Chromatography Forum

What's the relationship between respose and the scan range?

Say I'm looking for ion 94 in a standard. I run it twice, first while looking for m/zs from 50 to 550, and then while looking from 50 to 300. In the latter case, the range is half the size, so the amount of time spent on each ion is doubled... so my response should double?

Am I'm thinking about this incorrectly?

Might depend on how your instrument software controls the scan rate

If you keep the scan rate (Agilent, the time taken for the whole scan) constant then the dwell time and therefore the sensitivity will increase as the number of ions decreases.
If you keep the dwell time (time taken for each ion) constant, then the sensitivity will stay constant no matter how many ions you look at.

In practice, there's a "dead time" between scans as well, so halving the number of ions scans won't actually double the sensitivity, but if the length of one full scan length stays constant, then reducing the number of ions scanned should increase the sensitivity.

All for a normal quad MS too. Some MSs do different things when scanning...

If you work with SIM(Single Ion Monitoring) you will find that the ion abundance versus dwell time is not a linear relationship. Increasing dwell time will increase ion abundance but you hit a point of diminishing returns. Also if dwell time increases too much you lose scans per peak which can also cause problems with sensitivity.

Cutting the scan range in half will probably give you a 10-20% increase in response not the 100% it would seem it should.

One experiment you can try is to put it into manual tune mode and monitor the peaks of the tune gas as you alter the scan range and see how the sensitivity of the lighter ions changes.

Thanks guys,

James - I am seeing about a 20% increase, just like you said. Glad it at least makes sense.

There was one thing Peter wrote that I didn't quite understand.

"If you keep the dwell time (time taken for each ion) constant, then the sensitivity will stay constant no matter how many ions you look at."

In that case, wouldn't you get more scans over a peak? And if you have more scans at a given dwell time, isn't there more total time spent looking at each ion during the time that peak is eluting? I'm pretty sure I'm wrong but I'd like to see how I'm wrong...

Thanks guys,

James - I am seeing about a 20% increase, just like you said. Glad it at least makes sense.

There was one thing Peter wrote that I didn't quite understand.

"If you keep the dwell time (time taken for each ion) constant, then the sensitivity will stay constant no matter how many ions you look at."

In that case, wouldn't you get more scans over a peak? And if you have more scans at a given dwell time, isn't there more total time spent looking at each ion during the time that peak is eluting? I'm pretty sure I'm wrong but I'd like to see how I'm wrong...

If you look at each mass longer, then you get fewer scans per peak, if you look at each mass for less time then you get more scans per peak. If you lower the mass range you will have faster scans times from beginning to end of scan thus more scans per peak, if you increase the dwell time for each mass you can return the time it takes to make each scan to make the scan time from 35-300 equal to that of 35-550. This is done on a Agilent using the Samples setting in the Tune menu(Quadrupole Icon not Tuning Fork Icon). You can't set exact dwell time in scan mode, but you can in SIM mode.

There was one thing Peter wrote that I didn't quite understand.

"If you keep the dwell time (time taken for each ion) constant, then the sensitivity will stay constant no matter how many ions you look at."

In that case, wouldn't you get more scans over a peak? And if you have more scans at a given dwell time, isn't there more total time spent looking at each ion during the time that peak is eluting? I'm pretty sure I'm wrong but I'd like to see how I'm wrong...

You'll have more scans across the peak, and therefore the peak will be better defined, but it won't actually be any larger. The area of the peak is area underneath it (when the individual scan points are joined together) , not the sum of the individual measured responses.

Sorry for the clumsy explanation, but more scans across a peak gives a more accurate area, while longer dwell times gives a higher response.

Another thing to consider is that the longer the dwell times for each ion the worse is the distortion of ion ratios due to the steep rise and fall in concentration of a decently sharp capillary peak.

Peter