[14] What do Chromatograms tell us? Base line disturbance using Selective Sulfur Detection

Chromatograms are like fingerprints.  If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.

When running your analysis, sometimes a base line is observed as shown in fig. 1.  This was  a method where a selective “sulfur” detector was used. This could be a FPD, a sulfur cheluminescence detector or a PFPD. The basic idea of these methods is to “see” only the components we want to see.  When the base line is disturbed often one assumes a sulfur component is causing this.

Practically one needs to realize that “selective” detection devices always have a limited degree of selectivity. This means that if there is a selectivity of 5 magnitudes, if a matrix is injected that has no “selective “ response, the detector still will show a response. This is also what is seen in the chromatogram in figure 1.
Fig. 1 base line disturbance of a selective detector when matrix component elutes

If a component is eluting in the same area, the response of the selective detector will not be representing the real concentration

It is therefore very important to choose a column that separates the target analytes from the matrix. For example, Impurities in propane or propylene cannot be done on a Rtx-1 because the COS elutes at similar retention time as the propane/propylene. One need a column with different selectivity.  Silica Based adsorbents (Gaspro/SilicaPLOT), solve this, but COS and H2S response is a challenge.
Fig.2 separation of sulfurs using selective and retentive RT XL Sulfur phase in Siltek micropacked column, using selective detection

A unique solution is to use an inert porous polymer for this. The Rt XL Sulfur is already used for this application, but it’s a packed column and efficiency is low, see fig 2. This analysis was done on a 1m Rt-XL Sulfur, micropacked,  diameter 0.75mm. Sulfurs are at ppb level. The SS column material was Siltek deactivated, an essential treatment for trace sulfur quantification. The sulfurs are nicely separated from the hydrocarbons, eliminating risk of quenching.

Also on this column we see a base line disturbance when the large hydrocarbon matrix is eluting. Fig 3 show the trace analysis of COS in propylene and ethylene.
Fig. 3 Trace H2S and COS in ethylene/propylene using Rt-XL Sulfur and selective detection

For trace sulfur, one can also consider the Rt-U-BOND that separates nicely the COS and H2S from the matrix, see fig.4. This PLOT column shows to be very promising for polar and reactive analytes, especially sulfurs.
Fig. 4 separation of H2S and COS from C1-C3 hydrocarbons using Inert Rt U-BOND PLOT column

A series of columns was tested for sulfur separations in natural gas using PFPD, see this poster handout.Poster 2011-1 Sulfur in Natural gas HANDOUT