MS tune theory question

[jsupica]

I'm running an Agilent 6890-5975 and was curious about an aspect of the tune. One of the criteria is to ensure that when fragmenting PFTBA, the ratio of the mass 219 and 502 to 69 meets a certain threshold. I understand that this is to ensure the source is properly fragmenting and ionizing but why must this criteria be met?

The answer I've reasoned out is that if the ratio of 219 and 502 to 69 is too low it means there is too much fragmentation occurring and otherwise useful medium sized fragments are getting blasted down to 69. Is my thinking correct or do I need some cerebral re-calibration?

[MSCHemist]

I believe the ionization at the filament is pretty much a constant with tuning (70eV). The tuning is to calibrate the mass axis and to optimize the lenses, repeller and detector current for optimal sensitivity accross all masses. If there was a lot of variance with fragmentation then the libraries would be useless.

[jsupica]

Geeze, yes of course they would. I may have been a little hasty in posting this before asking around the lab more.

[Peter Apps]

As I understand it, quadrupoles have an inherent bias against high mass fragments, that is counteracted by the rest of the ion optics. So the high mass / low mass ratio is QC on that process.

In other words; what MSchemist said.

Peter

[James_Ball]

It also depends on the end use of the instrument. When doing analysis by EPA methods you must ensure that the instrument can also pass the criteria they list for either Bromofluorobenzene or DFTPP. This ensures that the generated spectra will match those in the library, which originally were based on magnetic sector MS spectra.

The tuning is for both optimum performance and adherence to standard spectral matches with older instruments, otherwise you would need libraries for each instrument, especially when selecting between quadrupole and ion trap types of mass specs.

[BZK]

Have you used Stune or Atune ? Agilent in the old instruments (i don't know the newer) as default running Atune, optimized for mid range mass. To get a standard spectrum you must use Stune.
As stated by previous answer tuning allow to compare the instrument MS spectrum to a 70 eV ( electron energy) library spectrum. All instrument parameters (lens voltage, quadrupole parameter, RF...) are tuned for this requirement. If calibration fails means that some parameter are unsuccesfull tuned and clean/repair action must be take.

Regards.

Robertino Barcarolo, Italy

[jsupica]

Thanks Robertino,

We are running Atune and don't often use Stune. I did some extra reading in the troubleshooting manual (link below) which gives some great information on every parameter used in Atune and Stune. I thought it was interesting that the "Wid219" parameter in the autotune printout is an adjustment is made to the quad voltage to optimize 219 since it's a prominent peak in the middle of the mass range.

http://www.chem.agilent.com/Library/tro ... -90037.pdf

[James_Ball]

Thanks Robertino,

We are running Atune and don't often use Stune. I did some extra reading in the troubleshooting manual (link below) which gives some great information on every parameter used in Atune and Stune. I thought it was interesting that the "Wid219" parameter in the autotune printout is an adjustment is made to the quad voltage to optimize 219 since it's a prominent peak in the middle of the mass range.

http://www.chem.agilent.com/Library/tro ... -90037.pdf

That helps balance the widths over the entire scan range. From the 5971 and up it was included as a setting in the software tune window, back before that it was a pot you adjusted on the quad driver board inside the instrument. You had to set the 69 and 502 width in software then while it was running repeat profiles use a small screwdriver and adjust 219 to the desired width. Often repeating the procedure many times as it tended to alter the high and low mass widths too. Fun days those were