ASTM D2887 Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
10. Calibration and Standardization
10.1 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 to the initial starting temperature, equilibration time, sample injection and system start, analysis, and final upper temperature hold time.
10.1.1 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.
10.1.2 During the cool down and equilibration time, ready the integrator/computer system. If a retention time or detector response calibration is being performed, use the peak detection mode. For samples and baseline compensation determinations, use the area slice mode of integration. The recommended slice rate for this test method is 1.0 Hz (one slice 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 (C44). 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.
10.1.3 At the exact time set by the schedule, inject either the calibration mixture 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 sequence protocol for all subsequent repetitive analyses or calibrations. Since complete resolution of sample peaks is not expected, do not change the detector sensitivity setting during the analysis.
10.2 Baseline Compensation Analysis - A baseline compensation analysis, or baseline blank, is performed exactly like an analysis except no injection is made. A blank analysis must be performed at least once per day. The blank analysis is necessary due to the usual occurrence of chromatographic baseline instability and is subtracted from sample analyses to remove any nonsample slice area from the chromatographic data. 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. Attention must be given to all factors that influence baseline stability, such as column bleed, septum bleed, detector temperature control, constancy of carrier gas flow, leaks, instrument drift, and so forth. Periodic baseline blank analyses should be made, following the analysis sequence protocol, to give an indication of baseline stability.
NOTE 5 - If automatic baseline correction (see Note 3) 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.
10.3 Retention Time Versus Boiling Point Calibration - A retention time versus boiling point calibration must 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 7.8) 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.
10.3.1 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 that will result in displacement of the peak apex relative to nonoverloaded peaks. 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 to avoid peak distortion.
10.3.2 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. n-Paraffin boiling point temperatures are listed in Table 2.
10.3.3 Plot the retention time of each peak versus the corresponding normal boiling point temperature of that component in degrees Celsius (or Fahrenheit) as shown in Fig. 2.
10.3.4 Ideally, the retention time versus boiling point temperature calibration plot would be linear, but it is impractical to operate the chromatograph such that curvature is eliminated completely. The greatest potential for deviation from linearity will be associated with the lower boiling point paraffins. They will elute from the column relatively fast and have the largest difference in boiling point temperature. In general, the lower the sample IBP, the lower will be the starting temperature of the analysis. Although extrapolation of the curve at the upper end is more accurate, calibration points must bracket the boiling range of the sample at both the low and high ends.
10.4 Reference Gas Oil Analysis - The Reference Gas Oil No. 1 sample is used to verify both the chromatographic and calculation processes involved in this test method. Perform an analysis of the gas oil following the analysis sequence protocol. Collect the area slice data and provide a boiling point distribution report as in Sections 12 and 13.
10.4.1 The results of this reference analysis must agree with the values given in Table 3 within the range specified by the test method reproducibility (see 14.1.2).
10.4.2 Perform this reference gas oil confirmation test at least once per day or as often as required to establish confidence in consistent compliance with 10.4.1.
