GC baseline problems - Agilent CP 7420 column

Question

Hello, I have a Agilient CP7420 column connected to the GCMS-QP2020NX also equipped with a FID detector and the column is connected to the FID (carrier gas helium 1mm/min). The column and the chromatograph are new, but the column was purchased from 3 years ago but was not unpacked

In terms of FAME separation, I am very pleased with the column. However, I have some problems with an unstable line at higher temperatures which generally make it very difficult and sometimes impossible to quantify the analytes.
Above 175C, irregularities begin which, with the appropriate enlargement, look like 'saw blades'.
Basically, all procedures related to the chromatograph were performed (column conditioning for many hours at different temperatures, cleaning of the injector and FID, the split filter was replaced, the silica gel in the hydrogen generator was dried, the septa and liner was replaced, the column was cut several times. and reinstallation, filters are installed on both the carrier gas and the detector gas lines).

The manufacturer's service after performing all the above-mentioned steps found that these instabilities came from the column. My question is whether it is possible to deal with the fluctuations attached in the picture or Is it just the column's fault? Or maybe the injections of various solvents, such as methanol or acetone, can help. so far, n-hexane has been used

I would be grateful for any help and suggestions

Dr. Know · Accepted Answer

Hi Domar, 
 this is textbook behaviour. 
 It is not related to the specific part number. What you see is the typical cyanopropyl-bleeding pattern of any stationary phase containing cyanopropyl groups. Normal bleeding is mainly hexamethylcyclotrisiloxane, a Si-O- six ring with 2 methyl groups on each Si (we call it D3). If you have a cyanopropyl phase one or more of the methyl groups is replaced by cyanopropyl groups (statistically the same percentage as cyanopropyl in the phase), the more CN-C3 the higher the molecular weight of the bleeding product and the lower is its volatility. 
 Bleeding starts above 200 &deg;C and you normally can see a rise in baseline. At the end of your temperature program you cool down (in most cases) ballistically, trapping the bleeding products in the phase until the next time you are above 200 &deg;C. Because these CN-D3s are big they have quite a long retention and don't travel immediately out of the column, then you cool down again and trap them again. With your next run the game starts again, that gives you the toothy look. 
 The only possibility to get rid of the problem is giving the bleeding products enough time to leave the column after your run by temperature-controlled cool-down. Cool with (-2) to (-5) &deg;C/min to 200 &deg;C at the end of a run. 
 For conditioning go to Tmax for 30 minutes and then cool down with -2 &deg;C/min to 200 &deg;C and then ballistically to your starting temperature. If it persists keep the column 60 minutes at Tmax. You need to experiment a little as the worst case is hexa cyanopropylcyclotrisiloxane, a molecule with molecular mass of 524 Da and a very long retention time. 
 Hope this helps 
 Regards, 
 Norbert

Dr. Know · Answer

Hi Piotr, 
 I assume to separate FAMEs. We saw that 0.5 &deg;C change in elution temperature of a component or critical pair can make or break the separation for this column. 1 &deg;C can switch critical peaks. 
 Temperature rates for some instruments go up to 100 &deg;C/sec and it works for some components, but might not for others. The advantage is less diffusion and therefore less ban broadening, but you will be out of the optimum in the Van-Deemter curve immediately and Nth goes down giving you more band broadening. Finding the right path here is the art of GC. 
 Equilibration time: as long as you have a stable baseline go for it. 
 Bleed is a function of column flow, twice the flow = twice the amount of bleeding, it still can be a stable baseline. It's more difficult when you use flow control in the GC. The flow is calculated, NOT measured and you use nominal column dimensions and not the real ones, with that you can have errors up to 20+% in flow. 
 Rule of thumb for the GC detector is at least 20 &deg;C higher than the max. oven temperature used. The same or lower temperatures can lead to condensation in the FID. So yes, the FID can have a higher temp than the Tmax of a column. I use 250 &deg;C as FID temperature as a default, when high boiling compounds are present I increase it to 270 &deg;C or 300 &deg;C, that's normally enough. You will always destroy a little amount of phase at the end of the column (some centimeters). 
 And finally speed up a GC analysis: better go for smaller IDs and the equivalent column dimensions than just use temperature. Temp is fine if you have a non-critical separation. Also here some experimenting helps. And use hydrogen as carrier gas if possible. Don't be afraid of hydrogen, it's at least twice as fast as helium and has a very broad optimum in the Van-Deemter curve (even with a little loss of plate count). 
 And sometimes you really need 100 m of column if the analysis is very complex.

GC baseline problems - Agilent CP 7420 column

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