Inconsistent solvent tail

[cjm]

OK here's my latest headache: I will try and upload chromatograms later today. I am running a 5890 split injection using an EPC..methylene chloride is the solvent. 90% of chromatograms have a very nice, even, low baseline over the course of a 12minute run, ramping from 50C to 320C, but about 10% (at random..) will show a "tail" (if that's even what this is..) for nearly the entire run. What makes this problem difficult for me to diagnose is that the response for all compounds of interest remain consistent. I am quantifying not only single target compounds, but also a hydrocarbon range and the added area is significant and unacceptable. If this is a split flow problem I would expect to see increased response during the bad runs. Could this be a symptom of an EPC valve starting to go bad? Could this be a problem with the detector board? Any thoughts? Thank you.

[chromatographer1]

Such variability suggests a particle on the column tip or some other contamination in the injector. It is certainly possible that with a vent partially blocked you could have solvent backflashing into the colder parts of the injector and continually bleeding solvent onto the column during the run.

Service the injector thoroughly. Something is dirty or blocked or you would not have this problem.

Check your column with a light to see there are no deposits or partilces inside the column.

best wishes,

Rod

[cjm]

Rod,

Thank you for the response. This problem has shown itself before, but only on the front setup. The chromatography on the back is always ok. Also I was noticing this problem for the past couple days and decided to do maintenance last night. I clipped a meter of guard, replaced the liner, wool, oring, and septa, and gold seal. The chromatography looks as it did before the maintenance. Wouldn't you think that if it were contamination that it would be more consistent? I like your idea that it may be solvent backflashing and continually bleeding onto the column. That is really the best way to describe the chromatography I see.

[chromatographer1]

Remove the trap on the split vent.

See if the problem goes away.

Reduce the amount injected by 5x or so. Does that solve the problem?

check manually the flows out the vent. Is the flow appropriate?

You may wish to replace the tubing from the flow controllers to the injector, or clean it thoroughly.

Check the flows on the septum purge. Are they correct?

Just follow your manual for the correct flows and cleaning procedures and you will find the problem.

GOOD LUCK,

ROD

[cjm]

So our lab director suggested first that it could be an electronic issue, something with the detector board..So to isolate inlet from detector I switched the columns in the two detectors (front inlet to back detector and vice versa). I ran 20 pairs of solvent blanks and there was some inconsistency with the baseline, but still on the front. So I ruled out the inlets, which was a surprise to me. I figured I would rule out the detectors..So I dismantled the front detector, cleaned all the parts, FID collector, insulation, jet, etc...replaced everything the way it should be, and....it seems to be working just fine. I suspect now perhaps a piece of junk in the jet or somewhere in the detector setup. Perhaps now I've seen everything? Ha, wishful thinking...Thanks Rod for your help. I'm glad it didn't come to replacing lines and traps unnecessarily..guess I will always assume simple contamination first.

[Peter Apps]

That is a surprise, thanks for the feedback - my money would have been on crud in the inlet. Presumably you are getting some soot in the detector from the chlorinated solvent. Increasing the air flow can help a bit, but at the cost of slightly reduced response.

Peter

[chromatographer1]

Piece of broken column or a particle dislodged from the injector can pass through the column until it reaches the HOT point at the detector where it can melt sticking in place. Then solvent hits it, it acts like a sponge and slowly desorbs the solvent.

I can believe your story.

It pays to trouble shoot. Learning how (in this case to eliminate the injectors) is valuable and satisfying when you solve the problem.

Glad for the happy ending.

Rod