ASTM D7096 Standard Test Method for Determination of the Boiling Range Distribution of Gasoline by Wide-Bore Capillary Gas Chromatography
9. Sampling
9.1 Sampling from Bulk Storage - Hydrocarbon liquids with vapor pressures of 16 psia or less may be sampled either into a floating piston cylinder or into an open container.
9.1.1 Piston Cylinder Sampling - Refer to Practice D3700 for instructions on transferring a representative sample of a hydrocarbon fluid from a source into a floating piston cylinder.

9.1.2 Open Container Sampling - Refer to Practice D4057 for instructions on manual sampling from bulk storage into open containers. Seal containers immediately after sampling and preserve the samples by storing at 0 °C to 4 °C and maintaining that temperature until prior to analysis.

9.2 Aliquoting Samples for Test:
9.2.1 Sampling from an Open Container - Cooled samples are transferred to a pre-cooled septum vial and sealed immediately. Obtain the test specimen for analysis directly from the sealed septum vial for automatic injection.

9.2.2 Sampling from a Floating Piston Cylinder - Samples contained in floating piston cylinders are transferred directly to a liquid sampling valve in the gas chromatograph by means of the ballast pressure in the cylinders. Before injection, verify that ballast pressure is sufficiently high to completely liquefy the sample.

9.3 Calibration Mixture - The calibration mixture should be stored in the refrigerator (0 °C to 4 °C) until ready for use. The calibration mixture shall be warmed to room temperature before sub-sampling (or analysis) to ensure that all components, particularly the C12 to C16 paraffins, are completely dissolved.

10. Preparation of Apparatus
10.1 Chromatographic Operating Conditions - Place in service in accordance with the manufacturer's instructions. Typical operating conditions are shown in Tables 1 and 2. Other conditions may be used provided they meet the criteria outlined in Section 11. Ensure that all components in the calibration mixture elute completely before the maximum oven temperature is reached.

10.2 Column Preparation - Follow Practice E1510 for recommended installation and conditioning procedures.

11. System Performance
11.1 Conformance with the performance criteria shall be established upon initial set-up ofthis test method and whenever any changes are made to the apparatus or the operating conditions. To check system performance, analyze in duplicate the calibration mixture (see 8.1 or 8.2), following the procedure described in Section 13. Using these results, confirm that the following criteria have been met.

11.2 Resolution - The system shall be able to identify the beginning and end of the elution of n-pentane and lighter saturated hydrocarbons from the column. The resolution (R) of dodecane and tridecane shall be between 6 and 10 when calculated according to Eq 1 (also see Fig. 1).
R = 2d/[1.699 (W1 + W2)]
where:
d = time between the peak maxima of dodecane and tridecane, s,
W1 = peak width at half height of dodecane, s, and
W2 = peak width at half height of tridecane, s.

11.3 Column Selectivity - Using a linear least squares fit of the data for only the n-paraffins (C 5 through C 16 ), establish the boiling point versus retention time relationship (see 12.1.1). From this relationship, calculate the apparent boiling point of each of the aromatics in the calibration mixture from their observed retention times. The apparent boiling point of each aromatic shall not differ from its actual boiling point by more than 2 °C (3 °F).

11.4 Peak Skewing - Peak skewing can result in retention time variance. Check skewness by calculating the ratio of the segments A/B as shown in Eq 2, on peaks in the calibration mixture. The ratio should be between 0.8 to 1.3. A graphical example of skew is given in Fig. 2.
S = A/B
where:
A = segment of the peak width (at 5 % of peak height) before the peak apex, and
B = segment of the peak width (at 5 % of peak height) after the peak apex.

11.5 Retention Time Repeatability - For consecutive analyses of the retention time mixture, the maximum difference in retention time for any component shall be 3 s (0.05 min), or less.

11.6 Minimum Propane Retention - Selection of column length and instrument operating conditions shall be such as to provide a minimum retention time for propane of at least 10 s (0.167 min).

11.7 Response Factor Validation - Refer to Practice D4626 for calculation of gas chromatographic response factors. To validate the experimental response factors, it is necessary to know the concentrations of the response factor standard components in both volume and mole percents. If conversion from one basis to another is required, a review of Practice D2421 is recommended. Appendix X4 provides sample calculations for response factor validation.
11.7.1 Volume response factors for each hydrocarbon component in the calibration mixture (not including the gaseous components) are calculated according to Eq 3. The values obtained shall agree within +/- 10 % of the theoretical volume response factors listed in Table 3.
RVRFi = (Vi x AC7)/(VC7 x Ai)
where:
RVRFi = volume response factor of component i, relative to the volume response factor of n-heptane,
AC7 = area of n-heptane peak,
VC7 = volume percent n-heptane,
Ai = area of component i, and
Vi = volume percent component i.

This same equation is used for the determination of the response factors of the oxygenate components that may be present in the gasoline.

