How to determine if peek elutes in dead time - ACQUITY UPLC-MS/MS System - minumum RT of peek?

mjvw

<p><span>I see in the forum there is a question asking what is the minimum gradient analysis time you have used with your acquity UPLC system and one of the options is 0.5 min. </span></p><p><span>I would have thought that analytes would elute in deadtime if your run was this short (when using MS as detector is there any practical way of knowing the dead point other than using a theoretical calculation)? I thought anything less than about 0.75 mins elutes in dead time (dependant on column used and flow rate). I have developed a method and put a hold of 0.5 mins and then start the gradient slowly to hold the compounds for nearly a minute so prevent the analytes eluting in dead time. What is the earliest time compounds can reliably elute? Is it different for MS detectors compared to for example a UV detector as with LC-UV you can see the solvent front and know if your peek elutes before/after/with this??</span></p>

AJA

I am not sure if I fully understand your question but I will try. Your MS is detecting as soon as the injection happens,it doesnt wait. Your compound of interest will elute sometime after that. If the compound is totally unretained it will elute at a time that is ~ equal to the volume from the injector to the MS. You generaly dont want to do this because the goal is to seperate compounds from suff (like salts) that could cause ion supression. There is a solvent front (there has to be), often it is not detecable with an MS. There are many customers doing very fast ballistic gradients of less than a minute at fairly high flow (2.1X50 mm columns at 1 mL/min often).

Think of it this way: If the volume of the injector, column, tubing is 200 uL (its less, but this is a nice round number), then at one mL/min the unretained material in the loop takes about 12 seconds to get to the MS.

when using MS as detector is there any practical way of knowing the dead point other than using a theoretical calculation - use an unretained peak that your ms can detect

I thought anything less than about 0.75 mins elutes in dead time - as you state, depends on the column and flow rate.

What is the earliest time compounds can reliably elute - they reliably elute at all times (even if they are in the void)

Is it different for MS detectors compared to for example a UV detector as with LC-UV you can see the solvent front - Nope

and know if your peek elutes before/after/with this?? Your peak wont come out before, it could come out at the solvent from if it is totally unretained by the column