diameters of 2 and 1 mm, respectively. The sample inlet system shall be installed and operated in a manner such that the chromatographic separation is not negatively affected.

6.1.4 Columns - Two columns of equal inside diameter are required:
6.1.4.1 A high-surface-area-silica column, capable of separating alkanes and olefins from aromatics as specified in Section 8. Typically, one or several 250-mm long columns are used. These columns are packed with particles having an average diameter of 5 µm or less, 600-nm (60-Å) pores, and a surface area of ≥350 m2/g.

NOTE 3 - Columns suitable for Test Method D 5186 are also suitable for the present method. A typical example is shown in Table 1.

6.1.4.2 A silver-loaded-silica column or a cation-exchange column in the silver form. Cation-exchange columns are claimed6 to yield more stable columns. Typically, one 50 or 100-mm long column packed with particles with an average diameter of 5 µm is used for the analysis.

NOTE 4 - Some columns that have been used successfully are shown in Table 1.

6.1.5 Column-temperature Control - The chromatograph shall be capable of column temperature control to within 0.5°C or less.

6.1.6 Computor or Electronic Integrator - Means shall be provided for the determination of accumulated peak areas. This can be done by means of a computer or electronic integrator. The computer or integrator shall have the capability of correcting for baseline shifts during the run.

6.1.7 Switching Valves - Two six-way switching valves are configured in accordance with the scheme shown in Fig. 1. This configuration allows four different valve positions, defined as follows:
6.1.7.1 Position A - Silica column (forward-flush mode) and silver-loaded column (forward-flush mode) connected in series. This position is used (a) to inject the sample on the two columns, (b) to elute the saturates, (c) to trap the olefins on the silver-loaded column, and (d) to retain the aromatics and oxygenates on the silica column.

6.1.7.2 Position B - Silica column (backflush mode) connected in-line; silver-loaded column not in flow path. This position is used to elute the aromatics and polar compounds.

6.1.7.3 Position C - Silica column not in flow path; silver-loaded column (backflush mode) connected in-line. This position is used to elute the olefins.

6.1.7.4 Position D - Silica column (forward-flush mode) connected in-line; silver-loaded column not in flow path. This position is used to optimize the separation. Also, this position allows Test Method D5186 to be performed without changing the system.

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 Air - Zero-grade (hydrocarbon-free) air is used as the FID oxidant. (Warning - Air is usually supplied as a compressed gas under high pressure, and it supports combustion.)

7.3 Calibration Solution - A mixture of hydrocarbons with a known mass % of olefins of the type and concentration found in typical gasolines. This olefin mixture can be diluted by weight with olefin-free components, such as alkylate, toluene, xylenes, and oxygenates, such as MTBE, as appropriate to approximate the composition of the fuels being tested.

7.4 Carbon Dioxide (CO2) - Supercritical-fluid-chromatographic grade, 99.99 % minimum purity, supplied pressurized in a cylinder with a dip tube for removal of liquid CO2. (Warning - Liquid at high pressure. Release of pressure results in production of extremely cold, solid CO2 and gas, which can dilute available atmospheric oxygen.)

7.5 Hydrogen - Hydrogen of high quality (hydrocarbon-free) is used as the fuel for the FID. (Warning - Hydrogen is usually supplied under high pressure and is extremely flammable.)

7.6 Loading-time Mixture - A mixture of a typical alkane and an olefin, which can be used to determine the loading time (see 8.2.2.3 (a) and 8.2.2.3 (b)) while protecting the silver-loaded column from exposure to aromatic compounds.

7.7 Performance Mixture - A mixture of a typical alkane, a mono-aromatic (usually toluene), and a typical mono-olefin can be used to fine-tune this test method and to check its performance. A mixture of n-heptane, toluene, and 3-methyl-2-pentene has been successfully used for this purpose.

7.8 Quality Control Sample - A motor gasoline containing olefins to be used to establish and monitor the precision of the analytical measurement system.