by

**Rndirk** » Tue Jan 03, 2017 9:05 am

Hi,

Welcome to the forum.

A peak that shows up in your chromatogram consists of data points which are connected by a line. If you have 3 data points over 1 peak, it will look like a distorted triangle. This is not enough for **quantitative** integration. On the other hand, 50 data points over 1 peak with give you a "noisy" peak with too much features. You see that there a **optimal amount of data points over 1 peak**: you want somewhere between 12-20.

With single or triple quadrupole MS detection, you typically want to look at more than 1 ion (transition) at a given moment. Your instrument can not measure them at the same time, it has to **switch** its filter (quad) rapidly between different masses. Here's where dwell and cycle time comes in.

Assume you want to measure 5 transitions over a certain period (also called *windows*).

Dwell time: the amount of time it measures 1 transition. Suppose you set this to 50 msec for each transition

Cycle time: The time it takes to aquire all transitions. This is 250 ms (50 x 5). (Note this is actually a little bit higher because it takes a few ms to switch between transitions).

What you should do with this information is optimize the amount of data points over your peak and give your instrument a minimal amount of dwell time per transition. Minimal dwell time is instrument dependent. For quantitative work, i use a minimum of 10 msec.

In the example above, it takes 250 ms for 1 cycle, so 5 transitions are measured once each during this time, then the cycle repeats. Each second, the cycle repeats 4 times (1 second/0.250 seconds). This means, if component X starts eluting at second 500 in your method , and ends eluting at second 503, it will have 3 sec x 4 = 12 data points over its peak.

The peak width is an important factor here, and depends on a lot of parameters in your method. You should check what peak widths you have and change the settings accordingly. In a given window, you want to optimize the settings for the narrowest peak. For example, for a narrow peak of 2 seconds, you want to lower the dwell time in the example above to 40 ms / transition (total dwell time 40*5 = 200ms), so you have 5 cycles per second => 10 data points over a 2 second peak.

Practice: You want to measure 8 components. Narrowest peak is 2.5sec. How much dwell time per component to get 12-20 data points per peak?