Chromatography Forum

I am using IC-Pak anion HC 4.6 x 150mm, isocratic with 70mM KH2PO4 10% ACN, pH 5.5, flow 1mL/min, to analyze nitrate, which comes out at 18 min.

To shorten the run, can I use 2mL/min flow to fast flush for 7min, then run with 1mL/min till the peak comes out?

Thanks in advance for your comment.

Why do you use flow gradient?
If i were you i would apply 2 ml/min for the whole injection..

I worry if I use 2mL/min all along, the interference peak around the target peak will be too close to it and peaks will not be sufficiently separated.

increase buffer concentration and decrease pH, use shorter column,use 3.2 mm column with 1 ml/min flow rate formally doubling flow rate relatively to cross section of the column

I worry if I use 2mL/min all along, the interference peak around the target peak will be too close to it and peaks will not be sufficiently separated.

How so?

I worry if I use 2mL/min all along, the interference peak around the target peak will be too close to it and peaks will not be sufficiently separated.

Don't worry and just try. You will see if it's possible...

Thanks for your all your comments.
Another point, doubling the flow rate means decreasing peak area by half?

Thanks for your all your comments.
Another point, doubling the flow rate means decreasing peak area by half?

For UV-detection? Yes.

Thanks for your all your comments.
Another point, doubling the flow rate means decreasing peak area by half?

For UV-detection? Yes.

Yes I use UV detection, so I cannot increase the flow rate and sacrifice sensitivity around my target RT.

Ok, I've got to jump in here to clear up some misconceptions.

First of all, when you double the flow rate, the resolution will decrease only slightly. Resolution is the ratio of retention time difference to average width. When you double the flow, both values are cut approximately in half. In practice, the width doesn't quite get cut in half because of lower efficiency, but the loss in resolution is typically only around 10-15%.

Second, sensitivity is a matter of signal to noise ratio. When you double the flow rate, the peak area changes only because of the lower residence time in the detector. The peak height -- which is what determines signal/noise -- stays approximately the same (as in the preceding paragraph, there is actually a slight loss due to lower efficiency). In practice, your detector will probably be a bit noisier at the higher flow, but there's no way to tell how much without doing the experiment (it may be only a minor effect).

Thanks for clarifying my misconception.
I would try by experiment.

Just out of curiosity, does anyone use flow gradient for HPLC?
What are the shortcomings or benefit (if any) of flow gradient?

people do other weird stuff to . In my opinion if you don't have correct tools you start inventing ways to use incorrect tools to do the job. This is one of the cases

What are the shortcomings or benefit (if any) of flow gradient?

It's been done. The major shortcoming is that peak area is essentially the product of absorbance and residence time in the flow cell. Changing the flow adds error to the "residence time" part . Benefit? Not much. As I indicated in my earlier post, today's columns really have a fairly shallow efficiency vs flow curve (at least for small molecules; biopolymers are a different story), so you don't gain much by slowing the flow at the beginning of the run. In our method development courses, i tell people to set the flow rate as high as possible consistent with their level of paranoia about pressure, baseline noise, and column lifetime.

I appreciate your explanation.