99% loss of sensitivity in GC/MS!!!

[MSCHemist]

I thought it might be the split valve stuck open so stuck a flowmeter on the split vent and it read 0 ml/min for 1 min while doing a splitless injection so no help there.

Due to the way that back-pressure regulated inlets (which I am pretty sure the Agilent has) are plumbed, there should be flow out of the split vent all the time. Splitless transfer to the column is generated by rerouting the gas flows inside the plumbing - this makes it infuriatinlgy difficult to troubleshoot back-pressure regulated inlets.

Try a split injection - about a 50:1 split of a 1 mg/ml solution will give you 20 ng per peak which should be easily detectable.

Peter

I vent, clipped and rehooked the column to the MS and tried it again. No Change.

I tried injecting a 4 ug/ml test standard 50:1 split and didn't really see anything (with a split liner). I also injected the same mixture 1ul splitless 1 min before purge and got pathetic whimpy little peaks I should see peak heights ~100,000 instead I am seeing barely 1000.


tune and air check still look fine. I have flow through the column (I dipped the end into a vial of methanol and saw a fast stream of small bubbles. I am running out of theories. THe inlet is clean and all the consumables are new (gold seal, septa, liner)

[Peter Apps]

The two possibilities are that the sample is not getting onto the column, and the MS is properly detecting what it sees, or that the MS's sensitivity is very poor.

To make sure that sample gets to the column; unscrew the inlet nut and feed 1 ul of the split test mix from a syringe direct into the column - all you need to do is to depreess the plunger with the needle tip touching the end of the column, the vacuum from the MS will suck the sample into the column. Then reconnect the nut. close the overn door and start the run. The oven door sensor on the Agilent GC is your mortal enemy. Because you have sucked air through the column, keep the filament switched off for a longer than usual time - aim to have it come on just in time for the first non-solvent peak in the mix. If you get huge peaks the MS is OK and there is very probably a problem in the inlet. If tiny peaks the MS is sick.

Peter

[MSCHemist]

The Agilent service engineers are as perplexed as I and everyone here. However, they think it has to be the column because of the tests I did on the inlet flows, ALS, and the MS tunes are fine with good abundances of all the PFTBA ions. He said he saw a column once that had a small fracture at the head and did not show an air leak on the MS due to the positive pressure on the top half. I am going to try a new column tomarrow.

He suggested I unhook the column and plug the head with a septa and leave it for 45 min then do the air/water check.

[Peter Apps]

If it is a leak it is causing a 100:1 split at least - I doubt that you can get a 200 ml/min flow out through an invisible crack in a column, but anyway - if there is a crack in the column it will break properly if you flex it - wiggle the coils around a bit.

Peter

[MSCHemist]

If it is a leak it is causing a 100:1 split at least - I doubt that you can get a 200 ml/min flow out through an invisible crack in a column, but anyway - if there is a crack in the column it will break properly if you flex it - wiggle the coils around a bit.

Peter

I think I found the issue. The outer repeller insulator on the MS was cracked all over the falanges. I decided to clean the source while conditioning the new column. I also previously noticed it was missing the belleville nut as well but disregarded it as it had been operating for some time without it. I will order one for next time. I have a replacement insulator in house.

[Peter Apps]

If it is a leak it is causing a 100:1 split at least - I doubt that you can get a 200 ml/min flow out through an invisible crack in a column, but anyway - if there is a crack in the column it will break properly if you flex it - wiggle the coils around a bit.

Peter

I think I found the issue. The outer repeller insulator on the MS was cracked all over the falanges. .

So it was a crack after all , just leaking electricity instead of gas !

Good work.

Peter

[cleh]

The 6890 split/splitless injector is a back pressure injector so there should be no change in flow out of the split vent when the solenoid switches. I wouldn't install a new column in the instrument until the split flow issue is solved. Check the split line from the weldment to the vent to make sure it is clean. Even if the flows are good, there may be enough build up there to act as an active site. The split line has a split vent trap in it that may have to be changed. Make sure the carrier gas and septum purge lines in the inlet weldment don't have pieces of septum in them. I'd do a complete check and cleaning of the injection port and lines. If you don't remove the seal, particles from the injection port walls may end up on the seal. I'd rather lose a seal than a new column.

[gcmswhisperer]

I'd go with the cold spot theory, in the 6890 and 7890 inlet there is a "thermal nut" that squeezes everything together giving good thermal conductivity throughout the inlet. Over time and repeated temperature cycles this nut can loosen up and cause a cold spot. You'll need to take off the cup and insulation where the column attaches to the inlet. Then remove the nut that holds the gold seal in place (I think this is a 1/2" fitting) and under that is the thermal nut (I forget the size), give it a good snug turn and then reassemble.

Also, (I've seen this before) where someone has a loop of column that unsprung from the coil and is hanging out of the oven.