Low Dispersion Injection

Dispersion is one of the reasons for peak tailing. You can reduce dispersion considerably during the injection.

In the middle of the loop, the liquid flow is faster than close to the capillary walls. Thus, the following injection profile appears at the loop outlet:

(* Solvent in the example - can be transport liquid or any a similar liquid, instead.)

You can either cut off or inject the tailing part completely, depending on when the injection valve is switched back to the Load position.

For the Dionex FAMOS and WPS-3000 micro autosamplers, Chromeleon supports the Low Dispersion Factor (LD Factor). This factor allows you to determine when the injection valve is switched back. Thus, the factor determines the amount of the original sample volume that is injected. A value of 1.0 means that the switching valve is switched back when the entire original sample volume was injected. This means that, in the middle of the capillary, the entire sample liquid is injected and that, on the capillary walls, sample liquid is cut-off:

(* Solvent in the example - can be transport liquid or any a similar liquid, instead.)

If the value is greater than 1.0, the injection valve is switched back after the injected volume is greater than the original sample volume. Thus, in the middle of the capillary, not only sample is injected. Simultaneously, the amount of sample that is cut off on the capillary walls is minimized.

If the value is less than 1.0, sample liquid is cut off in the middle of the capillary, too. In this way, a vertical injection profile appears at the end of the sample plug, i.e., the tailing part is completely cut off during the injection. However, this process can avoid neither tailing caused by dead volumes or overloading of the separation column nor chemical tailing during the separation. Nevertheless, with the same conditions, peak tailing is reduced by a factor of 1.2 to 1.4, compared to common injections.

The formula for calculating the time after which the injection valve is switched back to the Inject position is as follows:

Where: 

tI   = time [s] that the valve is in Inject position

f    = low dispersion factor

Vp = original sample volume in the loop [µl]

Fv = flow rate [µl/min]

The time that the valve is in Inject position must be at least 10 s. If the calculated switching time is less than this limit, it is reset to 10 s.

Low dispersion injections are possible for full-loop and partial-loop injections as well as in user-defined programs. In Chromeleon, the low dispersion factor can be any value between 0.01 and 100. For example, if the value is 1.3, the valve is switched back when 130 % of the value of the original sample plug has been injected. Meaningful values are usually between 0.7 and 2.0, i.e., between 70 % and 200 % of the original sample volume.