11. Procedure
11.1 Sample Preparation:
11.1.1 The amount of sample injected must not overload the column stationary phase nor exceed the detector linear range. A narrow boiling range sample will require a smaller amount injected than a wider boiling range sample.
11.1.1.1 To determine the detector linear range, refer to Practice E 594 for flame ionization detectors or Practice E 516 for thermal conductivity detectors.
11.1.1.2 The column stationary phase capacity can be estimated from the chromatogram of the calibration mixture (see 9.3.2). Different volumes of the calibration standard can be injected to find the maximum amount of a component that the stationary phase can tolerate without overloading (see 10.3.1). Note the peak height for this amount of sample. The maximum sample signal intensity should not exceed this peak height.
11.1.2 Samples that are of low enough viscosity to be sampled with a syringe at ambient temperature may be injected neat. This type of sample may also be diluted with CS2 to control the amount of sample injected to comply with 11.1.1.
11.1.3 Samples that are too viscous or waxy to sample with a syringe may be diluted with CS2.
11.1.4 Typical sample injection volumes are listed below.
11.2 Sample Analysis - Using the analysis sequence protocol, inject a sample aliquot into the gas chromatograph. Collect a contiguous time slice area record of the entire analysis.
12. Calculation
NOTE 6 - See Appendix X3 for the recommended calculation algorithm.
12.1 Correct the sample area slices for nonsample detector response by subtracting each blank analysis area slice (as determined in 10.2) from each sample area slice at the equivalent slice time. Also see Note 5 regarding automatic baseline correction. Sum the corrected area slices to obtain the cumulative corrected areas for each time interval during the run.
12.2 At the point on the chromatogram where the baseline at the end of the run first becomes steady as the total area point, record the total cumulative corrected area counts. Move back along the chromatogram until a cumulative area equals 99.5 % of the total area. Mark this point as the FBP.
NOTE 7 - Location of the FBP may be the most difficult step in this test method. Some samples have extremely long tailing end portions due to gradually decreasing quantities of heavy materials. This fact, coupled with the natural tendency of the chromatographic baseline to rise at the end of the run due to septum or column bleed or elution of traces of heavy compounds from previous samples, can preclude the possibility of the chromatogram returning precisely to the original baseline established prior to the IBP of the sample. Thus, the most satisfactory procedure is to inspect the chromatogram and the area counts at each interval near the end of the run to determine the point at which the rate of change per second of the chromatographic signal has reached a constant low value of no greater than 0.00001 % of the total area counts. In some rather unusual cases, a sample may have individual peaks, separated at the end of the run, which return to baseline between the peaks. In such cases, the total area point of the sample obviously is somewhere beyond the last detectable peak.
12.3 Observe the area counts at the start of the run, until the point is reached where the cumulative area count is equal to 0.5 % of the total area (see 12.2). Mark this point as the IBP of the sample.
12.4 Divide the cumulative area at each interval between the initial and FBPs by the total area (see 12.2) and multiply by 100. This will give the cumulative percent of the sample recovered at each time interval.
12.5 Tabulate the cumulative percent recovered at each interval and the retention time at the end of the interval. Using linear interpolation where necessary, determine the retention time associated with each percent between 1 and 99.
12.6 For each percent and its associated retention time, determine the corresponding boiling temperature from the calibration table (see 10.3.2).