Inconsistent Relative Response Factor

Discussions about GC and other "gas phase" separation techniques.

6 posts Page 1 of 1
Good day All,
I am trying to see what effects the relative response factor (RRF) of the FID on a GC-FID. I have purchases a gravimetric standard containing n-C10 to n-C50 that I use to measure the RRF with respect to n-eicosane (one of the components in the standard). The RRF increases dramatically for the final component n-C50. The standard is dissolved in carbon disulfide, an effort is made to make sure all the heavier components in the standard are completely dissolved in the solution prior to injection (the vial is warmed-up prior to the injection on the autosampler), but I still suspect the C50 is falling out of solution within the GC syringe prior to injection, and I believe this to be the cause of the RRF increase.

I would like to give you some details about the GC setup; if need be I can upload the entire method parameters. The method uses a COC inlet and a Restek MXT- HT simdist column (https://www.fishersci.com/shop/products/restek-mxt-1ht-sim-dist-metal-siltek-treated-capillary-columns/06713284).

Any help on this subject would be appreciated. I am unable to find any papers/articles regarding RRF dependence on method parameters such as FID flows and temperatures, inlet temp etc.
Are you looking for the RRF for C10 and C50 to be nearly the same, or are you having trouble with the RRF of C50 changing over time?

Chain length can affect the response on the FID and the ability to vaporize it can also affect the response. The higher the boiling point the more difficult to get it on column consistently.

Does the peak shape change when you see the change in RRF? (wider, shorter, ect)

Does the RRF change after injecting samples or just between injections of the standard?
The past is there to guide us into the future, not to dwell in.
Thanks for the reply James,
Per ASTM D7169 Hight temp SimDist, I am verifying the linearity of the FID detector. All the components of the purchases standard have a RRF less than 10% different (with respect to C20) except C50; 10% is the max difference of the method.

After running multiple injections on the same method, the C50 peak is consistently the same shape through out the runs and doesn't differ too much with the others. I have also performed multiple runs varying one method condition (that I believe could be related) at a time. Keep in mind, these runs are being performed on the same standard, being heated prior to each injection and cooled and re-capped after each.

Condition 1 - Default

Condition 2 - Inlet Temp Decreased to 50C

Condition 3 - Inlet temp increased to 150C

Condition 4 - Inlet temp increased to 150C and inj. volume increased from 1 to 1.5 uL

Image

As you can see, the RRF of the C50 is not changing at all.

I am starting to think that, even though I've taken every precaution, maybe I am not getting a complete transfer from the standard ampule to the GC vial - in other words C50 is falling out of the solution before I can transfer it.
Thanks for the reply James,
Per ASTM D7169 Hight temp SimDist, I am verifying the linearity of the FID detector. All the components of the purchases standard have a RRF less than 10% different (with respect to C20) except C50; 10% is the max difference of the method.

After running multiple injections on the same method, the C50 peak is consistently the same shape through out the runs and doesn't differ too much with the others. I have also performed multiple runs varying one method condition (that I believe could be related) at a time. Keep in mind, these runs are being performed on the same standard, being heated prior to each injection and cooled and re-capped after each.

Condition 1 - Default

Condition 2 - Inlet Temp Decreased to 50C

Condition 3 - Inlet temp increased to 150C

Condition 4 - Inlet temp increased to 150C and inj. volume increased from 1 to 1.5 uL

Image

As you can see, the RRF of the C50 is not changing at all.

I am starting to think that, even though I've taken every precaution, maybe I am not getting a complete transfer from the standard ampule to the GC vial - in other words C50 is falling out of the solution before I can transfer it.
Have you tried sonicating the vial prior to injection? Some things that fall out of solution can be difficult to get back into solution.
The past is there to guide us into the future, not to dwell in.
SimDist setup with capillary column must have FID makeup gas. Hydrocarbon vapor may significantly shift partial gas pressure in a flame over the jet.
Makeup is required for maintaining constant ionization effectivity.
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