Limitations of Alumina: What you should NOT inject on alumina / Al2O3 columns

Alumina (Al2O3) capillary PLOT columns have been available for about 35 years. They are best suited for separation of C1-C10 hydrocarbons and especially the C1-C5 range. The alumina has unique selectivity making it possible to separate all the unsaturated hydrocarbons at temperatures above ambient, see figure 1. While it's an excellent choice for unsaturated hydrocarbons, special attention is required for components like pentadiene and 1,2-butadiene which can show reactivity on alumina.

Fig 1 C1-C4 hydrocarbons separated on Al2O3 PLOT

Compounds that elute are basically all hydrocarbons up to C12 and aromatics up to the xylenes. Permanent and noble gases like Nitrogen, Oxygen, Hydrogen, Neon, Argon, Krypton, Xenon will elute virtually without retention. Also CO elutes from alumina together with methane and the permanent gases. These gases can be separated, but the oven temperature has to be lowered to -100C or lower.

Gases like N₂O are well retained on the Alumina column, and show good peak shape.

Several chlorofluorocarbons are retained, however the type of alumina and deactivation used will determine the (re)activity, see below.

We will discuss here the components we should NOT inject starting with the ones that have the biggest impact: WATER

Traces of water will deactivate the alumina and the result is that retention decreases see figure 2.  Water influences overall retention and changes the selectivity of the column.  This can be seen when analyzing polar hydrocarbons, like methyl acetylene.

What can be done?

• Remove the water from the sample. This is difficult and adds a lot of time.
• Elute the water after every injection. This can be done by heating the alumina at high temperature (200/250) for 8-10 minutes. The AluminaBOND MAPD can be heated up to 250C, speeding up this process.
• If water is present at trace level, give the column a conditioning after XX analysis. Usually XX is determined when the target peak runs out of the integration window.
• Use a Rtx-Wax pre-column (thick film) via a valve in series with the Alumina. Water will be retained on this column, while C1-C6 will elute first. When the C1-C6 have entered the alumina, the water peak is switched to vent or sent to a second detector (if it needs to me monitored)

Fig. 2 Impact of water on retention of Al2O3

POLAR COMPOUNDS (alcohols, amines, acids, amino alcohols, ammonia)

Polar compounds will not elute as a peak from the alumina column. If they elute, they will elute as a “blob” or a raised baseline and once they are baked out of the column the column will continue to perform well.

If polarity of component decreases, the impact on the alumina will also decrease. So, ketones, ethers and aldehydes will still be adsorbed but are easier to elute at higher temperatures.

HALOGENATED COMPOUNDS

Halogenated compounds may elute OK from the alumina column. We can measure several CFCs using Alumina BOND CFC columns, see figure 3. Reactivity can occur with some halogenated components, Figure 4 shows examples for 1,2 dichloroethane and 2-chloropropane, which decompose into propylene and vinyl chloride while splitting off a HCl molecule.

Fig. 3 CFC separations on AluminaBOND CFC

Fig. 4 Reactivity of alumina

CARBON DIOXIDE

Carbon dioxide will be adsorbed completely, but it takes significant amounts of CO2 to impact retention times. If the retention times decrease, condition the column for a few hours at its maximum temperature limit.

SULFUR COMPOUNDS

Sulfur compounds will be strongly adsorbed. Hydrocarbon samples containing Sulfur compounds in the sub ppm range (COS/H2S) can be analyzed with minor changes in retention time.

HEAVIER HYDROCARBONS

Hydrocarbons C12 and above will not elute as discrete peaks and will manifest themselves as a raised baseline. The alumina column will appear to “bleed”.  If it bleeds, it is an indication that there is something adsorbed on the column. Make sure you periodically condition the column at maximum programmable temperature until the baseline is flat.

SILOXANES

Be aware that siloxanes can also stay on the column and will produce a raised baseline. This is often caused by the septum. For light hydrocarbons, try to use a moderate injection port temperature (80-100°C). You do not need a 250°C injection port temperature for mixes that contain C1-C6 compounds.

BEST SOLUTION

In all cases when there are components present that are adsorbed on alumina, and that do impact your retention and separations, the best way is to use a pre-column that can be back-flushed. This way the adsorbed component do not make it onto the analytical alumina column and retention times for hydrocarbons will be reproducible. You can choose to use a short (1-2m) alumina column as a guard, and do the back flushing using valve switching or using a Deans (flow) switching. Today these systems are very easily to setup and operate.