Is there a peak coming in higher retention time with large intensity? Can you post such chromatograms.

Hi, Udaya Jayasundara ! Here is a chromatogram overlay. I am trying to look for a compound that elutes at ~4.3 min in the shorter chromatogram in a chromatogram with a longer run time. Checking the standard, the compound should elute at around 15.2 min. However, when I run my samples, I see a compound eluting at 4.3 min in the short GC but not at 15.2 min in the long GC. (Pls see pictures attached.) The peak at around 13.8 min is the internal standard. Hope you can help me resolve this.

The chromatogram looks like a lot has changed between the 2 GC methods. Temperature profile, flow/pressure, injection conditions? From the chromatogram it looks like your injected a lot less material on your column in the pink chromatogram. Furthermore, there is a dip in the chromatogram from 7-12 min, did you reduce the flow there? If you could share the GC settings of both chromatograms, I think I could help you out, but there's not enough information now to do anything but guessing.

Hi, Luc Alders ! Thanks for the input.

I am using a headspace autosampler for sampling VOCs in my sample. I did make it a point to run them under constant extraction conditions-- same amount of sample, extraction time and temperature, and even the sampling conditions. I am using a loop transfer sampler.

I shortened the run time to look at the presence of a target compound in a retention window. The temp program is indeed different from the GC with the longer run time. Aside from this, only the FID gas flow rates were changed (short run time: 32 mL/min H2 and 200 mL/min air; long run time: 32mL/min H2 nd 320 mL/min air).

I was actually expecting a higher sensitivity for the longer GC from the modified H2:air flow rate ratio in the FID. Instead of finding a GC with more number of peaks and a higher resolution, I got that pink GC.

Btw, the dip in the chromatogram probably is due to the air compressor sucking in air.

Thanks!

Dear Joyce,

You should think about whether the peak is so low that it is below the noise level in longer running times. Every chromatographic peak spreads and has a lower height in longer times. This can be what is happening in your chromatogram.

Thanks, Ademario Junior ! This explanation makes a lot of sense. The peak height for one of my target compounds is indeed very low when seen through a short GC run time.