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How to get a near Symmetrical Water Peak..

30 Jun 2012


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This seems to be a challenging question for many chromatographers, while it is not that special. Many times water is present in a sample, but we are not interested in the quantification. Last conference in Riva del Garda, I joined a presentation from professor Armstrong who was extremely excited about getting a good water-peak from an ion-liquid coated column, I was thinking:  what so special about this? Maybe because most ion-liquid phases show quite strong adsorption on polar analytes like alcohol, diol, acid and amine.

 


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Fig. 1 water elutes very well from a Rtx-1. One must use thicker films to get enough capacity

Water elutes very well from most capillary columns as long as they are well deactivated. Fig. 1 shows a water peak from a Rtx-1 type column which was operated at 40C. Here  the water eluted at 40C as an impurity in a ethanol matrix. The water peak was expanded and looks very good. If water must be measured it is recommended to use thicker films for 2 reasons: for capacity (loadability). This because its a very polar compound on a non-polar phase. Also with thicker films, components elute at higher temperature (when using programmed analysis), which will also benefit the peak symmetry.


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Fig 2 Water peak on a Rt-U-BOND. This polar porous polymer has high inertness

Alternatively, water can be done using porous polymers. On Q or U-BOND type porous polymers water also elutes as a sharp peak, see fig 2. Because of hydrophobicity of Rtx-1 as well as porous polymers, the water peak elutes almost as first peak.  Especially if done at higher temperatures this application runs smoothly.(fig 3). As we use a TCD, the first peak is always the air-peak, also using the Rtx-1 thick-film in figure 1.


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Fig. 3 water impurities in methanol on a Rt-Q-BOND. Water elutes very fast due to hydrophobic surface

This type of GC analysis can also replace Karl Fisher titrations.

Of course, for detection of water one need to use a universal detector like a TCD, MS or a PDD.

Bigger challenge is to eliminate matrix effects as water is present almost everywhere. (:)).