ASTM D7096 Standard Test Method for Determination of the Boiling Range Distribution of Gasoline by Wide-Bore Capillary Gas Chromatography
7. Apparatus
7.1 Gas Chromatograph - Any gas chromatograph (GC) designed for use with wide-bore (0.53 mm inside diameter) capillary columns, that meets the performance criteria specified in Section 11, and has the following features may be used. Typical operating conditions are shown in Table 1.
7.1.1 Column Oven Temperature Programming - The gas chromatograph shall be capable of linear temperature-programmed operation from −40 °C to 280 °C at rates up to 25 °C/min.
7.1.2 Injection Port - The injection port shall be capable of operation at temperatures required to completely volatize and transfer the sample to the GC column. Non-splitting or split/splitless vaporizing sample ports optimized for use with wide-bore capillary columns are acceptable. If using a split inlet port, it should be designed to provide a linear sample split injection.
7.1.3 Flame Ionization Detector - The detector shall be optimized for the use of wide-bore capillary gas chromatographic columns and shall conform to the specifications as described in Practice E594.
7.1.4 Carrier Gas Controls - The associated carrier gas controls shall be ofsufficient precision to produce reproducible column flows in order to maintain analytical integrity.
7.1.5 Baseline Correction - The gas chromatograph (or another component of the gas chromatographic system) shall be capable of subtracting the area slice of a blank run from the corresponding area slice of a sample run. This can be done internally on some gas chromatographs (baseline compensation) or externally by subtracting a stored, digitized signal from a blank run.
7.2 Sample Introduction - Sample introduction may be by means of a constant volume liquid sample valve or by injection with a micro syringe through a septum. An automatic sample introduction device is essential to the reproducibility of the analysis. Manual injections are not recommended. Poor injection technique can result in poor resolution. Ifcolumn overload occurs, peak skewing may result, leading to variation in retention times.
7.2.1 Samples with a vapor pressure (VP) of less than 16 psia as measured by Test Method D5191, or equivalent, may be introduced into the gas chromatograph by syringe injection into a heated, vaporizing inlet. Samples with vapor pressures between 12 psia and 16 psia should be kept chilled (refrigerated or in a cooled sample tray) and may require injection with a cooled syringe. Samples with a vapor pressure above 16 psia should be introduced by way of a constant volume liquid sampling valve. Refer to 9.1 for sampling practices.
7.3 Column - Any wide bore (0.53 mm inside diameter) open tubular (capillary) column, coated with a non-polar (100 % polydimethylsiloxane) phase that meets the performance criteria of 11.3 may be used. Columns of 15 metre to 30 metre lengths with a stationary phase film thickness of 5.0 µm have been successfully used. With either of these columns, initial cryogenic temperatures are not necessary.
7.4 Data Acquisition System - A computer provided with a monitor, printer, and data acquisition software is necessary to carry out this analysis. The computer should have sufficient hardware capacity and random access memory in order to run the data acquisition program while acquiring data at a frequency of 2 Hz to 5 Hz. The software should also be able to store the data for future recall, inspection, and analysis. The data acquisition software should be capable ofpresenting a real time plot. It may also be capable of controlling the operating variables of the gas chromatograph. Specialized software is necessary to obtain the boiling point distribution.
7.5 Bulk Sample Containers, floating piston cylinders (see 9.1.1); epoxy phenolic-lined metal cans; glass bottles with polytetrafluoroethylene-lined screw caps.
8. Reagents and Materials
8.1 Calibration Mixture - A synthetic mixture ofpure liquid hydrocarbons with boiling points that encompass the boiling range of the sample shall be used for retention time determination and response factor validation. Response factors for propane, isobutane, and n-butane are extrapolated from the relative molar response of the n-paraffins. An example of a relative response factor mixture with suggested nominal composition is given in Table 3. This mixture shall be accurately prepared on a mass basis using Practice D4307 or equivalent.
8.1.1 Asingle calibration standard may be used for retention time-boiling point determination and response factor validation provided isopentane and heavier components are known quantitatively. Gaseous components propane, isobutane, and n-butane are added in small quantities (< 0.2 volume % each). These small quantities do not significantly change the concentrations of the remaining hydrocarbons. This standard would also be used for measuring performance criteria in Section 11. It may be practical to generate this standard by bubbling a small amount of gaseous propane, isobutane, and n-butane (Warning - Extremely flammable gases.) into a quantitative mixture of isopentane and heavier components.
8.1.2 A combination of two calibration standards may also be used. A quantitative standard, containing known concentrations of isopentane and heavier compounds, is used to determine response factors. A qualitative standard, containing a wide boiling range of compounds including propane, n-butane, and isobutane is used for measuring the retention time-boiling point relationship and establishing the performance criteria outlined in Section 11.
8.2 Calibration Mixture with Oxygenates - When samples to be measured contain oxygenates, the calibration mixture (see 8.1) shall also contain the oxygenates. Therefore, the identity of the oxygenate(s) shall be known prior to analysis of the sample. Oxygenate content may be determined by Test Method D4815, Test Method D5599, or equivalent. Oxygenates, such as ethanol, should be added to the calibration mixture at an approximate concentration as that in the samples. This mixture is used to define the retention time boundary and relative volume response of the oxygenate to be applied to this region. For gasoline containing other oxygenates, determine if the oxygenate coelutes with any of the hydrocarbons listed in Table 3. If a coelution occurs, the coeluting hydrocarbon should not be included in the blend. Typical compositions of oxygenated blends are given in Table 4. Typical relative volume response factors, molecular weights, and densities for various oxygenated compounds are provided in Table 5.
8.3 Carrier Gas - Helium, 99.999 mol% pure. (Warning - Compressed gas under high pressure.)
8.4 Detector Gasses:
8.4.1 Fuel - Hydrogen, 99.999 mol% pure. (Warning - Extremely flammable gas under pressure.)
8.4.2 Oxidant - Air, 99.999 % free of hydrocarbons and water. (Warning - Compressed gas under high pressure. Supports combustion.)
8.5 Reference Gasoline - A gasoline sample that has been analyzed by laboratories participating in a test method cooperative study. (Warning - Extremely flammable liquid. Vapors are harmful if inhaled.)
