MRM question

MikeT

<p>Hello</p><p></p><p>It's me again trying to figure out my IR problem with this PG method I've been working on. The question is on MRM analysis how does the instrument "know" what precursor a product ion belongs to? I am trying to measure 16:0-18:1 PG. Precursor is 747, product ions 255 and 281. But there is this other PG (18:1-18:1), precusor 773, product ions 281 and 281 (R1 and R2 chains) that co-elutes with 16:0-18:1 PG. Is it possible some of the 281 I'm measuring could be from the 18:1-18:1 PG? I think not since I thought only precursors I specify reach the collision cell but at this point I'm grasping at any possibility.</p><p></p><p>TIA</p><p></p><p>Mike</p>

EdS

Hi Mike,

if you're running in MRM mode, the first scanning quadrupole is fixed on the precursor ion that you select and therefore will only let ions of that mass to charge ratio into the collision cell. So in your case only ions with a mass of 747 would be allowed into the collision cell and therefore it it only ions of that mass to charge ratio that would be fragmented to give you product ions at m/z 255 and 281. I've attached a graphic which might help you visualise exactly how this acquisition mode works.

You should not be getting contamination from a compound that is 26 Da heavier than the precursor that you're interested in unless there is a possibility that your mass 773 precursor could potentially fragment before the first quadrupole to give a mass of 747? It would be relatively easy to check this out if you had a standard of the 773 material, just run a product ion scanning (daughter scan) experiment at mass 773 using the cone voltage that you use for your component at 747 and a collision energy of 2V. this should allow you to see whether the cone voltage alone gives a fragmenty of 747 from your 773 ion.

Hope this helps.

All the best,

Ed

MikeT

Thanks Ed for your reply, I will try your suggestion to see if the 773 fragments to 747.

regards

Mike

Lin

Hi Ed,

To check for potential fragment of 747 from, I thought we should run a MS scan using the cone voltage that was used for 747, instead of running a product ion scanning, am I right?

Regards

Lin

EdS

My apologies Lin, you are correct. I must have been having an off day when I wrote that post. I would indeed just use and MS scanning experiment as you describe. The experiment that I described would test for whether the 773 ion spontaneously broke down to 747 in the collision cell, of course, if you are using MRM, the 773 ion shouldn't get to the collision cell in the first place...

Thanks for your correction and apologies if I have caused any confusion.

All the best,
Ed