ASTM D2427 Method for Determination of C2 through C5 Hydrocarbons in Gasolines
ASTM D2427 Standard Test Method for Determination of C2 through C5 Hydrocarbons in Gasolines by Gas Chromatography
8. Procedure
8.1 Calibration - Determine the relative area response of the compounds to be reported by injecting known quantities of the pure compounds or by using synthetic blends of known composition. For those compounds that are normally gases at room temperature it is advantageous to use commercially available certified light hydrocarbon blends. Sample light hydrocarbon blends contained in pressure containers from the liquid phase (Warning - Extremely flammable gas under pressure.) Blends of those hydrocarbons that are normally liquid at room temperature are easily prepared by volume with sufficient accuracy to establish relative response factors (Warning - Extremely flammable liquids.) If measurement of the C6 and heavier material by reverse flow through the detector is intended, an average calibration factor for these heavy materials must be determined. Gasolines that have been depentanized by laboratory distillation may be used as calibrants for this purpose (Warning - Extremely flammable.) If use of an internal standard is contemplated, the internal standard selected should be included in the calibration program.
8.2 Analysis:
8.2.1 Backflush Method - When the backflush technique is used, add a known quantity of internal standard equal to about 5 % to the sample. The internal standard can be added on either a weight or volume basis depending upon the method of reporting. One method of adding the internal standard that has been found convenient is given in Annex A1. Alternatively, quantitative results can be obtained by injecting repeatable quantities of the sample and of a known blend, and comparing the peak areas obtained for the sample with those obtained for the known concentration of components in the blend.
8.2.1.1 Precut Column - If a precut column is used, adjust the valving so that carrier gas is flowing in the normal direction through both the precut and analysis columns. Using a chilled syringe, charge sufficient sample to ensure a minimum of 10 % recorder deflection for a 0.1 % sample concentration of 2-methylbutene-2 at the most sensitive setting of the instrument. When all of the C5 and lighter hydrocarbons plus internal standard, if used, have entered the analyzer column, position the valves so that backflushing of the precut column is initiated. The time at which backflushing is commenced is critical and may have to be determined by trial and error. If properly done, it results in the elimination of any interference from low-boiling six-carbon paraffins and produces a chromatogram that exhibits peaks for C2 through C5 paraffins and olefins only (Fig. 1). When the last compound has been eluted, remove the chromatogram and proceed as described in 9.1.1.
8.2.1.2 Single Column - If a single column is used, it may be backflushed if an appropriate valving system has been installed. The operations described above are performed except that backflushing is commenced only when all the C5 and lighter hydrocarbons and internal standard have been eluted. The purpose of backflushing in this case is not to improve the separation, but merely to shorten the total analysis time and avoid passage of higher boiling hydrocarbons through the detector.
8.2.2 Reverse Flow Method - If reverse flow of the C6 and heavier portion through the detector is employed, the addition of an internal standard is unnecessary if adequate calibration has been performed and the composition of the C6 and heavier portion does not differ significantly from that of the depentanized gasolines used as calibrants. An internal standard can be used periodically to assure that analytical accuracy is maintained (Note 2). Adjust the valving so that carrier gas is flowing through the instrument in the normal direction. Charge sufficient sample to ensure a minimum of 10 % recorder deflection for a 0.1 percent sample concentration of 2-methylbutene-2 at the highest sensitivity. As soon as the last pentene peak (2-methylbutene-2) has been eluted, position the valving so that the carrier gas flow is reversed. After the flow has stabilized, adjust the base line. Attenuate for the C6 + portion as necessary. The run is complete when the recorder returns to the base line after elution of the C6 portion. This part of the sample will generally emerge as one broad peak with only slight indications of any separation. Proceed as described in 9.1.2.
NOTE 2 - All reverse flow determinations, including the C6 and heavier calibration runs should be made in the same carrier gas flow direction. All single-peak determinations and corresponding calibrations will then be made in the opposite carrier gas flow direction. The column should be kept free of high-boiling residues by periodically sweeping with carrier gas for several hours in the reverse direction. A level base line will quickly be attained in either flow direction when the column is clean and the system is free of leaks.
