ISO 3924 Petroleum products - Determination of boiling range distribution - Gas chromatography method
9 Calibration
9.1 Analysis sequence protocol
9.1.1 Define and use for all runs a predetermined schedule of analysis events to achieve maximum reproducibility. The schedule shall include cooling the oven to the initial starting temperature, equilibration time, sample injection and system start, analysis and final temperature hold time.
9.1.2 After the chromatographic conditions have been set to meet performance requirements, programme the column temperature upward to the maximum temperature being used and hold at that temperature for the selected time. Following the analysis sequence protocol, cool the column to the initial starting temperature.
9.1.3 During the cool-down and equilibration time, prepare the integrator/computer system for data acquisition. 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 method is 1 Hz (one slice per second).
9.1.4 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 this analysis sequence protocol for all subsequent analysis, blanks and calibrations.
9.2 Baseline compensation analysis
A baseline compensation analysis, or baseline blank, shall be performed at least once each day that the test is run, using the same technique for a sample analysis except that no injection is made.
NOTE 1 The blank analysis is necessary due to the normal occurrence of chromatographic baseline rise near the maximum column temperature. Factors that influence baseline stability are column bleed, septum bleed, detector temperature control, constancy of carrier and detector gas flows, leaks, instrument drift, etc.
Subtract the blank analysis from the sample analysis to remove any non-sample slice area from the chromatographic data.
NOTE 2 The blank analysis is typically performed prior to sample analysis, but can 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 analysis.
Carry out periodic baseline blank analysis in accordance with the analysis sequence protocol to give an indication of baseline stability.
9.3 Retention time versus boiling point calibration
9.3.1 A retention-time versus boiling-point calibration shall be performed at least once each day that the test is run. Inject an appropriate aliquot (0.2 µl to 2.0 µl) of the calibration mixture into the chromatograph following the analysis sequence protocol.
9.3.2 Prepare a calibration table based on the results of the analysis of the calibration mixture by recording the retention time and the normal boiling point temperature for each component in the mixture. Boiling point temperatures of alkanes are listed in Table 1.
9.3.3 Plot the retention time of each peak versus the corresponding boiling point temperature for that component. A typical calibration curve is shown in Figure 3.
9.3.4 Ensure that the calibration points bracket the boiling range of the sample at both the low and high ends. Ideally, the calibration plot of retention-time versus boiling-point temperature should be linear, but it is impractical to operate the chromatograph such that curvature is eliminated completely.
NOTE The greatest potential for deviation from linearity is associated with the lower-boiling-point alkanes, which elute from the column relatively fast and have the largest difference in boiling point temperatures. In general, the lower the sample initial boiling point, the lower the starting point of the analysis.
9.4 Analysis of reference material
9.4.1 The reference material (5.5) is used to verify both the chromatographic and calculation processes involved in this method.
A secondary reference material may be used, providing it satisfies the following criteria.
a) It is similar in nature and boiling range to the samples being analysed.
b) The boiling-range distribution values assigned to it are obtained by averaging multiple analysis of the secondary reference material on a system that is first shown to be operating properly with the primary reference material (5.5).
9.4.2 Analyse the primary reference material (5.5) or a secondary reference material at least once each day that the test is run. Perform an analysis of the reference material following the analysis sequence protocol (see 9.1). Collect the area slice data and provide a boiling-point distribution report in accordance with 12.1.
9.4.3 The results of the analysis of the reference material (5.5) (either batch 1 or batch 2 may be used) shall not deviate from the values for that batch given in Table 4 by more than the range specified by the reproducibility given in 13.3.
