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
8. System Performance
8.1 Resolution - For samples containing isopentane and lighter materials, the system must be able to identify the beginning and end of isopentane and lighter saturated compounds as they elute from the column. Individual peaks must be resolved from adjacent peaks so that the height at the valley above the baseline is not more than 5 % of the height of the smaller peak adjacent to it. The resolution, R, between nC12 and nC13 must be between 2 and 4 when calculated in accordance with the following equation as shown in Fig. 1:
R = 2D/(Y1 + Y2)
where:
D = time, s, between nC12 and nC13 apexes,
Y1 = peak width of nC12, s, and
Y2 = peak width of nC13, s.
8.2 Sensitivity and Noise - These criteria test the sensitivity and noise of the total system. From the first 20 readings or time intervals of the blank run, calculate the noise as the difference between the maximum area reading per second minus the minimum reading per second. From the measurements on the calibration mixture, calculate the signal/noise ratio, as follows:
A/(N x S)
where:
A = total area of the hexane peak,
N = noise, and
S = width of the hexane peak in seconds.
This value must not be less than 10 for each 0.05 volume % of hexane in the calibration mixture, for example, 200 for 1 %. If the noise is undetectable, assume the noise to be 1 count per second.
8.3 Drift - From the blank run, calculate by the following test procedure, a total area measured after the start of the run until the end of the run. Adjust the apparatus so that all measurements can be read whether positive or negative. On some equipment such as integrators, readings will need to be positive and increasing in value. Obtain the absolute difference between the average area reading in the first five time intervals and the individual readings in each time interval from the start of the blank run until the end. Sum these differences to obtain the total area for the blank. The total area measurement from the blank run must not be greater than 2.0 % of the total area measurement of the calibration mixture.
8.4 Skewing of Peaks - Calculate the ratio A/B on peaks in the calibration mixture as shown on Fig. 2. A is the width in seconds of the part of the peak ahead of the time of the apex at 5 % of peak height, and B equals the width in seconds of the part of the peak after the time of the apex at 5 % of peak height. This ratio must not be less than 0.5 nor more than 2.0.
NOTE 11 - A ratio of more than 2.0 is probably due to overloading of the column. This can be corrected by smaller sample size, higher loading of the liquid phase on the packing, or larger diameter column. A ratio of less than 0.5 probably indicates tailing, overloading the detector, or loss of liquid substrate. The column must be changed when tailing becomes excessive. It is possible that the peak shape can be distorted due to a combination of these reasons.
8.5 Retention Time - The system must be sufficiently repeatable when testing the calibration mixture to obtain peak maxima retention time repeatability (maximum difference between duplicate results) of 3 s for isopentane and lighter compounds, if present. The maximum difference between duplicate results of retention times of the normal pentane and heavier compounds must not be greater than a time equivalent to 3°C (2°F). In addition, the retention time of the apex of the first peak in the calibration mixture should be at least 15 s.
8.6 Polarity - Calculate the boiling point retention time relationship specified in 10.2.2, using only the n-paraffins. Using the observed retention time of the aromatic compounds, calculate their apparent boiling points. Compare the apparent boiling points of the aromatics with their known boiling points. The apparent boiling point of the aromatic compounds must not deviate more than 10°F (6°C) from linearity or normal paraffins in the calibration mixture.
8.7 Area Measurement - The area measurement may be made by an electronic integrator or an analog-to-digital converter in conjunction with a computer. As the run progresses, the amount of material eluted from the column is measured from time zero in time slice areas or counts at specified time intervals. The counts are summed continuously, and the time intervals are equated to equivalent temperatures using the calibration curve generated in 10.2.2. Continue measurement for 2 min after the apex of the last peak or until the chromatogram returns to a constant baseline at the end of the run. Duplicate results on consecutive runs on the area percent of the compounds in the calibration mixture must not differ by more than 0.1 %.
8.7.1 Time intervals need not be uniform throughout the run. However, it is important that all measurement be on the same basis for the blank, calibration, and sample. No interval shall be greater than 0.5 % of the total length of the run. In addition, in order to facilitate the measurement of light ends, the size of the time intervals for the isopentane and lighter compounds should be small enough to allow measurement of their areas and times to peak maxima.
NOTE 12 - The end of the run can be defined by using the following algorithm: find the time where the rate of change of the chromatographic signal is less than or equal to a specified value (0.05 mV/min and 0.001 % of the total area under the chromatogram have been used successfully). Then search for 1 min before and after that time. The point where the number of counts per slice is at a minimum in that 2-min period is defined as the end of the run.
8.8 Difference from Calibration Mixture - Multiply the area of each peak in the calibration mixture by the liquid volume response factor calculated in 11.2, and normalize the volume percent of each compound so that the volumes of all compounds heavier than n-butane add up to 100.0. Compare the volume percent of each compound heavier than n-butane with the known percent. The difference between the calculated and known percentages must not be greater than 0.5.
9. Sampling
9.1 Sampling from Bulk Storage:
9.1.1 Cylinder Refer to Practice D1265 for instructions on introducing samples into a cylinder from bulk storage. The cylinder should be pressurized with carrier gas to a pressure of at least 345 kPa (50 psi) above the vapor pressure of the sample (Warning - Gasoline is extremely flammable. Vapors are harmful if inhaled). If the sample is to be transferred to another vessel such as a vial with septum, the cylinder must be cooled to a temperature between 32 and 40°F (0 and 4°C).
9.1.2 Open Containers - Refer to Practice D4057 for instructions on introducing samples into open-type containers from bulk storage. Cool the container and its contents to 32 to 40°F (0 to 4°C) before removing any sample from it.
9.2 Sampling from Open-Type Containers - Follow the instructions in Test Method D323 for transferring material from an open-type container.
