Water, can’t live without it so how can we deal with it (and adsorbent columns)

Often the gas samples contain some water vapor. Although we are usually not interested in the amount of moisture present in the sample, if we don’t dry it, the water will be injected onto the column with our sample. Alan, in his blog, has previously discussed the effects of water on capillary columns with liquid stationary phases and adsorbent (PLOT) columns. But as they say: a picture is worth a thousand words, so I want to share a few examples I gathered while working with adsorbent columns and the impact of water/water vapor.

Molecular Sieve 5A

Molecular Sieves 5A are zeolites that are part of the family of hydrated aluminosilicate minerals. They are known adsorbents with a unique, tunnel-like crystalline structure and a well-defined pore size. The analytes separate on molecular sieves based on two adsorption mechanisms. First, how well the molecules fit into the pores of the material, a separation based on the size of the molecules. Second, the physical interactions between the molecules and the MSieve 5A crystal, a separation based on the polarity.  For example, nitrogen and oxygen are both small enough to fit in the pores of 5A mineral, but oxygen, a smaller molecule than nitrogen, will navigate through the “tunnels” faster because it has less interactions with the pore surface and elutes from the column before nitrogen. In the case of carbon monoxide, polarity plays a more important role and the MSieve 5A strongly retains these molecules. Adsorption on molecular sieves is reversible and the adsorption/desorption process is easily regulated with the temperature. We know that we can’t analyze water using the MSieve 5A columns because of the high temperature required to desorb polar water from the pores of the molecular sieve. What happens if we are injecting water onto the column? How much water can we inject before it’s “too much”? How does “too much” look like? Can we regenerate the MSieve 5A column? And how long does it take to regenerate it to its original performance?

To find the answers, I have performed a quick study where I injected water onto a Rt-MSieve 5A column, 30m x 0.53mm x 50µm (Cat#19723) in between the injections of permanent gases. I kept the system at 40°C the entire time, simulating isothermal analysis of gases. Right after 1µl injection of water I noticed carbon monoxide’s retention time shifted, eluting earlier (Figure 1).

 

Figure 1: Overlay of chromatograms of permanent gases, black overlay – initial analysis, blue chromatogram – analysis after injecting 1µl of water onto the column.

I finally stopped injecting water after carbon monoxide’s capacity factor (k’) was at half of its original value. Until then, 100µl of water was injected onto the column under the described conditions. Analysis of permanent gases at that point shows that adsorption sites are packed with water, analytes start to tail indicating sample loading capacity is decreased, and gases elute faster or even co-elute (Figure 2, Chromatogram B). 100µl of water on the column may not seem like that much until we put it into perspective. Let’s say our sample has 50% relative humidity. With every 1ml injection of gas, we are introducing ~0.01µl of water onto the column. That means, to lose 50% of retention for carbon monoxide we can make 10,000 injections.

 

Figure 2: Analysis of permanent gases (in order of elution: Argon, Oxygen, Nitrogen, Methane, Carbon Monoxide, concentration 3-5mol%, 100µl split 60:1 injection)), 40°C Isothermal, flow He 4 ml/min Chromatogram A: Analysis using a new column, Chromatogram B: Analysis of permanent gases after 200 injections of 0.5µl water at 40°C Isothermal, Chromatogram C: Analysis of gases after fast 20 min conditioning, Chromatogram D: After 2h conditioning, the water was removed from the column.

To regenerate the column the water can be desorbed by conditioning at the column’s maximum temperature, 300°C.  I wanted to track the time required for the column to recover by monitoring the capacity factor of carbon monoxide every 20 min.  After 20 min of conditioning, the water “distributed” through the column, and the peak shape of the analytes improved (Figure 2, Chromatogram C). The original column performance was restored after 2 h of conditioning (Figure 2, Chromatogram D and Chart Figure 3).

 

Figure 3: Graph of water desorption process relative to the k’ of carbon monoxide. The orange plot is the capacity factor of CO at the beginning, and the blue plot is the capacity factor of CO after conditioning at 300°C relative to the time.

Continuous injections of water on the MSieve column without elevating the temperature of the analysis will affect the chromatography, resulting in peak tailing, loss of retention, and resolution. Just remember, even when 50% of the column performance is lost the column can be restored to the original performance by conditioning it at the maximum temperature.

Porous Polymer Columns (Rt-Q, QS, S and U BOND)

Water does not affect the porous polymers and will elute from the column as a peak. Depending on the polarity of the polymer water will be more or less adsorbed and elution time will change accordingly. Figure 4 are chromatograms of permanent and hydrocarbon gases containing water vapor. On the nonpolar divinylbenzene Rt-Q BOND column, water elutes faster, and naturally, water is more retained on the most polar column, Rt-U- BOND. Bear in mind that water will not be detected with the FID detector.

 

Figure 4: Analysis of permanent and hydrocarbon gases on 30m x 0.53 mm x 20µm Rt-Q and Rt-U BOND column (Carrier gas: He@5ml/min, Oven: 40°C (3min) then 10°C/min to 190°C, Detector: TCD)

ShinCarbon Column

ShinCarbon column can adsorb a very limited amount of water. The amount of adsorbed water is dependent on the column dimensions (amount of the material in the column). The adsorbed water has no impact on the retention times of the compounds. However, eventually, if the water is not conditioned out of the column or if the injected concentration of the water is high, it will show up on the chromatogram as an overloaded peak with an almost “never-ending” tail and interfere with the integration of the analytes. Water will always show as “overloaded”. Therefore, depending on the injected concentration retention time of the water peak will shift.

 

Figure 5: Overlay of three chromatograms on a 2m x 1mm 100/120mesh ShinCarbon column– 30µg (green), 80µg (red) of water injection, and permanent gas with C1-C2 hydrocarbon standard (black) – showing where the water will elute from the column. (Carrier gas: He@10ml/min, Oven: 40°C (2min) then 20°C/min to 200°C, Detector: TCD)

Moisture present in the carrier gas will over time have a similar effect on the column performance. Mainly MSieve and ShinCarbon columns act as a carrier gas scrubber, trapping the moisture present in the carrier gas (Figure 6). Note that these materials are hygroscopic and will also attract water when the column is not sealed upon storage.

 

Figure 6: Overlay of the first and second instrument blank injection after the instrument has set idle with the ShinCarbon column installed at 40°C for 48 h. (I think it was time to change my carrier gas filter.)

Don’t forget, carrier gas filters, which will remove the moisture and other impurities, are essential accessory with this type of analysis.