How to get a near Symmetrical Water Peak..

2011-jaap-pasfoto4This 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.

 

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.

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.

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. (:)).

6 Responses to “How to get a near Symmetrical Water Peak..”

  1. Todd McMillan says:

    I quantify water for an industrial plant on a packed column GC. I am very interested on switching to a capillary column, do you have any more info on this?

  2. Hi
    You can do this 2 ways:
    using Thick-film Rtx-1 coated fused silica columns at 50-60C, or run a Rt-Q-BOND at high temperature (200C).

    Important is to use the right liner. Need good evaporation of the sample. Precision liner works nicely. If you need trace H2O , you may not succeed with TCD. PDD will help. Make sure that you have extreme pure helium to run the PDD.

    regards,
    Jaap de zeeuw, Restek Corporation

  3. Alan Sensue says:

    A Rt-U-Bond also produces a nice looking water peak. http://www.restek.com/chromatogram/view/GC_CH01137

  4. lim miaw yee says:

    sorry,there is mistaken in my first comment…
    i m using rt-Q-bond to analyse ethanol with 1-propanol as my internal standard.however i get three retention peak which is on 1.195,1.381 and 1.621..may i know how to confirm which retention time should be belong to the peak for ethanol?
    is”t possible the retention time on 1.195 is belong to water?

  5. Hi
    to confirm, inject a small amount of pure ethanol. it will tell you what peak is ethanol.

    You can do the same for water. Also verify elution of air.

    Water will elute BEFORE the alcohols, so it should be the first peak, so 1.195 can be water (or air). Note that H2O and air are not detected by FID.

    regards
    j de zeeuw

  6. Oscar Sanchez says:

    GC analysis can also replace Karl Fisher titrations in 1.2 EDC

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