ASTM D3710 for Boiling Range Distribution of Gasoline and Gasoline Fractions
ASTM D3710 Standard Test Method for Boiling Range Distribution of Gasoline and Gasoline Fractions by Gas Chromatography
10. Procedure
10.1 Blank - After conditions have been set to meet performance requirements, program the column temperature upward to the maximum temperature to be used. Following a rigorously standardized schedule, cool the column to the selected starting temperature. At the exact time set by the schedule, without injecting a sample, start the column temperature program. Measure and record the area in each time interval from the start of the run until the end of the run as specified in 8.7. Make a blank run at least daily.
10.1.1 In order for the blank run to be valid, it must meet the drift requirement specified in 8.3. In addition, no peaks must be found such that the difference in area readings per second in consecutive time intervals be greater than five times the noise. If the noise is not detectable, assume it to be 1 count per second.
NOTE 13 - The identification of a constant baseline at the end of the run is critical to this test method. Constant attention must be given to all factors that influence baseline stability, such as substrate bleed.
NOTE 14 - Some gas chromatographs have an algorithm built into their operating software, which causes a mathematical model of the column bleed profile to be stored in memory. This profile is subtracted automatically from the detector signal on subsequent runs to compensate for the column bleed.
10.2 Calibration:
10.2.1 Using the same conditions described in 10.1, inject the calibration mixture into the chromatograph. Record the data in such a manner that retention time of peak maxima and peak area of the individual components are obtained. As noted in 8.7, this can be done by means of a computer or integrator.
NOTE 15 - When determination of peak maxima and peak area is done by the time slice technique, the following algorithms can be used to verify the start of peak, end of peak, and peak maxima: A peak is defined as starting in that time slice in which the rate of change of the chromatographic signal is greater than a specified value (0.05 mV/min and 0.001 %/s have been used successfully). This criterion must be confirmed for two consecutive time segments in order to be valid. Once a peak is detected, the end is determined by one of two criteria. The first applies when there is good resolution between peaks. The peak can be defined as ending when the rate of change of the chromatographic signal is less than the value specified above. The second criterion applies when resolution between peaks is not complete. The first peak ends when, after the apex has passed, the area per time segment reaches a minimum and starts to increase. The retention time of peak maxima can be determined by the following equation, as shown in Fig. 3:
tmax = ti + (ti+1 - ti) Ai+1/(Ai-1 + Ai+1)
where:
tmax = retention time of peak maxima,
ti = time to start of segment i,
ti+1 = time to start of segment i + 1,
Ai+1 = area of segment that starts at ti+1, and
Ai-1 = area of segment that starts at ti-1.
For systems in which the output is in units other than millivolts, an equivalent measure of the slope may be used.
10.2.2 Plot the retention time of the maxima of each peak versus the corresponding normal boiling point in degrees Celsius (or Fahrenheit) as shown in Fig. 4. If the sample is known to contain less than 5.0 % aromatics, do not include aromatic compounds in the retention time calibration curve.
NOTE 16 - For best precision, the calibration curve should be essentially a linear plot of boiling point versus retention time. In general, the lower the initial boiling point of the sample, the lower will be the starting temperature of the chromatographic column. If the starting temperature is too high, there will be considerable curvature at the lower end of the curve, and loss of precision in that boiling range. Since it is impractical to operate the column so as to eliminate curvature completely at the lower end of the curve where initial boiling points below ambient temperature are encountered, at least one point on the curve should have a boiling point lower than or equal to the initial boiling point of the sample. Extrapolation of the curve at the upper end is more accurate, but for best accuracy, calibration points should bracket the boiling range of the sample at both the low and high ends.
10.2.3 The boiling point retention time calibration curve must be checked at least daily by either the calibration mixture or a secondary standard of known boiling point characteristics.
NOTE 17 - If peaks in the sample are used as boiling point calibration marks, the calibration mixture need not be run. However, it may prove helpful in establishing identity of peaks in the sample to run the calibration mixture once. Furthermore, precision may be improved in some cases by adding to the sample an n-paraffin, selected so as to be resolved completely from the sample, to serve as an additional boiling point calibration. Plot the retention times of the peaks versus the corresponding atmospheric boiling points to obtain the calibration curve.
10.3 Sampling:
10.3.1 Using the exact conditions and time basis as were used in the blank and calibration, inject the sample into the chromatograph. Disregarding peaks (if any) before propane, measure and record the area of each time segment at time intervals as specified in 8.7.
