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Can we see oxygen on FID?

26 Jun 2022

The Flame Ionization Detector (FID) is a popular detector mainly due to its sensitivity and extensive dynamic calibration range. However, it is a selective detector. It only responds to organic compounds, compounds containing carbon and hydrogen atoms. Inorganic molecules, for example, oxygen, nitrogen, water, H2S, CO, CO2, and others, cannot be seen with an FID. It is common to take advantage of the FID's selective performance when analyzing organic compounds in an inorganic matrix. This eliminates the large matrix peak from the chromatogram, and only a slight disturbance in the baseline is observed if we inject large volumes.

One of our customers analyzes ethylene in air, a compound given off by ripe apples using the Rt-Q BOND column and an FID detector. The Rt-Q BOND column resolves any disturbance in the baseline caused by the large volume of air passing through the detector from the ethylene. However, he noticed a large peak tailing into his analyte even though air can't be detected. After significant troubleshooting, it was determined a tailing peak is an oxygen.  

So, what was happening there?

The described phenomenon is known yet not well understood. The most plausible explanation is that the oxygen reacts with the Q-BOND polymer in the hot detector liner and generates FID detectable fragments, which we see on the chromatogram (Figure below).

To explore this further, we studied the effects that could induce the reaction of oxygen with the porous polymer. Variables like carrier gas options and temperature were investigated. We concluded that the quantity of reaction fragments generated is only temperature-dependent, and the threshold for this process is clearly observed above 200°C. Since the end of the column is inside the detector’s heated zone, it is exposed to elevated temperatures. At an FID temperature of 300°C, the response for "oxygen" was 15x larger than at 250°C and barely noticeable at 200°C (Figure). We evaluated several manufacturers of Q-type porous polymers and observed the same effect.

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Figure: Chromatogram overlay of 500µl injection of oxygen using Rt-Q BOND column at FID temperatures of 200˚C, 250˚C, and 300˚C.

Do we recommend you stop using the porous polymer columns to analyze oxygen? Absolutely not. The Rt-BOND column family offers unique selectivity and tremendous retention for light analytes. However, to reduce the appearance of an oxygen-induced peak, follow these tips:

  1. Use a particle trap or a deactivated transfer line
    Many users of porous polymer PLOT columns already use a particle trap or a transfer line connected to the end of the PLOT column using a PressTight connector. The plot column should not be inserted into a hot FID for this analysis. A particle trap offers an additional layer of protection and stops fugitive particles from reaching the detector.
  1. Temperature of the FID and inlet
    The FID (or any other detector) and inlet temperature are reduced to 200°C if oxygen has to be measured using a porous polymer stationary phase. That shouldn’t be a problem when the final temperature of the analysis is below 200°C.

But to answer the title question: can we use the FID to see oxygen? Can the FID be used by adding an in-situ reactor based on porous polymer for selective quantification of trace oxygen? Maybe, but so far, this is just an idea.

Recommended Reading

  1. Refinery Gas on Rt-Q-BOND (https://www.restek.com/chromatogram-detail/GC_PC1342)
  2. Ghost Peaks in Gas Chromatography Part 4: Reactivity in The Column While Doing Separations (https://content.restek.com/content/published/api/v1.1/assets/CONT176B548D2DB9420985CC52A82E907E81/native/GhostPeaks_Part4.pdf?channelToken=6428daeb0e4e4341beecd8e2b41b4d46&download=false)
  3. GC Particle Traps (https://www.restek.com/videos/gc-particle-traps)
  4. What do Chromatograms tell us? Peak Shape: Overload using Adsorbents in PLOT (https://www.restek.com/chromablography/4-what-do-chromatograms-tell-us-peak-shape-overload-using-adsorbents-in-plot)