By Anonymous on Friday, January 3, 2003 - 11:20 am:
Is it acceptable to change the flow rate within a single gradient method?
By Anonymous on Friday, January 3, 2003 - 03:11 pm:
If this is the only change you make, you may see peaks moving around
By bill tindall on Friday, January 3, 2003 - 04:04 pm:
I am curious why you would want to change the flow rate...
The answer to your qustion is of course you can change anything if changing it imporves something without sacrificing reproducibility.
for best reproducibility one usually chages as little as possible
By readski on Saturday, January 4, 2003 - 05:47 am:
We have done this to produce quick response for a customer. There are some issues with baseline and I would not recommend this to others unless time is an issue.
By B.Buglio on Wednesday, January 8, 2003 - 04:38 pm:
Be aware that changing the flow rate can result in
an effect similar to a change in capacity factor -
changing the resolution and in the worst case
changing the elution order of some peaks.
By Alex on Thursday, January 9, 2003 - 07:13 am:
Do you just want to change the flow rate ? That call for trouble: Dwell time will change and gradient slope as well - thatwill result in moving retention times and resolution.
Or are you going to change flow rate AND gradient duration? If the volume of solvent pumped during the gradient is the same, the chromatogramms might look similar.
By Russ on Thursday, January 9, 2003 - 10:40 am:
Are the peaks eluted during the gradient quantitated? If the purpose of the gradient is to elute strongly retained material which is not quantitated, increasing the total flow rate would help elute this material sooner which could decrease analysis time. If the later eluting peaks are quantitated there could be problems, as the others have already mentioned.
By E. ESTIME on Tuesday, July 29, 2003 - 11:51 am:
Does anyone know what can be the cause of the base line ramp that I’m having when I try a gradient run of acetonitrile(ACN)/water 40:60 to 100:0 ? The column that I use is a C18 4.6mm X 15cm, dp 5um.
The baseline ramp seems to follow the increase in concentration of the ACN. I don’t want to believe that it’s impurity because I filter the solvents and the ramp doesn’t show when the run is isocratic no matter the solvents proportion that I use (40:60, 50:50, … )
Thank you.
By Anonymous on Tuesday, July 29, 2003 - 01:16 pm:
E. Estime -
What is your detector wavelength? I believe the uv cutoff for ACN is around 190nm but any impurities (filtering won't necessarily remove dissolved organics) can easily increase it to the 215 - 220 nm range. If possible try a higher wavelength. Good luck.
-Jeff
By K.H.W. on Tuesday, July 29, 2003 - 10:26 pm:
Just go to the spectroscopy lab and have a UVis run of your ACN. This will clearly tell you the cutoff.
By Anonymous on Wednesday, July 30, 2003 - 05:12 am:
Different brands-batches of AcN differ in the UV cutoff mentioned above.
You will have some baseline drift as you increase the AcN concentration as water does not have the same UV spectrum as AcN.
The amount depends upon the quality of the AcN and the wavelength of UV light you use.