ASTM D6730 Test Method for Individual Components in Spark Ignition Engine Fuels
ASTM D6730 Standard Test Method for Determination of Individual Components in Spark Ignition Engine Fuels by 100-Metre Capillary (with Precolumn) High-Resolution Gas Chromatography
11. Column Evaluation
11.1 In order to establish that a column will perform as required, the following specifications shall be determined for new column acceptability and are useful for periodic evaluation of column deterioration. These specification determinations can be made with or without a precolumn, since the precolumn will have little effect on their values. See Annex A1, Fig. A1.1, for examples of these determinations. After performing the steps in Sections 9 and 10, analyze the column performance mixture (7.5.5) at 35°C isothermal, at least through heptane. The remainder of the analysis may be ignored, but the remaining components must be eluted from the column prior to performing another analysis. Setting the column temperature to 220°C for an additional 20 min will be sufficient.
11.2 Calculate the retention factor (k) for pentane at 35°C:
k = (tR - tM)/tM
where:
tM = gas holdup time (methane), and
tR = retention time for pentane, min.
11.2.1 The retention factor must be between 0.45 and 0.50 for proper application of this test method.
11.3 Calculate the column efficiency using the pentane peak:
n = 5.545 (tR/w1/2h)2
where:
n = column efficiency (theoretical plates),
tR = retention time of pentane, and
w1/2h = peak width at half height.
11.3.1 The column efficiency must be at least 400 000 plates for proper application of this test method.
11.4 The selectivity of apparently identical columns toward hydrocarbons may vary regarding oxygenated compounds; either due to extraneous materials in the liquid phase, or due to activity of the column wall surface. The addition of a precolumn has little if any affect on the selectivity toward oxygenates (see Annex A1, Fig. A1.4). The relative resolution of oxygenates is inherent to the quality of the primary 100-m column, and is specified by the resolution of t-butanol from 2-methylbutene-2 at 35°C. Calculate the resolution:
R = 2(tR2-M-Butene-2 - tRTBA)/1.699(w1/2h2-M-Butene-2 + w1/2hTBA)
11.4.1 The resolution for this pair at 35°C must be between 3.25 and 5.25.
11.5 Extraneous column effects, or instrumental effects such as an active injector liner, may cause adsorption of oxygenated compounds, commonly seen and referred to as tailing, and may increase their retention. If this effect is caused by instrumental activity, the problem should be corrected. If the column is inherently active, a new column should be obtained. A measure of the tailing can be made and specified by applying a skewness calculation, which determines a ratio of the distances from the peak apex perpendicular to the front and back of the peak at 5 % of the peak height. See Annex A1, Fig. A1.3 for an example of this calculation.
skewness = B/A
11.5.1 This test method shall be made using the t-butanol peak (0.5 %) in the analysis of the column performance mixture (7.5.5) at 35°C isothermal. The skewness ratio must be greater than 1.0 and not more than 5.0.
