Makeup Gas Pressure Effects on GC-VUV

In Jack’s blog, Dilution Is Not Always The Solution: The Impact Of Makeup Gas Pressure For A VUV Detector For GC, he briefly talks about how VUV spectrometers are equipped with makeup gas regulators.  These regulators allow the user to control the makeup gas pressure (MGP), dictating the amount of flow going through the flow cell.  Now, the GC column already provides flow, but consider the MGP as insurance, allowing the flow cell to be properly swept at low GC column flow rates. We mostly use nitrogen as the makeup gas because it’s cost effective and a low absorber in VUV radiation. Likewise, helium or argon can be used as alternate makeup gases.

MGP can be used to adjust the sensitivity of a VUV detector.  By increasing the MGP, the analyte residence time in the cell decreases, resulting in smaller peaks (peak height, peak area).  This can be used to an advantage.  For example, in my previous blog post, Dilution Is The Solution: Handling Analyte Concentration Differentials With Makeup Gas Pressure Programming For GC-VUV, I used  increased MGP to reduce solvent peak intensity over a portion of a chromatogram to avoid spectral saturation.

This blog post simply highlights how peak area is affected by MGP, which can be in seen in Figure 1.

Figure 1. As the makeup gas pressure (MGP) increases, the peak area drops like a power function similar to 1/x . When the MGP is set to the maximum (~4.00 psi), the peak area levels off.

Hopefully this helps with setting an MGP! Otherwise, we recommend setting the MGP to 0.25 psi for a VGA-100 detector and 0.35 psi for the VGA-101, allowing for maximum residence time in the flow cell and the best detectability, while maintaining good chromatographic peak shapes.

Figure 2. Plotting the makeup gas pressure (psi) versus the retention time (min) for a GC peak gives insight to the rate of change for the retention time as the makeup gas pressure increases. The rate of change is about 0.066 minutes/psi.

As I wrap up this blog post, you may be asking yourself, “Wait …. why does the retention time increase when you clearly stated as you increase the makeup gas pressure, the residence time within the flow cell decreases? In other words, wouldn’t the retention time be less as the MGP increases?”  Great question, but of course the answer isn’t trivial.  Due to the design of the VUV flow cell, there is a backpressure exerted on the GC column as the MGP increases, which results in a small increase in GC retention time.

As a quick recap:

• MGP effects GC peak area to a power function similar to 1/x.
• The default MGP pressures for the VGA-100 and VGA-101 are 0.25 and 0.35 psi, respectively.
• Slightly longer GC retention times occur when you go to higher MGP.