8. Procedure
8.1 Adjust the operating variables to optimum conditions. Temperatures should be as follows: Injection port and splitter 150 to 250°C, column at optimum temperature and detector greater than 100°C. Adjust the excess gas flow through the splitter to provide a proper sample size to the column.
8.2 Using the microsyringe, inject sufficient sample containing the internal standard. Both the sample volume and the split ratio must be considered in choosing the correct volume of sample to inject. Volumes entering the column in the range of 0.002 to 0.005 µL have been found satisfactory.
8.3 The various impurities present in the primary reference standards can be identified from retention time data obtained at the same gas chromatographic conditions. Typical chromatograms of ASTM n-heptane and ASTM isooctane are shown in Fig. 2. Relative retention time data for a number of hydrocarbons over squalane at 30°C are given in Table 1. The retention time data of Table 1 are corrected for the gas holdup of the column and are relative to n-heptane. Argon was used as the carrier gas.
8.4 Hydrocarbons that are commonly found as impurities in ASTM n-heptane and ASTM isooctane are listed in Table 2.
9. Calculation
9.1 After identifying the various impurities, measure the peak area of each impurity peak and that of the internal standard, cyclohexane, by ball and disk integrator or electronic integrator. Calculate the volume percent of each impurity as follows:
where:
VI = volume percent of the impurity to be determined,
VS = volume percent of the internal standard, cyclohexane,
PAI = peak area of the impurity to be determined,
PAS = peak area of the internal standard, cyclohexane, and
SI = the response per unit volume of the hydrogen flame ionization detector to the impurity relative to the response per unit volume to cyclohexane.
9.2 Hydrogen flame ionization detector response is given for several hydrocarbons relative to cyclohexane in Table 3. Report the volume percent of each impurity.
9.3 Total the concentrations of the individual impurities and then calculate the purity of the n-heptane or isooctane sample by difference.