air tune

[mdus]

Hi,

Should a BFB calibration and tune work well for mass below 69 (atmospheric gases)? Otherwise how could I go about generating an "air tune" with mass range 4 to 50, for example, or is that a bad idea?

FYI: PerkinElmer Clarus600 gc/ms

Thanks.

Martin

[carl.nott]

Not really sure what you're looking to do, but I scan down to 41 for my 8260 range of compounds. If you're trying to see how much air is in a sample of a gas... I guess that should be doable, but you'll need to account for any air in the system and, depending on how low you're looking to go, the background may dwarf what you're looking for.

[mdus]

We are looking at CO2, CH4, N2O, O2, N2, Ar, Kr, etc. in air samples. So the question is whether we should/can optimize tuning to these masses...rather than typical peaks at 69 through 502 for the BFB-based tune...let me know if I'm completely off base.

Thanks.

M

[thohry]

I think you can do that but you have to find a calibration compound that has stable fragment ions in the range you are interested (like PFTBA in the range 69-502)

[JI2002]

We are looking at CO2, CH4, N2O, O2, N2, Ar, Kr, etc. in air samples. So the question is whether we should/can optimize tuning to these masses...rather than typical peaks at 69 through 502 for the BFB-based tune...let me know if I'm completely off base.

Thanks.

M

You are missing the point why a BFB tune is required. The BFB tune is to assure the spectrum is searchable to NIST library and comparable between different labs. For the compounds you are looking for, there's no need for "air tune".

[mdus]

Excellent, so then, if the tune is optimized to BFB etc. can I assume that that is the best response I can get for lower m/z or can I modify the tune so that my ms response is optimized to the range of m/z of interest?

I have noticed that when the BFB valve is opened for tuning, the air peaks (N2, CO2, etc.) increase slightly; a leak I suppose. Can I use those leak-peaks as my reference gas?

Thanks again

[mdus]

I think I am finally seeing the error of my ways. I gather tuning has little to do with sensitivity, and more to do with accuracy in mass identification, and relative intensity of target peaks in ref. gas chromatogram?

Thanks for all your comments.

M

[Don_Hilton]

The tune of a mass spectrometer adjusts mass accuracy, sensitivity, AND consistancy of ionization and fragmentation - thus the issue of matching peak ratio on BFB or some other tune standard.

If you are measuring the various gasses with mass spec, you want to adjust all of these factors (sensitivty, accuracy, consistant ionization) - even if the fragmentation of specific gasses is not interesting, to ensure that the ionization is consistant over time. Changes in fragmentation molecules will give rize to fariation in response. Even though you will run a check standard on a regular basis, and calibrate as needed, you want to maximize the stability of your measurement. And you have factors like the age of the filiment - which will change temperature over time as you try to maintain current - and energy distribution of emitted electrons - that work against this stability.

Unless you have a regulatory requirement that specifies what you need - you can optimize where it makes sense. But be sure that you cover consistancy of the ionization process along with sensitivity and mass accuracy. In some instruments, it is most convienit to use the built in tune (like on PFTBA) and keep the instrument optimized to that. And that may work well enough for you.