Chromatography Forum

Been experiencing a pretty significant and consistent retention time shift of a volatile analyte when toggling between two Agilent GC's. The column is the same column, same inlet liner, same operating conditions, but on one instrument the analyte has a retention time of 3 minutes and on the other GC the peak of interest has retention times of 4.8 to 5 minutes. Thought the differences may be due to a leak somewhere, but this has been going on for months, with many column installations and consumables being replaced. Leak test shows no leak in inlet, gold seal, etc. Split vent trap and split vent line replaced. I have checked all column dimensions entered. Any ideas of what could be responsible for this consistent retention time shift?

"Once upon a time .................. and the driver said, officer, I was only going 55."

Just because you set a number into a keypad connected to a computing device controlling a piece of hardware, doesn't mean the hardware is actually doing what you set it to do.

Check flows. Check temperature. {Both by a hand-held, third party device.}

[ I assume the GCs are in the same lab, at the same distance above sea level. ]

When you assume something to be factual, and you don't tell me the 'facts' are assumptions, then
it is possible to make an ASS of U + ME.

Never ASSUME.

best wishes,

Rod

I'll second what Rod says - verify all the operating conditions.

This exact problem came up on this forum in the not so distant past, try searching the archives.

And it is difficult to offer good trouble shooting advice when you do not tell us what the analyte is, the column(s) or any of the operating conditions - the only hard facts you give are the manufacturer of the instruments and the two retention times.

Peter

Are you sure its the GC conditions and not because one column has degraded.

Have you tried swapping columns between kits?

same column

same inlet liner

same operating conditions

toggling between two Agilent GC's.

consistent retention time shift

Also, what analyte?

Two different instruments will not always perform identically.

John

Stupid question same gc method? Is one constant flow while the other is constant pressure.

And have you checked the calibration of the inlets for readign pressure exactly. If yoy cannot do that measure the dead time of the column (methane with a warm GC oven and inlet - or if you do not have natural gas available, you can use butane from a cigarett ligher - if you can find one, but use a failry hot GC oven.) and see if an unretained compund moves through the instruments at the same speed.

When I checked the flow using butane, the unretained compound had a retention time almost double what it should be for a 15M .530mm column. With the GC set to a linear velocity of 24, the actual average linear velocity was found to be 13.5 cm/sec. This explains the almost doubling of retention times. Could this large of a discrepancy be corrected by simply zeroing the EPC module?

First check to be sure that the column configuration inclulding detector is correct for the column. It is not that hard to accidentaly turn vacuum correction off or on in the wrong place. And be sure that columns one and two are set to different inlets. Off the top of my head this seems to be a fairly good sized jump for a re-zero. (I've seen both columns set to one inlet accidentally - and it can really mess things up, even though the settigns for the one column actually installed i nthe instrument are correct.)

Also check that you have set the correct carrier gas in the GC control software - different gasses need different pressures to get them flowing at the same velocity.

Peter

Thanks for everyone's input. I had some time to check some configurations and still can't pinpoint the cause of the excessive retention times for the analyte (EtOAc) / unretained compound. Using the same column and inlet liner on 3 different instruments - Agilent 5890, 6890, and 7890... It is the 6890 that is experiencing the excessive retention times. I went over the methods on all 3 instruments in hopes of finding an operating condition I may have overlooked. So far I've checked:
leaks (none using a pressure test), column dimensions, correct carrier gas selected (He), correct configuration of inlets/detectors, vacuum correction off, constant flow on all methods, same temperature program. Changed the split vent trap and split line.

The unretained compound, butane, has a retention time of almost double on the 6890 as compared to the 5890 and 7890. On the 6890, linear velocity was set at 24 and was calculated to be 13.5.

Could there be a delay in integration, and if so where can I check for that in Chemstation?

Could there be a valve of some sort that is getting partially stuck upon injecton?

I have not fooled around with recalibrating the EPC because I am unfamiliar with this process. Is this something that I should look into next? What are the caveats when re-zeroing EPC? I'm scratching my head on this one so any insight is much appreciated.

Note the pressure reading on each GC at start run time. If they show as being the same, get a pressure gauge and measure the head pressure. (They make - or at least used to make pressure guages for GC's).

An other thing to help sort this all out. The 6890 should gie you the linear velocity of the column. Since you know the length of the column, you can compute the elution time of the butane, assuming that it unretained. My guess is that one GC is close to the expected retention time (assuming a warm GC oven to keep from significantly retaining the butane) and the other will be off. This will point a finger at a GC.

Don,
I did calculate the linear velocity for the 6890 and it was 13 whereas the gc display read 24. The question is why...

Just to be absolutely sure - when you say the same column, same inlet liner etc etc do you mean that you have put the same physical column into each instrument, or that each instrument has its own column, and that they are the same length, diameter and stationary phase ?

What type of inlet iner(s) are you using. ?

Peter

The same physical column. The liner is agilent 4mm with glass wool...#5183-4647 and is used in all 3 instruments.