GC FID Temp

[jpie]

If max col temperature for a particular GC column is for example 250°, can I set the det temp above this, say 270°

Inlet temp is set to 250°

Literature says do not go above the max col temp, but on the other hand, says go 10-20° above the inlet temp, this would be greater than the max col temp .

The question then is it better to go 10-20° above the inlet temp, or stay below the max col temp, which would not be 10-20° above the inlet temp.

[chromatographer1]

The column phase limit is not a OFF - ON condition situation. It is not: OK at 250C, but at 251C all is destroyed. If the limit is 250C then you should not exceed having the oven at 250C even if your injector is set at 250C. Having the detector at 250C is also theoretically OK, but realize both ends of your column are at the limit and you ARE DAMAGING the phase. How much depends upon the phase and the oxygen level of your carrier gas (Fittings and septa leak).

If your phase limit is 250C then try to keep your oven, and both your injector and detector below that temperature.

For example, the TCEP phase has a limit of 250C, but it begins to degrade and bleed with gusto at 120C. It bleeds off faster at the higher temperature than it degrades, but it is still degrading at 100C and the longer it stays at that temperature the higher the bleed levels become. The bleed levels decrease as the column is conditioned above 200C but that is only because the phase is bled off faster than it is decomposing, even at the higher temperature.

For a longer life and for less reactive the column surface, use lower temperatures. If you need to use the higher temperatures, so be it, but understand the column will not last as long as it might if the temperatures used were more moderate.

best wishes,

Rod

[uzman]

Hello ,

According to my experience , using the injector or detector temperature over the maximum operating temperature of the column makes no harm for capillary columns.
There is only a few centimeters of the column inserted in the injector and detector , and in the worst case , the phase may strip off from these few centimeters but it will not have a negative effect on the chromatogram.
The polyimide coating and the fused silica will stand well above 350 degrees.

For the packed columns , the manufacturers often leave a space for both the injector and detector side to prevent the packing from burning and also for the solvent to expand at the injector side.

[Don_Hilton]

A word of caution on overheating a few centimeters of a column: You may be able to get away with this on some analyses, but remember that the stationary phase does not just magicaly go away. It is damaged by the heat and depolymerized. This process generates active sites - and if you compunds of interst are particularly sensitive, you may create a problem in your separation.

While you need an inlet hot enough to move the compunds of interst onto the column, it is often a good idea to keep the inlet at that temperature and no higher. This way non-volatile compunds will collect on the inlet liner rather than the column - and with frequent liner changes, the column lasts longer.

[chromatographer1]

The polyimide coating will not tolerate temperatures in excess of 350C for long. After a few hours the darkening and the brittle nature of the tubing WILL become obvious.

Don's comment on the phase destruction (not the burning) is very accurate. Packed columns can quickly demonstrate activity that can ruin samples. Even bonded capillary columns can degrade and become active. What you don't see may cause you to think, ah, it isn't present in the sample when all it means is that you left it on the column. Don't be foolish in trying to brute force samples onto a column when finesse is called for.

A fool and his column are soon replaced.

Don't let it be YOU.

best wishes,

Rod

[aldehyde]

I haven't observed problems running an FID hotter than the max temp on capillary columns but better safe than sorry for sensitive applications.

[jdezeeuw]

T max of phases is in general not the issue. The phase may bleed off, but the column surface should still be inert. Many columns can be heated to much higher temperatures, as long as the deactivation stands. You only pay in life time (and bleed).
There is a bigger concern at higher temperatures and that is development of reactivity. For those interested, we did a study on the activity of FID flame tip and it was amazing how active FID liners can be. http://www.sepscience.com/emails/sepsci1211eu.pdf
Question is: how is this activity developed? Not very strange if systems are opened at high temp and there is direct contact with air and metal surfaces.

Note this can also happen inside the column, for instance if a water peak elute in a 300C FID, what will happen to the phase? Probably hydrolysis, but we won't see it because adsorption at 300C will not show itself, until we get an extreme situation like in the flame tip. For sensitive components at low levels this should be a concern.

At the injection side: If phase is stripped from first 5 cm, there is a good chance it will be partly deposited below the injection port. I have seen this many times with TCEP. The challenge is, when phase is accumulating in the first part of the column, you get a "phase-droplet" which reduces efficiency immediate 50-80%. In wax columns you see this oft an appearance of droplets via dark spots on the inlet side. Cutting off a section fully restores the column efficiency, but it can happen again. a short guard column, PDMS deactivated solves that issue.

jaap