ASTM D6352 test method for boiling range distribution of petroleum distillates
ASTM D6352 standard test method for boiling range distribution of petroleum distillates in boiling range from 174 to 700°C by gas chromatography
9. Procedure
9.1 Analysis Sequence Protocol - Define and use a predetermined schedule of analysis events designed to achieve maximum reproducibility for these determinations. The schedule shall include cooling the column oven and injector to the initial starting temperature, equilibration time, sample injection and system start, analysis, and final high temperature hold time.
9.1.1 After chromatographic conditions have been set to meet performance requirements, program the column temperature upward to the maximum temperature to be used and hold that temperature for the selected time. Following the analysis sequence protocol, cool the column to the initial starting temperature.
9.1.2 During the cool down and equilibration time, ready the integrator/computer system. If a retention time calibration is being performed, use the peak detection mode. For samples and baseline compensation (with or without solvent injection), use the area slice mode operation. The recommended slice rate for this test method is 1.0 Hz (1 s). Other slice rates may be used if within the limits from 0.02 to 0.2 % of the retention time of the final calibration component (C90). Larger slice rates may be used, as may be required for other reasons, if provision is made to accumulate (bunch) the slice data to within these limits prior to determination of the boiling range distribution.
9.1.3 At the exact time set by the schedule, inject either the calibration mixture, solvent, or sample into the chromatograph; or make no injection (perform a baseline blank). At the time of injection, start the chromatograph time cycle and the integrator/computer data acquisition. Follow the analysis protocol for all subsequent repetitive analyses or calibrations. Since complete resolution of sample peaks is not expected, do not change the sensitivity setting during the analysis.
9.2 Baseline Blank - A blank analysis (baseline blank) shall be performed at least once per day. The blank analysis may be without injection or by injection of an equivalent solvent volume as used with sample injections, depending upon the subsequent data handling capabilities for baseline/solvent compensation. The blank analysis is typically performed prior to sample analyses, but may be useful if determined between samples or at the end of a sample sequence to provide additional data regarding instrument operation or residual sample carry over from previous sample analyses.
NOTE 4 - If automatic baseline correction (see Note 2) is provided by the gas chromatograph, further correction of area slices may not be required. However, if an electronic offset is added to the signal after baseline compensation, additional area slice correction may be required in the form of offset subtraction. Consult the specific instrumentation instructions to determine if an offset is applied to the signal. If the algorithm used is unclear, the slice area data can be examined to determine if further correction is necessary. Determine if any offset has been added to the compensated signal by examining the corrected area slices of those time slices that precede the elution of any chromatographic unretained substance. If these corrected area slices (representing the true baseline) deviate from zero, subtract the average of these corrected area slices from each corrected area slice in the analysis.
9.3 Retention Time versus Boiling Point Calibration - A retention time versus boiling point calibration shall be performed on the same day that analyses are performed. Inject an appropriate aliquot (0.2 to 2.0 µL) of the calibration mixture (see 7.5) into the chromatograph, using the analysis schedule protocol. Obtain a normal (peak detection) data record to determine the peak retention times and the peak areas for each component. Collect a time slice area record if a boiling range distribution report is desired.
9.3.1 Inspect the chromatogram of the calibration mixture for evidence of skewed (non-Gaussian shaped) peaks. Skewness is often an indication of overloading the sample capacity of the column, which will result in displacement of the peak apex relative to non-overloaded peaks. Skewness results obtained by one laboratory are presented in Table 6. Distortion in retention time measurement and, hence, errors in boiling point temperature determination will be likely if column overloading occurs. The column liquid phase loading has a direct bearing on acceptable sample size. Reanalyze the calibration mixture using a smaller sample size or a more dilute solution if peak distortion or skewness is evident.
9.3.1.1 Skewness Calculation - Calculate the ratio A/B on specified peaks in the calibration mixture as indicated by the designations in Fig. 3. A is the width in seconds of the portion of the peak eluting prior to the time of the apex peak and measured at 10 % of peak height (0.10-H), and B is the width in seconds of the portion of the peak eluting after the time of the peak apex at 10 % of peak height (0.10-H). This ratio for the n-pentacontane (normal C50) peak in the calibration mixture shall not be less than 0.5 or more than 2.0. Results of analysis in one laboratory are presented in Table 6.
9.3.2 Prepare a calibration table based upon the results of the analysis of the calibration mixture by recording the time of each peak maximum and the boiling point temperature in °C (or °F) for each component in the mixture. A typical calibration table is presented in Table 7. n-Paraffin boiling point (atmospheric equivalent temperatures) are listed in Table 1. Fig. 1 illustrates a graphic plot of typical calibration data.
9.4 Sample Preparation - Sample aliquots are introduced into the gas chromatograph as solutions in a suitable solvent (for example, CS2).
9.4.1 Place approximately 0.1 to 1 g of the sample aliquot into a screw-capped or crimp-cap vial.
9.4.2 Dilute the sample aliquot to approximately 1 weight % with the solvent.
9.4.3 Seal (cap) the vial, and mix the contents thoroughly to provide a homogeneous mixture. It may be necessary to warm the mixture initially to affect complete solution of the sample. However, the sample shall be in stable solution at room temperature prior to injection. If necessary, prepare a more dilute solution.
9.5 Sample Analysis - Using the analysis sequence protocol, inject a diluted sample aliquot into the gas chromatograph. Collect a contiguous time slice record of the entire analysis.
9.5.1 Be careful that the injection size chosen does not exceed the linear range of the detector. The typical sample size ranges from 0.2 to 2.0 µL of the diluted sample. The maximum sample signal amplitude should not exceed the maximum calibration signal amplitude found in 9.3.1. A chromatogram for round robin sample 95-3 is presented in Fig. 4.
9.5.2 Ensure that the system's return to baseline is achieved near the end of the run. If the sample chromatogram does not return to baseline by the end of the temperature program, the sample apparently has not completely eluted from the columns, and the sample is considered outside the scope of the test method.
