OPTIMISING OF THE COLUMN, TEMPERATURE, AND PRESSURE FOR INCREASED RESOLUTION AND PERFORMANCE FOR THE SEPARATION OF ALKANES AND AROMATICS FOR ASTM D5186 AND D6550 USING SFC
SFC is well established for enantiomeric separations and purifi cations in the pharmaceutical industry as well as for fuel analysis of gasoline and diesel using the ASTM D5186 and D6550 methods. Both ASTM methods use a silica column for the separation, while D6550 secondarily employs a silver column for retention of the olefi ns after the alkanes and aromatics are separated. In addition to these methods, GC is often used for analysis for the alkanes, but this application will show this is also possible using SFC.
The fi rst step in optimising the resolution for the ASTM methods is evaluating the available silica columns. Recent advances in column technology have provided numerous new phases and particles including small particle HILIC columns. These new columns were compared to traditional 5um columns to determine which is the best for this analysis.
The second step in improving the system performance is evaluating the retention behavior by adjusting system parameters on the best columns as determined in the previous step. As with HPLC
the column temperature is controlled in SFC, but this change in temperature alters the density of CO2 compared to a change in viscosity in HPLC producing a different effect on the separation in SFC. One additional and unique parameter in SFC is the back pressure, which is not controlled in HPLC, but can be adjusted in SFC to optimise a separation. The parameters of pressure and temperature were evaluated below to determine their effect on the resolution.
With the optimised column, temperature and pressure determined, the system was evaluated for ASTM 5186 and ASTM 6550 methods. In these methods the individual components within each class (non-aromatics, mono-aromatics, poly-aromatics) are not identifi ed, but simply grouped to determine the percentage of each group. Recent interest in further separation and identifi cation of the components within those compound classes has led to additional research.
Non-aromatic hydrocarbons are very diffi cult to retain as seen in the above-mentioned methods. In order to identify the components in the non-aromatics, a signifi cant increase in retention is required. Unique to SFC is the ability to join columns inline to increase the effective length of the separation column. This will be performed to increase the retention and resolution of the non-aromatics for further identifi cation.
Experimental
Equipment CO2
Delivery Pump: Autosampler:
PU-2080-CO2 XLC-3159AS
Column/FID Splitter Oven: GC-FID Back Pressure Regulator:
Results Column Evaluation
Figure 1 shows the comparison of the 6 silica columns from various manufacturers at 120bar back pressure. The JASCO column is the only one that shows any separation of hexadecane and cyclohexane and clearly shows the most resolution between those peaks and the toluene and naphthalene peaks. Figure 2 shows this same comparison at 200bar and again the separation of hexadecane and cyclohexane is the best and the resolution between those peaks and toluene and naphthalene is the largest.
BP-2080
Conditions CO2
Flow rate: 2.0 – 3.0 mL/min
Column/FID Splitter Temp.: 200ºC FID Temperature: Injection volume:
350ºC 0.5 - 1 µL
Figure 1: Silica Column Comparison. Back Pressure
120bar.Blue line = FID, Pink line = UV at 225nm. Peaks: 1. Hexadecane, 2. Cyclohexane, 3. Toluene, 4. Naphthalene.
Figure 2: Silica Column Comparison. Back Pressure
200bar.Blue line = FID, Pink line = UV at 225nm. Peaks: 1. Hexadecane, 2. Cyclohexane, 3. Toluene, 4. Naphthalene
AUGUST / SEPTEMBER •
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