Makeup gas for FID

[wulisheng2]

The rate of makup gas flow affects the FID sensitivity as we commonly know. Would anyone share his/her hands-on experience on how the makeup gas helps to improve the sensitivity or what the significance of its applications. Thanks!

[AICMM]

wulisheng2,

I have heard make-up gas type affecting sensitivity but not make-up flow (within limits.) FID is mass sensitive and relatively immune to changes in flow (one of the reasons it is so popular.) If using capillary and peak shape is poor, make-up will improve sensitivity by improving peak shape but otherwise.....

Perhaps if you provide more details.

Best regards,

AICMM

[Yama001]

You may see some apparent sensitivity - the make up gas can reduce peak broadening as they pass through the detector, giving a sharper peak.

The intricacies of flow dependant verse concentration dependant detectors (did I say that right?)has always escaped me; I know the distinction exists, but have always had trouble recalling which is which and how to use that info.

[Peter Apps]

Make up flow in an FID has two functions. First to flush the sample quickly from the tip of the column into the flame - this is now largely redundant since capillary columns are typically installed with their tips inside the flame jet, which is efficiently flushed by the fuel hydrogen. Second - it increases response per unit quantity, so peak areas (not only heights) are about 30% larger with nitrogen make up than with no make up (this on a Carlo Erba Fractovap many years ago, mileage may differ on more modern designs). How it does this is uncertain, since the mechanism of the FID is not fully understood. While make up vs no make up makes a measurable difference I do not think that it is true to say that "The rate of makup gas flow affects the FID sensitivity as we commonly know" .

Peter

[Johnny Rod]

I don't claim to have much understanding of the exact goings-on but presumably the make-up gas flow varies the flame hieght (so position in the collector) and also its temperature? As do fuel and air flows.

[wulisheng2]

We know the makeup gas flow rate is adjustable. So there must be a reason for the instrument designer to give this option.
In pharmaceutical industry, the measure of sensitivity is definded as "Detection Limit (DL)" at which point the component concentration carries out its peak signal and noise ratio (S/N) at about 3 (most analysts use peak hight to calculate it). If we accept the assumption that makeup gas flow rate can affect the peak shape by changing peak hight (the single), then the DL (sensitivity) would reduce while lowering the makeup gas flow rate. Is it logical? I might do some experiments to find out it when I get a chance.

Thanks a lot.

[Peter Apps]

We know the makeup gas flow rate is adjustable. So there must be a reason for the instrument designer to give this option interesting logic - or you have a lot more faith in instrument designers that I have. Any GC on the market has features that are sub-optimal due to legacy design, patent restrictions, deliberate locking out of in-house modification etc. Also, people like to think that they are in control, they do not like having anything fixed, although as standard methods with specified QC become more and more common I have already predicted a GC with no external adjustments whatsoever - software driven standard methods are downloaded from the internet and bar-coded samples are loaded to the autoinjector by a low skills technician, and that's it !.
In pharmaceutical industry, the measure of sensitivity is definded as "Detection Limit (DL)" at which point the component concentration carries out its peak signal and noise ratio (S/N) at about 3 (most analysts use peak hight to calculate it). If we accept the assumption that makeup gas flow rate can affect the peak shape by changing peak hight (the single), then the DL (sensitivity) would reduce while lowering the makeup gas flow rate. Is it logical? Logical, yes (on the untested assumption that slower make up flows increase peak height), practical no, there are far more effective ways of increasing peak height than fiddling with make up flow, and it is quite likely that changing make up flow will change noise just as much (or more) than it changes peak heightI might do some experiments to find out it when I get a chance good idea, please post the results here, or even better write them up as a paper.

Thanks a lot.

Peter