Chromatography Forum

Hi!
I'm new in the Forum and in liquid chromatography.My task is to analyse PAH in drinking water but the retention times are changing every injections. The separation temperature is ambient. I do not have any temperature control device. Maybe this is the problem.
Have any idea to solve this retention time fluctuation?
Thanks

P.S.: Sorry for my english!! I have to practise a lot!

This is a relatively straightforward analysis. Are you using an established method?

More info please - good for the practice

There are many possibilities, we really need more information to know where to start. It may be as simple as putting in a longer time to re-equilibrate the column after each run, or it could be a comples issue.

Thanks for the replies!
The method I'm using: sample preparation: Resprep-C18-47 SPE Disk 5ml MeOH soaked for 5 minutes
5ml Water pulled through
1L sample pulled through, then allowing the disk to dry
5ml acetonitrile pulled through.

Analytical measurement conditions:
Column:Lichrospher PAH 250*4.6, 5um
Mobile phase:acetonitrile
Flow: 1ml/min, isocratic
Temperature: ambient
Detection: Fluorescent, program:initial: Ex Em
280 450
1' 280 450
6' 290 430
12' 290 410
14' 300 500
31' 280 450
I was given this method but the results I get aren't the same as they were by my "anchestor". Moreover each of my injections are different from each other and I don't know why.
I hope you can help me according to these details.

how much do retention times change?
are you really using isocratic 100% ACN?

HI!
By my colleague one run took 10 minutes. But in my case it takes 21 minutes.
For example when I do a run on Monday the Rt=10.7 minutes for one component, but on Tuesday the same Rt=5.6 minutes (the same component).
Yes, I really use 100% ACCN isocratic. Is there any problem with that?

100% ACN is very strong eluent - I would think that all componenets should be eluted at about dead time, I think that there is some error that has been passed along the way

i would suggest to do a literature search and use one of the method found with a gradient elution preferrably and using new column

methods i found using google search use gradient elution

then you could compare results

if you can post these 100% ACN chromatograms, maybe it would be useful

wrapping it up - i think there has been a mistake made somwhere

Which PAH are you analysing?
Usually 16 PAH's are analysed but with your elution beginning with 100 % Ch3CN is not possible! The first 8 components (till pyrene) will elute in void volume (or nearly). Heaviest components (from benzo(a)pyrene to indeno(1,2,3)cd pyrene) can be separated with your isocratic program.
A difference in rentention time from 10.7 ' to 5.6' cannot (only) be due to a temperature difference (Personnaly, I saw retention time shift less than 1 minute between 30 and 40°C).
I think it should have been a chromatagraphic problem. I advice you to compare the pressure between your different injections. It should be stable. If not the system is not enought conditonned or perhaeps your sample can clog the column or frits. Also perhaeps your column is simply dead!
It's not possible to elute the 16 PAH's from your column with your conditions in 10 minutes. 21 minutes is more reallistic (a bit high, I would expect 18 min). I don't understand your fluorescent program. You speak about a run in 10 minute and the fluorescent program change at 12, 14 and 31 minutes! Please check your data, it should be a mistake somewhere.