ASTM D6296 Standard Test Method for Total Olefins in Spark-ignition Engine Fuels by Multi-dimensional Gas Chromatography
10. Procedure
10.1 Sample Preparation - Because the ether/alcohol and olefin traps have specific capacities that must not be exceeded, some samples will need to be diluted to stay within the working concentration range limits of the system.
10.1.1 Generally, the total olefin concentration must be lower than 5 %. When relatively large amounts of C4 and C5 olefins are present, a lower final concentration is required. In addition, the maximum total ether concentration is limited to 5 %. When any of these conditions is exceeded, a dilution of the sample is required prior to analysis.
10.2 Samples are diluted using isooctane as a diluent. Fig. 3 is a Sample Dilution Decision Diagram that shows examples of dilutions that can be used to obtain appropriate dilution for specific samples. Other dilutions may be necessary depending on the capacity of the olefin trap as determined by monitoring breakthrough (see Note 13) of olefins using the QC sample. Bring samples and calibration mixture to ambient temperature prior to dilution during sample preparation.
NOTE 13 - Breakthrough can be recognized in the chromatogram by the signal not returning to baseline within the first 10 min.
10.2.1 To prepare dilutions, tare to the nearest 0.1 mg, a glass container that can be sealed, preferably with a polyethylene screw cap or equivalent, and that is large enough to contain both the specimen and isooctane. Pipette a minimum of 1 mL of sample into the container and reweigh. Pipette a sufficient volume of isooctane into the container to obtain the desired dilution ratio. For example, if a 1:3 dilution is desired, pipette 5 mL of sample followed by 15 mL of isooctane. Calculate a dilution factor:
where:
Ddil = dilution factor,
Vsam = volume of sample added, mL, and
Vdil = volume of isooctane diluent, mL.
10.2.2 Calculate the density of the sample to three decimal places:
where:
D = density, kg/L,
M = mass of sample pipetted, g, and
V = volume of sample pipetted, mL.
10.3 Sample Analysis - Load the necessary system set-point conditions, which include initial system component temperatures, times at which column and trap temperatures are changed, the initial positions of switching valves, and times when valve switches occur (see Note 10).
10.4 When all component temperatures have stabilized at the analysis conditions, inject a reproducible 0.2-µL aliquot of a representative sample (or calibration test mixture), and start the analysis.
10.4.1 Starting the analysis should begin the timing function that controls all of the various programmed temperature changes and valve switching. It should also initiate the acquisition of peak area integration data for the C4 to C10 olefin peaks.
10.4.2 Upon completion of its programmed cycle, the system should automatically stop, generate a chromatogram, and print a report of total olefin concentration. A typical sample chromatogram is shown in Fig. 4.
NOTE 14 - If results are to be calculated by the system computer, load the dilution factor (if any) and the sample density, if mass % is to be calculated.