Ion Count vs Ionization Time in LCMS IonTrap?

[raghafr]

Hi everybody,

I'm wondering if Ion time and Ion counts relatioship is always constant. As far as I know, Ion time reflects how long the trap is "open" letting ions pass in, until it is filled. Consequently, for larger Ion counts, time is reduced. However, I find myself in the situation having dramatically different ionization times for the same Ion Counts.

Is this relationship expected to be constant (for the same conditions, of course, same mass ranges, same ionization paramenters etc)? Or does it depend on other factors?

Following with this idea, which one is a better indicator to quantify solvent purity or background noise?

Thanks in advance for clarifying!

[lmh]

as far as I know, you're correct. I only know the situation for a relatively elderly Thermo trap, so (a) what I write might not apply to other manufacturers; (b) I might get it wrong because I don't work for Thermo.

The sequence of events is that the mass spec first admits a tiny puff of ions into the trap, which it scans out ridiculously fast, such that there's no real resolution of masses, but the total count gives a measure of the ion flow arriving in the trap at the moment. Based on this, the mass spec calculates how long the entrance lens should remain open to achieve a set number of ions in the trap.

But here's the important bit: The time the entrance lens remains open depends on the sum of all ions arriving, not just your target ion, or the most abundant ion. If your target ion is the only ion present at the moment, the trap will be filled entirely with this target. If your target is a bit weak, the trap may be half full (or more!) of background ions. A fuzzy background can add up to a very substantial signal...

This means that the ion time may look as though it isn't linearly related to the ion count, because the ion count on a Thermo instrument is the y-axis maximum of the most abundant ion, not the sum of all masses present.

[raghafr]

Thanks for your input, lmh. I get your point. But if we stick to Total Ion Count, what do you think may be happening if same TIC provide different Ion Times? For me it doesnt make sense, but maybe i'm missing something!

[lmh]

I'm afraid that's one that I can't easily answer, because in Thermo-world, total ion count is most easily visible when viewing the data in chromatogram form after they've been collected, while ion time is really only visible while the data are being collected, in the tune software. Further, total ion count after the event is already corrected for varying ion time, otherwise the total ion chromatograms wouldn't necessarily have peaks, they could just be flattish lines with unevenly spaced points (subject to the fact that there is a maximum ion time when the entrance lens will close whether or not enough ions are likely to be in the trap).

[raghafr]

I see. Well, in my Varian 500 IonTrap both Ion time and TIC can be monitored at real time and there is where I noticed different behaviours in IT vs TIC relationship.
Anyway, thanks again for your comments, let's see if someone can throw some light on this