ASTM D7213 Test Method for Boiling Range Distribution of Petroleum Distillates
ASTM D7213 Standard Test Method for Boiling Range Distribution of Petroleum Distillates in the Boiling Range from 100 to 615°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 will include cooling the column oven and injector to the initial starting temperature, equilibration time, sample injection and system start, analysis, and final 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 retentiontime calibration is being performed, use the peak detection mode. For samples and baselinecompensation (with or without solvent injection), use the area slice mode operation. The recommendedslice rate for this test method is 1.0 Hz (3.3 slices per second). Other slice rates may be used if within the limits of 0.02 % and 0.2 % of the retention time of the final calibration component (C60). Faster slicerates 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 (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 - Perform a blank analysis (baseline blank) 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 carryover from previous sample analyses.
NOTE 3 - If automatic baseline correction (see Note 1) 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 which 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 7.6) into the chromatograph, using the analysis sequence protocol. Obtain a normal (peak detection) data record in order 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. Fig. 1 illustrates a graphical plot of a calibration analysis.
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. Distortion in retention time measurement and hence errors in boiling point temperature calibration 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 to avoid peak distortion.
9.3.1.1 Skewness Calculation - Calculate the ratio A/B on specified peaks in the calibration mixutre as indicated by the designations in Fig. 2. A is the width in seconds of the portion of the peak eluting prior to the time of the peak apex 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 n-pentacontane (n-C50) peak in the calibration mixture shall not be less than 0.5 nor more than 2.0.
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 degrees Celsius (or Fahrenheit) for every component in the mixture. Normal paraffin boiling point temperatures (atmospheric equivalent temperatures) are listed in Table 1. An example of a typical calibration report, showing retention times and boiling points for each n-paraffin, is found in Table 3.
9.4 Sample Preparation - Sample aliquots are introduced into the gas chromatograph as solutions in a suitable solvent (for example, carbon disulfide or cyclohexane).
9.4.1 Dilute the sample to approximately 2 wt % with the solvent.
9.4.2 Seal (cap) the vial, and mix the contents thoroughly to provide a homogeneous mixture. It may be necessary to warm the mixture initially to effect complete solution of the sample. However, the sample shall be in stable solution at room temperature prior to injection.
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 (area slice mode).
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 mL of the diluted sample. The maximum sample signal amplitude should not exceed the maximum calibration signal amplitude. A sample chromatogram is found in Fig. 3.
