Explanation of why ratio of 69 to 219 would be 1:1 ?

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Hi all, I recently adjusted my column going from the GC(6890) to the MS(5973), and the ratios of 69, 219, and 502 are now off. Prior to the adjustment (which was conducted in an attempt to resolve an air leak), the ratios were about 100:50:3, but after alteration the ratios are now 100:100:40, with counts around 8.3x10^6 for both 69 and 219.

Let me know if anyone knows why/how this could happen, and how to resolve? I am planning on re-adjusting the column length again to make sure it is the appropriate distance into the MS source, but unsure what else to try if that doesn't work.

Thanks for your thoughts!
GC tech (under construction).
The autotune functions trie to set the 69 abundance to a set value. If it has the wrong peak or some such error it will crank up the gain and EM voltage to achieve the set point. One way to get an idea what is wrong is to watch the screens and see if the various ramps maximize or behave in unusual manners. Of course it is helpful if you know what normal is.
If you haven't done so already, load an old tune file and start from there. Especially watch to see if the 69 ,219, 502 peaks are found right away or if the routine has to search for them.
Note the EM, ion focus and repeller voltages. Compare them to the last good tune.
If you vented to adjust the column, how long did you wait before tuning?

If the column tip is too far into the source it can affect the ratios, but not far enough won't change anything with tunes, only possibly with analyte breakdown.

I normally open the door and set the column by holding one of the scoring wafers against the end of the transfer line, then pushing the column in until I feel it touch the wafer, then remove the wafer and tighten the nut, which will push the column about 0.5-1mm farther in which is the correct depth.

I then let it equilibrate overnight to make sure temperatures are stable and all the moisture is gone. If moisture is high, then the high mass will be biased high. Strange but has always happened on mine. Once the moisture pumps out, the 502 will drop.
The past is there to guide us into the future, not to dwell in.
Other than the USEPA forcing you to adjust the PFTBA to meet an archaic, artificial criteria I can't think of any reason that you wouldn't want 219 high unless you're dealing only with compounds <100 daltons. Since I no longer have to deal with USEPA methods (yeah, James, I feel your pain) I typically try to get as much 219 signal as possible.

One reason your 219/502 may have gone high is your source temperature. Did you check your tune when the source was cold? That will radically affect the ion ratios.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
I would immediately lower your EM to get a more appropriate response for 69. 8.3million must be nearly maxing out the response for 69, giving you a false ratio of 69:219 and 69:502. Bring that 69 back down to earth, say around 600,000 counts by nothing but lowering your EM and compare the ratios.
Regards,

Christian
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