Retention Time Issues

[bdh]

I am running an HP 5890 II with an FID detector for DRO analysis. The chromatography is great, but I am having retention time shift problems. I can run standards all day with great reproducibility within that day. The next day I come in and run the same standards and the retention times have shifted to the right about 0.10 to 0.15 minutes. Again the analysis within that days period are great, but again I come in the next day and it shifts another 0.10 to 0.15 min to the right.

Any ideas??

[Schmitty]

What are your instrument conditions?
Are you following a specific published method?

Off hand I would guess that you have junk on your liner that is not removed effectively by your instrument. Do you routinely change the liner or septa?

[Peter Apps]

If all the peaks are shifting by more or less the same amount it is probably a carrier gas flow control problem, or the oven temperature control.

What do you do with the gas overnight ?, is the inlet carrier pressure constant from day to day ?

When did you last check for leaks ?

What temperature programme are you using ?, while you are telling us that you might as well give all the instrument conditions and what the analytes and solvent are - "DRO" does not mean anything to me.

Thanks

Peter

[Chromatocrat]

bdh:


There could follwing few reasons for this problems:
1. Minute leaks at the carrier lines- at septa, o-rings, column ferrules.- <B>Replace septa & o-rings if required, tighten ferrules.</B>
2. Improper gas reulation- <B>Check & verify the flow.</B>



regards

[bdh]

OK...let's see. The method is 8015B. Column RTX-5MS 30mX0.32mmX0.25um.

Injector Temp - 250C
Detector Temp - 325C
Initial temp - 60C, hold for 3 minutes
Program - 60C to 95C at 10C/min hold 2.5 min.
95C to 270c at 33C/min hold 2 min.
270C to 335C at 32C/min hold 5 min.

Helium is used as the carrier gas
Total Flow - 30 ml/min
Septum Purge - 1-2ml/min
Head Pressure - 16 psi
H2 - 30 ml/min
Air - 370 ml/min

Solvent is DCM
I am looking for gas, diesel, and oil range organics.

I have checked for leaks numerous times because that was my first thought. Inlet insert has been changed along with the septum. The H2 and Air are turned off over night, but the helium is left on all the time without changing any of the flow regulators.

[AICMM]

BDH,

Always to the right (later?) From your post I am assuming analog pressure control and not EPC?

Long shot, leave the flame on overnight. Also, where are you turning off the gases, at the manifold block or at the cylinder?

No decrease in sensitivity, only RT, right?

Best regards.

[bdh]

Yes, always to the right and it is the entire chromatogram that shifts to the right and the same amount.

Yes, analog pressure control.

No decrease in sensitivity.

I am turning the H2 and air off at the cylinders and manifold block. The He stays on all the time.

[Peter Apps]

Hi BDH

You have some pretty steep temperature ramps in your programme that the oven might have trouble reproducing, but I would expect that to give random variability, not a day to day change with stability in between.

What is your carrier gas linear velocity ? (inject methane, propane or butane and measure the retention). Does this change when the amalyte retention time changes ? If the RTs always shift later, are you seeing a progressive increase in RT, and by how much over how long ?

It is a long shot, but with the heavy samples and high temperatures you might be crudding up the flame tip, which would give a bit of extra back-pressure on the column.

Peter

[chemikas]

Peter, you right (at least in my case)

I had similar problem. In 2 month RT of solvent CH2Cl2 inreased about 1 min. for program 50 C - 250 C, 20 C/min, and 0.6 min for program 120 C- 330 C, 30/min.
My samples usually heavy to very heavy.
I've changed every changeble part in inlet, washed the split inlet vent line, but nothing helped until I chaned the jet.
Butane before changing the jet had very big RT - 2-3 times greater than it should.
The old jet was absolutely blocked - I hardly brushed it with het brusher.
Now I am OK (and I feel good)

Good luck everybody.

[bdh]

Linear velocity is 24 cm/sec and appears to remain constant from day to day.

It is a progressive increase of about 0.10 to 0.15 minutes per day. I don't know how long this would continue for.

I have cleaned the FID detector recently. I don't know how fast it would "crud" up.

[AICMM]

BDH,

Considering everything on the front sounds basically okay, the only other comment I would add is to add to what was said earlier. Anything that causes a back pressure, however slight, will increase the retention times of the analytes of interest. This is why a jet becoming clogged would cause an increase in retention times.

Best regards.

[Peter Apps]

Somewhere there is a blackage or a source of back-pressure - the inlet pressure is high for a 30 m column (even for the high temps you are using), yet the linear flow is slower than optimum.

When you clean the detector, do you clean iside the jet ? Also try pulling the colum back about 2 mm out of the detector - its tip might be sitting too close tothe detector tip orifice. Some 5890 jets had small i.d. of the stem that is too tight a fit to the column - it should be a loose fit.

Peter