I have an identified impurity in my method. This is partly overlaid with matrix peaks and there is not enough resolution for quantitation. I don't have a standard either. I can separate the peak by changing the gradient but at the same time I move my active further on and the peak gets too broad for quantitation. I need to start to quantify this impurity. What can I do? I could make two injections for each sample; one for active using the old gradient and one for impurity using the modified gradient. But I can't quantify my impurity using the response factor of the active, cause it is a different method, can I?
Do I need to start looking for an internal standard for the impurity method?

If the impurity elutes before the parent, you could change the method to get a sharper peak for the parent.

I can get a separation for the impurity by slightly slowing it. At the same time I move the parent further on and the peak gets too broad.

Haven't managed to get a separation by speeding things up.

Isn't gradient an option? Or adding some THF to your mobile phase? Or changing your organic compound? Changing your column may work in some cases,.. Maybe there are some options to work on.

I don't know what kind of separation you are working with, maybe you can gives us some more details.

It appears that once your impurity elutes, you can speed up the gradient to sharpen the peak of the parent. You nedd to think about the following: the width of the peak depends on the gradient slope at the point of elution of the peak. The gradient slope is generated in your gradient mixer, but your peak is generated at the end of the column. You need to determine the time difference between the gradient mixing and the exit of the column. Once you knwo this, you can program the gradient to do what you want to do.

Some more details will be really helpful! What kind of column do you use at the moment? I think you need a other kind of column with a high selectivity for substances with nearly equal chemical and physical behavior. May be you can't solve your problem only about the different solubility between the mobile and the stationary phase. You need a structural discrimination.

I have allways assumed that any further retained peak gets broad (at some point)...Are you saying that it is possible to get a sharp peak for higly retained peak by adjusting the gradient slope only at the point of elution? Isn't peak broadening a phenomena that takes plase IN a column??

In a gradient the most important think that determines the sharpness of the peak is the gradient slope at the point where the peak elutes. Peak broadening in a gradient is determined for the most part by the solvent conditions when the peak elutes.