11.7.2 The relative volume response factors of the gases are obtained by first determining the relative molar response factors of the C5 - C16 n-paraffins as calculated by Eq 4.
RMRi = (Ai x MC7)/(AC7 x Mi)
where:
RMRi = molar response factor of component i, relative to molar response factor of n-heptane,
AC7 = area of n-heptane peak,
MC7 = molar percent n-heptane,
Ai = area of component i, and
Mi = molar percent component i.

11.7.3 The relative molar response factor (RMR) is a linear function of the molecular weight for the n-paraffins. Thus, the RMRi is plotted versus the molecular weight. The data for the linear plot is subjected to a least squares fit. The plot should have a minimum least square fit (r2) of 0.99. By extrapolation, the RMRi for propane and n-butane are calculated from the resulting equation. Since the molecular weight of isobutane is the same as that of n-butane, both compounds have the same RMR; however, since their densities are not the same, their relative volume response factors will be different. Because of the low boiling point of isopentane and the difficulty in handling it on a balance, this compound is sometimes considered a gaseous component.

11.7.4 Convert the relative molar response factors of the gases to relative volume response factors utilizing the following equation (Eq 5):
RVRi = (MWi x RMRC7 x DC7)/(MWC7 x RMRi x Di)
where:
RVRi = relative volume response factor for the gas i,
MWi = the molecular weight of ith gas,
MWC7 = the molecular weight n-heptane,
Di = the density of the ith gas, and
DC7 = the density of n-heptane.

RMRi and RMR C7 are the relative molar response factors for ith gas and for n-heptane, respectively, as determined by Eq 4.

12. Calibration and Standardization
12.1 Non-oxygenated Gasoline - Prior to the analysis of samples, the analyzer should be calibrated to establish the boiling point versus retention time relationship. Calibration is carried out by analyzing the retention time and qualitative calibration mix(es) (see 8.1) using the procedure outlined in Section 13. Results from the calibration analyses are used to determine the following:
12.1.1 Boiling Point - Retention Time Correlation - Tabulate the retention time of each peak maximum and atmospheric boiling point in degrees Celsius (or Fahrenheit) of each component in the calibration mixture. Plot the retention times ofthe hydrocarbon components versus the corresponding atmospheric boiling point temperatures, as shown in Fig. 3. Visually verify that the calibration curve is essentially a straight line with slight curvature for the lowest boiling components.

12.1.2 Relative Volume Response Factors Calibration - Tabulate, for all components in the calibration mix, the retention time, area, and volume percent for each component. Utilize Eq 3 to calculate the relative volume response factor of each hydrocarbon heavier than n-butane. Calculate the molar percent composition of each n-paraffin in the mix. Plot the molar percent of each component versus the molecular weight, as described in 11.7. Fig. 4 shows a relative molar response plot. Using Eq 4 and 5, calculate the relative volume response factors for propane, isobutane, and n-butane. Tabulate the relative volume response factors and compare them to the theoretical volume response factors listed in Table 3. If agreement between experimental and theoretical response factors is within 10 %, the theoretical values should be used to determine the distillation results.

NOTE 1 - If the concentrations of propane, n-butane, and isobutane in the calibration mixture are known, differences noted between the observed and calculated molar response factors (MRF) indicate loss of light components. Ifa fresh calibration mixture is used, these differences can be indicative of sampling problems. Deviation of the molar response factors of the heavier components from the linear relationship could indicate problems in volatilizing the sample. Possible reasons include injection port temperature being too low, insufficient carrier gas flow, or lack of homogeneity during sampling. Fig. 4 illustrates these effects.

12.2 Oxygenated Gasoline - To measure gasoline blends containing oxygenates, the oxygenate shall be accounted for in the calibration. Calibration is carried out by analyzing the retention time and qualitative calibration mix(es) (see 8.2) using the procedure outlined in Section 13. Example chromatograms are presented in Figs. 5 and 6. Results from the calibration analyses are used to determine the following:
12.2.1 Boiling Point - Retention Time Correlation - Follow 12.1.1, omitting the oxygenate from the boiling point-retention time plot.

12.2.2 Relative Volume Response Factor Calibration - Follow 12.1.2, to calculate the experimental relative volume response factors of all components in the calibration mixture. Verify that the hydrocarbon response factors are within 10 % of theoretical. The oxygenate response factors should be similar to those listed in Table 5. Theoretical hydrocarbon response factors and experimental oxygenate response factors should be used for determining the distillation results.

12.3 Control Standard Analysis - A reference gasoline sample is used to verify both the chromatography and calculation process involved in this test method. Reference material should be analyzed daily to validate the distillation results. Following the procedures described in Section 13, analyze the reference gasoline (see 8.5). The boiling range distribution of the reference gasoline is calculated using the algorithm specified in Section 14.