By Anonymous on Thursday, April 3, 2003 - 04:08 am:
I have a following question.
When running a gradient with acetonitryle and water (20:80 to 70:30) what is the best solution:
1. bottle A 20:80; bottle B 70:30; gradient 0-100% B
2. bottle A 10:90; bottle B 80:20; gradient 15-85% B (more or less – I didn’t calculate it)
3.A pure ACN; B pure H2O; gradient 20-70% B
From what I know number 3 shouldn’t be used (at least not pure ACN), but what about the other two? Does it depend on equipment I use? I work on Merck LaChrom low pressure gradient pomp L-7100.
Regards
By Jan on Thursday, April 3, 2003 - 05:09 am:
Anonymous,
I do not have any experience with Merck pumps but normally number 1 should work fine. Do not bother to calculate the number 2 proportions...
Jan
By Anonymous on Thursday, April 3, 2003 - 06:17 am:
eternal question...
use no 3 if you don't need a perfect baseline and if you want to avoid differences between analysts etc. Pump will do it for you almost identically every day. You'll also get an advantage that you can add mobile phases during the run. Just dont pump 100% water to the column and you should be fine.
use no 1 or 2 if you need a perfect baseline and low limit of quantification.
By Alex on Thursday, April 3, 2003 - 06:46 am:
I have used the L7100 with 30% MeOH and (MeOH or 70% MeOH). My problem was not mixing quality. It was degassing of solvents. If you have an in-line degasser or use helium, everthing should be fine. Try to run a gradient with low wavelength detection (200 nm for acetonitrile) and look what the baseline looks like. Even stepgradients 1, 2, 5, 49, 50, 51, 95, 98, 99% (Methanol at 205 nm) were fine on the L-7100. Trying to reproduce this on other equipment I soon went to 0.1% acetone solutions.
By wanda on Friday, April 4, 2003 - 07:49 am:
There is always a question about the ability of pumps to start up at 0% in a gradient. In general, I design gradients so that the organic and aqueous are mixed. This is important when the methods are for a QC lab that has a large number of instruments in various states of maintence.
By judd on Tuesday, April 8, 2003 - 12:35 pm:
I've done it using method #3 for years with good success - this works well for systems using He sparging as it keeps conditions constant.
I usually only use the other methods when I have a very shallow gradient or one at the extreme high end (>90% of any one componenet) that I want to have precisely metered.
Chris