Signal respone in NCI

[sky]

My friend told me that GC/MS/NCI is a good detector for halo compound but they said that the stability of signal response is not good, I mean signal vary much in each injection. Is it true?

[chhubert]

My experience is yes but I don't know why. Probably because of source contamination during sample injection which leads to signal fluctuation. Nonetheless, NCI is less prone to matrix interference for sure. I suggest you to use good I.S. such as isotopically labelled standards for monitoring the changes.

[Reda HTDS]

I think that it depend also of reagent gaz quality & flow fluctuations...

[sky]

Thanks for your information.

Reda:
I read the data and noticed that the flow of reagent gas (CH4) is quite steady and I guess it is regulated by the GC in certain "psi".

chhubert:
Great to hear that you have same observation. At least I think the problem is not caused by inproper setting. Our situation is that the signal fluctuation also observed just by injecting the standard solution. And I would like to use it for quantitation. So I quite worry about the possibility although many literature papers said it can. Also, do you find the linearity range is very narrow in NCI?
Its very expensive to use isotopes for IS, any cheaper & best alteratives?

[chhubert]

Sky,
I did have similar observation when simply injecting standard solution. However, as you may know that CI source is more prone to contamination, probably due to presence of methane at relative high pressure and low temperature in the ion source, injecting standard solution may still make the source dirty to a certain extent. In my analyses, I also use homologues as I.S. but performance is not as good as isotopically labelled ones. It's just a compromise. Linear range also depends on whether a good I.S. is used, which two orders of magnitude is not a problem.