Alumina porous layer open tubular (PLOT) columns have long been used for the analysis of volatile hydrocarbons. Due to the highly selective retention mechanisms at work in alumina PLOT columns, saturated and unsaturated hydrocarbons can be fully resolved in a variety of applications. To enhance the performance of alumina PLOT columns when performing quantitative work at trace-level concentrations, alumina adsorbents must be deactivated with salts like KCl and Na2SO4.
The practical separation behavior of alumina PLOT columns is highly dependent on a number of factors. The columns are susceptible to water contamination and may require periodic thermal conditioning to restore performance. Repeated cycling to high temperatures can alter the selectivity of the adsorbent material coated in alumina PLOT columns.
Instrument conditions can also strongly affect the separation characteristics of alumina PLOT columns. Variations in carrier gas flow rate, initial oven temperatures, initial oven hold times, and oven ramp rates can significantly shift retention times and elution orders for volatile hydrocarbons. The position of propadiene and acetylene can be tuned by changing the settings of temperature, flow, and program, allowing simple optimization.
Despite the fact that alumina activity can be controlled by using deactivation salts, it is still very sensitive for water. Any water in the carrier gas or the sample, or introduced via a small leak, will be adsorbed by the alumina surface. The water molecules will act as a deactivation agent and cover the most active sites. As a result, the alumina column will demonstrate reduced retention, and a polarity change will be observed as well. An Rt®-Alumina BOND/KCl column (Restek Corp., Bellefonte, PA) was tested isothermally; following this, water was injected onto the column. In total, 4.500.000 ng water was injected onto the column. Figure 1 depicts the impact on the retention. All peaks eluted much more quickly because water was covering the active sites on the alumina surface.
Figure 1 - Impact of water on retention and peak position of Alumina BOND/KCl. Alumina becomes less polar when water is adsorbed.
A change in selectivity is also evident, especially for the polar hydrocarbons. The alumina column behaves in a much less “polar” way because the polar hydrocarbons elute relatively faster. After water exposure, propyne(methylacetylene) elutes very closely to iso-pentane. Heating the column for 3 hr at 200 ºC removes the majority of the water, and the column has almost its full original retention. Here, there was an excessive amount of water. To achieve reproducible retention times when there is water in the sample, the following approaches can be used:
- After each analysis program the oven is set to 200–250 ºC and all the water is eluted during the same run; this adds 5–10 min to every analysis.
- If water concentrations are low (< 3 ppm), one can do 20–30 analytical runs. Once the hydrocarbon peaks move out of the integration window, the column can be conditioned for a few hours at Tmax.
- A polar, thick-film (Rtx®-Wax or Stabilwax ® [Restek]) pre-column and a valve/Deans switch. The polar “wax”-type pre-column will retain the water as water elutes around nonane, but hydrocarbons will pass almost without retention. After all hydrocarbons have passed the wax pre-column, the column is switched to vent or to a second detector (if water is to be measured). This type of system also performs well at lower temperatures since water will not reach the alumina columns and retention times will be stable.
It may be unusual to see retention times shift based on water exposure, but water will never damage an alumina column because the alumina can always be regenerated.
More polar alumina PLOT columns, such as Na2SO4-deactivated columns, are more sensitive to moisture exposure. Polar hydrocarbons show larger shifts in retention. Since the alumina column behaves in a less polar way, the acetylene and propadiene can move into the iso-butane/n-butane. The KCl-deactivated alumina is the least sensitive for water impact because it has the lowest polarity. KCl-, Na2SO4-, and MAPD-deactivated alumina can be regenerated by conditioning for a few hours at 200 ºC (KCl and Na2SO4) and 250 ºC (MAPD).
Effect of oven temperature and flow on selectivity
In GC, every stationary phase shows a certain dependency of peak elution order with changing temperature. Liquid stationary phases usually become more polar when used at a higher temperature. Adsorbents work the opposite way; their behavior is more polar if the temperature is reduced. For alumina this is very interesting, since separations can be improved considerably by starting at a lower temperature.
Figure 2 - Effect of temperature on peak position of Alumina BOND/Na2SO4 when operated under different isothermal conditions. Alumina becomes more polar at lower temperatures, moving acetylene away from n-butane.
Figure 2 shows the elution order for C1–C5 hydrocarbons when the alumina column is kept isothermally at 140, 120, 100, and 80 ºC. At 140 ºC, the propadiene elutes before the isobutane. With lower temperatures, the propadiene peak shifts to the back. At 100 ºC, propadiene coelutes with n-butane. At 80 ºC, both components elute after n-butane. The impact is also evident on propyne and 1,3-butadiene, for which there is a strong shift away from the pentanes.
Because the analysis time increases rapidly with lower temperature, very often a temperature program is used to elute the upper hydrocarbons within an acceptable time window. Lower starting temperatures have a considerable influence on the position of the polar hydrocarbons. When using a temperature-programmed analysis, this effect is even more pronounced. Figure 3 shows temperature-programmed analysis and the impact of different initial times at 60 ºC. An analysis that is a few minutes longer at 60 ºC moves the propadiene/acetylene far behind the butane. Adjusting the time at the starting oven temperature gives a completely different separation.
Figure 3 - Impact of initial time at 60 ºC on peak elution of alumina. A longer time at 60 ºC significantly makes the alumina behave polar, resulting in higher retention for propadiene and acetylene. Column: 30 m × 0.32 mm Alumina BOND/Na2SO4; carrier: He, 100 kPa; oven: 60 ºC, 0, 1, 3, and 5 min, 30 ºC/min→ 200 ºC.
This effect is related to the elution temperatures for the polar hydrocarbons. The lower the elution temperature, the more the polar hydrocarbon will be moved to the back of the chromatogram. The column flow can also be used to attain this effect. With increased column flow, the polar components will elute at a lower temperature and will shift relative to the back of the chromatogram.
Figure 4 - Impact of flow on elution profile. Higher flows reduce elution temperature, again making the alumina behave more polar. Column: 30 m × 0.32 mm Rt-Alumina BOND/Na2SO4; carrier: He, 100, 150, and 200 kPa; oven: 60 ºC, 3 min, 30 °C/min→ 200 °C.
Figure 4 shows the effect of using higher helium flow at 100, 150, and 200 kPa inlet pressures, but with similar oven temperature programming. This can be an interesting approach since the optimum carrier gas velocities for PLOT columns are on the average higher than those found with liquid stationary phases. For example, for 1,3-butadiene, one can work at 80 cm/sec using hydrogen with a minor loss in efficiency.