Sinusoidal baseline.

Hi ccbarnes5,

I have updated the tags for better visibility. Have you tried a new jet in the FID? It could be restricted with carbon. Also when you reply, it would be helpful if you could reply with your method parameters .

Regards

James

Oops.  The HS conditions are:

Curtis [Personal information removed by moderator]

At first glance I don't see much to be concerned about here with method parameters other than the gas saver flow is 15ml/min, 5ml below the suggested of 20ml/min. Has there been any maintenance done to the FID, jet change etc?

James

No.  There has NOT been any change with the detector; yet.  We’re going to try changing the FID.  I’ll take a look at the collector when the jet is changed.  The issue that’s really throwing us off is that it only happens AFTER the elution of the solvent (Dimethyl acetamide), when the oven temperature gets over 160°C.  AND it’s so consistent.  Subsequent chromatograms can be overlaid and that baseline will look EXACTLY the same over that time period (about 15.5 minutes – 20.0 minutes).

Curtis [Personal information removed by moderator.]

I checked with one of our column chemists and he doesnt feel that dimethyl acetamide should cause you any concerns. The amplitude flucuation is low, doesnt appear to be electronic. It maybe something else coming off. One test to try would be an injection without sample or solvent. Essentially a blank run from the headspace with a vial of just air to see what happens.

James

Ok. Now we’re cookin’ with gas.  After changing the jet we now have an improved chromatogram:

                BUT, we still have the sinusoidal issue.  I’ve asked the analyst to remove the collector and sonicate for a few minutes in acetone and MeOH.  HOWEVER, I was told we saw this ‘sinusoidal’ effect with this method on more than one instrument.  Can the dimethyl acetamide be affecting the stationary phase to produce this?

Curtis [Personal information removed by moderator.]

Def second the blank injection idea.  See if systemic or chemical.  Almost looks like something that builds up...  like the bulk of the material comes off but the remainder builds up pressure, then releases, builds up, then releases.. etc.  Perhaps a dead volume issue in your system plumbing?

I agree.  It looks very much like a pressure increase then sudden release repeating over and over.  But WHY during this one particular period?  Why don’t we see it throughout the chromatogram?  This pattern is seen only over this particular time/temperature range.

Curtis Barnes

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I agree.  It looks very much like a pressure increase then sudden release repeating over and over.  But WHY during this one particular period?  Why don’t we see it throughout the chromatogram?  This pattern is seen only over this particular time/temperature range.

Curtis Barnes

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Because it has to reach that temp/pressure in order for the material to appear in the first place.  Hence the reason you don’t see it prior to your large peak.  Once you’ve met that T/P “sweet spot” for this material, it is continually building up, or off-gassing, or whatever it’s doing.  If you don’t inject anything but monitor the signal as you gradually increase the oven temp manually, do you still see it?  You don’t even have to do a blank injection for this.  Just set your oven at the temp where your main peak there comes out, then gradually nudge it up.  What do you see?  If it’s something in your system, you’ll start seeing junk come off.  Though if you have a “pocket” somewhere in your plumbing, it could still be systemic but related to the chemical simply because you have to introduce it for the material to build up.

If you run a series of sequential blanks, do you see the same intensity or does this diminish over time?  I had a “similar” situation where I had a contaminant I could ID but not determine the source.  My first injection had a HUGE peak, but all following runs had a significantly smaller peak, but was VERY consistent.  It ended up being residue from new gas lines that built up in my system overnight, over the weekend—while my system was dormant and cool, but released when I fired it up Monday morning.  The steady peak I saw afterward was what was able to build up in my system during the cool-down btwn runs.

Have you introduced any new plumbing/gas/etc recently?

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