Analytical Instrumentation
As the Princeton 60A and the JASCO columns were the only ones to show separation of hexadecane and cyclohexane, those were further evaluated at a few other pressures. As the resolution of those peaks increased when running at 200bar compared to 120bar, 150bar and 250bar were run to confirm the trend and also that higher pressure led to the best separation.
13
Figure 7 shows the retention behavior of toluene at various temperatures and various temperatures. The retention time does not vary between 35°C and 45°C with the exception at 100bar yielding a slightly longer retention. Although the retention times do not vary, the peak shape is certainly sharper at 35°C. At 55°C the peak shape mimics that broader peak shape seen at 45°C, but the retention time decreases slightly, with the exception of 100bar where the retention time increases. This same data set was repeated using benzene as the sample and the exact same retention behavior was displayed. With benzene, peak broadening was also evident at 45°C and 55°C compared to 35°C, but was much less significant.
The retention time increases as the pressure is increased from 150 bar, up to 300 bar with both 100% CO2
and 90% CO2 , and toluene has : 10% methanol as plotted in figure 5 and seen in figures 3 and 4. The retention time
trend is identical for both with 10% methanol as expected producing earlier elution. At 100 bar, however, the retention time does not follow the trend when the mobile phase is 100% CO2
longer retention as the temperature is increased from 35°C to 45°C to 55°C. This suggests that there is a significant difference in the mobile phase (100% CO2
very different retention times. The presence of 10% methanol, which is in the liquid phase, inhibits the density change leading to retention times that follow the trends seen at higher pressures.
) density at 100bar for 35°C and 55°C producing
Figure 3: Princeton and JASCO Silica Column Comparison. Blue line = FID, Pink line = UV at 225nm. Peaks: 1. Hexa- decane, 2. Cyclohexane, 3. Toluene, 4. Naphthalene.
As illustrated in figure 3, an increase in pressure led to better resolution and 250bar was proven to be the best back pressure on both columns, with the JASCO column proving to be the best. As the pore size of the JASCO column is 30A, a Princeton 30A was compared as well as a Phenomenex Kinetex HILIC column using a simplified 3 component mix. As shown in figure 4 the difference between the Princeton 30A and Princeton 60A is significant as the separation of hexadecane and cyclohexane is drastically improved. The Kinetex®
column did not show much retention of any of the peaks. As
the pore size was 100A that was likely a factor in the insufficient resolution of hexadecane and cyclohexane, but also the toluene retention time was less than the Halo Penta HILIC.
Figure 4: Silica Column Comparison. Back Pressure 250bar.. Peaks: 1. Hexadecane, 2. Cyclohexane, 3. Toluene Figure 7: Toluene Retention Time Comparison at Various Temperatures and Various Back Pressures. ASTM D6550 Results
Standards were purchased from Spectrum Quality Standards and consisted of individual ampules of 1.0%, 3.5%, 6.0%, 8.5%, 12.0%, 17.0% and 25.0% olefin in a mixture of 75% isooctane : 25% toluene.
Figure 5: Hexadecane Retention Time Comparison at 35C and Various Back Pressures. Temperature and Pressure
Figure 5 and figure 6 show that the retention time increases as the back pressure is increased for both hexadecane and cyclohexane. However the magnitude of this increase is different for each. Hexadecane at 100bar elutes at 1.54 minutes and has poor peak shape, while cyclohexane elutes earlier at 1.46 minutes. The peak shape sharpens as the back pressure is increased for hexadecane, but notice the retention time only increases to 1.64 minutes at 300bar compared to later cyclohexane elution at 1.70 minutes. Effectively if these two compounds were in the same mixture their elution order would reverse showing the importance of the back pressure.
Figure 8: ASTM D6550 Olefin Analysis. Sample is 25% olefins. Overlay of 20 injections.
The overlay of 20 chromatograms of the 25% olefin sample is shown in Figure 8. The zoomed olefin overlay of those 20 injections is shown in Figure 9. The overlay of the olefin peak of the various standards, 1.0%, 3.5%, 6.0%, 8.5%, 12.0%, 17.0% and 25%, is shown in figure 8 and the corresponding calibration curve from those standards is also shown. The linearity of the calibration curve had a correlation coefficient (R) of 0.997.
Figure 6: Cyclohexane Retention Time Comparison at 35C and Various Back Pressures.
Figure 9: Overlay of 1.0%, 3.5%, 6.0%, 8.5%, 12%, 17%, and 25% Olefins. Calibration curve correlation coefficient (R) was 0.997.
AUGUST / SEPTEMBER •
WWW.PETRO-ONLINE.COM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68
