ASTM D5134 Standard Test Method for Detailed Analysis of Petroleum Naphthas through n-Nonane by Capillary Gas Chromatography
10. Column Evaluation
10.1 In order to establish that a column will perform the required separation, certain specifications must be met with respect to efficiency, resolution, and polarity. Determine the following data for new columns. Check older columns on a periodic basis to ensure that column deterioration has not occurred. A column which does not meet these specifications is unsuitable for use.
10.2 Set oven temperature parameters for isothermal operation. Under isothermal conditions at 35°C, inject ;25 µL of methane and record the retention time. Also at 35°C, analyze the column evaluation mixture described in 7.12. Record the retention times and the peak widths at half height of each of the components.
10.2.1 Calculate efficiency of the column using Eq 1. The number of theoretical plates (n) must be greater than 225 000.
10.2.2 Calculate resolution (R) between 2-methylheptane and 4-methylheptane using Eq 2. R must be at least 1.35.
where:
R = resolution,
tR(A) = retention time of 4-methylheptane,
tR(B) = retention time of 2-methylheptane,
Wh(A) = peak width at half-height of 4-methylheptane, and
Wh(B) = peak width at half-height of 2-methylheptane
10.2.3 Determine relative polarity of the column using the difference in Kovats Retention Indices (see Annex A1) of toluene and 2,3,3-trimethylpentane. The relative polarity of the column I(2.3,3-TMP)-I(Toluene) must be 0.4 more or less 0.4 at 35°C.
NOTE 2 - Caution: This specification is critical. Seemingly slight differences in the polarity have a significant effect on the relative order of elution of components, thus making peak identifications difficult.
10.2.3.1 Kovats Retention Index is given by:
10.2.3.2 Adjusted retention time of a peak is determined by subtracting the retention time of an unretained substance (methane) from the retention time of the peak.
10.2.3.3 If 2,3,3-trimethylpentane and toluene are not resolved, run separate mixtures, each containing only one of these compounds along with n-C7 and n-C8 in 2-methylpentane solvent.