Increase in FID response when switching makeup gas

[Rhodium]

From what I understand, FIDs are generally not affected by changes in makeup gas composition. However, I often read that a switch to nitrogen would cause an increase in sensitivity, but is that only true when using other detectors like TCD? When switching from helium to nitrogen as a makeup gas, and all flow rates kept constant, would you still see an increase in response or sensitivity in an FID? If so, has anyone with experience in switching makeup gases from helium to nitrogen seen any significant changes in sensitivity or S/N ratio?

[aldehyde]

I have heard anecdotal reports that N2 make up on FID gives better sensitivity, but I have no data to back this up.

[Rhodium]

I seem to be running across conflicting literature on the subject. Some literature states that no change in FID response occurs when switching from helium to nitrogen. Other literature plainly states you can expect to see 10-20% increase in FID sensitivity when switching to nitrogen.

The Troubleshooting and Maintenance Guide For Gas Chromatographers, page 146, reads: "Using nitrogen as the makeup gas increases detector (FID)sensitivity by 10-20% over helium"

So I am wondering if this book, although updated in 2007 is outdated or even flat out wrong, since most other literature points to no change in FID response when switching makeup gas from helium to nitrogen.

[CE Instruments]

Not all GCs even need a make up gas. Surely a well designed FID for capillary does not need an extra 30ml/min of make up gas for it to work when it already has 300-400ml of gas going to it ?

Make up was usually needed when going from Packed columns 30-50ml/min flows to capillary 1-2ml/min flows.
Adding Helium make up should make minimal difference to the sensitivity of the detector as you are changing the total flows by less than 10%. When switching to Nitrogen, a UK gas supplier used (still does ?) to make special FID air that had a raised level of Nitrogen. The higher level supposedly gave a better response, but from memory this was only claimed as 3-5% I can't confirm either if this was due to better sensitivity or lower noise.

As a vendor in the past of a GC that did not need make up gas I always used it as a selling point that our detector was designed for capillary and did not require a make up gas. Why pay to have Nitrogen piped to the system or waste another 30 ml/min of expensive Helium.

I would be delighted to see if anyone has data or can point to published data that supports the need for added make up and/or enhanced response from raised Nitrogen levels.

[Peter Apps]

Traditionally make up gas had one main function; to replace the flow of carrier gas that used to come through packed columns, and which flushed the column effluent quickly through the detector. That function become largely obsolete as soon as it became standard practice to install the column with its tip inside the FID jet - a practice that came in only after silica columns were widely adopted. With glass columns it was usually impossible or very difficult to install the column close to the jet, and an extra flow was needed to flush the potential dead volumes.

In addition, part of the ionisation process in an FID involves hydrogen radicals from the combustion zone diffusing against the gas flow to fragment incoming analyte molecules. This might be where the presence of nitrogen make up could significantly affect flame structure, and where swapping to helium from nitrogen would produce a change in response.

Many years ago the lab I was in tried adding makeup gas to a Carlo-Erba Fractovap FID, which usually ran without make up. Response increased by 20 - 30%.

Peter

[chromatographer1]

My experience agrees with Peter's.

Nitrogen make-up did increase the sensitivity of the FID. Depending upon the design of the FID it may not have been necessary, but depending upon the gas flows used it usually improved the sensitivity, while on occasion increased the 'noise' level of the detector.

I would certainly suggest that using nitrogen is a cheaper solution than helium for makeup if makeup were to be implemented.

Leaning the flame mixture and increasing the conductivity of the flame by replacing helium with nitrogen makes a lot of sense to me. I encourage experimentation in the lab instead of references in books or oral histories.

What works best, works best.

Rod