no retention on new column

[AICMM]

Comments to toss in the hat. One, at very low pressures, I have observed EFC units pulsing on some detectors. In my opinion, they don't do really well at really low pressures. When we installed a much bigger downstream restrictor, the EFC unit did much better. Second, ECE, the point is to measure linear velocity of the gas through the column. Thus, you want an unretained peak that moves along at the speed of the mobile phase. Volumetric flows can be decieving at very low flows. And, it is very easy to do as Peter points out. Third, you had a narrow heptane peak and you worked to widen it. Not what we traditionally look to do in chromatography. We generally always want to maintain as narrow a peak width as we can! So, the earlier flow rates were probably more appropriate. Fourth, you have not said whether you are running split but I assume you are, so your heptane peak should be wickedly narrow, not broad. Finally, have your tried anything heavier at a decent level, say octadecane at 200 ug/mL or so? See what kind of peak you get for that. Nice, heavy compound, not really reactive, good responder, ...

Best regards.

[GasMan]

AICMM,

You guessed correctly in your first posting, this is biodiesel analysis, and both the ASTM and EN methods call out Cool On Column as the injector.

Gasman

[jf]

I cannot think of any way of accurately volumetrically measuring a volume flow of 3 ml/min through a column that is connected to a detector with the detector gasses flowing to match the back pressure that you get when running an analysis. This is the advantage of measuring linear flow rate in terms of the retention time of an unretained compound - you can do it with the column installed to the detector and under exactly the same conditions as you use for the actual analysis.

Peter

we actually use a bubble flowmeter. how close do you think this is compared to a column with no bubble flowmeter attached?

[Peter Apps]

The back-pressure from the bubble flow meter is not the problem. The problem is that to use a bubble flow meter you have to turn the detector gasses off (or disconnect the column from the detector), so you lose the back-pressure from the detector. With the very low inlet pressures that you have, the back pressure from the detector can make a significant difference to the pressure drop across the column, and therefore the flow through it. To convince yourself that detector back-pressure can influence column flow you can do an experiment (that does not require any extra equipment) - turn on the detector gasses, disconnect the column from the inlet and measure the flow coming backwards through the column.

Peter

[ece]

thanks peter! i see now, back pressure with detector gases flowing i can see how it would definately influence column flow in real time.

i also tried measuring the flow rate additively yesterday, by measuring all the flows individually and making sure they were correct, then measuring them all together and seeing if the flow was the same. the combined flow varied, which suggested to me that there are pockets of dead volume at times, and that is not the best way to accurately measure or determine the flow.

i understand now, thanks for the advice as always! happy holidays!

[ece]

at what temperature should those linear velocities be set at for the FID? 50 deg. C?

[Bigbear]

I will throw my hat in the ring by agreeing that your flow is way to low. I have Agilent's method converter and to get the equivalent chromatography from a 0.32 column @ 3cc/min, you would have to increase the flow to 6.2 cc/min on a 0.53 column.

[chromatographer1]

The flow for the method indicates that it should be 3ml per minute at the initial temperature.

That is for a 0.32mm ID column, not a 0.53mm ID column.

A 0.53mm ID column will require a much larger flow rate. It should not be kept at 3ml/min

Your pneumatics should also be set for constant flow, not constant pressure. Loss of analytes can be in the butt connector connecting the 0.53mm retention inlet gap to the analytical column. I assume you are using a direct injection method, and preferrably using a cold on column injection hardware with the slowest injection setting.

I hope your problems get sorted out.

best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rodney.george@sial.com