rough base line

[ym3142]

I am running a gradient method to detect unkown degradants from Meoh:1%HOAc = 10:90 to 80:20.



I am sorry I do not know how to upload my gram.
anyway, it seems that it was Ok before 30min but gave more than one noise peak each minutes after 30min(0.002 to 0.005Au).

Does anyone know the reason?
Thanks and Merry Christmas!

[tom jupille]

I am sorry I do not know how to upload my gram.

http://www.sepsci.com/chromforum/viewtopic.php?t=2617

A number of possible causes and cures for this type of problems are discussed in these threads:
http://www.sepsci.com/chromforum/viewtopic.php?t=2996
http://www.sepsci.com/chromforum/viewtopic.php?t=2946
http://www.sepsci.com/chromforum/viewtopic.php?t=2001
http://www.sepsci.com/chromforum/viewtopic.php?t=1706
http://www.sepsci.com/chromforum/viewtopic.php?t=891
http://www.sepsci.com/chromforum/viewtopic.php?t=110

[ym3142]

Happy Holidays, everyone.
Sorry, Tom and others:I reply late and I did not find the way to change my PDF chromatogram into a picture.

I've read all Tom's threads but not yet understand why.

I've injected just water many times. It repeated showed the rippled baseline after >50% methanol. It was perfect good before 50% MeOH. it looks that it has something to do with enough amount of methanol. would that due to outgassing?

I've tried different MWD and DAD paratmeters: band width, slit, with or without reference, and peak width. I havn't had much success.

Do you think it is due to MeOH higher compressibility (than water)?

Or I need to pre-degass MeOH though I do have degass with my aglient. Could I have hardware malfuctions?
Thanks,

[tom jupille]

Sorry, Tom and others:I reply late and I did not find the way to change my PDF chromatogram into a picture.

You need to copy the image you want from your pdf and then paste into a paint program (the free Paint progam that comes with Windows will work fine). Then save the image as a .gif file (that is one of the options for file type). From there, proceed per the instructions.

Now, on to the baseline problem. It might simply be UV-absorbing garbage building up on the head of the column and then being desorbed at particular MeOH levels. To test for this, run three "dummy" gradients (no injection). Equilibrate the column for your usual time before the second gradient, and then equilibrate for three times as lonag before the third gradient. Compare the size of your baseline ripples on the 2nd and 3rd chromatogram. If the peaks are larger for longer equilibration, then you have confirmed the cause. If they are comparable, then I would look for something like RI effects.

[ym3142]

http://tinypic.com/view/?pic=j8eb9d

this is my initial noisy gram . Hope this works. I thank Tom.

[ym3142]

sorry I did not try Tom's suggestion yet but paste my new resutls below.

This show two consecutive injections at different MP. Does this rule out the garbage theory?

[img][img]http://tinypic.com/j8fa5w.jpg[/img][/img]

this the best I obtained so far.

[img][img]http://tinypic.com/j8fcjl.jpg[/img][/img]

[tom jupille]

This show two consecutive injections at different MP. Does this rule out the garbage theory?

No, if anything it tends to support it. The only way to confirm the garbage hypothesis (I wouldn't go so far as to call it a theory! ) is to run the three gradients with differing equilibration times as previously suggested.

[ym3142]

after many injections of H2O under 1%HOAc:MeOH=50:50 TO 20:80 for 30min, either I have gotton rid of the garbage in the column or I had better detector parameters(band width 4; peak width 0.85; slit 4 and with reference 360nm on) I obtained a baseline even better than last one I posted.

Thanks, Tom. I will try your method when I got rippled base line next time.

[tom jupille]

Glad you got it working!